Montrose Environmental (MEG) 10-Q12 May 212021 Q1 Quarterly reportFinancial data

Company Profile

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2021	May 07, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2021
Document Quarterly Report	true
Document Transition Report	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Trading Symbol	MEG
Entity Registrant Name	Montrose Environmental Group, Inc.
Entity Central Index Key	0001643615
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		26,055,727
Entity Interactive Data Current	Yes
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	001-39394
Entity Tax Identification Number	46-4195044
Entity Incorporation State Country Code	DE
Entity Address Address Line1	1 Park Plaza
Entity Address, Address Line Two	Suite 1000
Entity Address City Or Town	Irvine
Entity Address State Or Province	CA
Entity Address Postal Zip Code	92614
City Area Code	949
Local Phone Number	988-3500
Security12b Title	Common Stock, par value $0.000004 per share
Security Exchange Name	NYSE

UNAUDITED CONDENSED CONSOLIDATE

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL POSITION - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
CURRENT ASSETS:
Cash and restricted cash	$ 10,641	$ 34,881
Accounts receivable—net	65,771	54,102
Contract assets	61,636	38,576
Prepaid and other current assets	8,830	6,709
Total current assets	146,878	134,268
NON-CURRENT ASSETS:
Property and equipment—net	34,941	34,399
Goodwill	282,199	274,667
Other intangible assets—net	157,315	154,854
Other assets	3,896	4,538
TOTAL ASSETS	625,229	602,726
CURRENT LIABILITIES:
Accounts payable and other accrued liabilities	52,431	34,877
Accrued payroll and benefits	22,564	21,181
Business acquisitions contingent consideration, current	50,364	49,902
Current portion of long-term debt	6,214	5,583
Total current liabilities	131,573	111,543
NON-CURRENT LIABILITIES:
Business acquisitions contingent consideration, long-term	16,971	4,565
Other non-current liabilities	2,514	2,523
Deferred tax liabilities—net	2,591	2,815
Long-term debt—net of deferred financing fees	169,425	170,321
Total liabilities	344,562	312,653
COMMITMENTS AND CONTINGENCIES
STOCKHOLDERS’ EQUITY:
Additional paid-in-capital	261,588	259,427
Accumulated deficit	(133,949)	(122,353)
Accumulated other comprehensive income	100	71
Total stockholders’ equity	127,739	137,145
TOTAL LIABILITIES, CONVERTIBLE AND REDEEMABLE SERIES A-2 PREFERRED STOCK AND STOCKHOLDERS’ EQUITY	625,229	602,726
Conversion Option
NON-CURRENT LIABILITIES:
Embedded derivative	21,488	20,886
Convertible And Redeemable Series A-2 Preferred Stock
NON-CURRENT LIABILITIES:
REDEEMABLE SERIES PREFERRED STOCK	$ 152,928	$ 152,928

UNAUDITED CONDENSED CONSOLIDA_2

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL POSITION (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Common stock, par value	$ 0.000004	$ 0.000004
Common stock, shares authorized	190,000,000	190,000,000
Common stock, shares issued	25,438,857	24,932,527
Common stock, shares outstanding	25,438,857	24,932,527
Convertible And Redeemable Series A-2 Preferred Stock
Temporary equity, par value	$ 0.0001	$ 0.0001
Temporary equity, shares authorized	17,500	17,500
Temporary equity, shares issued	17,500	17,500
Temporary equity, shares outstanding	17,500	17,500
Temporary equity, aggregate liquidation preference	$ 182.2	$ 182.2

UNAUDITED CONDENSED CONSOLIDA_3

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
REVENUES	$ 133,817	$ 61,031
COST OF REVENUES (exclusive of depreciation and amortization shown below)	95,316	44,398
SELLING, GENERAL AND ADMINISTRATIVE EXPENSE	25,000	20,519
INITIAL PUBLIC OFFERING EXPENSE		531
FAIR VALUE CHANGES IN BUSINESS ACQUISITIONS CONTINGENT CONSIDERATION	11,064
DEPRECIATION AND AMORTIZATION	10,769	7,560
LOSS FROM OPERATIONS	(8,332)	(11,977)
OTHER EXPENSE
Other expense	(574)	(29,830)
Interest expense—net	(2,688)	(2,593)
Total other expenses—net	(3,262)	(32,423)
LOSS BEFORE EXPENSE (BENEFIT) FROM INCOME TAXES	(11,594)	(44,400)
INCOME TAXES EXPENSE (BENEFIT)	2	(3,152)
NET LOSS	(11,596)	(41,248)
EQUITY ADJUSTMENT FROM FOREIGN CURRENCY TRANSLATION	29	(3)
COMPREHENSIVE LOSS	(11,567)	(41,251)
ACCRETION OF REDEEMABLE SERIES A-1 PREFERRED STOCK		(5,415)
CONVERTIBLE AND REDEEMABLE SERIES A-2 PREFERRED STOCK DIVIDEND	(4,100)
NET LOSS ATTRIBUTABLE TO COMMON STOCKHOLDERS	$ (15,696)	$ (46,663)
WEIGHTED AVERAGE COMMON SHARES OUTSTANDING— BASIC AND DILUTED	25,117	8,904
NET LOSS PER SHARE ATTRIBUTABLE TO COMMON STOCKHOLDERS— BASIC AND DILUTED	$ (0.62)	$ (5.24)

UNAUDITED CONDENSED CONSOLIDA_4

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF REDEEMABLE SERIES A-1 PREFERRED STOCK, CONVERTIBLE AND REDEEMABLE SERIES A-2 PREFERRED STOCK AND STOCKHOLDERS' (DEFICIT) EQUITY - USD ($) $ in Thousands	Total	Redeemable Series A-1 Preferred Stock	Convertible And Redeemable Series A-2 Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive (Loss) Income
Beginning balance at Dec. 31, 2019	$ (26,291)				$ 38,153	$ (64,404)	$ (40)
Beginning balance, shares at Dec. 31, 2019				8,370,107
Beginning balance at Dec. 31, 2019		$ 128,822
Beginning balance, shares at Dec. 31, 2019		12,000
Net loss	(41,248)					(41,248)
Accretion of the redeemable series A-1 preferred stock to redeemable value	(5,415)				(5,415)
Accretion of the redeemable series A-1 preferred stock to redeemable shares		$ 5,415
Stock-based compensation	1,150				1,150
Accumulated other comprehensive income	37						37
Ending balance at Mar. 31, 2020	(71,767)				33,888	(105,652)	(3)
Ending balance, shares at Mar. 31, 2020		12,000
Ending balance at Mar. 31, 2020		$ 134,237
Ending balance, shares at Mar. 31, 2020				8,370,107
Beginning balance at Dec. 31, 2020	137,145				259,427	(122,353)	71
Beginning balance, shares at Dec. 31, 2020				24,932,527
Beginning balance at Dec. 31, 2020			$ 152,928
Beginning balance, shares at Dec. 31, 2020			17,500
Net loss	(11,596)					(11,596)
Stock-based compensation	1,805				1,805
Dividend payment to the Series A-2preferred shareholders	(4,100)		$ (4,100)		(4,100)
Common stock issued	$ 4,456				4,456
Common stock issued, shares	506,330			506,330
Accumulated other comprehensive income	$ 29						29
Ending balance at Mar. 31, 2021	$ 127,739				$ 261,588	$ (133,949)	$ 100
Ending balance, shares at Mar. 31, 2021			17,500
Ending balance at Mar. 31, 2021			$ 152,928
Ending balance, shares at Mar. 31, 2021				25,438,857

UNAUDITED CONDENSED CONSOLIDA_5

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
OPERATING ACTIVITIES:
Net loss	$ (11,596)	$ (41,248)
Adjustments to reconcile net loss to net cash used in operating activities:
Provision for bad debt	508	6,333	$ 4,532
DEPRECIATION AND AMORTIZATION	10,769	7,560
Stock-based compensation expense	1,805	1,150
Fair value changes in embedded derivatives	602	29,627
FAIR VALUE CHANGES IN BUSINESS ACQUISITIONS CONTINGENT CONSIDERATION	11,064
Deferred income taxes	2	(3,152)
Other	50	(180)
Changes in operating assets and liabilities—net of acquisitions:
Accounts receivable and contract assets	(29,029)	(319)
Prepaid expenses and other current assets	787	(683)
Accounts payable and other accrued liabilities	3,183	(5,005)
Accrued payroll and benefits	(2,058)	(2,458)
Other assets		(603)
Net cash used in operating activities	(13,913)	(8,978)
INVESTING ACTIVITIES:
Purchases of property and equipment	(922)	(1,558)
Proprietary software development and other software costs	(204)	(102)
Cash paid for acquisitions—net of cash acquired	(6,272)
Net cash used in investing activities	(7,398)	(1,660)
FINANCING ACTIVITIES:
Proceeds from line of credit		50,453
Payments on line of credit		(37,275)
Repayment of term loan	(547)	(1,250)
Payment of contingent consideration and other purchase price obligations		(4,703)
Repayment of capital leases	(625)	(685)
Payments of deferred offering costs		(1,175)
Debt issuance costs		(127)
Proceeds from issuance of common stock for exercised stock options	2,185
Dividend payment to the Series A-2 shareholders	(4,100)
Net cash (used in) provided by financing activities	(3,087)	5,238
CHANGE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(24,398)	(5,400)
Foreign exchange impact on cash balance	158	36
Beginning of year	34,881	6,884	6,884
End of period	10,641	1,520	$ 34,881
SUPPLEMENTAL DISCLOSURES OF CASH FLOWS INFORMATION:
Cash paid for interest	2,500	1,745
Cash paid for income tax	305	64
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES:
Accrued purchases of property and equipment	594	613
Property and equipment purchased under capital leases	670	1,493
Accretion of the redeemable series A-1 preferred stock to redeemable value		5,415
Common stock issued to acquire new businesses	2,271
Acquisitions unpaid contingent consideration	$ 67,335	4,082
Offering costs included in accounts payable and other accrued liabilities		$ 49

Description of the Business and

Description of the Business and Basis of Presentation	3 Months Ended
Description of the Business and Basis of Presentation	Mar. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Description of the Business and Basis of Presentation	1. DESCRIPTION OF THE BUSINESS AND BASIS OF PRESENTATION Description of the Business —Montrose Environmental Group, Inc. (“Montrose” or the “Company”) is a corporation formed on November 2013, under the laws of the State of Delaware. The Company has approximately 70 offices across the United States, Canada and Australia and over 2,000 employees as of March 31, 2021. Montrose is an environmental services company serving the recurring environmental needs of a diverse client base, including Fortune 500 companies and federal, state and local governments through the following three segments: Assessment, Permitting and Response — Through its Assessment, Permitting and Response segment, Montrose provides scientific advisory and consulting services to support environmental assessments, environmental emergency response, and environmental audits and permits for current operations, facility upgrades, new projects, decommissioning projects and development projects. The Company’s technical advisory and consulting offerings include regulatory compliance support and planning, environmental, ecosystem and toxicological assessments and support during responses to environmental disruption. Montrose helps clients navigate regulations at the local, state, provincial and federal levels. Measurement and Analysis — Through its Measurement and Analysis segment, Montrose’s teams test and analyze air, water and soil to determine concentrations of contaminants as well as the toxicological impact of contaminants on flora, fauna and human health. Montrose’s offerings include source and ambient air testing and monitoring, leak detection and repair (“LDAR”) and advanced analytical laboratory services such as air, storm water, wastewater and drinking water analysis. Remediation and Reuse — Through its Remediation and Reuse segment, Montrose provides clients with engineering, design, implementation and operations and maintenance services, primarily to treat contaminated water, remove contaminants from soil or create biogas from waste. The Company does not own the properties or facilities at which it implements these projects or the underlying liabilities, nor does it own material amounts of the equipment used in projects; instead, the Company assists clients in designing solutions, managing projects and mitigating their environmental risks and liabilities at their locations. Initial Public Offering —On July 27, 2020, the Company completed its initial public offering (“IPO”) of common stock, in which it sold 11,500,000 shares, including 1,500,000 shares issued pursuant to the underwriters full exercise on July 24, 2020 of the underwriters’ option to purchase additional shares, at a price to the public of $15.00 per share, resulting in net proceeds to the Company of approximately $161.3 million after deducting underwriting discounts of $11.2 million. Additionally, the Company offset $4.4 million of deferred IPO costs against IPO proceeds recorded to additional paid in capital. These deferred IPO costs were directly attributable to the IPO offering in accordance with Staff Accounting Bulletin Topic 5: Miscellaneous Accounting. The Company’s common stock began trading on the New York Stock Exchange on July 23, 2020. Basis of Presentation —The unaudited condensed consolidated financial statements include the operations of the Company and its wholly-owned subsidiaries. These unaudited condensed consolidated financial statements are presented in conformity with accounting principles generally accepted in the United States (“U.S. GAAP”) and have been prepared pursuant to the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) that permit reduced disclosure for interim periods. The unaudited condensed consolidated financial statements include all accounts of the Company and, in the opinion of management, include all recurring adjustments and normal accruals necessary for a fair statement of the Company’s financial position, results of operations and cash flows for the dates and periods presented. These unaudited condensed consolidated financial statements should be read in conjunction with the Company’s audited financial statements as of and for the year ended December 31, 2020. Results for interim periods are not necessarily indicative of the results to be expected during the remainder of the current year or for any future period. All intercompany transactions, accounts and profits, have been eliminated in the unaudited condensed consolidated financial statements.

Summary of New Accounting Prono

Summary of New Accounting Pronouncements	3 Months Ended
Summary of New Accounting Pronouncements	Mar. 31, 2021
New Accounting Pronouncements And Changes In Accounting Principles [Abstract]
Summary of New Accounting Pronouncements	2. SUMMARY OF NEW ACCOUNTING PRONOUNCEMENTS Recently Adopted Accounting Pronouncements —The Company qualifies as an emerging growth company as defined in the Jumpstart Our Business Startups Act of 2012 (“JOBS Act”) and therefore intends to take advantage of certain exemptions from various public company reporting requirements, including delaying adoption of new or revised accounting standards until those standards apply to private companies. The Company has elected to use this extended transition period under the JOBS Act. The effective dates shown below reflect the election to use the extended transition period. In December 2019, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. ASU 2019-12 removes certain exceptions to the general principles in Accounting Standard Codification (“ASC”) 740 and clarifies and amends certain guidance to promote consistent application. The standard was adopted as of January 1, 2021 and did not have a material impact on the Company’s unaudited condensed consolidated financial statements. In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation: Improvements to Nonemployee Share-Based Payment Accounting. Under the revised guidance, the accounting for awards issued to non-employees will be similar to the accounting for employee awards. The new guidance is effective for fiscal years beginning after December 15, 2019. The standard was adopted as of January 1, 2020 and did not have a material impact on the Company’s unaudited condensed consolidated financial statements. In January 2017, the FASB issued ASU 2017-04, Intangibles—Goodwill and Other (Topic 350): Simplifying the Accounting for Goodwill Impairment. The revised guidance eliminates Step 2 of the current goodwill impairment analysis test, which requires hypothetical purchase price allocation to measure goodwill impairment. A goodwill impairment loss will instead be measured at the amount by which a reporting unit’s carrying amount exceeds its fair value, not to exceed the carrying amount of goodwill. The revised guidance was adopted as of January 1, 2020 and did not have a material impact on the Company’s unaudited condensed consolidated financial statements. Recently Issued Accounting Pronouncements Not Yet Adopted —In August 2020, the FASB issued ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40)—Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity. The ASU simplifies accounting for convertible instruments by removing major separation models required under current U.S. GAAP. Consequently, more convertible debt instruments will be reported as a single liability instrument with no separate accounting for embedded conversion features. The ASU removes certain settlement conditions that are required for equity contracts to qualify for the derivative scope exception, which will permit more equity contracts to qualify for the exception. The ASU also simplifies the diluted net income per share calculation in certain areas. The new guidance is effective for fiscal years beginning after December 15, 2023, including interim periods within those fiscal years, and early adoption is permitted. The Company is currently evaluating the impact of the adoption of the standard on the unaudited condensed consolidated financial statements. In March 2020, the FASB issued ASU 2020-04, Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting. ASU 2020-04 provides optional expedients and exceptions for applying U.S. GAAP to contracts, hedging relationships and other transactions affected by the expected transition away from reference rates that are expected to be discontinued, such as LIBOR. ASU 2020-04 was effective upon issuance. The Company may elect to apply the guidance prospectively through December 31, 2022. The Company is currently evaluating the impact of the adoption of the standard on the unaudited condensed consolidated financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326). The standard introduces a new model for recognizing credit losses on financial instruments based on an estimate of current expected credit losses and will apply to trade receivables. The new guidance will be effective for the Company’s annual and interim periods beginning after December 15, 2022. The Company does not anticipate the adoption of this standard to have a material impact on the unaudited condensed consolidated financial statements. In February 2016, the FASB issued ASU 2016-2, Leases (Topic 842), to improve financial reporting regarding leasing transactions. The ASU primarily affects the accounting by the lessee in that it requires a lessee to recognize lease assets and liabilities, initially measured at the present value of the lease payments, on the balance sheets for those leases classified as operating leases under previous guidance. The new leasing standard is effective for the Company’s annual and interim periods beginning after December 15, 2021. The Company anticipates a material increase in assets and liabilities due to the recognition of the required right-of-use asset and corresponding liability for all significant lease obligations that are currently classified as operating leases.

Revenues and Accounts Receivabl

Revenues and Accounts Receivable	3 Months Ended
Revenues and Accounts Receivable	Mar. 31, 2021
Revenues And Accounts Receivable [Abstract]
Revenues and Accounts Receivable	3. REVENUES AND ACCOUNTS RECEIVABLE The Company’s main revenue sources derive from the following revenue streams: Assessment, Permitting and Response Revenues —Assessment, Permitting and Response revenues are generated from multidisciplinary environmental consulting services. The majority of the contracts are fixed-price or time and material based. Measurement and Analysis Revenues —Measurement and Analysis revenues are generated from emissions sampling, testing and reporting services, leak detection services, ambient air monitoring services and laboratory testing services. The majority of the contracts are fixed-price or time-and-materials based. Remediation and Reuse Revenues — Remediation and Reuse revenues are generated from operating and maintenance (“O&M”) services (on biogas and waste water treatment facilities), as well as remediation, monitoring and environmental compliance services. Services on the majority of O&M contracts are provided under long-term fixed-fee contracts. Remediation, monitoring and environmental compliance contracts are predominantly fixed-fee and time-and-materials based. Disaggregation of Revenue —The Company disaggregates revenue by its operating segments The Company believes disaggregating revenue into these categories achieves the disclosure objectives to depict how the nature, amount, and uncertainty of revenue and cash flows are affected by economic factors. Disaggregated revenue disclosures are provided in Note 19. Contract Balances —The Company presents contract balances for unbilled receivables (contract assets), as well as customer advances, deposits and deferred revenue (contract liabilities) within contract assets and accounts payable and accrued expenses, respectively, on the unaudited condensed consolidated statements of financial position. Amounts are generally billed at periodic intervals (e.g., weekly, bi-weekly or monthly) as work progresses in accordance with agreed-upon contractual terms. The Company utilizes the practical expedient to not adjust the promised amount of consideration for the effects of a significant financing component as the period between when the Company transfers services to a customer and when the customer pays for those services is one year or less. Amounts recorded as unbilled receivables are generally for services the Company is not entitled to bill based on the passage of time. Under certain contracts, billing occurs subsequent to revenue recognition, resulting in contract assets. The Company sometimes receives advances or deposits from customers before revenue is recognized, resulting in contract liabilities. The following table presents the Company’s contract balances: March 31, December 31, 2021 2020 Contract assets $ 61,636 $ 38,576 Contract liabilities 6,476 6,114 Contract assets acquired through business acquisitions amounted to $0.5 million and $6.5 million as of March 31, 2021 and December 31, 2020, respectively. Contract liabilities acquired through business acquisitions amounted to $0.5 million and zero as of March 31, 2021 and December 31, 2020, respectively. Revenue recognized during the three months ended March 31, 2021, included in the contract liabilities balance at the beginning of the year was $1.2 million. The revenue recognized from the contract liabilities consisted of the Company satisfying performance obligations during the normal course of business. The amount of revenue recognized from changes in the transaction price associated with performance obligations satisfied in prior periods during the three months ended March 31, 2021 was not material. Remaining Unsatisfied Performance Obligations —Remaining unsatisfied performance obligations represent the total dollar value of work to be performed on contracts awarded and in progress. The amount of remaining unsatisfied performance obligations increases with new contracts or additions to existing contracts and decreases as revenue is recognized on existing contracts. Contracts are included in the amount of remaining unsatisfied performance obligations when an enforceable agreement has been reached. As of March 31, 2021 and December 31, 2020, the estimated revenue expected to be recognized in the future related to performance obligations that are unsatisfied was approximately $25.4 million and $24.4 million, respectively. As of March 31, 2021, the Company expected to recognize approximately $22.3 million of this amount as revenue within the next year and $3.1 million the year after. Accounts Receivable, Net —Accounts receivable, net consisted of the following: March 31, December 31, 2021 2020 Accounts receivable, invoiced $ 64,314 $ 57,228 Accounts receivable, other 5,766 1,139 Allowance for doubtful accounts (4,309 ) (4,265 ) Accounts receivable—net $ 65,771 $ 54,102 The Company extends non-interest-bearing trade credit to its customers in the ordinary course of business. Accounts receivable are shown on the face of the unaudited condensed consolidated statements of financial position, net of an allowance for doubtful accounts. In determining the allowance for doubtful accounts, the Company analyzes the aging of accounts receivable, historical bad debts, customer creditworthiness and current economic trends. During the first quarter of 2020, there was a global outbreak of a new strain of coronavirus, COVID-19. The COVID-19 pandemic has added uncertainty to the collectability of certain receivables, particularly in industries hard hit by the pandemic. As a result, the Company recorded a $6.3 million bad debt reserve during the first quarter of 2020. The bad debt adjustment included a $5.5 million reserve for one customer in the Company’s Remediation and Reuse segment in which management concluded to discontinue select service lines as of March 31, 2020 (Note 19). As of March 31, 2021 and December 31, 2020, the Company had one customer who accounted for 15.9% and 10.2%, respectively, of our gross accounts receivable. For the three months ended March 31, 2021, the Company had three customers who accounted for 16.3%, 12.7% and 10.2% of revenue. The Company did not have any customers that exceeded 10.0% of revenue as of March 31, 2020. The Company performs ongoing credit evaluations, and accordingly, believes that the balances from these largest customers do not represent a significant credit risk. The allowance for doubtful accounts consisted of the following: Beginning Balance Bad Debt Expense Charged to Allowance Other ( 1) Ending Balance Three months ended March 31, 2021 $ 4,265 $ 508 $ (473 ) $ 9 $ 4,309 Year ended December 31, 2020 1,327 4,532 (2,633 ) 1,039 4,265 ____________________ (1) This amount consists of additions to the allowance due to business acquisitions.

Prepaid and Other Current Asset

Prepaid and Other Current Assets	3 Months Ended
Prepaid and Other Current Assets	Mar. 31, 2021
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid and Other Current Assets	4. PREPAID AND OTHER CURRENT ASSETS Prepaid and other current assets consisted of the following: March 31, December 31, 2021 2020 Deposits $ 796 $ 708 Prepaid expenses 5,613 3,510 Supplies 2,421 2,491 Prepaid and other current assets $ 8,830 $ 6,709

Property and Equipment, Net

Property and Equipment, Net	3 Months Ended
Property and Equipment, Net	Mar. 31, 2021
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. PROPERTY AND EQUIPMENT, NET Property and equipment are stated at cost or estimated fair value for assets acquired through business combinations. Depreciation and amortization are provided using the straight-line method over the estimated useful lives of the assets. Leasehold improvements are amortized using the straight-line method over the shorter of the remaining lease term, including options that are deemed to be reasonably assured, or the estimated useful life of the improvement. Property and equipment, net, consisted of the following: Estimated March 31, December 31, Useful Life 2021 2020 Lab and test equipment 7 years $ 19,017 $ 18,631 Vehicles 5 years 14,047 13,320 Equipment 3–7 years 33,263 32,177 Furniture and fixtures 7 years 3,023 2,938 Leasehold improvements 7 years 6,884 6,767 Aircraft 10 years 834 834 Building 39 years 2,975 2,975 80,043 77,642 Land 725 725 Construction in progress 489 219 Less accumulated depreciation (46,316 ) (44,187 ) Total property and equipment— net $ 34,941 $ 34,399 Total depreciation expense included in the unaudited condensed consolidated statements of operations was $2.2 million and $2.0 million for the three months ended March 31, 2021 and March 31, 2020, respectively.

Business Acquisitions

Business Acquisitions	3 Months Ended
Business Acquisitions	Mar. 31, 2021
Business Combinations [Abstract]
Business Acquisitions	6. BUSINESS ACQUISITIONS In line with the Company’s strategic growth initiatives, the Company acquired one business during the three months ended March 31, 2021 and several businesses during the year ended December 31, 2020. The results of each The identifiable intangible assets for acquisitions are valued using the excess earnings method discounted cash flow approach for customer relationships, the relief from royalty method for trade names, the patent and external proprietary software, the “with and without” method for covenants not to compete and the replacement cost method for the internal proprietary software by incorporating Level 3 inputs as described under the fair value hierarchy of ASC 820. These unobservable inputs reflect the Company’s own assumptions about which assumptions market participants would use in pricing an asset on a non-recurring basis. These assets will be amortized over their respective estimated useful lives. Other purchase price obligations (primarily deferred purchase price liabilities and target working capital liabilities or receivables) are included on the unaudited condensed consolidated statements of financial position in accounts payable and other accrued liabilities, other non-current liabilities or accounts receivable-net in the case of working capital deficits. Contingent consideration outstanding from acquisitions are included on the unaudited condensed consolidated statements of financial position in business acquisition contingent consideration, current or in business acquisitions contingent consideration, long-term. These obligations are scheduled to be settled if certain performance thresholds are met. The Company considers several factors when determining whether or not contingent consideration liabilities are part of the purchase price, including the following: (i) the valuation of its acquisitions is not supported solely by the initial consideration paid, (ii) the former stockholders of acquired companies that remain as key employees receive compensation other than contingent consideration payments at a reasonable level compared with the compensation of the Company’s other key employees and (iii) contingent consideration payments are not affected by employment termination. The Company reviews and assesses the estimated fair value of contingent consideration at each reporting period. Transaction costs related to business combinations totaled $0.2 million and $1.3 million Acquisitions Completed During the Three Months Ended March 31, 2021 MSE Group —In January 2021, the Company completed the acquisition of MSE Group (“MSE”) by acquiring 100.0% of its membership interests. MSE is a provider of environmental assessment, compliance, engineering, and design services primarily to the U.S. federal government. MSE is based in Orlando, FL with additional offices in Tampa, Orlando, Jacksonville, San Antonio, TX, and Wilmington, NC, and satellite locations nationwide. The following table summarizes the elements of the purchase price of MSE: Cash Common Stock Other Purchase Price Components Current Other Purchase Price Components Long Term Contingent Consideration Current Contingent Consideration Long Term Total Purchase Price MSE $ 9,082 $ 2,271 $ 10,146 $ — $ — $ 1,804 $ 23,303 The other purchase price components of the MSE purchase price consist of a target working capital amount, a 338 election tax liability, 2020 and 2021 purchase price true ups and contingent consideration. The 2020 and 2021 purchase price true up elements are based on MSE’s actual 2020 and 2021 results. The contingent consideration element is related to earn-outs which are based on the expected achievement of revenue or earnings thresholds as of the date of the acquisition and for which the maximum potential amount is limited. The Company paid the target working capital amount and the 2020 purchase price true up in April 2021 (Note 22). The Company may be required to make up to $6.2 million in aggregate true up and earn-out payments in 2022 and 2023. The preliminary purchase price attributable to the MSE acquisition was allocated as follows: MSE Cash $ 2,810 Accounts receivable 3,068 Other current assets 31 Current assets 5,909 Property and equipment 513 Customer relationships 8,720 Trade names 521 Covenants not to compete 922 Goodwill 7,532 Total assets 24,117 Current liabilities (814 ) Total liabilities (814 ) Purchase price $ 23,303 MSE results of operations have been combined with those of the Company since the date of acquisition. The Company’s unaudited condensed consolidated statement of operations for the three months ended March 31, 2021 includes revenue and pre-tax loss of $4.0 million and $0.3 million, respectively. MSE is included in the Company’s Remediation and Reuse segment. The weighted average useful lives for the acquired customer relationships and related backlog for the MSE acquisition are 7 years and 2 years, respectively. The weighted average useful lives for the acquired tradenames and covenants not to compete for the MSE acquisition are 2 years Goodwill associated with the MSE acquisition is deductible for income tax purposes. The Company has not yet completed the initial purchase price allocation for this acquisition due to the timing of the close of the transaction. Acquisitions Completed During the Year Ended December 31, 2020 The Center for Toxicology and Environmental Health, L.L.C. —In April 2020, the Company completed the acquisition of The Center for Toxicology and Environmental Health, L.L.C. (“CTEH”) by acquiring 100.0% of its membership interests. CTEH is an environmental consulting company headquartered in Arkansas that specializes in environmental response and toxicology. The cash payment made to acquire CTEH was funded through the issuance of the Convertible and Redeemable Series A-2 Preferred Stock (Note 16) and the common stock portion of the purchase price was funded through the issuance of 791,139 shares of common stock. Leed Environmental Inc .— In September 2020, the Company acquired certain testing assets, and operations from Leed Environmental Inc. (“LEED”). LEED provides environmental project management and coordination services. LEED expands the Company’s remediation capabilities in the Northeast region of the United States. The cash payment made to acquire LEED was funded via cash on hand. American Environmental Testing Co . — In September 2020, the Company acquired certain assets and operations of American Environmental Testing Co. (“AETC”), a stack testing company in Utah. AETC expands the Company’s air measurement and analysis capabilities in the West Coast region. The cash payment made to acquire AETC was funded via cash on hand. The following table summarizes the elements of purchase price of the acquisitions completed during the year ended December 31, 2020: Cash Common Stock Other Purchase Price Components Current Other Purchase Price Components Long Term Contingent Consideration Current Contingent Consideration Long Term Total Purchase Price CTEH $ 175,000 $ 25,000 $ (1,939 ) $ — $ 34,451 $ 10,543 $ 243,055 All other acquisitions 450 — 50 100 210 — 810 Total $ 175,450 $ 25,000 $ (1,889 ) $ 100 $ 34,661 $ 10,543 $ 243,865 The contingent consideration elements of the purchase price of the acquisitions is related to earn-outs which are based on the expected achievement of revenue or earnings thresholds as of the date of the acquisition and for which the maximum potential amount is limited. The CTEH first year earn-out was calculated at twelve times CTEH’s 2020 EBITDA (as defined in the purchase agreement) in excess of $18.3 million, with a maximum first year earn-out payment of $50.0 million, which was fully achieved. The second year earn-out is to be calculated at ten times CTEH’s 2021 EBITDA in excess of actual 2020 EBITDA (with actual 2020 EBITDA subject to a minimum of $18.3 million and a maximum of $22.5 million), with a maximum second year earn-out payment of $30.0 million. The 2020 earn-out was initially payable 100.0% in common stock, but as a result of the completion of the Company’s IPO (Note 1), 50.0% was payable in cash. In April 2021, the 2020 earn-out payment was made with 50.0% paid in cash and the remaining 50.0% paid in common stock of the Company (Note 22). The 2021 earn-out, if any, is payable 100.0% in cash. The purchase price attributable to the acquisitions was allocated as follows: CTEH All Other Acquisitions Total Cash $ 1,527 $ — $ 1,527 Accounts receivable 17,059 — 17,059 Other current assets 1,265 — 1,265 Current assets 19,851 — 19,851 Property and equipment 7,042 75 7,117 Customer relationships 56,000 — 56,000 Trade names 4,200 — 4,200 Covenants not to compete 4,000 109 4,109 Proprietary software 14,700 — 14,700 Goodwill 146,983 626 147,609 Total assets 252,776 810 253,586 Current liabilities 9,721 — 9,721 Total liabilities 9,721 — 9,721 Purchase price $ 243,055 $ 810 $ 243,865 CTEH, LEED and AETC are included in the Company’s Assessment, Permitting and Response, Remediation and Reuse and Measurement and Analysis segments, respectively. The weighted average useful lives for the acquired customer relationships and internal proprietary software for the CTEH acquisition are 15 years and 3 years, respectively. The weighted average useful lives for the acquired tradenames, covenants not to compete and external proprietary software for the CTEH acquisition is 5 years. The weighted average useful lives for the acquired covenants not to compete for the other acquisitions is 4 years. Goodwill associated with the CTEH, LEED and AETC acquisitions is deductible for income tax purposes. Supplemental Unaudited Pro-Forma —The unaudited condensed consolidated financial information summarized in the following table gives effect to the 2021 and the 2020 acquisitions discussed above assuming they occurred on January 1, 2020. These unaudited consolidated pro forma operating results do not assume any impact from revenue, cost or other operating synergies that are expected or may have been realized as a result of the acquisitions. These unaudited consolidated pro forma operating results are presented for illustrative purposes only and are not indicative of the operating results that would have been achieved had the acquisitions occurred on January 1, 2020, nor does the information purport to reflect results for any future period. For the Three Months Ended March 31, 2021 2020 As reported Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) As reported Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) Revenues $ 133,817 $ — $ 133,817 $ 61,031 $ 36,197 $ 97,228 Net (loss) income (11,596 ) — (11,596 ) (41,248 ) 11,352 (29,896 ) During the first quarter of 2020, the Company determined to reduce the footprint of its environmental lab in Berkeley, California, and to exit its non-specialized municipal water engineering service line and its food waste biogas engineering service line, (together, “the Discontinued Service Lines”). Revenues from Discontinued Service Lines included in revenues in the above table for the three months ended March 31, 2021 and March 31, 2020 were zero and $2.5 million, respectively

Goodwill and Intangible Assets

Goodwill and Intangible Assets	3 Months Ended
Goodwill and Intangible Assets	Mar. 31, 2021
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	7. GOODWILL AND INTANGIBLE ASSETS Amounts related to goodwill are as follows: Assessment, Permitting and Response Measurement and Analysis Remediation and Reuse Total Balance as of December 31, 2020 $ 162,156 $ 69,054 $ 43,457 $ 274,667 Goodwill acquired during the period — — 7,532 7,532 Balance as of March 31, 2021 $ 162,156 $ 69,054 $ 50,989 $ 282,199 Amounts related to finite-lived intangible assets are as follows: March 31, 2021 Estimated Useful Life Gross Balance Accumulated Amortization Total Intangible Assets—Net Finite lived intangible assets Customer relationships 2–15 years $ 173,502 $ 58,294 $ 115,208 Covenants not to compete 4–5 years 30,864 22,471 8,393 Trade names 1–5 years 17,459 13,445 4,014 Proprietary software 3–5 years 21,260 7,368 13,892 Patent 16 years 17,479 1,671 15,808 Total other intangible assets —net $ 260,564 $ 103,249 $ 157,315 December 31, 2020 Estimated Useful Life Gross Balance Accumulated Amortization Total Intangible Assets—Net Finite lived intangible assets Customer relationships 7–15 years $ 164,782 $ 53,446 $ 111,336 Covenants not to compete 4–5 years 29,942 21,469 8,473 Trade names 1–5 years 16,938 12,849 4,089 Proprietary software 3–5 years 21,007 6,132 14,875 Patent 16 years 17,479 1,398 16,081 Total other intangible assets —net $ 250,148 $ 95,294 $ 154,854 Intangible assets with finite lives are stated at cost, less accumulated amortization and impairment losses, if any. These intangible assets are amortized using the straight-line method over the estimated useful lives of the assets. Future amortization expense is estimated to be as follows for each of the five following years and thereafter: December 31, 2021 (remaining) $ 22,667 2022 26,545 2023 21,276 2024 18,096 2025 11,854 2026 and thereafter 56,877 Total $ 157,315

Accounts Payable and Other Accr

Accounts Payable and Other Accrued Liabilities	3 Months Ended
Accounts Payable and Other Accrued Liabilities	Mar. 31, 2021
Payables And Accruals [Abstract]
Accounts Payable and Other Accrued Liabilities	8. ACCOUNTS PAYABLE AND OTHER ACCRUED LIABILITIES Accounts payable and other accrued liabilities consisted of the following: March 31, December 31, 2021 2020 Accounts payable $ 15,470 $ 15,481 Accrued expenses 19,555 11,469 Other business acquisitions purchase price obligations 9,511 50 Contract liabilities 6,476 6,114 Other current liabilities 1,229 1,507 Income tax payable 190 256 Total accounts payable and other accrued liabilities $ 52,431 $ 34,877

Accrued Payroll and Benefits

Accrued Payroll and Benefits	3 Months Ended
Accrued Payroll and Benefits	Mar. 31, 2021
Statement Of Financial Position [Abstract]
Accrued Payroll and Benefits	9. ACCRUED PAYROLL AND BENEFITS Accrued payroll and benefits consisted of the following: March 31, December 31, 2021 2020 Accrued bonuses $ 1,800 $ 5,416 Accrued paid time off 2,355 2,067 Accrued payroll 13,273 9,133 Accrued other 5,136 4,565 Total accrued payroll and benefits $ 22,564 $ 21,181

Income Taxes

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	10. INCOME TAXES The Company calculates its interim income tax provision in accordance with ASC Topic 270, Interim Reporting (“ASC 270”), and ASC 740. The Company’s effective tax rate (“ETR”) from continuing operations was (0.12%) and 7.11% for the three months ended March 31, 2021 and March 31, 2020, respectively. Income tax expense recorded by the Company during the three months ended March 31, 2021 was not material. The Company recorded an income tax benefit of $3.2 million during the three months ended March 31, 2020. The difference between the ETR and federal statutory rate of 21.0% is primarily attributable to items recorded for U.S. GAAP but permanently disallowed for U.S. federal income tax purposes, recognition of a U.S. federal and state valuation allowance, state and foreign income tax provisions and Global Intangible Low Taxed Income (“GILTI”). A valuation allowance is recorded when it is more-likely-than-not some of the Company’s deferred tax assets may not be realized. Significant judgment is applied when assessing the need for a valuation allowance and the Company considers future taxable income, reversals of existing deferred tax assets and liabilities and ongoing prudent and feasible tax planning strategies, in making such assessment. As of March 31, 2021, the Company’s U.S. federal, state and various foreign net deferred tax assets are not more-likely-than-not to be realized and a full valuation allowance is maintained. The Company records uncertain tax positions in accordance with ASC 740, on the basis of a two-step process in which (i) the Company determines whether it is more likely than not a tax position will be sustained on the basis of the technical merits of such position and (ii) for those tax positions meeting the more-likely-than-not recognition threshold, the Company would recognize the largest amount of tax benefit that is more than 50.0% likely to be realized upon ultimate settlement with the related tax authority. The Company has determined it has no uncertain tax positions as of March 31, 2021. The Company classifies interest and penalties recognized on uncertain tax positions as a component of income tax expense.

Warrant Options

Warrant Options	3 Months Ended
Warrant Options	Mar. 31, 2021
Warrants And Rights Note Disclosure [Abstract]
Warrant Options	11. WARRANT OPTIONS In October 2018, in connection with the issuance of the Redeemable Series A-1 Preferred Stock, the Company issued a detachable warrant to acquire 534,240 shares of common stock at a price of $0.01 per share at any given time during a period of ten years beginning on the instrument’s issuance date. For the three months ended March 31, 2020, fair value gains/(losses) recorded in other expense on the unaudited condensed consolidated statements of operations related to the Redeemable Series A-1 warrant were not material. In April 2020, in connection with the issuance of the Convertible and Redeemable Series A-2 Preferred Stock, the Company issued a detachable warrant to acquire 1,351,960 shares of common stock at a price of $0.01 per share at any time following the occurrence of a qualifying IPO, a sale of the Company, or a redemption in full of the Series A-2 preferred stock (each, an “Adjustment Event”), with an expiration date of ten years from the instrument’s issuance date. The number of shares underlying the warrant and issuable upon exercise was subject to adjustment based upon the price per share of common stock upon the occurrence of an Adjustment Event (Note 16) to reflect an aggregate value of $30.0 million. As a result of the $15.00 per share public offering price in the IPO, the warrant issued in connection with the issuance of the Convertible and Redeemable Series A-2 Preferred Stock was adjusted pursuant to its terms and, upon closing of the IPO, represented a warrant to purchase 1,999,999 shares of common stock (an increase of 648,039 shares). On July 30, 2020, the Redeemable Series A-1 Preferred Stock and the Convertible and Redeemable Series A-2 Preferred Stock warrants were

Debt

Debt	3 Months Ended
Debt	Mar. 31, 2021
Debt Disclosure [Abstract]
Debt	12. DEBT Debt consisted of the following: March 31, December 31, 2021 2020 Term loan facility $ 173,359 $ 173,906 Capital leases 3,348 3,088 Equipment line of credit 2,803 3,018 Less deferred debt issuance costs (3,871 ) (4,108 ) Total debt 175,639 175,904 Less current portion of long-term debt (6,214 ) (5,583 ) Long-term debt, less current portion $ 169,425 $ 170,321 Deferred Financing Costs —Costs relating to debt issuance have been deferred and are presented as discounted against the underlying debt instrument. These costs are amortized to interest expense over the terms of the underlying debt instruments. Revolving Line of Credit and Term Loan Facility — On April 13, 2020, the Company entered into a Unitranche Credit Agreement (the “2020 Credit Facility”) providing for a new $225.0 million credit facility comprised of a $175.0 million term loan and a $50.0 million revolving credit facility and repaid all amounts outstanding under the prior senior secured credit facility. T he 2020 Credit Facility matures in April 2025. Up until October 6, 2020, the term loan and the revolver bore interest at LIBOR plus 5.0% with a 1.0% LIBOR floor or the base rate plus 4.0% and LIBOR plus 3.5% or the base rate plus 2.5% , respectively. Effective October 6, 2020, the Company amended the 2020 Credit Facility to provide for a reduction on the applicable interest rate on the term loan from LIBOR plus 5.0% with a 1.0% LIBOR floor to LIBOR plus 4.5% with a 1.0% LIBOR floor. The revolver interest rate remained unchanged. The revolver is also subject to an unused commitment fee of 0.35%. The Term Loan has quarterly repayments that started on September 30, 2020 of $0.5 million, increasing to $1.1 million on September 30, 2021 and further increasing to $1.6 million on September 30, 2022, with the remaining outstanding principal amount due on the maturity date. The Company has the option to borrow incremental term loans up to an aggregate principal amount of $100.0 million subject to satisfaction of certain conditions, including the borrower’s pro forma compliance with the financial covenants under the 2020 Credit Facility. Immediately after giving effect to the incurrence of any such incremental term loans, the lenders must collectively hold at least 70.0% of all pari passu debt of all lenders under the credit facility. The existing lenders are not obligated to participate in any incremental term loan facility. The 2020 Credit Facility includes a number of covenants imposing certain restrictions on the Company’s business, including, among other things, restrictions on the Company’s ability to incur indebtedness, prepay or amend other indebtedness, create liens, make certain fundamental changes including mergers or dissolutions, pay dividends and make other payments in respect of capital stock, make certain investments, sell assets, change the Company’s lines of business, enter into transactions with affiliates and other corporate actions. The 2020 Credit Facility also contains financial covenants requiring the Company to remain below a maximum consolidated total leverage ratio of 4.25 times, which steps down to 4.00 times beginning December 31, 2021 and then to 3.75 times beginning December 31, , and a minimum consolidated fixed charge coverage ratio of 1.25 times. The 2020 Credit Facility contains mandatory prepayment features upon a number of events, including with the proceeds of certain asset sales, proceeds from the issuance of any debt and proceeds of the capital contribution amounts contributed to cure a financial covenant default. The 2020 Credit Facility also includes mandatory prepayments of 50.0% of excess cash flow minus voluntary prepayments of the term loan and, solely to the extent accompanied by a permanent reduction in the revolving commitment, the revolver, if the Company’s consolidated total leverage ratio for the year ending December 31, 2020 is greater than or equal to 3.25 times and, for any year thereafter, the amount of any such mandatory prepayment shall be reduced to 25.0% of excess cash flow if the leverage ratio is less than 3.00 times. As of March 31, 2021 and December 31, 2020, the Company’s consolidated total leverage ratio (as defined in the credit agreement) was 3.1 times and 2.7 times, respectively. The weighted average interest rate on the 2020 Credit Facility as of March 31, 2021 was 5.5%. The 2020 Credit Facility contains a number of customary events of default related to, among other things, the non-payment of principal, interest or fees, violations of covenants, inaccuracy of representations or warranties, certain bankruptcy events, default in payment under or the acceleration of other indebtedness and certain change of control events. In the event of a default, subject to varying cure periods and rights for certain events of default, the required lenders may, at their option, declare the commitments to fund the credit facility to be terminated. The Company’s obligations under the 2020 Credit Facility are guaranteed by certain of the Company’s existing and future direct and indirect subsidiaries, and such obligations are secured by substantially all of the Company’s assets, including the capital stock or other equity interests in those subsidiaries. Prior Senior Secured Credit Facility —The Company’s Prior Senior Secured Credit Facility (the “Prior Senior Credit Facility”), which was paid in full in April 2020 via proceeds from the issuance of the 2020 Credit Facility, consisted of a $50.0 million term loan and a $130.0 million revolving credit facility. Borrowings under the Prior Senior Credit Facility bore interest at either (i) LIBOR plus the applicable margin or (ii) a base rate (equal to the highest of (a) the federal funds rate plus 0.5%, (b) Lender A’s prime rate and (c) Eurodollar Rate, which is based on LIBOR, (using a one-month period plus 1.0%), plus the applicable margin, as the Company elects. The applicable margin means a percentage per annum determined in accordance with the following table: Pricing Tier Consolidated Leverage Ratio Commitment Fee Eurodollar Rate Loans and LIBOR Letter of Credit Fee Daily Floating Rate Loans Rate Loans 1 > 3.75 to 1.0 0.50 % 4.00 % 4.00 % 3.00 % 2 ≤ 3.75 to 1.0 but > 3.00 to 1.0 0.50 3.50 3.50 2.50 3 ≤ 3.00 to 1.0 but > 2.25 to 1.0 0.40 3.00 3.00 2.00 4 < 2.25 to 1.0 0.30 2.50 2.50 1.50 Equipment Line of Credit —On March 12, 2019, the Company increased its equipment line of credit facility for the purchase of equipment and related freight, installation costs and taxes paid for an additional amount not to exceed $2.0 million. On May 16, 2019, the Company entered into a Canadian equipment line of credit facility for an amount not to exceed $1.0 million Canadian dollars. Interest on the line of credit is determined based on a three-year swap rate at the time of funding. Capital Lease Obligations —The assets and liabilities under capital lease agreements are recorded at the lower of the present value of the minimum lease payments or the fair value of the asset. The assets are being amortized over the shorter of their related lease terms or their estimated useful lives ranging from three to seven years. The gross amount of assets under capital leases as of March 31, 2021 and December 31, 2020 was $6.2 million and $6.1 million, respectively. The amortization of assets under capital leases was $0.6 million and $0.2 million for the three months ended March 31, 2021 and March 31, 2020, respectively, and was included in depreciation and amortization on the unaudited condensed consolidated statements of operations. All capital leases (including those purchased through the Company’s equipment line of credit) mature by 2025 as follows: March 31, Payments Interest Principal 2022 $ 2,727 $ 341 $ 2,386 2023 2,203 227 1,976 2024 1,403 98 1,305 2025 507 23 484 Total $ 6,840 $ 689 $ 6,151 The following is a schedule of the aggregate annual maturities of long-term debt presented on the unaudited condensed consolidated statement of financial position, based on the terms of the 2020 Credit Facility, capital lease obligations and equipment line of credit: March 31, 2022 $ 6,214 2023 7,992 2024 7,868 2025 7,047 2026 150,389 Total $ 179,510

Fair Value of Financial Instrum

Fair Value of Financial Instruments	3 Months Ended
Fair Value of Financial Instruments	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	13. FAIR VALUE OF FINANCIAL INSTRUMENTS The following financial liabilities are measured at fair value on a recurring basis using significant unobservable inputs (Level 3): March 31, December 31, 2021 2020 Business acquisitions contingent consideration, current $ 50,364 $ 49,902 Business acquisitions contingent consideration, long-term 16,971 4,565 Conversion option 21,488 20,886 Total $ 88,823 $ 75,353 The estimated fair value amounts shown above are not necessarily indicative of the amounts that the Company would realize upon disposition, nor do they indicate the Company’s intent or ability to dispose of the financial instrument. The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis: Business Acquisitions Contingent Consideration, Current Business Acquisitions Contingent Consideration, Long-term Conversion Option Contingent Put Option Warrant Option Total Balance—at January 1, 2020 $ 8,614 $ 379 $ — $ 7,100 $ 16,878 $ 32,971 Changes in fair value included in earnings — — — 29,627 (1 ) 29,626 Payment of contingent consideration payable (4,703 ) — — — — (4,703 ) Foreign currency translation of contingent consideration payment (208 ) — — — — (208 ) Balance—at March 31, 2020 $ 3,703 $ 379 $ — $ 36,727 $ 16,877 $ 57,686 Balance—at January 1, 2021 $ 49,902 $ 4,565 $ 20,886 $ — $ — $ 75,353 Acquisitions — 1,804 — — — 1,804 Changes in fair value included in earnings 462 10,602 602 — — 11,666 Balance—at March 31, 2021 $ 50,364 $ 16,971 $ 21,488 $ — $ — $ 88,823 Quantitative Information about Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3): Business Acquisitions Contingent Consideration —The fair value of the contingent consideration payable associated with the acquisition of CTEH and MSE was determined using a Monte Carlo simulation of earnings in a risk-neutral Geometric Brownian Motion framework. The fair values of the contingent consideration payables for the other acquisitions were calculated based on expected target achievement amounts, which are measured quarterly and then subsequently adjusted to actuals at the target measurement date. The method used to price these liabilities is considered level 3 due to the subjective nature of the unobservable inputs used to determine the fair value. The input is the expected achievement of earn-out thresholds. Conversion Option —The fair value of the embedded derivative associated with the issuance of the Convertible and Redeemable Series A-2 Preferred Stock (Note 16) was estimated using a “with-and-without” method. The “with-and-without” methodology considers the value of the security on an as-is basis and then without the embedded conversion premium. The difference between the two scenarios is the implied fair value of the embedded derivative. The unobservable input is the required rate of return on the Series A-2. The considerable quantifiable inputs in the valuation relate to the timing of conversions or redemptions. Contingent Put Option —The fair value of the contingent put option associated with the issuance of the Redeemable Series A-1 Preferred Stock was estimated using a “with-and-without” method. The “with-and-without” methodology considers the value of the security on an as-is basis and then without the embedded contingent put option. The difference between the two scenarios is the implied fair value of the embedded derivative, recorded as the contingent put option liability. In this case the Series A-1 was redeemed on the date of value so the value of the “with” scenario is known. The unobservable input is the required rate of return on the Series A-1 through to maturity in the “without” scenario. The contingent put option was redeemed in July 2020 (Note 15). Warrant Option — The warrant option was exercised on July 30, 2020 (Note 11). The fair value of the warrant option associated with the issuance of the Redeemable Series A-1 Preferred Stock was calculated based on the Black-Sholes pricing model using the following assumptions: March 31, 2020 Common stock value (per share) $ 31.60 Expected volatility 35.93 % Risk-free interest rate 0.70 % Expected life (years) 10 The method used to price this liability is considered Level 3 due to the subjective nature of the unobservable inputs (common stock value and expected volatility) used to determine the fair value.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
Commitments and Contingencies	Mar. 31, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	14. COMMITMENTS AND CONTINGENCIES Operating Leases —The Company leases office facilities over various terms expiring through 2030. Certain of these operating leases contain rent escalation clauses. The Company also has office equipment leases that expire through 2025 March 31, Rent Office Equipment Total 2022 $ 7,105 $ 371 $ 7,476 2023 5,115 268 5,383 2024 3,598 90 3,688 2025 1,757 22 1,779 2026 and thereafter 4,896 — 4,896 Total $ 22,471 $ 751 $ 23,222 Total rent expense under operating leases was $2.5 million and $2.1million for the three months ended March 31, 2021 and March 31, 2020, respectively. Other Commitments —The Company has commitments under the 2020 Credit Facility, its equipment line of credit and its capital lease obligations (Note 12). Contingencies —The Company is subject to purchase price contingencies related to earn-outs associated with certain acquisitions (Note 6). Legal —In the normal course of business, the Company is at times subject to pending and threatened legal actions. In management’s opinion, any potential loss resulting from the resolution of these matters is not expected to have a material effect on the unaudited condensed consolidated results of operations, financial position or cash flows of the Company.

Redeemable Series A-1 Preferred

Redeemable Series A-1 Preferred Stock	3 Months Ended
Redeemable Series A-1 Preferred Stock	Mar. 31, 2021
Temporary Equity Disclosure [Abstract]
Redeemable Series A-1 Preferred Stock	15. REDEEMABLE SERIES A-1 PREFERRED STOCK On October 19, 2018, the Company issued 12,000 shares of Redeemable Series A-1 Preferred Stock with a par value of $0.0001 per share and a detachable warrant to purchase 534,240 shares of the Company’s common stock. Each preferred share was issued as part of a unit, which consisted of one share of the Redeemable Series A-1 Preferred Stock at $0.01 million per share. On April 13, 2020, the Company amended and restated the certificate of designation of the Company’s Redeemable Series A-1 Preferred Stock. The most significant changes in the amendment included (i) the Redeemable Series A-1 Preferred Stock became pari passu with the Convertible and Redeemable Series A-2 Preferred Stock (Note 16), (ii) the maturity was extended to October 2024; (iii) the Company could use up to $50.0 million of indebtedness or cash on hand to redeem the Redeemable Series A-1 Preferred Stock, and (iv) upon an IPO, up to 50.0% of accumulated dividends could be paid in shares of common stock and (v) the Company could elect to reduce the three year make whole penalty to a two year make whole penalty if the warrant issued in connection with the issuance of the Redeemable Series A-1 Preferred Stock was redeemed in full at a share price of no less than $31.60. Following a partial redemption of outstanding Redeemable Series A-1 Preferred Stock, the dividend rate of the remaining Redeemable Series A-1 Preferred Stock would be reduced proportionally (between 15.0% and 9.0%) in relation to the proportion of Redeemable Series A-1 Preferred Stock redeemed, with the rate increasing by an additional 1.0% for dividends that are accrued versus paid in cash. Based on a qualitative assessment performed by the Company, the Redeemable Series A-1 Preferred Stock amendments did not represent a significant long-term change to the original terms of the instrument and, therefore, there was no change in the accounting of the instrument. On July 27, 2020, the Company redeemed in full the Redeemable Series A-1 Preferred Stock, including the guaranteed minimum two-year dividend. The Redeemable Series A-1 Preferred Stock contained restrictive covenants. Prior to its redemption, the Company was subject to a consolidated total leverage ratio (including the outstanding principal and accrued dividend on the Redeemable Series A-1 Preferred Stock) limit of less than 10.0 times as of the end of any fiscal quarter ending until maturity. Before redemption, the Redeemable Series A-1 Preferred Stock accrued dividends quarterly at an annual rate of 15.0% with respect to dividends that were paid in cash and at an annual rate of 14.2% with respect to dividends that were accrued. At issuance the Company determined that the detachable warrant (Note 11) and the contingent put option were required to be accounted for separately. The contingent put option change in value of $29.6 million for the three months ended March 31, 2020 was recorded to other expense.

Convertible and Redeemable Seri

Convertible and Redeemable Series A-2 Preferred Stock	3 Months Ended
Convertible and Redeemable Series A-2 Preferred Stock	Mar. 31, 2021
Temporary Equity Disclosure [Abstract]
Convertible and Redeemable Series A-2 Preferred Stock	16. CONVERTIBLE AND REDEEMABLE SERIES A-2 PREFERRED STOCK On April 13, 2020, the Company entered into an agreement to issue 17,500 shares of the Convertible and Redeemable Series A-2 Preferred Stock with a par value of $0.0001 per share and a detachable warrant to purchase shares of the Company’s common stock with a 10-year life, in exchange for gross proceeds of $175.0 million, net of $1.3 million debt issuance costs. Before the Company’s IPO, each share of Convertible and Redeemable Series A-2 Preferred Stock accrued dividends at the rate of 15.0% per annum, with respect to dividends that were paid in cash, and 14.2% per annum, with respect to dividends that were accrued. The Company paid dividends on shares of the Convertible and Redeemable Series A-2 Preferred Stock of $4.1 million during the three months ended March 31, 2021. At issuance, the Company determined that Convertible and Redeemable Series A-2 Preferred Stock and the detachable warrant (Note 11), were required to be accounted for separately. Upon the Company’s IPO, following which the Redeemable Series A-1 Preferred Stock was fully redeemed, the Convertible and Redeemable Series A-2 Preferred Stock terms automatically updated to the following: (i) no mandatory redemption, (ii) no stated value cash repayment obligation other than in the event of certain defined liquidation events, (iii) only redeemable at the Company’s option, (iv) the instrument became convertible into common stock beginning on the four year anniversary of issuance at a 15.0% discount to the common stock market price (with a limit of $60.0 million in stated value of Convertible and Redeemable Series A-2 Preferred Stock eligible to be converted in any 60-day period prior to the seventh anniversary of issuance and the amount of stated value of the Convertible and Redeemable Series A-2 Preferred Stock eligible for conversion limited to $60.0 million during year 5 and $120.0 million (which includes the aggregate amount of the stated value of the Convertible and Redeemable Series A-2 Preferred Stock and any accrued but unpaid dividends added to such stated value of any shares of Convertible and Redeemable Series A-2 Preferred Stock converted in year 5) during year 6), (v) the dividend rate stepped down to 9.0% per year with required quarterly cash payments, (vi) in an event of noncompliance, the dividend rate shall increase to 12.0% per annum for the first 90-day period from and including the date the noncompliance event occurred, and thereafter shall increase to 14.0% per annum, (vii) the debt incurrence test ratio increased to 4.5 times, (viii) the total leverage cap covenant was removed, and (ix) minimum repayment amount dropped down from $50.0 million to $25.0 million. The Company may, at its option on any one or more dates, redeem all or a minimum portion (the lesser of (i) $25.0 million in aggregate stated value of the Convertible and Redeemable Series A-2 Preferred Stock and (ii) all of the Convertible and Redeemable Series A-2 Preferred Stock then outstanding) of the outstanding Convertible and Redeemable Series A-2 Preferred Stock in cash. With respect to any redemption of any share of the Convertible and Redeemable Series A-2 Preferred Stock prior to the third-year anniversary, the Company is subject to a make whole penalty in which the holders of the Convertible and Redeemable Series A-2 Preferred Stock are guaranteed a minimum repayment equal to outstanding redeemed stated value plus three years of dividends accrued or accruable thereon. The Convertible and Redeemable Series A-2 Preferred Stock does not meet the definition of a liability pursuant to “ASC 480- Distinguishing Liabilities from Equity.” However, as (i) the instrument is redeemable upon a change of control as defined in the certificate of designations governing the terms of the Convertible and Redeemable Series A-2 Preferred Stock, and (ii) the Company cannot assert it would have sufficient authorized and unissued shares of common stock to settle all future conversion requests due to the variable conversion terms, the instrument is redeemable upon the occurrence of events that are not solely within the control of the Company, and therefore the Company classifies the Convertible and Redeemable Series A-2 Preferred Stock as mezzanine equity. Subsequent adjustment of the carrying value of the instrument is required if the instrument is probable of becoming redeemable. As of March 31, 2021, the Company has determined that a change of control is not probable. Additionally, as of March 31, 2021, the Company has determined that it is not probable that there will be a future conversion request that the Company is unable to settle with authorized and issued shares based on the Company’s current stock price and available shares as well as the Company’s monitoring efforts to ensure there are a sufficient number of shares available to settle any conversion request. Therefore, as of March 31, 2021, the Company has determined that the instrument is not probable of becoming redeemable, and does not believe subsequent adjustment of the carrying value of the instrument will be necessary. The Convertible and Redeemable Series A-2 Preferred Stock had an aggregate liquidation preference of $182.2 million as of March 31, 2021. The Convertible and Redeemable Series A-2 Preferred Stock contains embedded features that are required to be bifurcated and are subject to separate accounting treatment from the instrument itself. At issuance, these embedded features consisted of (i) a contingent dividend feature associated with the decrease in the dividend rate upon an IPO and (ii) a conversion option of the preferred shares to shares of common stock beginning on the fourth-year anniversary of the issuance date. Upon the Company’s IPO, the embedded derivative only consisted of the conversion option. As of March 31, 2021, this conversion embedded feature had a net fair value of $21.5 million. The change in value of $0.6 million for the three months ended March 31, 2021 was recorded to other expense.

Stockholder's Equity (Deficit)

Stockholder's Equity (Deficit)	3 Months Ended
Stockholder's Equity (Deficit)	Mar. 31, 2021
Equity [Abstract]
Stockholders' Deficit	17. STOCKHOLDERS’ EQUITY (DEFICIT) Authorized Capital Stock— The Company was authorized to issue 190,000,000 shares of common stock, with a par value of $0.000004 per share as of March 31, 2021 and December 31, 2020. Warrants— In May 2015, the Company issued warrants to acquire 116,350 shares of Common Stock at a price of approximately $17.19 per share to the placement agent as consideration for backstopping the financing completed in May 2015. These warrants were exercised in full as a cashless transaction during the three months ended March 31, 2021. As a result of this cashless transaction, the resulting number of shares issued was 67,713 shares. Common Stock Issuances —The Company issued the following shares of common stock: Three Months Ended March, 2021 2020 Shares Average Price per Share Shares Average Price per Share Acquisitions 71,740 $ 31.65 — $ — Exercise of warrants 67,713 17.19 — — Exercise of options 330,060 6.62 — — Restricted shares, net 36,817 29.88 — — Total 506,330 $ 13.27 — $ — Employee Equity Incentive Plans —The Company has two plans under which stock-based awards have been issued: (i) the Montrose Amended & Restated 2017 Stock Incentive Plan (“2017 Plan”) and (ii) the Montrose Amended & Restated 2013 Stock Option Plan (“2013 Plan”) (collectively the “Plans”). As of March 31, 2021 and March 31, 2020, there was $18.8 million and $7.2 million, respectively, of total unrecognized stock compensation expense related to unvested options and restricted stock granted under the Plans. Such unrecognized expense is expected to be recognized over a weighted-average three year period March 31, 2021 2017 Plan 2013 Plan Total Shares authorized to be issued 3,944,750 2,047,269 5,992,019 Shares available for grant 1,618,996 — 1,618,996 March 31, 2020 2017 Plan 2013 Plan Total Shares authorized to be issued 1,057,785 2,058,619 3,116,404 Shares available for grant 8,217 4,775 12,992 Total stock compensation expense for the Plans was as follows: Three Months Ended March 31, 2021 2020 2017 plan 2013 plan 2017 plan 2013 plan Options Restricted Stock Options Total Options Restricted Stock Options Total Cost of revenue $ 606 $ — $ 10 $ 616 $ 355 $ — $ 70 $ 425 Selling, general and administrative expense 1,004 182 3 1,189 287 370 68 725 Total $ 1,610 $ 182 $ 13 $ 1,805 $ 642 $ 370 $ 138 $ 1,150 Montrose Amended & Restated 2017 Stock Incentive Plan Restricted Stock —During the three months ended March 31, 2021 and March 31, 2020, the Company issued 14,532 and 33,229 shares of restricted stock with a fair market value of $30.96 and $31.60 per share, respectively, to certain 2017 Plan participants as Director’s compensation. These shares of restricted stock granted in the three months ended March 31, 2021 and March 31, 2020 vest one year from the date of grant, or, in each case, in full upon a change in control, subject to the participant’s continued service as a Director throughout such date, or upon retirement. Members of the Board of Directors that receive stock-based compensation are treated as employees for accounting purposes. There were 36,817 and zero shares of restricted stock that became fully vested and were released as unrestricted shares of common stock during the three months ended March 31, 2021 and March 31, 2020, respectively. There were no forfeitures of restricted shares during the three months ended March 31, 2021 and March 31, 2020. There were an aggregate of 281,705 and 273,122 restricted shares outstanding as of March 31, 2021 and March 31, 2020, respectively. Options —Options issued to all optionees under the 2017 Plan vest over four years from the date of issuance (or earlier vesting start date, as determined by the Board of Directors) as follows: one half on the second anniversary of date of grant and the remaining half on the fourth anniversary of the date of grant, with the exception of certain annual grants to certain executive officers, which vest annually over a 3-year and 1-year period. The following summarizes the options activity of the 2017 Plan: Options to Purchase Common Stock Weighted- Average Exercise Price per Share Weighted Average Grant Date Fair Value per Share Weighted Average Remaining Contract Life (in Years) Aggregate Intrinsic Value of In-The- Money Options (in Thousands) Outstanding at January 1, 2020 617,852 $ 24 $ 12 7.82 $ 4,696 Granted 158,062 32 12 — — Forfeited/ cancelled (2,000 ) 32 — — — Outstanding at March 31, 2020 773,914 26 12 8.50 4,693 Outstanding at January 1, 2021 1,840,229 23 12 9.09 15,598 Granted 224,270 38 20 — — Forfeited/ cancelled (10,000 ) 26 — — — Expired (1,250 ) 18 — — — Exercised (17,188 ) 16 — — 519 Outstanding at March 31, 2021 2,036,061 24 13 8.97 52,317 Exercisable at March 31, 2021 218,379 24 — 7.79 5,702 Options vested and expected to vest 2,036,061 24 — 8.97 52,317 The following weighted-average assumptions were used in the Black-Sholes option-pricing model calculation: March 31, March 31, 2021 2020 Common stock value (per share) $ 37.61 $ 31.60 Expected volatility 58.01 % 31.87 % Risk-free interest rate 0.71 % 1.78 % Expected life (years) 5.5-7.0 7.0 Forfeiture rate None None Dividend rate None None Montrose Amended & Restated 2013 Stock Option Plan —The following summarizes the activity of the 2013 Plan: Options to Purchase Common Stock Weighted- Average Exercise Price per Share Weighted Average Grant Date Fair Value per Share Weighted Average Remaining Contract Life (in Years) Aggregate Intrinsic Value of In-The- Money Options (in Thousands) Outstanding at January 1, 2020 1,855,469 $ 6 $ 1 6.14 46,617 Expired (725 ) 6 — — — Outstanding at March 31, 2020 1,854,744 6 1 6.14 46,521 Outstanding at January 1, 2021 1,787,869 6 — 5.40 43,867 Exercised (311,872 ) 6 — — 12,321 Outstanding at March 31, 2021 1,475,997 6 — 5.14 64,492 Exercisable at March 31, 2021 1,456,922 6 — 5.13 63,740 Options vested and expected to vest 1,475,997 6 — 5.14 64,492 Total shares outstanding from exercised options were 579,260 shares and 201,600 shares as of March 31, 2021 and March 31, 2020, respectively. Common Stock Reserved for Future Issuances —The Company has reserved certain stock of its authorized but unissued common stock for possible future issuance in connection with the following: March 31, 2021 2020 Warrants — 650,590 Montrose 2013 Stock Incentive Plan 2,047,269 2,058,619 Montrose 2017 Stock Incentive Plan 3,944,750 1,057,785 Common stock reserved for future issuance 5,992,019 3,766,994

Net Loss Per Share

Net Loss Per Share	3 Months Ended
Net Loss Per Share	Mar. 31, 2021
Earnings Per Share [Abstract]
Net Loss per Share	18. NET LOSS PER SHARE Basic net loss per share is computed by dividing net loss attributable to common stockholders by the weighted average number of common shares outstanding during each period. The Redeemable Series A-1 Preferred Stock, which was outstanding prior to its redemption on July 27, 2020, and the Convertible and Redeemable Series A-2 Preferred Stock are considered a participating security during the applicable period. Net losses are not allocated to the Redeemable Series A-1 Preferred stockholders nor the Convertible and Redeemable Series A-2 stockholders, as they were not contractually obligated to share in the Company’s losses. Diluted net loss per share is computed by dividing net loss attributable to common stockholders by the weighted average number of common and dilutive common equivalent shares outstanding for the period using the treasury-stock method or the as-converted method. During the three months ended March 31, 2020, shares issuable in connection with the warrant options (Note 11) were considered outstanding common shares for purposes of calculating net loss per share since they did not contain any conditions that must be satisfied for the holder to exercise the warrant. Potentially dilutive shares are comprised of restricted stock and shares of common stock underlying stock options outstanding under the Plans and warrants (other than warrant options) to purchase common stock. During the three months ended March 31, 2021 and March 31, 2020, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to the Company’s net loss and potentially dilutive shares being anti-dilutive. The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company: Three Months Ended March 31, (In thousands, except for net loss per share) 2021 2020 Net loss $ (11,596 ) $ (41,248 ) Accretion of redeemable series A-1 preferred stock — (5,415 ) Convertible and redeemable series A-2 preferred stock dividend (4,100 ) — Net loss attributable to common stockholders – basic and diluted (15,696 ) (46,663 ) Weighted-average common shares outstanding – basic and diluted 25,117 8,904 Net loss per share attributable to common stockholders – basic and diluted $ (0.62 ) $ (5.24 ) The following equity shares were excluded from the calculation of diluted net loss per share attributable to common stockholders because their effect would have been anti-dilutive: March 31, 2021 2020 Stock options 3,293,679 2,095,909 Restricted stock 26,201 19,059 Warrants — 116,350

Segment Information

Segment Information	3 Months Ended
Segment Information	Mar. 31, 2021
Segment Reporting [Abstract]
Segment Information	19. SEGMENT INFORMATION The Company has three operating and reportable segments: Assessment, Permitting and Response, Measurement and Analysis and Remediation and Reuse. These segments are monitored separately by management for performance against budget and prior year and are consistent with internal financial reporting. The Company’s operating segments are organized based upon primary services provided, the nature of the production process, their type of customers, methods used to distribute the products and the nature of the regulatory environment. Segment Adjusted EBITDA is the primary measure of operating performance for all three operating segments. Segment Adjusted EBITDA is the calculated Company’s Earnings before Interest, Tax, Depreciation and Amortization (“EBITDA”), adjusted to exclude certain transactions such as stock-based compensation, acquisition costs and fair value changes in financial instruments, amongst others. The Chief Operating Decision Maker (“CODM”) does not review segment assets as a measure of segment performance. Corporate and Other includes costs associated with general corporate overhead (including executive, legal, finance, safety, human resources, marketing and IT related costs) that are not directly related to supporting operations. Overhead costs that are directly related to supporting operations (such as insurance, software, licenses, shared services and payroll processing costs) are allocated to the operating segments on a basis that reasonably approximates an estimate of the use of these services. Segment revenues and Adjusted EBITDA consisted of the following: Three Months Ended March 31, 2021 2020 Segment Segment Segment Adjusted Segment Adjusted Revenues EBITDA Revenues EBITDA Assessment, Permitting and Response $ 75,262 $ 15,804 $ 4,530 $ 1,442 Measurement and Analysis 33,440 4,860 36,440 7,379 Remediation and Reuse 25,115 2,481 20,061 2,107 Total Operating Segments 133,817 23,145 61,031 10,928 Corporate and Other — (6,345 ) — (5,375 ) Total $ 133,817 $ 16,800 $ 61,031 $ 5,553 Presented below is a reconciliation of the Company’s segment measure to loss before (expense) benefit from income taxes: For the Three Months Ended March 31, 2021 2020 Total $ 16,800 $ 5,553 Interest expense, net (2,688 ) (2,593 ) Income tax (expense) benefit (2 ) 3,152 Depreciation and amortization (10,769 ) (7,560 ) Stock-based compensation (1,805 ) (1,150 ) Start-up losses and investment in new services (968 ) (379 ) Acquisition costs (237 ) (1,307 ) Fair value changes in financial instruments (602 ) (29,626 ) Fair value changes in business acquisitions contingent consideration (11,064 ) — Short term purchase accounting fair value adjustment to deferred revenue — (243 ) Initial public offering expense — (531 ) Discontinued services ( i ) — (6,417 ) Expenses related to financing transactions (50 ) — Other losses or expenses (211 ) (147 ) Net loss $ (11,596 ) $ (41,248 ) (i) During the first quarter of 2020, the Company determined to reduce the footprint of its environmental lab in Berkeley, California, and to exit its non-specialized municipal water engineering service line and its food waste biogas engineering service line. As a part of discontinuing service lines, the Company made the decision to book an additional bad debt reserve related to the uncertainty around the ability to collect on receivables related to these service lines (Note 3). It was determined that the discontinuation of these service lines did not represent a strategic shift that had (or will have) a major effect on the Company’s operations and financial results therefore did not meet the requirements to be classified as discontinued operations.

Related-Party Transactions

Related-Party Transactions	3 Months Ended
Related-Party Transactions	Mar. 31, 2021
Related Party Transactions [Abstract]
Related-Party Transactions	20. RELATED-PARTY TRANSACTIONS The Company engaged a related party to provide Quality of Earnings reports on acquisition targets. The Company paid this related party approximately zero and $0.1 March 31, 2020, respectively The Company ceased using the services of this related party during 2020. During the year ended December 31, 2020, the holder of the Redeemable Series A-1 Preferred Stock and Convertible and Redeemable A-2 Preferred Stock became a stockholder in the Company. On the redemption date of the Redeemable Series A-1 Preferred Stock (Note 15), the Company issued 1,786,739 shares of common stock as dividend payment. Additionally, this related party exercised its warrant options (Note 11), becoming the holder of 2,534,239 additional common shares.

Defined Contribution Plan

Defined Contribution Plan	3 Months Ended
Defined Contribution Plan	Mar. 31, 2021
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	21. DEFINED CONTRIBUTION PLAN On January 1, 2014, the Company established the Montrose Environmental Group 401(k) Savings Plan (the “401(k) Savings Plan”). As of March 31, 2021 and December 31, 2020, plan participants may defer up to 85.0% of their eligible wages for the year, up to the Internal Revenue Service dollar limit and catch up contribution allowed by law. Prior to May 22, 2020, the Company provided employer matching contributions equal to 100.0% of the first 3.0% of the participant’s compensation and 50.0% of the participant’s elective deferrals that exceed 3.0% but do not exceed 4.0% of the participant’s compensation. Beginning on May 22, 2020, the Company temporarily ceased making employer contributions. Employer contributions were reinstated beginning on April 23, 2021. Employer contributions under the 401(k) Savings Plan for the three months ended March 31, 2021 and March 31, 2020 were zero 0.8

Subsequent Events

Subsequent Events	3 Months Ended
Subsequent Events	Mar. 31, 2021
Subsequent Events [Abstract]
Subsequent Events	22. SUBSEQUENT EVENTS Business Acquisitions —On April 1, 2021, the Company issued 539,607 shares of common stock valued at a price per share of $46.33 and paid in cash $25.0 million as additional consideration for the acquisition of CTEH pursuant to the 2020 earn-out provision of the transaction agreement. On April 1, 2021, the Company issued 24,200 shares of common stock valued at a price per share of $44.81 and paid in cash $8.0 million as additional consideration for the acquisition of MSE pursuant to the purchase price true up and working capital surplus provision of the transaction agreement. Debt —Effective April 27, 2021, the Company entered into a new Senior Secured Credit Agreement providing for a new $300.0 million credit facility comprised of a $175.0 million term loan and a $125.0 million revolving line of credit (the “2021 Credit Facility”) , and used a portion of the proceeds from the 2021 Credit Facility to repay all amounts outstanding under the 2020 Credit Facility (Note 12). The 2021 revolving credit facility includes a $20.0 million sublimit for the issuance of letters of credit. Subject to certain exceptions, all amounts under the 2021 Credit Facility will become due on April 27, 2026. The company has the option to borrow incremental term loans or request an increase in the aggregate commitments under the revolving credit facility up to an aggregate amount of $150.0 million subject to the satisfaction of certain conditions. The 2021 Credit Facility term loan must be repaid in quarterly installments and Amortization Table Year 1 Year 2 Year 3 Year 4 Year 5 Term Loan 5.0 % 5.0 % 7.5 % 7.5 % 10.0 % The 2021 Credit Facility term loan and the revolver bear interest subject to the Company’s leverage ratio and LIBOR as follows: Pricing Tier Net Leverage Ratio Senior Credit Facilities Commitment Fee Letter of Credit Fee LIBOR Base Rate 1 ≥ 3.75x to 1.0 2.50 % 1.50 % 0.25 % 2.50 % 2 <3.75x to 1.0 but ≥ 3.25 to 1.0 2.25 1.25 0.23 2.25 3 <3.25 to 1.0 but ≥ 2.50 to 1.0 2.00 1.00 0.20 2.00 4 <2.50 to 1.0 but ≥ 1.75 to 1.0 1.75 0.75 0.15 1.75 5 <1.75 to 1.0 1.50 0.50 0.15 1.50 Additionally, the Company may receive an interest rate adjustment of up to 0.05% under the 2021 Credit Facility based on the Company’s performance against certain defined sustainability and environmental, social and governance related objectives. The 2021 Credit Facility includes a number of covenants imposing certain restrictions on the Company’s business, as well as financial covenants requiring the Company to remain below a maximum total net leverage ratio of 4.25 times, which steps down to 4.00 times beginning with the fiscal quarter ending December 31, 2022 through and including the fiscal quarter ending September 30, 2023 and then to 3.75 times beginning with the fiscal quarter ending December 31, 2023, and a minimum fixed charge coverage ratio of 1.25 times.

Description of the Business a_2

Description of the Business and Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation —The unaudited condensed consolidated financial statements include the operations of the Company and its wholly-owned subsidiaries. These unaudited condensed consolidated financial statements are presented in conformity with accounting principles generally accepted in the United States (“U.S. GAAP”) and have been prepared pursuant to the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) that permit reduced disclosure for interim periods. The unaudited condensed consolidated financial statements include all accounts of the Company and, in the opinion of management, include all recurring adjustments and normal accruals necessary for a fair statement of the Company’s financial position, results of operations and cash flows for the dates and periods presented. These unaudited condensed consolidated financial statements should be read in conjunction with the Company’s audited financial statements as of and for the year ended December 31, 2020. Results for interim periods are not necessarily indicative of the results to be expected during the remainder of the current year or for any future period. All intercompany transactions, accounts and profits, have been eliminated in the unaudited condensed consolidated financial statements.
Recently Adopted Accounting Pronouncements /Recently Issued Accounting Pronouncements Not Yet Adopted	Recently Adopted Accounting Pronouncements —The Company qualifies as an emerging growth company as defined in the Jumpstart Our Business Startups Act of 2012 (“JOBS Act”) and therefore intends to take advantage of certain exemptions from various public company reporting requirements, including delaying adoption of new or revised accounting standards until those standards apply to private companies. The Company has elected to use this extended transition period under the JOBS Act. The effective dates shown below reflect the election to use the extended transition period. In December 2019, the Financial Accounting Standards Board (“FASB”) issued Accounting Standard Update (“ASU”) 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. ASU 2019-12 removes certain exceptions to the general principles in Accounting Standard Codification (“ASC”) 740 and clarifies and amends certain guidance to promote consistent application. The standard was adopted as of January 1, 2021 and did not have a material impact on the Company’s unaudited condensed consolidated financial statements. In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation: Improvements to Nonemployee Share-Based Payment Accounting. Under the revised guidance, the accounting for awards issued to non-employees will be similar to the accounting for employee awards. The new guidance is effective for fiscal years beginning after December 15, 2019. The standard was adopted as of January 1, 2020 and did not have a material impact on the Company’s unaudited condensed consolidated financial statements. In January 2017, the FASB issued ASU 2017-04, Intangibles—Goodwill and Other (Topic 350): Simplifying the Accounting for Goodwill Impairment. The revised guidance eliminates Step 2 of the current goodwill impairment analysis test, which requires hypothetical purchase price allocation to measure goodwill impairment. A goodwill impairment loss will instead be measured at the amount by which a reporting unit’s carrying amount exceeds its fair value, not to exceed the carrying amount of goodwill. The revised guidance was adopted as of January 1, 2020 and did not have a material impact on the Company’s unaudited condensed consolidated financial statements. Recently Issued Accounting Pronouncements Not Yet Adopted —In August 2020, the FASB issued ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40)—Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity. The ASU simplifies accounting for convertible instruments by removing major separation models required under current U.S. GAAP. Consequently, more convertible debt instruments will be reported as a single liability instrument with no separate accounting for embedded conversion features. The ASU removes certain settlement conditions that are required for equity contracts to qualify for the derivative scope exception, which will permit more equity contracts to qualify for the exception. The ASU also simplifies the diluted net income per share calculation in certain areas. The new guidance is effective for fiscal years beginning after December 15, 2023, including interim periods within those fiscal years, and early adoption is permitted. The Company is currently evaluating the impact of the adoption of the standard on the unaudited condensed consolidated financial statements. In March 2020, the FASB issued ASU 2020-04, Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting. ASU 2020-04 provides optional expedients and exceptions for applying U.S. GAAP to contracts, hedging relationships and other transactions affected by the expected transition away from reference rates that are expected to be discontinued, such as LIBOR. ASU 2020-04 was effective upon issuance. The Company may elect to apply the guidance prospectively through December 31, 2022. The Company is currently evaluating the impact of the adoption of the standard on the unaudited condensed consolidated financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326). The standard introduces a new model for recognizing credit losses on financial instruments based on an estimate of current expected credit losses and will apply to trade receivables. The new guidance will be effective for the Company’s annual and interim periods beginning after December 15, 2022. The Company does not anticipate the adoption of this standard to have a material impact on the unaudited condensed consolidated financial statements. In February 2016, the FASB issued ASU 2016-2, Leases (Topic 842), to improve financial reporting regarding leasing transactions. The ASU primarily affects the accounting by the lessee in that it requires a lessee to recognize lease assets and liabilities, initially measured at the present value of the lease payments, on the balance sheets for those leases classified as operating leases under previous guidance. The new leasing standard is effective for the Company’s annual and interim periods beginning after December 15, 2021. The Company anticipates a material increase in assets and liabilities due to the recognition of the required right-of-use asset and corresponding liability for all significant lease obligations that are currently classified as operating leases.

Revenues and Accounts Receiva_2

Revenues and Accounts Receivable (Tables)	3 Months Ended
Revenues and Accounts Receivable (Tables)	Mar. 31, 2021
Revenues And Accounts Receivable [Abstract]
Schedule of Contract Balances	The following table presents the Company’s contract balances: March 31, December 31, 2021 2020 Contract assets $ 61,636 $ 38,576 Contract liabilities 6,476 6,114
Schedule of Accounts Receivable, Net	Accounts receivable, net consisted of the following: March 31, December 31, 2021 2020 Accounts receivable, invoiced $ 64,314 $ 57,228 Accounts receivable, other 5,766 1,139 Allowance for doubtful accounts (4,309 ) (4,265 ) Accounts receivable—net $ 65,771 $ 54,102
Schedule of Allowance for Doubtful Accounts	The allowance for doubtful accounts consisted of the following: Beginning Balance Bad Debt Expense Charged to Allowance Other ( 1) Ending Balance Three months ended March 31, 2021 $ 4,265 $ 508 $ (473 ) $ 9 $ 4,309 Year ended December 31, 2020 1,327 4,532 (2,633 ) 1,039 4,265 ____________________ (1) This amount consists of additions to the allowance due to business acquisitions.

Prepaid and Other Current Ass_2

Prepaid and Other Current Assets (Tables)	3 Months Ended
Prepaid and Other Current Assets (Tables)	Mar. 31, 2021
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Schedule of Prepaid and Other Current Assets	Prepaid and other current assets consisted of the following: March 31, December 31, 2021 2020 Deposits $ 796 $ 708 Prepaid expenses 5,613 3,510 Supplies 2,421 2,491 Prepaid and other current assets $ 8,830 $ 6,709

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	3 Months Ended
Property and Equipment, Net (Tables)	Mar. 31, 2021
Property Plant And Equipment [Abstract]
Property and Equipment, Net	Property and equipment, net, consisted of the following: Estimated March 31, December 31, Useful Life 2021 2020 Lab and test equipment 7 years $ 19,017 $ 18,631 Vehicles 5 years 14,047 13,320 Equipment 3–7 years 33,263 32,177 Furniture and fixtures 7 years 3,023 2,938 Leasehold improvements 7 years 6,884 6,767 Aircraft 10 years 834 834 Building 39 years 2,975 2,975 80,043 77,642 Land 725 725 Construction in progress 489 219 Less accumulated depreciation (46,316 ) (44,187 ) Total property and equipment— net $ 34,941 $ 34,399

Business Acquisitions (Tables)

Business Acquisitions (Tables)	3 Months Ended
Business Acquisitions (Tables)	Mar. 31, 2021
Business Acquisition [Line Items]
Summary of Elements of Purchase Price of Acquisitions	The following table summarizes the elements of purchase price of the acquisitions completed during the year ended December 31, 2020: Cash Common Stock Other Purchase Price Components Current Other Purchase Price Components Long Term Contingent Consideration Current Contingent Consideration Long Term Total Purchase Price CTEH $ 175,000 $ 25,000 $ (1,939 ) $ — $ 34,451 $ 10,543 $ 243,055 All other acquisitions 450 — 50 100 210 — 810 Total $ 175,450 $ 25,000 $ (1,889 ) $ 100 $ 34,661 $ 10,543 $ 243,865
Summary of Purchase Price Attributable to Acquisitions	The purchase price attributable to the acquisitions was allocated as follows: CTEH All Other Acquisitions Total Cash $ 1,527 $ — $ 1,527 Accounts receivable 17,059 — 17,059 Other current assets 1,265 — 1,265 Current assets 19,851 — 19,851 Property and equipment 7,042 75 7,117 Customer relationships 56,000 — 56,000 Trade names 4,200 — 4,200 Covenants not to compete 4,000 109 4,109 Proprietary software 14,700 — 14,700 Goodwill 146,983 626 147,609 Total assets 252,776 810 253,586 Current liabilities 9,721 — 9,721 Total liabilities 9,721 — 9,721 Purchase price $ 243,055 $ 810 $ 243,865
Summary of Supplemental Unaudited Pro-Forma Information	The unaudited condensed consolidated financial information summarized in the following table gives effect to the 2021 and the 2020 acquisitions discussed above assuming they occurred on January 1, 2020. These unaudited consolidated pro forma operating results do not assume any impact from revenue, cost or other operating synergies that are expected or may have been realized as a result of the acquisitions. These unaudited consolidated pro forma operating results are presented for illustrative purposes only and are not indicative of the operating results that would have been achieved had the acquisitions occurred on January 1, 2020, nor does the information purport to reflect results for any future period. For the Three Months Ended March 31, 2021 2020 As reported Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) As reported Acquisitions Pro-Forma (Unaudited) Consolidated Pro-Forma (Unaudited) Revenues $ 133,817 $ — $ 133,817 $ 61,031 $ 36,197 $ 97,228 Net (loss) income (11,596 ) — (11,596 ) (41,248 ) 11,352 (29,896 )
MSE Group
Business Acquisition [Line Items]
Summary of Elements of Purchase Price of Acquisitions	The following table summarizes the elements of the purchase price of MSE: Cash Common Stock Other Purchase Price Components Current Other Purchase Price Components Long Term Contingent Consideration Current Contingent Consideration Long Term Total Purchase Price MSE $ 9,082 $ 2,271 $ 10,146 $ — $ — $ 1,804 $ 23,303
Summary of Purchase Price Attributable to Acquisitions	The preliminary purchase price attributable to the MSE acquisition was allocated as follows: MSE Cash $ 2,810 Accounts receivable 3,068 Other current assets 31 Current assets 5,909 Property and equipment 513 Customer relationships 8,720 Trade names 521 Covenants not to compete 922 Goodwill 7,532 Total assets 24,117 Current liabilities (814 ) Total liabilities (814 ) Purchase price $ 23,303

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	3 Months Ended
Goodwill and Intangible Assets (Tables)	Mar. 31, 2021
Goodwill And Intangible Assets Disclosure [Abstract]
Schedule of Amounts Related to Goodwill	Amounts related to goodwill are as follows: Assessment, Permitting and Response Measurement and Analysis Remediation and Reuse Total Balance as of December 31, 2020 $ 162,156 $ 69,054 $ 43,457 $ 274,667 Goodwill acquired during the period — — 7,532 7,532 Balance as of March 31, 2021 $ 162,156 $ 69,054 $ 50,989 $ 282,199
Schedule of Amounts Related to Finite-Lived Intangible Assets	Amounts related to finite-lived intangible assets are as follows: March 31, 2021 Estimated Useful Life Gross Balance Accumulated Amortization Total Intangible Assets—Net Finite lived intangible assets Customer relationships 2–15 years $ 173,502 $ 58,294 $ 115,208 Covenants not to compete 4–5 years 30,864 22,471 8,393 Trade names 1–5 years 17,459 13,445 4,014 Proprietary software 3–5 years 21,260 7,368 13,892 Patent 16 years 17,479 1,671 15,808 Total other intangible assets —net $ 260,564 $ 103,249 $ 157,315 December 31, 2020 Estimated Useful Life Gross Balance Accumulated Amortization Total Intangible Assets—Net Finite lived intangible assets Customer relationships 7–15 years $ 164,782 $ 53,446 $ 111,336 Covenants not to compete 4–5 years 29,942 21,469 8,473 Trade names 1–5 years 16,938 12,849 4,089 Proprietary software 3–5 years 21,007 6,132 14,875 Patent 16 years 17,479 1,398 16,081 Total other intangible assets —net $ 250,148 $ 95,294 $ 154,854
Schedule of Future Amortization Expense	Future amortization expense is estimated to be as follows for each of the five following years and thereafter: December 31, 2021 (remaining) $ 22,667 2022 26,545 2023 21,276 2024 18,096 2025 11,854 2026 and thereafter 56,877 Total $ 157,315

Accounts Payable and Other Ac_2

Accounts Payable and Other Accrued Liabilities (Tables)	3 Months Ended
Accounts Payable and Other Accrued Liabilities (Tables)	Mar. 31, 2021
Payables And Accruals [Abstract]
Summary of Accounts Payable and Other Accrued Liabilities	Accounts payable and other accrued liabilities consisted of the following: March 31, December 31, 2021 2020 Accounts payable $ 15,470 $ 15,481 Accrued expenses 19,555 11,469 Other business acquisitions purchase price obligations 9,511 50 Contract liabilities 6,476 6,114 Other current liabilities 1,229 1,507 Income tax payable 190 256 Total accounts payable and other accrued liabilities $ 52,431 $ 34,877

Accrued Payroll and Benefits (T

Accrued Payroll and Benefits (Tables)	3 Months Ended
Accrued Payroll and Benefits (Tables)	Mar. 31, 2021
Statement Of Financial Position [Abstract]
Schedule of Accrued Payroll and Benefits	Accrued payroll and benefits consisted of the following: March 31, December 31, 2021 2020 Accrued bonuses $ 1,800 $ 5,416 Accrued paid time off 2,355 2,067 Accrued payroll 13,273 9,133 Accrued other 5,136 4,565 Total accrued payroll and benefits $ 22,564 $ 21,181

Debt (Tables)

Debt (Tables)	3 Months Ended
Debt (Tables)	Mar. 31, 2021
Debt Disclosure [Abstract]
Schedule of Debt	Debt consisted of the following: March 31, December 31, 2021 2020 Term loan facility $ 173,359 $ 173,906 Capital leases 3,348 3,088 Equipment line of credit 2,803 3,018 Less deferred debt issuance costs (3,871 ) (4,108 ) Total debt 175,639 175,904 Less current portion of long-term debt (6,214 ) (5,583 ) Long-term debt, less current portion $ 169,425 $ 170,321
Schedule of Applicable Percentages	The applicable margin means a percentage per annum determined in accordance with the following table: Pricing Tier Consolidated Leverage Ratio Commitment Fee Eurodollar Rate Loans and LIBOR Letter of Credit Fee Daily Floating Rate Loans Rate Loans 1 > 3.75 to 1.0 0.50 % 4.00 % 4.00 % 3.00 % 2 ≤ 3.75 to 1.0 but > 3.00 to 1.0 0.50 3.50 3.50 2.50 3 ≤ 3.00 to 1.0 but > 2.25 to 1.0 0.40 3.00 3.00 2.00 4 < 2.25 to 1.0 0.30 2.50 2.50 1.50
Schedule of Capital Leases Mature by 2025	All capital leases (including those purchased through the Company’s equipment line of credit) mature by 2025 as follows: March 31, Payments Interest Principal 2022 $ 2,727 $ 341 $ 2,386 2023 2,203 227 1,976 2024 1,403 98 1,305 2025 507 23 484 Total $ 6,840 $ 689 $ 6,151
Schedule of Aggregate Annual Maturities of Long-Term Debt	The following is a schedule of the aggregate annual maturities of long-term debt presented on the unaudited condensed consolidated statement of financial position, based on the terms of the 2020 Credit Facility, capital lease obligations and equipment line of credit: March 31, 2022 $ 6,214 2023 7,992 2024 7,868 2025 7,047 2026 150,389 Total $ 179,510

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	3 Months Ended
Fair Value of Financial Instruments (Tables)	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Summary of Financial Liabilities Measured at Fair Value on Recurring Basis	The following financial liabilities are measured at fair value on a recurring basis using significant unobservable inputs (Level 3): March 31, December 31, 2021 2020 Business acquisitions contingent consideration, current $ 50,364 $ 49,902 Business acquisitions contingent consideration, long-term 16,971 4,565 Conversion option 21,488 20,886 Total $ 88,823 $ 75,353
Summary of Financial Instruments Measured at Fair Value on Recurring Basis	The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis: Business Acquisitions Contingent Consideration, Current Business Acquisitions Contingent Consideration, Long-term Conversion Option Contingent Put Option Warrant Option Total Balance—at January 1, 2020 $ 8,614 $ 379 $ — $ 7,100 $ 16,878 $ 32,971 Changes in fair value included in earnings — — — 29,627 (1 ) 29,626 Payment of contingent consideration payable (4,703 ) — — — — (4,703 ) Foreign currency translation of contingent consideration payment (208 ) — — — — (208 ) Balance—at March 31, 2020 $ 3,703 $ 379 $ — $ 36,727 $ 16,877 $ 57,686 Balance—at January 1, 2021 $ 49,902 $ 4,565 $ 20,886 $ — $ — $ 75,353 Acquisitions — 1,804 — — — 1,804 Changes in fair value included in earnings 462 10,602 602 — — 11,666 Balance—at March 31, 2021 $ 50,364 $ 16,971 $ 21,488 $ — $ — $ 88,823
Summary of Fair Value of Warrant Options Associated with Issuance of Redeemable Preferred Stock	The fair value of the warrant option associated with the issuance of the Redeemable Series A-1 Preferred Stock was calculated based on the Black-Sholes pricing model using the following assumptions: March 31, 2020 Common stock value (per share) $ 31.60 Expected volatility 35.93 % Risk-free interest rate 0.70 % Expected life (years) 10

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
Commitments and Contingencies (Tables)	Mar. 31, 2021
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments under Leases	The following is a schedule of the future minimum lease payments by year under the leases: March 31, Rent Office Equipment Total 2022 $ 7,105 $ 371 $ 7,476 2023 5,115 268 5,383 2024 3,598 90 3,688 2025 1,757 22 1,779 2026 and thereafter 4,896 — 4,896 Total $ 22,471 $ 751 $ 23,222

Stockholder's Equity (Deficit)

Stockholder's Equity (Deficit) (Tables)	3 Months Ended
Stockholder's Equity (Deficit) (Tables)	Mar. 31, 2021
Summary of Issued Shares of Common Stock	The Company issued the following shares of common stock: Three Months Ended March, 2021 2020 Shares Average Price per Share Shares Average Price per Share Acquisitions 71,740 $ 31.65 — $ — Exercise of warrants 67,713 17.19 — — Exercise of options 330,060 6.62 — — Restricted shares, net 36,817 29.88 — — Total 506,330 $ 13.27 — $ —
Schedule of Share Authorized to be Issue and Available for Grant	The following number of shares were authorized to be issued and available for grant: March 31, 2021 2017 Plan 2013 Plan Total Shares authorized to be issued 3,944,750 2,047,269 5,992,019 Shares available for grant 1,618,996 — 1,618,996 March 31, 2020 2017 Plan 2013 Plan Total Shares authorized to be issued 1,057,785 2,058,619 3,116,404 Shares available for grant 8,217 4,775 12,992
Schedule of Stock Compensation Expense	Total stock compensation expense for the Plans was as follows: Three Months Ended March 31, 2021 2020 2017 plan 2013 plan 2017 plan 2013 plan Options Restricted Stock Options Total Options Restricted Stock Options Total Cost of revenue $ 606 $ — $ 10 $ 616 $ 355 $ — $ 70 $ 425 Selling, general and administrative expense 1,004 182 3 1,189 287 370 68 725 Total $ 1,610 $ 182 $ 13 $ 1,805 $ 642 $ 370 $ 138 $ 1,150
Summary of Weighted Average Assumptions Used in Black-Sholes Option-pricing Model	The following weighted-average assumptions were used in the Black-Sholes option-pricing model calculation: March 31, March 31, 2021 2020 Common stock value (per share) $ 37.61 $ 31.60 Expected volatility 58.01 % 31.87 % Risk-free interest rate 0.71 % 1.78 % Expected life (years) 5.5-7.0 7.0 Forfeiture rate None None Dividend rate None None
Schedule of Common Stock Reserved for Future Issuance	Common Stock Reserved for Future Issuances —The Company has reserved certain stock of its authorized but unissued common stock for possible future issuance in connection with the following: March 31, 2021 2020 Warrants — 650,590 Montrose 2013 Stock Incentive Plan 2,047,269 2,058,619 Montrose 2017 Stock Incentive Plan 3,944,750 1,057,785 Common stock reserved for future issuance 5,992,019 3,766,994
Montrose Amended & Restated 2017 Stock Incentive Plan
Summary of Stock Option Activity	The following summarizes the options activity of the 2017 Plan: Options to Purchase Common Stock Weighted- Average Exercise Price per Share Weighted Average Grant Date Fair Value per Share Weighted Average Remaining Contract Life (in Years) Aggregate Intrinsic Value of In-The- Money Options (in Thousands) Outstanding at January 1, 2020 617,852 $ 24 $ 12 7.82 $ 4,696 Granted 158,062 32 12 — — Forfeited/ cancelled (2,000 ) 32 — — — Outstanding at March 31, 2020 773,914 26 12 8.50 4,693 Outstanding at January 1, 2021 1,840,229 23 12 9.09 15,598 Granted 224,270 38 20 — — Forfeited/ cancelled (10,000 ) 26 — — — Expired (1,250 ) 18 — — — Exercised (17,188 ) 16 — — 519 Outstanding at March 31, 2021 2,036,061 24 13 8.97 52,317 Exercisable at March 31, 2021 218,379 24 — 7.79 5,702 Options vested and expected to vest 2,036,061 24 — 8.97 52,317
Montrose Amended and Restated 2013 Stock Option Plan
Summary of Stock Option Activity	The following summarizes the activity of the 2013 Plan: Options to Purchase Common Stock Weighted- Average Exercise Price per Share Weighted Average Grant Date Fair Value per Share Weighted Average Remaining Contract Life (in Years) Aggregate Intrinsic Value of In-The- Money Options (in Thousands) Outstanding at January 1, 2020 1,855,469 $ 6 $ 1 6.14 46,617 Expired (725 ) 6 — — — Outstanding at March 31, 2020 1,854,744 6 1 6.14 46,521 Outstanding at January 1, 2021 1,787,869 6 — 5.40 43,867 Exercised (311,872 ) 6 — — 12,321 Outstanding at March 31, 2021 1,475,997 6 — 5.14 64,492 Exercisable at March 31, 2021 1,456,922 6 — 5.13 63,740 Options vested and expected to vest 1,475,997 6 — 5.14 64,492

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
Net Loss Per Share (Tables)	Mar. 31, 2021
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders	The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company: Three Months Ended March 31, (In thousands, except for net loss per share) 2021 2020 Net loss $ (11,596 ) $ (41,248 ) Accretion of redeemable series A-1 preferred stock — (5,415 ) Convertible and redeemable series A-2 preferred stock dividend (4,100 ) — Net loss attributable to common stockholders – basic and diluted (15,696 ) (46,663 ) Weighted-average common shares outstanding – basic and diluted 25,117 8,904 Net loss per share attributable to common stockholders – basic and diluted $ (0.62 ) $ (5.24 )
Equity Shares Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders	The following equity shares were excluded from the calculation of diluted net loss per share attributable to common stockholders because their effect would have been anti-dilutive: March 31, 2021 2020 Stock options 3,293,679 2,095,909 Restricted stock 26,201 19,059 Warrants — 116,350

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
Segment Information (Tables)	Mar. 31, 2021
Segment Reporting [Abstract]
Components of Segment Revenues and Adjusted EBITDA	Segment revenues and Adjusted EBITDA consisted of the following: Three Months Ended March 31, 2021 2020 Segment Segment Segment Adjusted Segment Adjusted Revenues EBITDA Revenues EBITDA Assessment, Permitting and Response $ 75,262 $ 15,804 $ 4,530 $ 1,442 Measurement and Analysis 33,440 4,860 36,440 7,379 Remediation and Reuse 25,115 2,481 20,061 2,107 Total Operating Segments 133,817 23,145 61,031 10,928 Corporate and Other — (6,345 ) — (5,375 ) Total $ 133,817 $ 16,800 $ 61,031 $ 5,553
Reconciliation of Segment Measure to Loss Before (Expense) Benefit from Income Taxes	Presented below is a reconciliation of the Company’s segment measure to loss before (expense) benefit from income taxes: For the Three Months Ended March 31, 2021 2020 Total $ 16,800 $ 5,553 Interest expense, net (2,688 ) (2,593 ) Income tax (expense) benefit (2 ) 3,152 Depreciation and amortization (10,769 ) (7,560 ) Stock-based compensation (1,805 ) (1,150 ) Start-up losses and investment in new services (968 ) (379 ) Acquisition costs (237 ) (1,307 ) Fair value changes in financial instruments (602 ) (29,626 ) Fair value changes in business acquisitions contingent consideration (11,064 ) — Short term purchase accounting fair value adjustment to deferred revenue — (243 ) Initial public offering expense — (531 ) Discontinued services ( i ) — (6,417 ) Expenses related to financing transactions (50 ) — Other losses or expenses (211 ) (147 ) Net loss $ (11,596 ) $ (41,248 ) (i) During the first quarter of 2020, the Company determined to reduce the footprint of its environmental lab in Berkeley, California, and to exit its non-specialized municipal water engineering service line and its food waste biogas engineering service line. As a part of discontinuing service lines, the Company made the decision to book an additional bad debt reserve related to the uncertainty around the ability to collect on receivables related to these service lines (Note 3). It was determined that the discontinuation of these service lines did not represent a strategic shift that had (or will have) a major effect on the Company’s operations and financial results therefore did not meet the requirements to be classified as discontinued operations.

Subsequent Events (Tables)

Subsequent Events (Tables)	3 Months Ended
Subsequent Events (Tables)	Mar. 31, 2021
Subsequent Events [Abstract]
Summary of Term Loan Amortization	The 2021 Credit Facility term loan must be repaid in quarterly installments and Amortization Table Year 1 Year 2 Year 3 Year 4 Year 5 Term Loan 5.0 % 5.0 % 7.5 % 7.5 % 10.0 %
Summary of 2021 Credit Facility Interest Rate Subject to Leverage Ratio and LIBOR	The 2021 Credit Facility term loan and the revolver bear interest subject to the Company’s leverage ratio and LIBOR as follows: Pricing Tier Net Leverage Ratio Senior Credit Facilities Commitment Fee Letter of Credit Fee LIBOR Base Rate 1 ≥ 3.75x to 1.0 2.50 % 1.50 % 0.25 % 2.50 % 2 <3.75x to 1.0 but ≥ 3.25 to 1.0 2.25 1.25 0.23 2.25 3 <3.25 to 1.0 but ≥ 2.50 to 1.0 2.00 1.00 0.20 2.00 4 <2.50 to 1.0 but ≥ 1.75 to 1.0 1.75 0.75 0.15 1.75 5 <1.75 to 1.0 1.50 0.50 0.15 1.50

Description of the Business a_3

Description of the Business and Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Millions	Jul. 27, 2020USD ($)$ / sharesshares	Mar. 31, 2021EmployeeOffice$ / sharesshares	Mar. 31, 2020$ / shares
Description Of Business And Basis Of Presentation [Line Items]
Entity formation, month and year		2013-11
Number of offices in which entity operates \| Office		70
Entity number of employees \| Employee		2,000
Common stock issued, shares \| shares		506,330
Share price \| $ / shares		$ 37.61	$ 31.60
IPO
Description Of Business And Basis Of Presentation [Line Items]
Common stock issued, shares \| shares	11,500,000
Share price \| $ / shares	$ 15
Net proceeds from sale of common stock \| $	$ 161.3
Underwriting discounts \| $	11.2
Deferred offset \| $	$ 4.4
Underwriters
Description Of Business And Basis Of Presentation [Line Items]
Common stock issued, shares \| shares	1,500,000

Summary of New Accounting Pro_2

Summary of New Accounting Pronouncements - Additional Information (Details)	Mar. 31, 2021
Accounting Standards Update 2019-12
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2021
Change in accounting principle, accounting standards update, immaterial effect	true
Accounting Standards Update 2018-07
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2020
Change in accounting principle, accounting standards update, immaterial effect	true
Accounting Standards Update 2017-04
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2020
Change in accounting principle, accounting standards update, immaterial effect	true

Revenues and Accounts Receiva_3

Revenues and Accounts Receivable - Schedule of Contract Balances (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Revenues And Accounts Receivable [Abstract]
Contract assets	$ 61,636	$ 38,576
Contract liabilities	$ 6,476	$ 6,114

Revenues and Accounts Receiva_4

Revenues and Accounts Receivable - Additional Information (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021USD ($)Customer	Mar. 31, 2020USD ($)Customer	Dec. 31, 2020USD ($)
Accounts Notes And Loans Receivable [Line Items]
Contracts assets acquired through business acquisitions	$ 500		$ 6,500
Contract liabilities acquired through business acquisitions	500		0
Contract with customer liabilities, revenue recognized	1,200
Revenue remaining performance obligations	25,400		24,400
Bad debt reserve	$ 508	$ 6,333	$ 4,532
Remediation and Reuse Segment
Accounts Notes And Loans Receivable [Line Items]
Bad debt reserve		$ 5,500
Customer Concentration Risk \| Accounts Receivable \| One Customer
Accounts Notes And Loans Receivable [Line Items]
Concentration risk percentage	15.90%		10.20%
Customer Concentration Risk \| Revenue
Accounts Notes And Loans Receivable [Line Items]
Concentration risk percentage		10.00%
Number of customers \| Customer	3	0
Customer Concentration Risk \| Revenue \| One Customer
Accounts Notes And Loans Receivable [Line Items]
Concentration risk percentage	16.30%
Customer Concentration Risk \| Revenue \| Customer Two
Accounts Notes And Loans Receivable [Line Items]
Concentration risk percentage	12.70%
Customer Concentration Risk \| Revenue \| Customer Three
Accounts Notes And Loans Receivable [Line Items]
Concentration risk percentage	10.20%

Revenues and Accounts Receiva_5

Revenues and Accounts Receivable - Additional Information (Details1) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue remaining performance obligations	$ 25.4	$ 24.4
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-04-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue remaining performance obligations	$ 22.3
Revenue remaining performance obligations, satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2022-04-01
Revenue Remaining Performance Obligation Expected Timing Of Satisfaction [Line Items]
Revenue remaining performance obligations	$ 3.1
Revenue remaining performance obligations, satisfaction period

Revenues and Accounts Receiva_6

Revenues and Accounts Receivable - Schedule of Accounts Receivable, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues And Accounts Receivable [Abstract]
Accounts receivable, invoiced	$ 64,314	$ 57,228
Accounts receivable, other	5,766	1,139
Allowance for doubtful accounts	(4,309)	(4,265)	$ (1,327)
Accounts receivable—net	$ 65,771	$ 54,102

Revenues and Accounts Receiva_7

Revenues and Accounts Receivable - Schedule of Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Revenues And Accounts Receivable [Abstract]
Beginning Balance	$ 4,265	$ 1,327	$ 1,327
Bad Debt Expense	508	$ 6,333	4,532
Charged to Allowance	(473)		(2,633)
Other	9		1,039
Ending Balance	$ 4,309		$ 4,265

Prepaid and Other Current Ass_3

Prepaid and Other Current Assets - Schedule of Prepaid and Other Current Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Deposits	$ 796	$ 708
Prepaid expenses	5,613	3,510
Supplies	2,421	2,491
Prepaid and other current assets	$ 8,830	$ 6,709

Property and Equipment, Net - P

Property and Equipment, Net - Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 80,043	$ 77,642
Less accumulated depreciation	(46,316)	(44,187)
Total property and equipment— net	$ 34,941	34,399
Lab and test equipment
Property Plant And Equipment [Line Items]
Estimated Useful Life	7 years
Property and equipment, gross	$ 19,017	18,631
Vehicles
Property Plant And Equipment [Line Items]
Estimated Useful Life	5 years
Property and equipment, gross	$ 14,047	13,320
Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 33,263	32,177
Equipment \| Minimum
Property Plant And Equipment [Line Items]
Estimated Useful Life	3 years
Equipment \| Maximum
Property Plant And Equipment [Line Items]
Estimated Useful Life	7 years
Furniture and fixtures
Property Plant And Equipment [Line Items]
Estimated Useful Life	7 years
Property and equipment, gross	$ 3,023	2,938
Leasehold improvements
Property Plant And Equipment [Line Items]
Estimated Useful Life	7 years
Property and equipment, gross	$ 6,884	6,767
Aircraft
Property Plant And Equipment [Line Items]
Estimated Useful Life	10 years
Property and equipment, gross	$ 834	834
Building
Property Plant And Equipment [Line Items]
Estimated Useful Life	39 years
Property and equipment, gross	$ 2,975	2,975
Land
Property Plant And Equipment [Line Items]
Property and equipment, gross	725	725
Construction in Progress
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 489	$ 219

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Property Plant And Equipment [Abstract]
Depreciation expense	$ 2.2	$ 2

Business Acquisitions - Additio

Business Acquisitions - Additional Information (Details) - USD ($)	Apr. 01, 2021	Apr. 30, 2021	Jan. 31, 2021	Apr. 30, 2020	Mar. 31, 2021	Mar. 31, 2020
Business Acquisition [Line Items]
Transaction costs related to business combinations					$ 237,000	$ 1,307,000
Number of common stock shares issued in connection with business acquisition					71,740
Revenues from discontinued service lines					$ 0	$ 2,500,000
MSE Group
Business Acquisition [Line Items]
Percentage of interests acquired			100.00%
Number of common stock shares issued in connection with business acquisition					71,740
Revenue					$ 4,000,000
Pre-tax loss					$ 300,000
MSE Group \| Subsequent Event
Business Acquisition [Line Items]
Number of common stock shares issued in connection with business acquisition	24,200
MSE Group \| Customer Relationships
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets			7 years
MSE Group \| Related Backlog
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets			2 years
MSE Group \| Trade Names
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets			2 years
MSE Group \| Covenants Not to Compete
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets			5 years
MSE Group \| 2022 Earn Out
Business Acquisition [Line Items]
Business combination, earn-out payment, maximum			$ 6,200,000
MSE Group \| 2023 Earn Out
Business Acquisition [Line Items]
Business combination, earn-out payment, maximum			$ 6,200,000
CTEH
Business Acquisition [Line Items]
Percentage of interests acquired				100.00%
Number of common stock shares issued in connection with business acquisition				791,139
CTEH \| External Proprietary Software
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets				5 years
CTEH \| Subsequent Event
Business Acquisition [Line Items]
Number of common stock shares issued in connection with business acquisition	539,607
CTEH \| Customer Relationships
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets				15 years
CTEH \| Trade Names
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets				5 years
CTEH \| Covenants Not to Compete
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets				5 years
CTEH \| Proprietary Software
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets				3 years
CTEH \| 2020 Earn Out
Business Acquisition [Line Items]
Business combination, earn-out payment, maximum				$ 50,000,000
Business combination, contingent consideration arrangements, basis for amount					The CTEH first year earn-out was calculated at twelve times CTEH’s 2020 EBITDA (as defined in the purchase agreement) in excess of $18.3 million, with a maximum first year earn-out payment of $50.0 million, which was fully achieved.
Business combination, contingent consideration arrangements, description					The 2020 earn-out was initially payable 100.0% in common stock, but as a result of the completion of the Company’s IPO (Note 1), 50.0% was payable in cash. In April 2021, the 2020 earn-out payment was made with 50.0% paid in cash and the remaining 50.0% paid in common stock of the Company (Note 22).
Business combination, minimum EBITDA for contingent consideration arrangements				$ 18,300,000
Percentage of earn out payable in common stock				100.00%
CTEH \| 2020 Earn Out \| Subsequent Event
Business Acquisition [Line Items]
Percentage of earn out paid in cash, as result of completing of IPO		50.00%
Percentage of earn out paid in common stock, as a result of completion of IPO		50.00%
CTEH \| 2020 Earn Out \| Minimum
Business Acquisition [Line Items]
Business combination, actual EBITDA				$ 18,300,000
CTEH \| 2020 Earn Out \| Maximum
Business Acquisition [Line Items]
Business combination, actual EBITDA				22,500,000
CTEH \| 2021 Earn Out
Business Acquisition [Line Items]
Business combination, earn-out payment, maximum				$ 30,000,000
Business combination, contingent consideration arrangements, basis for amount					The second year earn-out is to be calculated at ten times CTEH’s 2021 EBITDA in excess of actual 2020 EBITDA (with actual 2020 EBITDA subject to a minimum of $18.3 million and a maximum of $22.5 million), with a maximum second year earn-out payment of $30.0 million.
Business combination, contingent consideration arrangements, description					The 2021 earn-out, if any, is payable 100.0% in cash.
Percentage of earn out payable in cash				100.00%
Other Acquisition \| Covenants Not to Compete
Business Acquisition [Line Items]
Weighted average useful lives for acquired intangible assets				4 years

Business Acquisitions - Summary

Business Acquisitions - Summary of Elements of Purchase Price of Acquisitions (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2021	Dec. 31, 2020
Business Acquisition [Line Items]
Cash		$ 175,450
Common Stock		25,000
Other Purchase Price Components Current		(1,889)
Other Purchase Price Components Long Term		100
Contingent Consideration Current		34,661
Contingent Consideration Long Term		10,543
Total Purchase Price		243,865
MSE Group
Business Acquisition [Line Items]
Cash	$ 9,082
Common Stock	2,271
Other Purchase Price Components Current	10,146
Contingent Consideration Long Term	1,804
Total Purchase Price	$ 23,303
CTEH
Business Acquisition [Line Items]
Cash		175,000
Common Stock		25,000
Other Purchase Price Components Current		(1,939)
Contingent Consideration Current		34,451
Contingent Consideration Long Term		10,543
Total Purchase Price		243,055
Other Acquisition
Business Acquisition [Line Items]
Cash		450
Other Purchase Price Components Current		50
Other Purchase Price Components Long Term		100
Contingent Consideration Current		210
Total Purchase Price		$ 810

Business Acquisitions - Summa_2

Business Acquisitions - Summary of Purchase Price Attributable to Acquisitions (Details) - USD ($) $ in Thousands	Jan. 31, 2021	Dec. 31, 2020
Business Acquisition [Line Items]
Cash		$ 1,527
Accounts receivable		17,059
Other current assets		1,265
Current assets		19,851
Property and equipment		7,117
Goodwill		147,609
Total assets		253,586
Current liabilities		9,721
Total liabilities		9,721
Purchase price		243,865
Customer Relationships
Business Acquisition [Line Items]
Intangible assets		56,000
Trade Names
Business Acquisition [Line Items]
Intangible assets		4,200
Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets		4,109
Proprietary Software
Business Acquisition [Line Items]
Intangible assets		14,700
MSE Group
Business Acquisition [Line Items]
Cash	$ 2,810
Accounts receivable	3,068
Other current assets	31
Current assets	5,909
Property and equipment	513
Goodwill	7,532
Total assets	24,117
Current liabilities	814
Total liabilities	814
Purchase price	23,303
MSE Group \| Customer Relationships
Business Acquisition [Line Items]
Intangible assets	8,720
MSE Group \| Trade Names
Business Acquisition [Line Items]
Intangible assets	521
MSE Group \| Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets	$ 922
CTEH
Business Acquisition [Line Items]
Cash		1,527
Accounts receivable		17,059
Other current assets		1,265
Current assets		19,851
Property and equipment		7,042
Goodwill		146,983
Total assets		252,776
Current liabilities		9,721
Total liabilities		9,721
Purchase price		243,055
CTEH \| Customer Relationships
Business Acquisition [Line Items]
Intangible assets		56,000
CTEH \| Trade Names
Business Acquisition [Line Items]
Intangible assets		4,200
CTEH \| Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets		4,000
CTEH \| Proprietary Software
Business Acquisition [Line Items]
Intangible assets		14,700
All Other Acquisitions
Business Acquisition [Line Items]
Property and equipment		75
Goodwill		626
Total assets		810
Purchase price		810
All Other Acquisitions \| Covenants Not to Compete
Business Acquisition [Line Items]
Intangible assets		$ 109

Business Acquisitions - Summa_3

Business Acquisitions - Summary of Supplemental Unaudited Pro-Forma Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Business Acquisition [Line Items]
Revenues	$ 133,817	$ 61,031
Net (loss) income	(11,596)	(41,248)
Acquisition Proforma
Business Acquisition [Line Items]
Revenues		36,197
Net (loss) income		11,352
Consolidated Proforma
Business Acquisition [Line Items]
Revenues	133,817	97,228
Net (loss) income	$ (11,596)	$ (29,896)

Goodwill and Intangible Asset_2

Goodwill and Intangible Assets - Schedule of Amounts Related to Goodwill (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
Goodwill [Line Items]
Balance	$ 274,667
Goodwill acquired during the period	7,532
Balance	282,199
Assessment, Permitting and Response
Goodwill [Line Items]
Balance	162,156
Balance	162,156
Measurement and Analysis
Goodwill [Line Items]
Balance	69,054
Balance	69,054
Remediation and Reuse Segment
Goodwill [Line Items]
Balance	43,457
Goodwill acquired during the period	7,532
Balance	$ 50,989

Goodwill and Intangible Asset_3

Goodwill and Intangible Assets - Schedule of Amounts Related to Finite-Lived Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Balance	$ 260,564	$ 250,148
Accumulated Amortization	103,249	95,294
Total Intangible Assets—Net	157,315	154,854
Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Gross Balance	173,502	164,782
Accumulated Amortization	58,294	53,446
Total Intangible Assets—Net	$ 115,208	$ 111,336
Customer Relationships \| Minimum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	2 years	7 years
Customer Relationships \| Maximum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	15 years	15 years
Covenants Not to Compete
Finite-Lived Intangible Assets [Line Items]
Gross Balance	$ 30,864	$ 29,942
Accumulated Amortization	22,471	21,469
Total Intangible Assets—Net	$ 8,393	$ 8,473
Covenants Not to Compete \| Minimum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	4 years	4 years
Covenants Not to Compete \| Maximum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	5 years	5 years
Trade Names
Finite-Lived Intangible Assets [Line Items]
Gross Balance	$ 17,459	$ 16,938
Accumulated Amortization	13,445	12,849
Total Intangible Assets—Net	$ 4,014	$ 4,089
Trade Names \| Minimum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	1 year	1 year
Trade Names \| Maximum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	5 years	5 years
Proprietary Software
Finite-Lived Intangible Assets [Line Items]
Gross Balance	$ 21,260	$ 21,007
Accumulated Amortization	7,368	6,132
Total Intangible Assets—Net	$ 13,892	$ 14,875
Proprietary Software \| Minimum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	3 years	3 years
Proprietary Software \| Maximum
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	5 years	5 years
Patent
Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	16 years	16 years
Gross Balance	$ 17,479	$ 17,479
Accumulated Amortization	1,671	1,398
Total Intangible Assets—Net	$ 15,808	$ 16,081

Goodwill and Intangible Asset_4

Goodwill and Intangible Assets - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
Amortization expense	$ 8.6	$ 5.6

Goodwill and Intangible Asset_5

Goodwill and Intangible Assets - Schedule of Future Amortization Expense (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
2021 (remaining)	$ 22,667
2022	26,545
2023	21,276
2024	18,096
2025	11,854
2026 and thereafter	56,877
Total Intangible Assets—Net	$ 157,315	$ 154,854

Accounts Payable and Other Ac_3

Accounts Payable and Other Accrued Liabilities - Summary of Accounts Payable and Other Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Payables And Accruals [Abstract]
Accounts payable	$ 15,470	$ 15,481
Accrued expenses	19,555	11,469
Other business acquisitions purchase price obligations	9,511	50
Contract liabilities	6,476	6,114
Other current liabilities	1,229	1,507
Income tax payable	190	256
Total accounts payable and other accrued liabilities	$ 52,431	$ 34,877

Accrued Payroll and Benefits -

Accrued Payroll and Benefits - Schedule of Accrued Payroll and Benefits (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Accrued bonuses	$ 1,800	$ 5,416
Accrued paid time off	2,355	2,067
Accrued payroll	13,273	9,133
Accrued other	5,136	4,565
Total accrued payroll and benefits	$ 22,564	$ 21,181

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended
Income Taxes - Additional Information (Details) - USD ($)	Mar. 31, 2021	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Annual effective tax rate ("ETR") from continuing operations	(0.12%)	7.11%
Federal statutory rate	21.00%
Income tax (expense) benefit	$ (2,000)	$ 3,152,000
Uncertain tax positions	$ 0

Warrant Options - Additional In

Warrant Options - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	Jul. 30, 2020	Jul. 27, 2020	Mar. 31, 2021	Apr. 30, 2020	Apr. 13, 2020	Mar. 31, 2020	Oct. 19, 2018
Class Of Warrant Or Right [Line Items]
Period between issuance and expiration of outstanding warrant					10 years
Share price			$ 37.61			$ 31.60
Redeemable Series A-1 Preferred Stock
Class Of Warrant Or Right [Line Items]
Warrants to purchase common stock							534,240
Warrant exercise price per share							$ 0.01
Period between issuance and expiration of outstanding warrant							10 years
Warrants exercised in full for cash, date			Jul. 30, 2020
Redeemable Series A-1 Preferred Stock \| Common Stock
Class Of Warrant Or Right [Line Items]
Warrant exercise price per share	$ 0.01
Exercise of redeemable A1 preferred stock warrant shares	2,534,239
Redeemable Series A-2 Preferred Stock
Class Of Warrant Or Right [Line Items]
Warrants to purchase common stock				1,351,960
Warrant exercise price per share				$ 0.01
Period between issuance and expiration of outstanding warrant				10 years
Aggregate value to determine number of shares underlying the warrant and issuable upon exercise of warrants				$ 30
Convertible And Redeemable Series A-2 Preferred Stock
Class Of Warrant Or Right [Line Items]
Warrants to purchase common stock		1,999,999
Share price		$ 15
Increase of shares		648,039

Debt - Schedule of Debt (Detail

Debt - Schedule of Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Apr. 13, 2020
Line Of Credit Facility [Line Items]
Total debt			$ 225,000
Capital leases	$ 3,348	$ 3,088
Less deferred debt issuance costs	(3,871)	(4,108)
Total debt	175,639	175,904
Less current portion of long-term debt	(6,214)	(5,583)
Long-term debt, less current portion	169,425	170,321
Term Loan Facility
Line Of Credit Facility [Line Items]
Total debt	173,359	173,906	$ 175,000
Equipment Line Of Credit
Line Of Credit Facility [Line Items]
Total debt	$ 2,803	$ 3,018

Debt - Additional Information (

Debt - Additional Information (Details) - USD ($)

Sep. 30, 2022

Sep. 30, 2021

Oct. 06, 2020

Sep. 30, 2020

Apr. 13, 2020

Mar. 31, 2021

Mar. 31, 2020

Oct. 06, 2020

Dec. 31, 2020

Dec. 31, 2022

Dec. 31, 2021

May 16, 2019

Mar. 12, 2019

Line Of Credit Facility [Line Items]

Line of credit

$ 225,000,000

Credit facility maturity date

Apr. 30,
2025

Debt instrument, repayments term

Quarterly

Debt instrument, repayments

$ 500,000

Maximum consolidated leverage ratio

425.00%

Minimum consolidated fixed charge coverage ratio

125.00%

Consolidated total leverage ratio

3.10%

2.70%

Weighted average interest rate

5.50%

Capital Lease Obligations

Line Of Credit Facility [Line Items]

Capital leased assets, gross

$ 6,200,000

$ 6,100,000

Capital leases, income statement, amortization expense

$ 600,000

$ 200,000

Minimum | Capital Lease Obligations

Line Of Credit Facility [Line Items]

Capital lease asset estimated useful lives

3 years

Maximum | Capital Lease Obligations

Line Of Credit Facility [Line Items]

Capital lease asset estimated useful lives

7 years

Forecast

Line Of Credit Facility [Line Items]

Debt instrument, repayments

$ 1,600,000

$ 1,100,000

Maximum consolidated leverage ratio

375.00%

400.00%

Incremental Term Loans

Line Of Credit Facility [Line Items]

Line of credit facility, maximum borrowing capacity

$ 100,000,000

Incremental Term Loans | Minimum

Line Of Credit Facility [Line Items]

Percentage of pari passu debt of all lenders to be held by unitranche lenders

70.00%

Leverage Ratio Greater Than Equal to 3.25

Line Of Credit Facility [Line Items]

Percentage of prepayment on excess cash flow less voluntary prepayments of term loan

50.00%

Consolidated fixed leverage ratio

3.25%

Leverage Ratio Less Than 3.00

Line Of Credit Facility [Line Items]

Percentage of prepayment on excess cash flow less voluntary prepayments of term loan

25.00%

Consolidated fixed leverage ratio

3.00%

Equipment Line Of Credit

Line Of Credit Facility [Line Items]

Line of credit facility, maximum borrowing capacity

$ 2,000,000

Canadian Equipment Line Of Credit

Line Of Credit Facility [Line Items]

Line of credit facility, maximum borrowing capacity

$ 1,000,000

LIBOR Plus | Prior Senior Secured Credit Facility

Line Of Credit Facility [Line Items]

Interest rate

1.00%

Federal Funds Rate Plus | Prior Senior Secured Credit Facility

Line Of Credit Facility [Line Items]

Interest rate

0.50%

Term Loan Facility

Line Of Credit Facility [Line Items]

Line of credit

$ 175,000,000

$ 173,359,000

$ 173,906,000

Term Loan Facility | Prior Senior Secured Credit Facility

Line Of Credit Facility [Line Items]

Line of credit

50,000,000

Term Loan Facility | LIBOR Plus

Line Of Credit Facility [Line Items]

Interest rate

4.50%

5.00%

Term Loan Facility | Base Rate Plus

Line Of Credit Facility [Line Items]

Interest rate

4.00%

Term Loan Facility | LIBOR Floor

Line Of Credit Facility [Line Items]

Interest rate

1.00%

Revolving Line of Credit

Line Of Credit Facility [Line Items]

Line of credit

$ 50,000,000

Line of credit facility unused commitment fee

0.35%

Revolving Line of Credit | Prior Senior Secured Credit Facility

Line Of Credit Facility [Line Items]

Line of credit

$ 130,000,000

Revolving Line of Credit | LIBOR Plus

Line Of Credit Facility [Line Items]

Interest rate

3.50%

Revolving Line of Credit | Base Rate Plus

Line Of Credit Facility [Line Items]

Interest rate

2.50%

Debt - Schedule of Applicable P

Debt - Schedule of Applicable Percentages (Detail)	3 Months Ended	12 Months Ended
Debt - Schedule of Applicable Percentages (Detail)	Mar. 31, 2021	Dec. 31, 2020
Line Of Credit Facility [Line Items]
Consolidated Leverage Ratio	3.10%	2.70%
Prior Senior Secured Credit Facility \| Pricing Tier 1
Line Of Credit Facility [Line Items]
Commitment Fee	0.50%
Prior Senior Secured Credit Facility \| Pricing Tier 1 \| Minimum
Line Of Credit Facility [Line Items]
Consolidated Leverage Ratio	375.00%
Prior Senior Secured Credit Facility \| Pricing Tier 1 \| Eurodollar Rate Loans and LIBOR Letter of Credit Fee
Line Of Credit Facility [Line Items]
Interest rate	4.00%
Prior Senior Secured Credit Facility \| Pricing Tier 1 \| Daily Floating Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	4.00%
Prior Senior Secured Credit Facility \| Pricing Tier 1 \| Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	3.00%
Prior Senior Secured Credit Facility \| Pricing Tier 2
Line Of Credit Facility [Line Items]
Commitment Fee	0.50%
Prior Senior Secured Credit Facility \| Pricing Tier 2 \| Maximum
Line Of Credit Facility [Line Items]
Consolidated Leverage Ratio	375.00%
Prior Senior Secured Credit Facility \| Pricing Tier 2 \| Minimum
Line Of Credit Facility [Line Items]
Consolidated Leverage Ratio	300.00%
Prior Senior Secured Credit Facility \| Pricing Tier 2 \| Eurodollar Rate Loans and LIBOR Letter of Credit Fee
Line Of Credit Facility [Line Items]
Interest rate	3.50%
Prior Senior Secured Credit Facility \| Pricing Tier 2 \| Daily Floating Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	3.50%
Prior Senior Secured Credit Facility \| Pricing Tier 2 \| Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	2.50%
Prior Senior Secured Credit Facility \| Pricing Tier 3
Line Of Credit Facility [Line Items]
Commitment Fee	0.40%
Prior Senior Secured Credit Facility \| Pricing Tier 3 \| Maximum
Line Of Credit Facility [Line Items]
Consolidated Leverage Ratio	300.00%
Prior Senior Secured Credit Facility \| Pricing Tier 3 \| Minimum
Line Of Credit Facility [Line Items]
Consolidated Leverage Ratio	225.00%
Prior Senior Secured Credit Facility \| Pricing Tier 3 \| Eurodollar Rate Loans and LIBOR Letter of Credit Fee
Line Of Credit Facility [Line Items]
Interest rate	3.00%
Prior Senior Secured Credit Facility \| Pricing Tier 3 \| Daily Floating Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	3.00%
Prior Senior Secured Credit Facility \| Pricing Tier 3 \| Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	2.00%
Prior Senior Secured Credit Facility \| Pricing Tier 4
Line Of Credit Facility [Line Items]
Commitment Fee	0.30%
Prior Senior Secured Credit Facility \| Pricing Tier 4 \| Maximum
Line Of Credit Facility [Line Items]
Consolidated Leverage Ratio	225.00%
Prior Senior Secured Credit Facility \| Pricing Tier 4 \| Eurodollar Rate Loans and LIBOR Letter of Credit Fee
Line Of Credit Facility [Line Items]
Interest rate	2.50%
Prior Senior Secured Credit Facility \| Pricing Tier 4 \| Daily Floating Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	2.50%
Prior Senior Secured Credit Facility \| Pricing Tier 4 \| Rate Loans
Line Of Credit Facility [Line Items]
Interest rate	1.50%

Debt - Schedule of Capital Leas

Debt - Schedule of Capital Leases Mature by 2025 (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
Line Of Credit Facility [Line Items]
Payments	$ 6,840
Interest	689
Principal	6,151
2022
Line Of Credit Facility [Line Items]
Payments	2,727
Interest	341
Principal	2,386
2023
Line Of Credit Facility [Line Items]
Payments	2,203
Interest	227
Principal	1,976
2024
Line Of Credit Facility [Line Items]
Payments	1,403
Interest	98
Principal	1,305
2025
Line Of Credit Facility [Line Items]
Payments	507
Interest	23
Principal	$ 484

Debt - Schedule of Aggregate An

Debt - Schedule of Aggregate Annual Maturities of Long-Term Debt (Details) $ in Thousands	Mar. 31, 2021USD ($)
Debt Disclosure [Abstract]
2022	$ 6,214
2023	7,992
2024	7,868
2025	7,047
2026	150,389
Total	$ 179,510

Fair Value of Financial Instr_3

Fair Value of Financial Instruments - Summary of Financial Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Recurring - Level 3 - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Level 3 Liabilities	$ 88,823	$ 75,353	$ 57,686	$ 32,971
Business Acquisitions Contingent Consideration, Current
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Level 3 Liabilities	50,364	49,902	3,703	8,614
Business Acquisitions Contingent Consideration, Long-Term
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Level 3 Liabilities	16,971	4,565	$ 379	$ 379
Conversion Option
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Level 3 Liabilities	$ 21,488	$ 20,886

Fair Value of Financial Instr_4

Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Details) - Fair Value, Recurring - Level 3 - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance	$ 75,353	$ 32,971
Acquisitions	1,804
Changes in fair value included in earnings	11,666	29,626
Payment of contingent consideration payable		(4,703)
Foreign currency translation of contingent consideration payment		(208)
Ending balance	88,823	57,686
Contingent Put Option
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance		7,100
Changes in fair value included in earnings		29,627
Ending balance		36,727
Business Acquisitions Contingent Consideration, Current
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance	49,902	8,614
Changes in fair value included in earnings	462
Payment of contingent consideration payable		(4,703)
Foreign currency translation of contingent consideration payment		(208)
Ending balance	50,364	3,703
Business Acquisitions Contingent Consideration, Long-Term
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance	4,565	379
Acquisitions	1,804
Changes in fair value included in earnings	10,602
Ending balance	16,971	379
Conversion Option
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance	20,886
Changes in fair value included in earnings	602
Ending balance	$ 21,488
Warrant Option
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance		16,878
Changes in fair value included in earnings		(1)
Ending balance		$ 16,877

Fair Value of Financial Instr_5

Fair Value of Financial Instruments - Summary of Fair Value of Warrant Options Associated with Issuance of Redeemable Preferred Stock (Details)	Apr. 13, 2020	Mar. 31, 2020	Oct. 19, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option	10 years
Redeemable Series A-1 Preferred Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option			10 years
Common Stock Value (per Share) \| Redeemable Series A-1 Preferred Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option		31.60
Expected Volatility \| Redeemable Series A-1 Preferred Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option		0.3593
Risk-free Interest Rate \| Redeemable Series A-1 Preferred Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option		0.0070
Expected Life (Years) \| Redeemable Series A-1 Preferred Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Assumptions used in calculating fair value of the warrant option		10 years

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Commitments And Contingencies [Line Items]
Lessee operating lease, expiration year	2030
Rent expense	$ 2.5	$ 2.1
Office Equipment
Commitments And Contingencies [Line Items]
Lessee operating lease, expiration year	2025

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Lease Payments under Leases (Details) $ in Thousands	Mar. 31, 2021USD ($)
Commitments And Contingencies [Line Items]
2022	$ 7,476
2023	5,383
2024	3,688
2025	1,779
2026 and thereafter	4,896
Total	23,222
Rent
Commitments And Contingencies [Line Items]
2022	7,105
2023	5,115
2024	3,598
2025	1,757
2026 and thereafter	4,896
Total	22,471
Office Equipment
Commitments And Contingencies [Line Items]
2022	371
2023	268
2024	90
2025	22
Total	$ 751

Redeemable Series A-1 Preferr_2

Redeemable Series A-1 Preferred Stock - Additional Information (Details) - USD ($)	Apr. 13, 2020	Mar. 31, 2021	Mar. 31, 2020	Oct. 19, 2018
Temporary Equity [Line Items]
Share price		$ 37.61	$ 31.60
Percentage of dividends paid in cash	14.20%
Redeemable Series A-1 Preferred Stock
Temporary Equity [Line Items]
Temporary equity, shares issued				12,000
Temporary equity, par value per share				$ 0.0001
Warrants to purchase common stock				534,240
Temporary equity, liquidation preference per share				$ 10,000
Temporary equity, maturity month and year	2024-10
Temporary equity, penalty term	3 years
Temporary equity, penalty term upon full exercise of warrant	2 years
Percentage of increase in accrued dividend	1.00%
Temporary equity, covenant description		The Redeemable Series A-1 Preferred Stock contained restrictive covenants. Prior to its redemption, the Company was subject to a consolidated total leverage ratio (including the outstanding principal and accrued dividend on the Redeemable Series A-1 Preferred Stock) limit of less than 10.0 times as of the end of any fiscal quarter ending until maturity.
Percentage of dividends paid in cash		15.00%
Percentage of dividends accrued		14.20%
Fair value changes in the contingent put option			$ 29,600,000
Redeemable Series A-1 Preferred Stock \| Maximum
Temporary Equity [Line Items]
Temporary equity, aggregate amount of redemption requirement	$ 50,000,000
Percentage of accumulated dividends payable in common stock upon IPO	50.00%
Share price	$ 31.60
Percentage of dividend payable in cash	15.00%
Redeemable Series A-1 Preferred Stock \| Minimum
Temporary Equity [Line Items]
Percentage of dividend payable in cash	9.00%

Convertible and Redeemable Se_2

Convertible and Redeemable Series A-2 Preferred Stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Apr. 13, 2020	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Temporary Equity [Line Items]
Period between issuance and expiration of outstanding warrant	10 years
Proceeds from the Series A-2 and Warrant	$ 175,000
Percentage of dividends accrued	14.20%
Preferred stock, dividends paid		$ 4,100
Compound embedded derivative, change in value		$ (602)	$ (29,627)
Convertible And Redeemable Series A-2 Preferred Stock
Temporary Equity [Line Items]
Number of shares issued	17,500	17,500		17,500
Par value per share	$ 0.0001	$ 0.0001		$ 0.0001
Debt issuance costs, net	$ 1,300
Percentage of dividends accrued	15.00%
Preferred stock, dividends paid		$ 4,100
Percentage of discount on common stock market price		15.00%
Percentage of dividend rate stepped downs per year		9.00%
Percentage of dividend rate increase per annum in the event of noncompliance		12.00%
Number of days dividend increase rate applicable noncompliance event occurred		90 days
Percentage of dividend rate increase per annum upon noncompliance occurred and thereafter		14.00%
Number of times increase in debt incurrence test ratio		4.5
Minimum repayment amount		$ 50,000
Minimum repayment amount drop down		$ 25,000
Temporary equity description		The Company may, at its option on any one or more dates, redeem all or a minimum portion (the lesser of (i) $25.0 million in aggregate stated value of the Convertible and Redeemable Series A-2 Preferred Stock and (ii) all of the Convertible and Redeemable Series A-2 Preferred Stock then outstanding) of the outstanding Convertible and Redeemable Series A-2 Preferred Stock in cash.
Aggregate stated value of stock redeemed		$ 25,000
Aggregate liquidation preference		182,200		$ 182,200
Compound embedded derivative, fair value net		21,500
Convertible And Redeemable Series A-2 Preferred Stock \| Other Expense
Temporary Equity [Line Items]
Compound embedded derivative, change in value		600
Convertible And Redeemable Series A-2 Preferred Stock \| 60-Day Period Prior to Seventh Anniversary
Temporary Equity [Line Items]
Temporary equity convertible into common stock		60,000
Convertible And Redeemable Series A-2 Preferred Stock \| Year 5
Temporary Equity [Line Items]
Temporary equity convertible into common stock		60,000
Convertible And Redeemable Series A-2 Preferred Stock \| Year 6
Temporary Equity [Line Items]
Temporary equity convertible into common stock		$ 120,000

Stockholder's Equity (Deficit_2

Stockholder's Equity (Deficit) - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	May 31, 2015
Class Of Stock [Line Items]
Common stock, shares authorized	190,000,000		190,000,000
Common stock, par value	$ 0.000004		$ 0.000004
Stock issued on exercise of warrants	67,713
Total unrecognized stock compensation expense related to unvested options and restricted stock granted under the Plans	$ 18.8	$ 7.2
Unrecognized expense expected to be recognized period	3 years
Montrose Amended & Restated 2017 Stock Incentive Plan
Class Of Stock [Line Items]
Number of restricted stock shares issued	14,532	33,229
Restricted stock fair value market price per share	$ 30.96	$ 31.60
Restricted shares became fully vested and released as common stock	36,817	0
Restricted shares outstanding	281,705	273,122
Restricted shares forfeited	0	0
Montrose Amended & Restated 2017 Stock Incentive Plan \| Restricted Stock
Class Of Stock [Line Items]
Shares of vested over period	1 year
Montrose Amended & Restated 2017 Stock Incentive Plan \| Stock Options \| Board of Directors
Class Of Stock [Line Items]
Shares of vested over period	4 years
Montrose Amended & Restated 2017 Stock Incentive Plan \| Stock Options \| Executive Officers
Class Of Stock [Line Items]
Shares of vested over period	3 years
Vesting frequency of period	1 year
Montrose Amended & Restated 2017 Stock Incentive Plan \| Stock Options \| Second Anniversary \| Board of Directors
Class Of Stock [Line Items]
Award vesting percentage	50.00%
Montrose Amended & Restated 2017 Stock Incentive Plan \| Stock Options \| Fourth Anniversary \| Board of Directors
Class Of Stock [Line Items]
Award vesting percentage	50.00%
Montrose Amended and Restated 2013 Stock Option Plan
Class Of Stock [Line Items]
Shares outstanding from exercised options	579,260	201,600
Private Offering
Class Of Stock [Line Items]
Warrants to purchase common stock				116,350
Warrant exercise price per share				$ 17.19

Stockholder's Equity (Deficit_3

Stockholder's Equity (Deficit) - Summary of Issued Shares of Common Stock (Details)	3 Months Ended
	Mar. 31, 2021$ / sharesshares
Equity [Abstract]
Acquisitions, Shares \| shares	71,740
Exercise of warrants, Shares \| shares	67,713
Exercise of options, Shares \| shares	330,060
Restricted shares, net, Shares \| shares	36,817
Total, Shares \| shares	506,330
Acquisitions, Average Price per Share \| $ / shares	$ 31.65
Exercise of warrants, Average Price per Share \| $ / shares	17.19
Exercise of options, Average Price per Share \| $ / shares	6.62
Restricted shares, net, Average Price per Share \| $ / shares	29.88
Total, Average Price per Share \| $ / shares	$ 13.27

Stockholder's Equity (Deficit_4

Stockholder's Equity (Deficit) - Summary of Number of Shares Authorized to be Issued and Available for Grant (Details) - shares	Mar. 31, 2021	Mar. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized to be issued	5,992,019	3,116,404
Shares available for grant	1,618,996	12,992
2017 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized to be issued	3,944,750	1,057,785
Shares available for grant	1,618,996	8,217
2013 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares authorized to be issued	2,047,269	2,058,619
Shares available for grant		4,775

Stockholder's Equity (Deficit_5

Stockholder's Equity (Deficit) - Schedule of Stock Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	$ 1,805	$ 1,150
2017 Plan \| Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	1,610	642
2017 Plan \| Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	182	370
2013 Plan \| Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	13	138
Cost of Revenue
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	616	425
Cost of Revenue \| 2017 Plan \| Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	606	355
Cost of Revenue \| 2013 Plan \| Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	10	70
Selling, General and Administrative Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	1,189	725
Selling, General and Administrative Expense \| 2017 Plan \| Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	1,004	287
Selling, General and Administrative Expense \| 2017 Plan \| Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	182	370
Selling, General and Administrative Expense \| 2013 Plan \| Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock compensation expense	$ 3	$ 68

Stockholder's Equity (Deficit_6

Stockholder's Equity (Deficit) - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Exercised	(330,060)
Montrose Amended & Restated 2017 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Outstanding Beginning Balance	1,840,229	617,852	617,852
Options to Purchase Common Stock Granted	224,270	158,062
Options to Purchase Common Stock Forfeited/cancelled	(10,000)	(2,000)
Options to Purchase Common Stock Expired	(1,250)
Options to Purchase Common Stock Exercised	(17,188)
Options to Purchase Common Stock Outstanding Ending Balance	2,036,061	773,914	1,840,229	617,852
Options to Purchase Common Stock Exercisable	218,379
Options to Purchase Common Stock Options vested and expected to vest	2,036,061
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price per Share Outstanding Beginning Balance	$ 23	$ 24	$ 24
Weighted-Average Exercise Price per Share Granted	38	32
Weighted-Average Exercise Price per Share Forfeited/cancelled	26	32
Weighted-Average Exercise Price per Share Expired	18
Weighted-Average Exercise Price per Share Exercised	16
Weighted-Average Exercise Price per Share Outstanding Ending Balance	24	26	23	$ 24
Weighted-Average Exercise Price per Share Exercisable	24
Weighted-Average Exercise Price per Share Options vested and expected to vest	24
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Weighted Average Grant Date Fair Value per Share Outstanding Beginning Balance	12	12	12
Weighted Average Grant Date Fair Value per Share Granted	20	12
Weighted Average Grant Date Fair Value per Share Ending Balance	$ 13	$ 12	$ 12	$ 12
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding	8 years 11 months 19 days	8 years 6 months	9 years 1 month 2 days	7 years 9 months 25 days
Weighted Average Remaining Contract Life (in Years) Exercisable	7 years 9 months 14 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest	8 years 11 months 19 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 52,317	$ 4,693	$ 15,598	$ 4,696
Aggregate Intrinsic Value of In-The-Money, Options Exercised	519
Aggregate Intrinsic Value of In-The-Money Options Exercisable	5,702
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 52,317
Montrose Amended and Restated 2013 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options to Purchase Common Stock Outstanding Beginning Balance	1,787,869	1,855,469	1,855,469
Options to Purchase Common Stock Expired		(725)
Options to Purchase Common Stock Exercised	(311,872)
Options to Purchase Common Stock Outstanding Ending Balance	1,475,997	1,854,744	1,787,869	1,855,469
Options to Purchase Common Stock Exercisable	1,456,922
Options to Purchase Common Stock Options vested and expected to vest	1,475,997
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-Average Exercise Price per Share Outstanding Beginning Balance	$ 6	$ 6	$ 6
Weighted-Average Exercise Price per Share Expired		6
Weighted-Average Exercise Price per Share Exercised	6
Weighted-Average Exercise Price per Share Outstanding Ending Balance	6	6	6	$ 6
Weighted-Average Exercise Price per Share Exercisable	6
Weighted-Average Exercise Price per Share Options vested and expected to vest	$ 6
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Weighted Average Grant Date Fair Value per Share Outstanding Beginning Balance		1	$ 1
Weighted Average Grant Date Fair Value per Share Ending Balance		$ 1		$ 1
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contract Life (in Years) Outstanding	5 years 1 month 20 days	6 years 1 month 20 days	5 years 4 months 24 days	6 years 1 month 20 days
Weighted Average Remaining Contract Life (in Years) Exercisable	5 years 1 month 17 days
Weighted Average Remaining Contract Life (in Years) Options vested and expected to vest	5 years 1 month 20 days
Aggregate Intrinsic Value of In-The-Money Options Outstanding	$ 64,492	$ 46,521	$ 43,867	$ 46,617
Aggregate Intrinsic Value of In-The-Money, Options Exercised	12,321
Aggregate Intrinsic Value of In-The-Money Options Exercisable	63,740
Aggregate Intrinsic Value of In-The-Money Options vested and expected to vest	$ 64,492

Stockholder's Equity (Deficit_7

Stockholder's Equity (Deficit) - Summary of Weighted Average Assumptions Used in Black-Sholes Option-pricing Model (Details) - $ / shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock value (per share)	$ 37.61	$ 31.60
Expected volatility	58.01%	31.87%
Risk-free interest rate	0.71%	1.78%
Expected life (years)		7 years
Forfeiture rate	0.00%	0.00%
Dividend rate	0.00%	0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected life (years)	5 years 6 months
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected life (years)	7 years

Stockholder's Equity (Deficit_8

Stockholder's Equity (Deficit) - Schedule of Common Stock Reserved for Future Issuance (Details) - shares	Mar. 31, 2021	Mar. 31, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	5,992,019	3,766,994
Warrants
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance		650,590
Montrose 2013 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	2,047,269	2,058,619
Montrose 2017 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	3,944,750	1,057,785

Net Loss Per Share - Computatio

Net Loss Per Share - Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Net loss	$ (11,596)	$ (41,248)
Accretion of redeemable series A-1 preferred stock		(5,415)
Convertible and redeemable series A-2 preferred stock dividend	(4,100)
Net loss attributable to common stockholders – basic and diluted	$ (15,696)	$ (46,663)
Weighted-average common shares outstanding – basic and diluted	25,117	8,904
NET LOSS PER SHARE ATTRIBUTABLE TO COMMON STOCKHOLDERS— BASIC AND DILUTED	$ (0.62)	$ (5.24)

Net Loss Per Share - Equity Sha

Net Loss Per Share - Equity Shares Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders (Details) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of loss per share amount	3,293,679	2,095,909
Restricted Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of loss per share amount	26,201	19,059
Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of loss per share amount		116,350

Segment Information - Additiona

Segment Information - Additional Information (Details)	3 Months Ended
Segment Information - Additional Information (Details)	Mar. 31, 2021Segment
Segment Reporting [Abstract]
Number of operating segments	3
Number of reportable segments	3

Segment Information - Component

Segment Information - Components of Segment Revenues and Adjusted EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Segment Revenues	$ 133,817	$ 61,031
Segment Adjusted EBITDA	16,800	5,553
Operating Segments
Segment Reporting Information [Line Items]
Segment Revenues	133,817	61,031
Segment Adjusted EBITDA	23,145	10,928
Assessment, Permitting and Response \| Operating Segments
Segment Reporting Information [Line Items]
Segment Revenues	75,262	4,530
Segment Adjusted EBITDA	15,804	1,442
Measurement and Analysis \| Operating Segments
Segment Reporting Information [Line Items]
Segment Revenues	33,440	36,440
Segment Adjusted EBITDA	4,860	7,379
Remediation and Reuse \| Operating Segments
Segment Reporting Information [Line Items]
Segment Revenues	25,115	20,061
Segment Adjusted EBITDA	2,481	2,107
Corporate and Other
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ (6,345)	$ (5,375)

Segment Information - Reconcili

Segment Information - Reconciliation of Segment Measure to Loss Before (Expense) Benefit from Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting [Abstract]
Total	$ 16,800	$ 5,553
Interest expense, net	(2,688)	(2,593)
Income tax (expense) benefit	(2)	3,152
Depreciation and amortization	(10,769)	(7,560)
Stock-based compensation	(1,805)	(1,150)
Start-up losses and investment in new services	(968)	(379)
Acquisition costs	(237)	(1,307)
Fair value changes in financial instruments	(602)	(29,626)
Fair value changes in business acquisitions contingent consideration	(11,064)
Short term purchase accounting fair value adjustment to deferred revenue		(243)
Initial public offering expense		(531)
Discontinued services		(6,417)
Expenses related to financing transactions	(50)
Other losses or expenses	(211)	(147)
Net loss	$ (11,596)	$ (41,248)

Related-Party Transactions - Ad

Related-Party Transactions - Additional Information (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Related Party Transaction [Line Items]
Amount due to related party	$ 0		$ 0
Number of shares issued as dividend payment			1,786,739
Number of additional common stock outstanding owned by related party			2,534,239
Acquisition Targets \| Selling, General and Administrative Expenses
Related Party Transaction [Line Items]
Payment to related party	$ 0	$ 100,000

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		5 Months Ended	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	May 22, 2020	Dec. 31, 2020
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, description	401(k) Savings Plan
Defined contribution plan, maximum annual contributions per employee, percent	85.00%			85.00%
Defined contribution plan, employer matching contribution, percent of match			100.00%
Defined contribution plan participant's compensation percent			3.00%
Defined contribution plan percentage of participant's elective deferrals			50.00%
Selling, General and Administrative Expenses
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employer discretionary contribution amount	$ 0	$ 0.8
Minimum
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan participant's compensation percent	3.00%
Maximum
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan participant's compensation percent	4.00%

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - USD ($)	Apr. 27, 2021	Apr. 01, 2021	Apr. 13, 2020	Jan. 31, 2021	Apr. 30, 2020	Mar. 31, 2021	Dec. 31, 2020
Subsequent Event [Line Items]
Number of common stock shares issued in connection with business acquisition						71,740
Cash							$ 175,450,000
Credit facility maturity date			Apr. 30, 2025
Incremental Term Loans
Subsequent Event [Line Items]
Line of credit facility, maximum borrowing capacity			$ 100,000,000
CTEH
Subsequent Event [Line Items]
Number of common stock shares issued in connection with business acquisition					791,139
Cash							$ 175,000,000
MSE Group
Subsequent Event [Line Items]
Number of common stock shares issued in connection with business acquisition						71,740
Cash				$ 9,082,000
Subsequent Event \| 2021 Credit Facility
Subsequent Event [Line Items]
Line of credit facility, maximum borrowing capacity	$ 300,000,000
Credit facility maturity date	Apr. 27, 2026
Percentage of interest rate adjustment	0.05%
Maximum net leverage ratio	425.00%
Maximum net leverage ratio year two	400.00%
Maximum net leverage ratio year three	375.00%
Minimum fixed charge coverage ratio	125.00%
Subsequent Event \| Incremental Term Loans
Subsequent Event [Line Items]
Line of credit facility, maximum borrowing capacity	$ 150,000,000
Subsequent Event \| Term Loan Facility \| 2021 Credit Facility
Subsequent Event [Line Items]
Line of credit facility, maximum borrowing capacity	175,000,000
Subsequent Event \| Revolving Line of Credit \| 2021 Credit Facility
Subsequent Event [Line Items]
Line of credit facility, maximum borrowing capacity	125,000,000
Subsequent Event \| Revolving Line of Credit \| Letter of Credit \| 2021 Credit Facility
Subsequent Event [Line Items]
Line of credit facility, maximum borrowing capacity	$ 20,000,000
Subsequent Event \| CTEH
Subsequent Event [Line Items]
Number of common stock shares issued in connection with business acquisition		539,607
Shares issued, price per share		$ 46.33
Cash		$ 25,000,000
Subsequent Event \| MSE Group
Subsequent Event [Line Items]
Number of common stock shares issued in connection with business acquisition		24,200
Shares issued, price per share		$ 44.81
Cash		$ 8,000,000

Subsequent Events - Summary of

Subsequent Events - Summary of Term Loan Amortization (Details) - Subsequent Event - 2021 Credit Facility - Term Loan Facility	Apr. 27, 2021
Subsequent Event [Line Items]
Amortization percentage, Year 1	5.00%
Amortization percentage, Year 2	5.00%
Amortization percentage, Year 3	7.50%
Amortization percentage, Year 4	7.50%
Amortization percentage, Year 5	10.00%

Subsequent Events - Summary o_2

Subsequent Events - Summary of 2021 Credit Facility Interest Rate Subject to Leverage Ratio and LIBOR (Details) - Subsequent Event - 2021 Credit Facility	Apr. 27, 2021
Pricing Tier 1
Subsequent Event [Line Items]
Commitment Fee	0.25%
Letter of Credit Fee	2.50%
Pricing Tier 1 \| Minimum
Subsequent Event [Line Items]
Net Leverage Ratio	375.00%
Pricing Tier 1 \| LIBOR
Subsequent Event [Line Items]
Interest rate	2.50%
Pricing Tier 1 \| Base Rate
Subsequent Event [Line Items]
Interest rate	1.50%
Pricing Tier 2
Subsequent Event [Line Items]
Commitment Fee	0.23%
Letter of Credit Fee	2.25%
Pricing Tier 2 \| Maximum
Subsequent Event [Line Items]
Net Leverage Ratio	375.00%
Pricing Tier 2 \| Minimum
Subsequent Event [Line Items]
Net Leverage Ratio	325.00%
Pricing Tier 2 \| LIBOR
Subsequent Event [Line Items]
Interest rate	2.25%
Pricing Tier 2 \| Base Rate
Subsequent Event [Line Items]
Interest rate	1.25%
Pricing Tier 3
Subsequent Event [Line Items]
Commitment Fee	0.20%
Letter of Credit Fee	2.00%
Pricing Tier 3 \| Maximum
Subsequent Event [Line Items]
Net Leverage Ratio	325.00%
Pricing Tier 3 \| Minimum
Subsequent Event [Line Items]
Net Leverage Ratio	250.00%
Pricing Tier 3 \| LIBOR
Subsequent Event [Line Items]
Interest rate	2.00%
Pricing Tier 3 \| Base Rate
Subsequent Event [Line Items]
Interest rate	1.00%
Pricing Tier 4
Subsequent Event [Line Items]
Commitment Fee	0.15%
Letter of Credit Fee	1.75%
Pricing Tier 4 \| Maximum
Subsequent Event [Line Items]
Net Leverage Ratio	250.00%
Pricing Tier 4 \| Minimum
Subsequent Event [Line Items]
Net Leverage Ratio	175.00%
Pricing Tier 4 \| LIBOR
Subsequent Event [Line Items]
Interest rate	1.75%
Pricing Tier 4 \| Base Rate
Subsequent Event [Line Items]
Interest rate	0.75%
Pricing Tier5
Subsequent Event [Line Items]
Commitment Fee	0.15%
Letter of Credit Fee	1.50%
Pricing Tier5 \| Maximum
Subsequent Event [Line Items]
Net Leverage Ratio	175.00%
Pricing Tier5 \| LIBOR
Subsequent Event [Line Items]
Interest rate	1.50%
Pricing Tier5 \| Base Rate
Subsequent Event [Line Items]
Interest rate	0.50%