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.

	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.

	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.

Beginning

Balance

Bad Debt

Expense

Charged to

Allowance

Other⁽¹⁾

Ending

Balance

Three months ended March 31, 2021

4,265

508

(473

)

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.