Balance Sheet Items

Allowance for doubtful accounts

The following table presents the changes in the allowance for doubtful accounts:



	Three Months Ended					Six Months Ended
	June 30,					June 30,
	2020			2019		2020			2019
Balance, beginning of period	$	254		$	—	$	146		$	—
Add: provision	—			—		119			—
Less: deduction and other adjustments	(5		)	—		(16		)	—
Balance, end of period	$	249		$	—	$	249		$	—

Property and equipment, net

The following table shows the components of property and equipment, net:



	June 30, 2020			December 31, 2019
Plant, machinery and equipment	$	79,046		$	75,578
Structural fill site improvements	55,760			55,760
Vehicles	19,263			19,163
Office equipment	2,785			2,741
Buildings and leasehold improvements	262			262
Structural fill sites	7,110			7,110
Construction in progress	7,519			12,324
Total property and equipment	$	171,745		$	172,938
Less: accumulated depreciation	(96,590		)	(87,644		)
Property and equipment, net	$	75,155		$	85,294

Depreciation expense was $4,692 and $3,285 for the three months ended June 30, 2020 and 2019, respectively, and $9,121 and $10,332 for the six months ended June 30, 2020 and 2019, respectively.

Accrued liabilities



	June 30, 2020		December 31, 2019
Accrued expenses	$	23,120	$	20,456
Accrued payroll and bonuses	12,812		13,273
Accrued dividends	850		—
Accrued interest	714		1,761
Accrued liabilities	$	37,496	$	35,490

Asset Retirement Obligations

The Company owns and operates two structural fill sites that will have continuing maintenance and monitoring requirements subsequent to their closure. As of June 30, 2020 and December 31, 2019, the Company has accrued $10,948 and 15,131, respectively, for the asset retirement obligation.

The following table reflects the activity for the asset retirement obligation:



	Three Months Ended						Six Months Ended
	June 30,						June 30,
	2020			2019			2020			2019
Balance, beginning of period	$	12,987		$	24,218		$	15,131		$	26,065
Liabilities incurred	—			—			—			1,017
Liabilities settled	(2,201		)	(3,575		)	(4,533		)	(6,782		)
Accretion	162			302			350			645
Balance, end of period	10,948			20,945			10,948			20,945
Less: current portion	(5,845		)	(14,126		)	(5,845		)	(14,126		)
Non-current portion	$	5,103		$	6,819		$	5,103		$	6,819