ASSET RETIREMENT AND ENVIRONMENTAL RECLAMATION OBLIGATIONS

12 Months Ended

Dec. 31, 2023

Asset Retirement Obligation And Accrual For Environmental Loss Contingencies Disclosure [Abstract]

ASSET RETIREMENT AND ENVIRONMENTAL RECLAMATION OBLIGATIONS

Coal mines: We have asset retirement obligations for the final reclamation costs and environmental obligations for post-reclamation monitoring related to the Colowyo Mine and the New Horizon Mine. The New Horizon Mine is currently in post-reclamation monitoring. Two pits at the Colowyo Mine are in final reclamation with the other pit still being actively mined.

Generation: We have asset retirement obligations related to equipment, dams, ponds, wells, ash landfill and underground storage tanks at the generating facilities.

Aggregate carrying amounts of asset retirement obligations and environmental reclamation obligations are as follows (dollars in thousands):



	2023		2022
Obligations at beginning of period	$	187,007	$	90,281
Liabilities incurred	—		—
Liabilities settled	(5,711)		(3,184)
Accretion expense	5,298		6,163
Change in estimate	30,607		93,747
Total obligations at end of period	$	217,201	$	187,007
Less current obligations at end of period	(21,635)		(5,419)
Long-term obligations at end of period	$	195,566	$	181,588

During 2023, we increased the asset retirement obligations related to two pits at the Colowyo Mine by $25.5 million due to revised cost estimates and timing adjustments, with an offsetting decrease in the asset retirement obligation related to the third pit of $1.4 million due to reclamation costs paid and a gain on settlement. In the second quarter of 2022, we increased the environmental reclamation obligation at the New Horizon Mine by $44.9 million due to revised cost estimates. The New Horizon Mine environmental remediation liability is $67.3 million as of December 31, 2023. Of this amount, $36.8 million is recorded on a discounted basis, using a discount rate of 3.25 percent, with total estimated undiscounted future cash outflows of $57.9 million. Environmental obligation expense is included in other operating expenses on our consolidated statement of operations. In the fourth quarter of 2023, we reversed the $44.9 million of environmental obligation expense that was recorded in 2022 as a regulatory item to be amortized to expense over 25 years and recovered from our Utility Members through rates. During 2022, we recorded an additional asset retirement obligation of $36.8 million related to a change in cost estimates for our pond, ash landfill and post-closure reclamation obligations at various generating facilities.

We also have asset retirement obligations with indeterminate settlement dates. These are made up primarily of obligations attached to transmission and other easements that are considered by us to be operated in perpetuity and therefore the measurement of the obligation is not possible. A liability will be recognized in the period in which sufficient information exists to estimate a range of potential settlement dates as is needed to employ a present value technique to estimate fair value.