OAR 340-043-0180: Guidelines for the Design, Construction, Operation and Closure of Chemical Mining Operations: Guidelines for Open-Pit Closure

(1) Open pits that will be left as a result of the mining operation shall be assessed prior to, and following, mining operations for the potential to contaminate water to the extent that it might not meet water-quality standards due to buildup of acid or toxic metals. (2) If the Department finds that the potential for water accumulation in the pit(s) exists, the permit applicant shall submit a closure plan for the pit that will address contamination prevention and possible remedial treatment of the water. The closure plan shall, at a minimum, examine the following alternatives: (a) Avoidance, during mining, of acid-generating materials that can be left in place, rather than being exposed to oxidation and weathering; (b) Removal from the pit and disposal, during or after the mining operation, of residual acid-generating materials that would otherwise be left exposed to oxidation and weathering; (c) Protective capping in-situ of residual acid-generating materials; (d) Treatment methods for correcting acidity and toxicity of accumulated water; (e) Installation of an impermeable liner under ponded water to prevent groundwater contamination; (f) Backfilling of the pit(s) to the level necessary to, in conjunction with other appropriate control measures, prevent oxidation of residual acid-generating materials.