AbstractThe potential for in situ bioremediation of cyanide (CN) and nitrate (NO₃) contamination within the extracted ore (residue pile) and downgradient groundwater at a CN heap leach mine in New Mexico was assessed through the following steps: (1) identification of the relative abundance of CN‐degrading microorganisms in the contaminated residue pile, (2) identification of amendments to enhance aerobic CN degradation and assimilatory NO₃reduction and determination of the optimal carbon‐to‐nitrogen concentration for degradation, (3) assessment of optimum amendment's influence on biodégradation of CN and NO₃ by experiments with large‐scale columns filled with residue pile material, and (4) evaluation of the potential for other adverse environmental effects, specifically acid rock drainage, due to application of the amendment. These investigations determined that application of a reduced carbon source (sucrose) significantly increased the rate of CN degradation and NO₃ immobilization without increasing the probability of acid rock drainage.