Comparison of Predicted and Actual Water Quality at Hard Rock Mines

This study reviewed the history and accuracy of water quality predictions in Environmental Impact Statements (EISs) for major hardrock mines in the United States. An analysis was performed to identify the most common causes of water quality impacts and prediction failures. In addition, an analysis was conducted to determine if there were inherent risk factors at mines that may predispose an operation to having water quality problems. A total of 183 major hardrock metal mines were identified as having operated since 1975, and 137 were subject to the National Environmental Policy Act (NEPA) with EISs or Environmental Assessments (EAs). We obtained, reviewed, and collected information on geologic, hydrologic, geochemical, and engineering characteristics from 104 NEPA documents for 71 of the 137 NEPA mines. We compared actual (operational) and predicted (from EISs) water quality at 25 case study mines. Case study mines were selected based on: the ease of access to information on operational water quality and the variability in geographic location, commodity type, extraction and processing methods, climate, proximity to groundwater and surface water resources, acid drainage potential, and contaminant leaching potential. The major characteristics of the case study mines were similar to those of all 71 mines with reviewed EISs. The most common contaminants of concern identified at the 71 mines were those with known adverse effects to aquatic biota, including metals (especially cadmium, copper, lead, and zinc), cyanide, and ammonia and nitrate (from blasting agents). The ameliorating effect of mitigation on water quality was overestimated in the majority of the case study mines. Of the 25 case study mines, 76% had mining-related exceedences in either surface water or groundwater. Mines with close proximity to water resources and with a moderate to high acid drainage or contaminant leaching potential (15/25 mines) had more frequent mining-related exceedences in surface water and groundwater. These results suggest that the combination of proximity to water resources (including discharges) and moderate to high acid drainage or contaminant leaching potential does increase the risk of water quality impacts and is a good indicator of future adverse water quality impacts. The water quality impacts were caused by failures in mitigation measures or geochemical or hydrologic characterization. Sixteen of 25 mines (64%) exhibited failures in mitigation measures, suggesting that improvements must be made in engineering solutions for preventing environmental contamination.