WATERSHED-SCALE MODELING APPROACH IN MINERALIZED AREAS

The U.S. Geological Survey Mineral Resources Program is developing methodologies for mineral-environmental assessments to complement mineral-resource assessments. Our analysis is focused on baseline water quality at the watershed scale with aquatic metal toxicity as a qualitative endpoint. We have adopted a broad, integrated approach for first-order prediction of likely baseline water-quality conditions and acid-rock drainage formation. The first-order prediction of baseline water quality will later be used as the basis for evaluating potential future mining impacts. Drawing on available information from previous studies we are discerning what master variables are needed to define baseline water-quality conditions in watersheds with limited available information. One of the most important master variables that control acid generation and acid neutralization in mineralized areas is the type and amount of hydrothermal alteration. Bedrock lithology can be an important factor in determining baseline water quality because weathering of bedrock, soil, and sediment are sources of chemical constituents that ultimately enter hydrological and biological systems, and different source rocks give rise to different water compositions. Consequently, various rock types can influence potential aquatic toxicological effects due to lithological controls on water composition. For example, water collected from watersheds underlain by sedimentary rock tends to have higher mean pH, alkalinity, and Ca concentrations (which tend to protect aquatic organisms from metal toxicity) than water collected from watersheds underlain by igneous rock. The relative importance of climatic and hydrologic variables in water composition in mineralized areas also is being evaluated.