MODELING METALS TMDLS IN A MINING-IMPACTED WATERSHED

Metals transport modeling was performed to help develop metals total maximum daily loads (TMDLs) and evaluate the effects of restoration alternatives on metals transport in a mountain watershed in Montana impacted by hundreds of abandoned hardrock metal mines. These types of watersheds are widespread in Montana and many other areas of the western U.S. Impacts from abandoned hardrock, or metal mines include loadings of sediment, metals, and other pollutants causing impairment of multiple beneficial uses and exceedances of water quality standards. The U.S. Environmental Protection Agency (EPA) Water Quality Analysis Simulation Program (WASP) was used to model and evaluate TMDLs for several heavy metals in Tenmile Creek, a mountain stream supplying drinking water to the City of Helena, Montana. The model was calibrated for baseflow conditions, validated using data collected by EPA and the U.S. Geological Survey, and used to assess existing metals loadings and losses, including interactions between metals in water and bed sediment, uncertainty, water quality standard exceedances, TMDLs, potential source areas, and required reductions in loadings. During baseflow conditions, adits and point sources contribute significant metals loadings to Tenmile Creek. Shallow groundwater and bed sediment also contribute metals loadings in some key locations under these conditions. Adsorption and precipitation onto bed sediments play a primary role in losses from the water column in some areas.Modeling results indicate that some uncertainty exists in the metal partition coefficients associated with sediment, significance of precipitation reactions, and in the specific locations of unidentified sources and losses of metals. Exceedances of standards are widespread throughout the stream under both baseflow and higher flow conditions. TMDLs were evaluated for conservative conditions assuming no reactions in the stream, as well as using the model incorporating the full range of instream processes. In most cases, considerable reductions in loadings are required to achieve TMDLs and water quality standards. The modeling showed that reductions in loadings from point sources, mine waste near watercourses, and streambed sediment can help improve water quality, but alteration of the water supply scheme and increasing baseflow will also be needed to achieve target loadings and concentrations.