Water Quality Simulation for Planning Restoration of a Mined Watershed

Water quality simulation was performed to evaluate the effects of restoration alternatives on metals transport in a mountainous watershed in Montana, U.S.A. impacted by hundreds of abandoned hardrock mines. The Water Quality Analysis Simulation Program (WASP5), developed by the U.S. Environmental Protection Agency (U.S. EPA), was used to assist in planning restoration of the Upper Tenmile Creek watershed, a major drinking water supply for the City of Helena. Synoptic survey data collected by U.S. EPA and the U.S. Geological Survey were used for model calibration and validation. The effectiveness of eight restoration alternatives was modeled under steady-state, low flow conditions. These alternatives ranged from removal of adit and point source discharges to modification of the water supply scheme to provide higher in-stream flows. The model was also used for a number of related purposes, including evaluation of metals loadings and losses, exceedances of water quality standards, interactions between metals in water and bed sediment, and model and data uncertainties. Although standards exceedances are common throughout the watershed, modeling results indicated that removal of point sources, mine waste near watercourses, and streambed sediment can help improve water quality. Alteration of the water supply scheme and increasing baseflow will also ultimately be required to meet standards for all metals. The model also showed that although adits and point sources contribute significant metals loadings to the stream during baseflow, in some areas shallow groundwater and bed sediment can also be sources of metals. Adsorption and precipitation onto bed sediments are also important loss mechanisms in some locations. The model helped to identify uncertainties in the metal partition coefficients associated with sediment, significance of precipitation reactions, and locations of unidentified sources and losses of metals.