WEBMOD SIMULATIONS OF WATER QUALITY IN GRANITIC AND HYDROTHERMALLY ALTERED WATERSHEDS IN COLORADO – POTENTIAL IMPACTS OF CLIMATE CHANGE

Managers need to better understand how to effectively manage drainage from abandoned and active mines, and minimize potential impacts of future mining operations in light of uncertainty in climate impacts on future water quantity and quality. Many streams in the Colorado mineral belt are naturally acidic as a result of the oxidation of sulfide minerals. This natural acidity can be exacerbated by mining activities that lower the water table, exposing previously anaerobic horizons to the air. For this project, the USGS Water, Energy, and Biogeochemical Model (WEBMOD) was applied to two watersheds that drain the continental divide in Colorado: Loch Vale, a 6.6 km² watershed underlain by granite; and Handcart Gulch, an unmined 3.9 km² watershed underlain by hydrothermally-altered rocks typical of the Colorado mineral belt. The simulations of fluxes of water, major ions, and trace elements through each watershed were calibrated to match observations of groundwater level, stream discharge, and chemical composition of samples retrieved from the vadose zone, groundwater, and stream water. Precipitation and temperature described by the baseline data were then modified based on maximum, likely, and minimum climate-change scenarios, and the potential effect of these climatic changes on stream-water quality were evaluated.