Geochemical balance of the Dillon Reservoir and investigation of the effects of acid rock-drainage in Summit County, Colorado

This study examines the effects of acid-mine drainage (AMD) and acid-rock drainage (ARD) in streams flowing into the Dillon Reservoir, Summit County, Colorado. The Dillon reservoir was formed in 1963 and has been accumulating metal-rich sediment since that time. The streams entering the reservoir transport trace metals that are dissolved in the water and sorbed to hydroxide particles.; Weathering of ore samples in the laboratory indicate that the pH and chemical composition of the solutions evolve and become similar to samples of AMD collected in the field. The results demonstrate that the mineral composition of the ore controls the chemical composition of the experimental mine effluent.; The distribution of trace metals between water and precipitates is controlled by sorption depending on the pH, which ranges from 2.8 to near 8.0 in the drainage. The dissolved metal concentrations in the water include Zn (up to 3050 ppb), Cu (up to 454 ppb), Pb (up to 30 ppb), and Mo (up to 437 ppb). The highest concentrations of Zn, Cu, and Pb occur in Peru Creek near the Pennsylvania Mine, whereas Tenmile Creek has the highest concentration of Mo. The hydroxides of Fe and Al that form at the confluence of the Snake River with Deer Creek are enriched in trace metals whose concentrations in the sediment of the Dillon Reservoir include Zn (up to 3217 ppm), Cu (up to 195 ppm), Pb (up to 401 ppm), and Mo (up to 83 ppm).; The total value of recoverable metals from a 1 cm thick layer of sediment in the Dillon Reservoir is estimated at {dollar}1,414,550, which can offset the cost of dredging the reservoir in the future.; The water in the Dillon Reservoir has higher concentrations of major elements and lower concentrations of trace metals than predicted based on the chemical composition of tributaries weighted by their respective monthly average discharges. The addition of Na and Ca is likely caused by road salt used by the towns surrounding the reservoir. The deficit of the trace metal concentrations can be explained by sorption in response to increases of pH as well as by uptake by aquatic organisms. The importance of pH in controlling trace-metal concentrations of the Dillon Reservoir is confirmed by the fact that these concentrations are strongly correlated to the pH of the water leaving the reservoir.