DEPENDENCE OF TRACE-METAL CONCENTRATIONS ON THE PH OF THE WATER IN THE DILLON RESERVOIR, SUMMIT COUNTY, COLORADO

The Dillon Reservoir, which provides drinking water to the City of Denver, receives water primarily from the Snake River, the Blue River, and Tenmile Creek. The water in these streams is variously contaminated by mine effluent and by weathering of mineralized bedrock. The concentrations of several metals (Zn, Mo, Cu, etc.) in the reservoir vary in response to seasonal changes of the pH (6.5-8.5). This phenomenon is attributable primarily to sorption of cations on hydroxide precipitates of Fe and Al, which form in the tributary streams and are transported into the Dillon Reservoir. The average concentrations of acid-leachable metals in sediment in the central basin of the reservoir include: Fe (20379 ppm), Al (8824 ppm), Pb (182 ppm), Cu (113 ppm), Mo (44 ppm), and Ag (4 ppm). The chemical composition of water in the reservoir was estimated from the chemical composition of the tributaries weighted in accordance with their monthly average discharges for two consecutive water years form September 1998 to October 2000. The Snake River is the principal contributor of Zn (203 ppb) and Cu (3.2 ppb), Tenmile Creek contributes Mo (191 ppb), and the Blue River has the lowest concentrations of all trace metals. The results demonstrate that the water of the Dillon Reservoir has lower than expected concentrations of trace metals. We demonstrate by use of sorption curves for transition metals in water of the Snake River that the deficit of trace metals is explainable by sorption to colloidal particles caused by a small increase of 0.2 pH units, which occurs when the water of the tributaries mixes with the water in the Dillon Reservoir. The water in the Dillon Reservoir meets the drinking water quality standards of the U.S. Environmental Protection Agency.