POTENTIAL METAL CONTRIBUTIONS FROM BOTTOM SEDIMENTS TO THE WATER IN THE DILLON RESERVOIR, COLORADO

The Dillon Reservoir has been accumulating sediment derived rom the Tenmile Creek, the Blue River, and the Snake River drainages since the construction of the Blue River dam in 1963. XRD patterns of the sediment indicate the presence of goethite, kaolinite, muscovite and quartz and the possible presence of ferryhydrite, which is consistent with the high concentrations of Fe (8948 – 31810 ppm) in the acid-soluble fraction of the sediment. The oxyhydroxide precipitates form upstream and contain metals such as Pb, Cu, Zn, Ni, Cd, and Co. The pH of the water in the Dillon Reservoir varies seasonally between 6.5 and 8.5 and is inversely related to discharge. In addition, the concentrations of metals in the water fluctuate as a function of pH. It is possible that the sediment at the bottom of the Dillon Reservoir could affect the overall chemical composition of the water if changes in pH were to occur in the reservoir. To test this hypothesis, a series of leaching experiments were performed on representative sediment samples collected from the bottom of the reservoir. Approximately 0.5000 g aliquots of sediment from four different locations were exposed to 75 mL solutions of pH 2, 3, 4, 5, and 6 and allowed to equilibrate for 48 hours. The pH of each solution was measured after equilibration and in all cases the pH increased by a minimum of 0.5 to a maximum of 3.89 pH units. In general the concentrations of Fe, Al, Mn, Zn, Ni, Cd, and Co decreased in solution with an increase in pH. Lead concentrations decreased as a function of increase in pH but increased near pH 7 possibly due to the formation of Pb hydroxide species at near neutral pH. Molybdenum concentrations increased with an increase in pH due to the formation of Mo oxyanionic species, which are exchanged for hydroxide ions that compete for sorption sights on the sediment. These experiments indicate that metals in the sediment in the Dillon Reservoir can potentially be released based on changes in pH and therefore may contribute to the overall chemical composition of the water in the Dillon Reservoir.