Effect of Georgetown Lake on the water quality of Clear Creek, Georgetown, Colorado, 1997-98
Georgetown Lake is a recreational reservoir located in the upper Clear Creek Basin, a designated Superfund site because of extensive metal mining in the past. Metals concentrations in Clear Creek increase as the stream receives runoff from mining-affected areas. In 1997, the U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency, began a study to determine the effect of the reservoir on the transport of metals in Clear Creek. A bathymetric survey determined the capacity of the reservoir to be about 440 acre-feet of water, which remained constant during the study. Average water residence time in the reservoir is about 1-3 days during high flow. During low flow (10 cubic feet per second), average residence is about 22 days without ice cover and about 15 days with a 3-foot-thick ice cover. Sediment samples collected from the bottom of Georgetown Lake contained substantial concentrations of iron (average 25,500 milligrams per kilogram), aluminum (average 12,300 milligrams per kilogram), zinc (2,830 milligrams per kilogram), lead (618 milligrams per kilogram), manganese (548 milligrams per kilogram), and sulfide minerals (average 602 milligrams per kilogram as S). Sediment also contained abundant sulfate-reducing bacteria, indicating anoxic conditions. Algae and diatoms common to cold-water lakes were identified in sediment samples; one genus of algae is known to adapt to low-light conditions such as exist beneath ice cover. Vertical profiles of temperature, specific conductance, pH, and dissolved-oxygen concentrations were measured in the reservoir on July 28, 1997, when inflow to the reservoir was about 170 cubic feet per second and average residence time of water was about 1.3 days, and on February 13, 1998, when the reservoir was covered with about 3 feet of ice, inflow was about 15 cubic feet per second, and average residence time was about 12 days. The measurements on July 28, 1997, showed that the reservoir water was well mixed, although pH and dissolved-oxygen concentrations were increased by photosynthesis near the bottom of the reservoir. Measurements on February 13, 1998, indicated thermal and chemical stratification with warmer water (about 4 degrees Celsius) beneath colder water and increases in pH and dissolved-oxygen concentrations generally occurring near the top of the warmer layer. Concentrations of dissolved oxygen were saturated to over-saturated throughout the water column on both dates, although the concentrations were greater on February 13, 1998, because of colder temperature and photosynthesis. Median pH was about 0.5 unit higher on February 13, 1998, than on July 28, 1997, largely because the longer residence time on February 13, 1998, allowed greater cumulative effects of photosynthesis. Samples of inflow and outflow water were collected from August 1997 to August 1998. Dissolved cadmium and dissolved lead in inflow and outflow samples exceeded acute and chronic water-quality standards during some of the sampling period, whereas dissolved zinc exceeded both standards in inflow and outflow samples during the entire sampling period. Chromium, nickel, and silver were detected in a few samples at small concentrations. Arsenic, selenium, and thallium were not reported in any water samples. Georgetown Lake removes some metals from inflow water and releases others to outflow water. From August 1997 to August 1998, Georgetown Lake estimated outflow loads were about 21 percent less than the inflow load of cadmium and about 11 percent less than the inflow load of zinc. Estimated inflow loads were about 18 percent less than the outflow load of copper, about 13 percent less than the outflow load of iron, and about 27 percent less than the outflow load of manganese. Inflow and outflow loads of lead were essentially balanced. The outflow load of nitrite plus nitrate was about 14 percent less than the inflow load, probably because of plant uptake.