Sources of High Trace-Element Concentrations in the French Gulch Drainage, Breckenridge, Colorado
By Lori Apodaca, U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO, 80225
Effects of metal mining on surface- and ground-water-quality are an important issue in the Upper Colorado River (UCOL) Basin, Colorado. As part of the U.S. Geological Survey's National Water- Quality Assessment (NAWQA) Program in the UCOL Basin, surface- and ground-water quality is being evaluated in the French Gulch drainage, which is located about 2 miles east of Breckenridge, Colorado, at altitudes ranging from about 9,700 to 11,000 ft. This drainage has undergone extensive placer and underground lode mining from the late 1850's to the 1960's, and large quantities of Ag, Au, Pb, and Zn and minor amounts of Cu were produced from the lode deposits. To determine sources of high trace-element concentrations in the French Gulch drainage, surface-water samples were collected during low and high flow conditions for trace- element analyses. Groundwater samples for trace-element analyses also were collected during high flow. High trace-element concentrations in the drainage are defined as those concentrations that are near or exceed the U.S. Environmental Protection Agency's water-quality criteria for drinking water (for example: concentrations of Zn >5 mg/L). Water in the area generally has a near neutral pH; however, some of the groundwater containing high trace-element concentrations has lower pH (pH values range from 4.7 to 8.1). Specific conductance in surface water generally increases from sites upstream from the mine-affected reach to the sites downstream from the mine along French Gulch. The specific conductance for groundwater associated with mining in the drainage is several thousand mS/cm. In the dissolved phase, Zn concentrations in the surface water ranged from 7 mg/L upstream to 3,100 mg/L downstream from mine affected areas. Groundwater samples associated with the mine tailings had Zn concentrations as large as 1,000 mg/L, and less affected areas had Zn concentrations of several hundred mg/L. A potential source of trace-element concentrations in the surface water is surface runoff, which can leach trace elements from tailings piles onsite. Groundwater, including seeps in the area, can acquire trace elements by flowing through underground mine workings or by percolation of surface water that leaches trace elements from the tailings and recharges the groundwater.