INFILTRATION THROUGH A MINE-WASTE DUMP AND TRANSPORT OF METALS TO A NEARBY STREAM, CLEAR CREEK COUNTY, COLORADO

Results of geophysical surveys, tracer studies, and synoptic sampling of the Waldorf mine site in Clear Creek County, Colorado, were used to evaluate the infiltration of adit water through a mine-waste dump to nearby Leavenworth Creek before and after remediation. The common objective of these integrated studies was to evaluate metal transport from several mining-affected areas. Trout are unable to survive in Leavenworth Creek downstream from the mine site due to elevated levels of zinc. Remediation by routing adit water around the waste dump was intended to improve water quality. Before the remediation, a NaCl tracer was used to measure the infiltration rate of adit water moving through the dump. The tracer indicated a maximum flow rate of 90 m/day. Flume measurements of braided surface channels flowing over the dump indicated that 43 percent of the adit discharge infiltrated the dump. Constituents that behaved conservatively in this system (Ca, Mg and Sr) helped to characterize different sources of water and the degree of mixing. Electromagnetic (EM) and direct current (DC) resistivity surveys conducted before and after remediation were used to map physical properties of the mine-waste dump, including preferential flowpaths of sub-surface water. Metal loading from several mining-affected inflows to Leavenworth Creek was determined from a LiBr tracer-injection study and synoptic water-quality sampling of the stream and tributary inflows. Water chemistry of selected constituents such as Ca and Sr was used to distinguish among different sources of mining-affected water, and to quantify the relative contribution from each of these sources to a 930 m stream reach. The pre-remediation sampling results indicate that the dump was actually a net sink for most metals, including copper and zinc. The metals are thought to have sorbed onto manganese-oxide precipitates. The wetland area below the dump did not appear to have a significant effect on zinc concentrations, although the pH generally decreased between the waste dump and the wetland area. This study indicates that the waste dump is not necessarily the largest source of metal loading to the stream. Repeated sampling indicates that metal loading from the waste dump and wetland area, and from the leaching of the dispersed mill tailings along the creek are approximately equal.