STREAMS AS INTEGRATORS OF MINE-DRAINAGE SOURCES AND INSTREAM PROCESSES

By its very nature, a stream is a physical and chemical integrator of a watershed. Physically, a stream receives water from all parts of the catchment. Chemically, constituents from all sources are mixed, promoting chemical reactions. By quantifying the physical processes with a tracer injection, we establish the hydrologic context to evaluate mass loading and chemical reactions. The integrated mass-loading profile identifies the locations of the greatest loading to a stream. In Mineral Creek, Colorado, the major locations of zinc loading correspond to drainage of inactive mines and to dispersed alteration suites. By quantifying these loads, we see that the amount attributed to mine-drainage sources was about 42 percent of the total zinc load. The amount attributed to dispersed geologic sources was about 58 percent. Detailed spatial sampling of Mineral Creek and its inflows within this hydrologic context allows us to quantify the dynamic instream reactions of aluminum. Upstream from the Middle Fork pH was 5.72, the load of aluminum was 66 kilograms per day (kg/day), and colloidal aluminum was 91 percent of the total aluminum load (defined by ultrafiltration). Downstream from Middle Fork, pH decreased to 4.76, the total load increased to 363 kg/day, and colloidal aluminum decreased to 44 percent, indicating the extent of the dissolution reaction. Downstream from the South Fork, which had a pH of 7.20, instream pH increased to 6.78, aluminum load remained constant, but colloidal aluminum became 99 percent of the total. After this transformation, 297 kg/day of the colloidal aluminum was removed from the water column to the streambed through physical processes. This deposited aluminum can greatly affect the recovery of stream ecosystems because of the effect it has on streambed habitat and on fish. Thus, we quantify the relative importance of metal sources and instream reactions as they are integrated by a stream like Mineral Creek.