Water and sediment study of the Snake River watershed, Colorado, Oct. 9-12, 2001

The Snake River watershed, located upstream from Dillon Reservoir in the central mountains of
Colorado, has been affected by historical base-metal mining. Trout stocked in the Snake River for
recreational purposes do not survive through the winter. Sediment cores analyzed by previous
investigators from the reservoir revealed elevated concentrations of base metals and mercury. We
collected 36 surface water samples (filtered and unfiltered) and 38 streambed-sediment samples from
streams in the Snake River watershed. Analyses of the sediment and water samples show that
concentrations of several metals exceed aquatic life standards in one or both media. Ribbon maps
showing dissolved concentrations of zinc, cadmium, copper, and manganese in water (0.45-micron
filtered and corrected for the ameliorating effect of hardness), and copper, cadmium, and zinc in sediment
indicate reaches where toxic effects on trout would be expected and stream reaches where toxicity
standards for rainbow, brown, and brook trout are exceeded.
Instantaneous loads for sulfate, strontium, iron, cadmium, copper, and zinc were calculated from
0.45-micron-filtered water concentrations and discharge measurements were made at each site. Sulfate
and strontium behave conservatively, whereas copper, cadmium, and zinc are reactive. The dissolved
copper load entering the reservoir is less than 20 percent of the value calculated from some upper reaches;
copper is transferred to suspended and or streambed sediment by sorption to iron oxyhydroxides. Higher
percentages of zinc and cadmium reach the reservoir in dissolved form; however, load calculations
indicate that some of these metals are also precipitated out of solution. The most effective remediation
activities should be concentrated on reducing the dissolved loads of zinc, cadmium, and copper in two
reaches of lower Peru Creek between the confluence with the Snake River and Cinnamon Gulch.
We analyzed all streambed sediment for mercury and selected streambed-sediment and reservoir
core samples for lead isotope signatures. Results indicate that the mercury anomaly in the reservoir
sediment was not from any known source in the Snake River, Blue River, or Tenmile Creek watersheds.
Its source remains an enigma.