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Science for Watershed Decisions on Abandoned Mine Lands: Review of Preliminary Results, Denver, Colorado, February 4-5, 1998

Ground-Water Input of Zinc to a Watershed Affected by Acidic-Mine Drainage: Simulation Results and Implications for Remediation--Cement Creek, Upper Animas River Watershed, Colorado

By Katherine Walton-Day,¹ R.L. Runkel,² B.A. Kimball,³ and K.E. Bencala⁴

Solute-transport models can be used to help interpret results from tracer-injection studies in streams. The models can help provide a physically quantitative interpretation of spatial profiles of metal concentrations in streams affected by acidic-mine drainage. Separating the physical (transport) effects on instream-concentration profiles from the chemical (reactive) effects aids in interpreting dominant processes controlling these profiles and in predicting effects of different remediation approaches. The OTIS (One-dimensional Transport with Inflow and Storage) solute-transport model was used to simulate results from spatially intensive synoptic sampling with tracer injection along a 7.5-mile (12-kilometer) reach of Cement Creek upstream from Silverton, Colorado, in the upper Animas River watershed. Simulation results for zinc, in which zinc is assumed to be nonreactive at current instream pH values (pH = 6.5 decreasing to 4.2 over the first half mile of the reach, and thereafter decreasing to about 3.5 at the bottom of the reach), indicate instream concentrations of zinc are greater than can be accounted for by inputs of zinc from measurable surface inflows to the stream. The discrepancy is greatest in the stream reaches between Fairview Gulch and Minnesota Gulch, in the vicinity of Ohio Gulch, and downstream from Niagara Gulch. This result suggests that ground-water discharge probably is an important source of zinc to Cement Creek. Simulations of remediating, or "turning off," zinc-rich inflows at Prospect Gulch and the Main Fork of Cement Creek upstream from the South Fork confluence predict a zinc concentration of about 0.6 milligram per liter at the mouth of Cement Creek, a decrease of about 25 percent compared to measured concentrations. In designing future simulation studies, the following will be the working hypothesis: actual remediation probably would cause instream pH values to increase to a range in which zinc is more chemically reactive. This increased reactivity of zinc could cause zinc concentrations to decrease more than the current remediation simulations indicate. The existence of ground-water sources of zinc, however, limits the effectiveness of any engineered remediation of surface inflows of zinc to Cement Creek because the ground-water inputs may keep instream concentrations of zinc greater than clean-up standards.

¹U.S. Geological Survey, MS 415, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (kwaltond@usgs.gov)

²U.S. Geological Survey, MS 415, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (runkel@usgs.gov)

³U.S. Geological Survey, 1745 West 1700 South, Salt Lake City, UT 84104 (bkimball@usgs.gov)

⁴U.S. Geological Survey, MS439, 345 Middlefield Road, Menlo Park, CA 94025 (kbencala@usgs.gov)

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