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

Integration of Mine-Drainage Effects in Watersheds Using Tracer Injections and Synoptic Sampling

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

Effects of acid-mine drainage occur on different scales. An individual mine might affect a single hillside, a small tributary, or an entire watershed. Streams in mineralized regions typically drain both mines and natural sources of metal-rich water that contribute to the metal load of a watershed. The spatial pattern of toxic concentrations results from the contributions of all the sources combined with the interaction of the hydrologic, biologic, and geochemical processes that influence metal transport. Injecting a tracer at a constant rate for a sufficient time provides discharge measurements for spatially intensive synoptic sampling. Three calculations help to integrate all the sources and processes of the watershed: (1) the measured instream load is calculated for each stream site as the product of concentration and discharge, (2) a minimum total loading to the stream comes from summing all positive changes of instream load between stream sites, and (3) a maximum estimate of loading from visible inflows comes from summing the product of measured inflow concentrations and the change in discharge between stream sites around the inflow. Comparison of the three calculations for a tracer-injection study in Cement Creek, Colorado, indicates that 65 percent of the zinc load was from nonpoint sources at low flow. Although 54 percent of the zinc load was transported to the Animas River as dissolved load, the other 46 percent of the zinc load was transferred to colloidal iron and transported as colloidal load or was stored in streambed coatings. Streambed coatings are flushed by snowmelt runoff and contribute to high zinc loads in the Animas River. The perspective gained from using tracer injections and synoptic sampling facilitates planning of remediation efforts.

¹U.S. Geological Survey, 1745 West 1700 South, Salt Lake City, UT 84104 (bkimball@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, MS 415, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (kwaltond@usgs.gov)

⁴U.S. Geological Survey, MS 439, 345 Middlefield Road, Menlo Park, CA 94025 (kbencala@usgs.gov)

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