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

Transport and Partitioning of Zinc Among Water, Colloids, and Bed Sediments During Low-Flow Conditions, Animas River Watershed, Colorado

By B.A. Kimball¹ and S.E. Church²

Metals from mine drainage near Silverton, Colorado, affect water quality and aquatic life in the Animas River for more than a hundred miles downstream. Samples of water, colloids, and bed sediments were collected in October 1995 to give an overview of the sources, transport, and partitioning of toxic metals in the Animas River watershed. The transport and partitioning of zinc illustrates how metals move downstream from the upstream sources. Zinc enters the streams as a dissolved solute in water draining source rocks and also as part of the sediments that are eroded from outcrops, waste-rock piles, and tailings. Dissolved zinc is transformed to colloidal zinc as it sorbs to colloidal-iron solids during transport, especially in the mixing zones downstream from Cement and Mineral Creeks. The colloids move with the water but progressively aggregate and settle in calm areas of the stream. In this way, the originally dissolved zinc is added to the zinc in the bed sediments as zinc sorbed to aggregated, colloidal particles. This colloidal zinc occurs in the most easily digested fraction of stream-bed sediments and may be the most readily available zinc for aquatic organisms. The majority of zinc in the noncolloidal fraction of the bed sediments, however, is present as sphalerite, even at great distances from Silverton. Lead-isotopic data from bed-sediment samples indicate that 172 kilometers downstream at Aztec, New Mexico, 57 percent of the lead in bed sediments was derived from sources upstream from Silverton. The annual flushing of zinc during snowmelt runoff greatly increases the colloidal and sediment loads, transporting zinc to these downstream sites. Understanding these physical and chemical processes will help managers make decisions about water-quality management options for the Animas River watershed.

¹U.S. Geological Survey, 1745 West 1700 South, Salt Lake City, UT 84104 (bkimball@usgs.gov)

²U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (schurch@usgs.gov)

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