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

Effects of Selected Mine Dump Piles on Dissolved-Constituent Loads in the Cement Creek Basin, Upper Animas River Watershed, Colorado—A Preliminary Assessment

By Winfield G. Wright,¹ Kenneth J. Leib,² and M. Alisa Mast³

In a watershed approach, the effects of dissolved-constituent discharges from mine entrances, mine-dump piles, and natural-background sources need to be integrated in order to prioritize methods of improving water quality in a watershed affected by abandoned mine lands. An important factor in the upper Animas River watershed is the effect of dissolved-constituent discharges from mine-dump piles on water quality. Two mine-dump piles were chosen for study in the Cement Creek basin upstream from Silverton, Colorado---one at the May Day Mine, which has no water discharging from the adit (a "dry dump"), and one at the Yukon Mine, which has water discharging from the adit (a "wet dump"). Wells were installed in the May Day Mine dump pile to study geochemical processes that occur as ground water infiltrates and percolates through the dump pile. Preliminary estimates of dissolved-constituent loads from the two study sites were obtained by measuring Cement Creek upstream and downstream from the sites during April 1997 and by performing a streamflow-tracer test at the May Day Mine site during October 1997. The gains in dissolved-zinc loads (which may include natural inputs) through the reaches of Cement Creek that pass each site and the percentage of the dissolved-zinc load at the mouth of Cement Creek were as follows: May Day Mine, April 1997, 7 pounds per day (lb/d), 4 percent of Cement Creek; Yukon Mine, April 1997, 6 lb/d, 3 percent of Cement Creek; May Day Mine, October 1997, 4 lb/d, 3 percent of Cement Creek. In comparison, two large natural springs in the Cement Creek basin contribute dissolved-zinc loads of 2 lb/d (1 percent of Cement Creek) and 9 lb/d (5 percent of Cement Creek) during low flow. The principal sources of water to the dry dump include hillslope runoff from the small mountain catchment uphill from the dump (which only flows during snowmelt and rainfall-runoff periods) and ground water that is discharging into the dump pile from beneath the dump. The principal sources of water to the wet dump are discharge from the adit, hillslope runoff from the mountain catchment uphill from the dump, and ground water that is discharging into the dump pile from beneath the dump. Direct rainfall that infiltrates and percolates through the dump piles is not considered to be a large contributor to dissolved-constituent loads from the dump piles; however, rainfall runoff in the form of overland flow from the dump piles might have short-term effects on water quality in streams. The effects of snowmelt on dissolved-constituent loads from the mine-dump piles are currently (1997) being assessed. The dissolved constituents in water discharging from the mine-dump piles result from hillslope runoff and adit discharge that percolate through the dump piles, interact with the dump material, and become rich in dissolved constituents. Hydrologic controls to reroute hillslope runoff and adit discharges around mine-dump piles are the first step in preventing dissolved-constituent discharges from mine-dump piles.

¹U.S. Geological Survey, P.O. Box 3367, Durango, CO 81302 (wgwright@usgs.gov)

²U.S. Geological Survey, P.O. Box 3367, Durango, CO 81302 (kjleib@usgs.gov)

³U.S. Geological Survey, MS 415, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (mamast@usgs.gov)

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