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

Comparison of Surface-Water Chemistry in Undisturbed and Mining-Impacted Areas of the Cement Creek Watershed, Colorado

By M. Alisa Mast,¹ Winfield G. Wright,² and Kenneth J. Leib³

Many tributaries in the upper Animas River watershed are acidic and have elevated concentrations of dissolved metals as a result of historical mining and natural weathering of mineralized volcanic rocks. To gain a better understanding of the relative importance of natural-background and mining-related sources of dissolved metals, more than 70 streams, springs, and draining adits were sampled during summer 1997 in the Cement Creek watershed, a 50-square-kilometer subbasin of the upper Animas River. Although prospect pits and abandoned mines are scattered throughout the study area, much of the watershed, particularly at higher elevations, is relatively undisturbed. Preliminary results indicate that natural-background and mining-impacted areas of the basin produce acidic and neutral surface water, however, concentrations of major ions and dissolved metals generally are higher at the impacted sites than at the natural-background sites. The median pH of samples collected at the mining-impacted sites was 4.28 (range 2.79 to 7.32) compared to the median value of 6.59 (range 2.97 to 7.96) at the natural-background sites. Sulfate concentrations at the sampling sites ranged from 1 to 450 milligrams per liter (mg/L) and were generally higher at the mining-impacted sites (median value 138 mg/L) than at the background sites (median value 56 mg/L). Dissolved zinc concentrations were highly variable among the sampling sites, ranging from <10 to 14,600 micrograms per liter (µg/L). Results from the natural-background springs and streams revealed a geographic pattern in surface-water chemistry that appears to be related to the degree of bedrock alteration. The eastern part of the basin is primarily underlain by propylitically altered lavas, which produce neutral surface water (pH 6.40 to 7.96) with relatively low concentrations of dissolved metals, except for zinc (as much as 230 µg/L). The western part of the basin is more intensely altered than the eastern part, and includes pervasive argillic-type alteration in the northwest quadrant and quartz-sericite-pyrite alteration localized along structures. Water draining these areas is more acidic (pH ranging from 3.25 to 4.61) and has elevated concentrations of dissolved metals. The results of this study should provide useful information for establishing water-quality standards and characterizing sources of metals loads for surface waters in the upper Animas River Basin.

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

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

³U.S. Geological Survey, P.O. Box 3367, Durango, CO 81302 (kjleib@usgs.gov)

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