Open-File Report 1998–0297
Science for Watershed Decisions on Abandoned Mine Lands: Review of Preliminary Results, Denver, Colorado, February 4-5, 1998
The relative contributions of dissolved constituents from natural and mining-related ground-water sources in a watershed affected by abandoned mines need to be determined to prioritize basins for remediation and to assist with the establishment of water-quality standards. The oxygen isotopes of dissolved sulfate can help distinguish between natural and mining-related sources of dissolved constituents. In hydrogeochemical systems where sulfide-mineral oxidation is the main geochemical mechanism for the production of dissolved sulfate (such as in the upper Animas River watershed), the reaction chemistry of the sulfur and oxygen molecules in dissolved sulfate can differ between natural and mining-related hydrogeochemical systems. The reaction chemistry can be affected by: (1) higher concentrations of atmospheric oxygen in mine workings compared to natural ground-water systems; (2) the presence of ferric iron (an oxidant for the oxidation of sulfide minerals); and (3) the presence of sulfide-mineral-oxidizing and sulfate-reducing bacteria. When the oxygen-isotope data are graphically related to dissolved constituents such as sulfate or zinc, water samples from natural springs, open mines, and collapsed mines plot in three separate zones, indicating that different proportions of atmospheric oxygen and water-molecule oxygen have been incorporated into the sulfate molecules or that the sulfate has been affected by sulfate reduction or gypsum dissolution. Methods to determine the relative percentage of natural and mining-related dissolved constituents using the oxygen isotopes of dissolved sulfate include: (1) the isotope dilution equation for simple mixing zones (two sources and one receiving stream); (2) the isotope mass-balance equation for streams receiving dissolved sulfate from multiple sources; and (3) graphical relations and the mathematical solution of simultaneous equations. Integrating oxygen-isotope data from natural and mining-related ground-water sites on a watershed scale can illustrate the relative contributions from the different site types.
1U.S. Geological Survey, P.O. Box 3367, Durango, CO 81302 (wgwright@usgs.gov)
2U.S. Geological Survey, MS 415, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (mamast@usgs.gov)
3U.S. Geological Survey, P.O. Box 3367, Durango, CO 81302 (kjleib@usgs.gov)
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