S.E. Church, J.D. Causey, and T.C. Sole, U.S. Geological Survey, Denver, Colorado
Geospatial analysis of data on the distribution of historic mines, significant past-producers, and current measures of water quality as indicated by climate and available water quality data have been evaluated for the contiguous U.S. west of 102°. Of the 65,900 metal mining entries in the Materials Availability System (MAS) database from the study area, only about 25,000 have had historic production of metals, 2,569 of which are placer deposits. The majority of the disturbances listed in MAS are exploration workings from which no minerals have been produced.
Acidic drainage results from the oxidation of pyrite and other sulfide minerals and is the natural consequence of exposure of pyritic rocks in response to surface water interaction. This process of weathering produces acidic drainage in areas underlain by hydrothermally altered rock, in areas underlain by pyritic shale or their metamorphic equivalents, or in areas underlain by coal-bearing pyritic shale. Disturbances caused by man in search of extractable resources, construction activities, and road building can expose large volumes of pyritic rock to air and water, accelerating the rate of weathering and resulting in development of acidic drainage.
USGS studies of the environmental response of produced mineral deposits to weathering have shown that the geology of the deposit is an important constraint on the development of acidic mine drainage (AMD). The factors that contribute most to the development of AMD are the mineral deposit type, geologic setting, size of the disturbance, and the climate. A mineral deposit most likely to cause environmental impact: 1) contains a large amount of fine-grained pyrite, 2) contains large tonnages of disturbed material, 3) occurs in rocks that have a low acid-neutralizing capacity, and 4) is located in relatively moist climates. Plots of the distribution of significant mineral producers by mineral deposit type on regional maps showing climatic variation provide a basis for evaluation of regions within the western U.S. where AMD poses a threat to the nation's surface and ground water resources. Geospatial analysis of the distribution of produced mineral deposit types within the ecoregions as outlined by Bailey (1995) can be used to identify areas where AMD may be most likely to occur.
Measures of water quality have been conducted near major population centers rather than in the sparsely populated areas, largely on federal lands in the western U.S. where historic mining has occurred. Thus, many of the watersheds that may have been impacted by historic mining have not been adequately characterized by either state or federal water-quality programs. Geospatial analysis of available data indicate that measures of water quality have not been made sufficiently close to the sites of historic mining to characterize water quality in the watersheds. Activities by local stake-holder groups and some federal and state research programs have focused on these data gaps in some of these watersheds. The Abandoned Mine Lands Initiative is such a cooperative watershed effort between the U.S. Bureau of Land Management, U.S. Forest Service, and the U.S. Geological Survey. Two studies are currently ongoing: 1) in the Animas watershed in southwestern Colorado, and 2) in the Boulder watershed in west central Montana.
Paper presented at the Western U.S. Mining-Impacted Watersheds: Joint Conference on Remediation and Ecological Risk Assessment Technologies, Adams' Mark Hotel, Denver, Colo., Oct. 26-29, 1998, Hosted by the U.S. Environmental Protection Agency.