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Abandoned Mine Lands Initiative — Providing Science for Watershed Issues

USGS AML Science Team

Historical mining activities in the western U.S. have resulted in contaminated stream reaches in the headwaters of more than forty percent of the watersheds in and west of the Rocky Mountains. Acidic drainage and elevated metal levels affect water quality and the biota that live in these impacted streams, thereby damaging aquatic and riparian habitats, impacting human health, limiting recreational use, and lessening the aesthetic qualities of our public lands. Whereas the rocks exposed in the watershed control water chemistry, precipitation, evapo-transpiration, and temperature, depicted by ecoregions in the western U.S., are the primary variables that control the rate of weathering reactions and subsequent release of metals and acidity. There are more than 25,000 inactive mine sites in the western U.S. and numerous exploration prospects, many of which are on Federal lands. Weathering of waste rock associated with inactive mines is one of the major sources of metals, but natural sources also affect stream reaches.

The geology of the watershed, that is the composition of the rocks and minerals exposed in the surface and near-surface environment within the watershed, control the surface and ground water chemistry through natural weathering processes. Exposure of pyrite to the atmosphere by past mining activity greatly accelerates the rate at which pyrite weathers to form sulfuric acid which, in turn, reacts with other sulfide minerals to release metals into the environment. The type of mineral deposit as well as the surrounding geologic setting within the watershed determine the acidity and metals released to the environment upon weathering. The relationship between pH and total metals in acidic drainage is a function of the mineral deposit characteristics. Weathering of altered rocks that surround mineral deposits may result in dispersed sources of natural acidic drainage within the watershed, or may contribute carbonate which will naturally buffer the acidity caused by weathering. Both processes must be evaluated when setting watershed-scale restoration goals.

The AML Initiative is a collaborative effort between the U.S. Geological Survey, Bureau of Land Management and National Park Service in the U.S. Department of the Interior, and the U.S. Forest Service in the U.S. Department of Agriculture. Two pilots were selected for study in 1996: the Animas River watershed in southwestern Colorado, and the Boulder River watershed in west central Montana. The USGS goals of the AML Initiative are:

• Develop multi-disciplinary methods of investigation that integrate geologic, hydrologic, geochemical and ecological data that can be transferred to Federal land managers,
• Provide Federal land managers with a scientific basis for successful watershed-scale mitigation strategies, and
• Create an integrated science database for use by the public, private entities, and government agencies to make land management decisions.

The science strategy for the AML Initiative is:

• Identify major sources of contaminants within the watershed,
• Determine the physical, chemical, hydrologic, and biological processes that control dispersion of contaminants,
• Determine the adverse effects on aquatic and riparian ecosystems, and
• Determine pre-mining watershed conditions to allow establishment of realistic restoration goals.

These studies are being conducted by a multi-disciplinary team of USGS scientists. AML project reports on these two pilot watersheds will be completed in 2000-2001.

The USGS AML science team is developing new multi-disciplinary methods to address fundamental watershed characterization needs:

• Characterize surface and ground water flow,
• Determine sources of acidic drainage,
• Evaluate impacts on aquatic and riparian habitats and biota,
• Assess suitable repository sites,
• Define pre-mining conditions in the watershed, and
• Use geospatial analysis to integrate science.

These tools are being developed to provide a scientific basis for effective mitigation actions at a watershed scale. The technology will be transferred to the Federal land managers for their use in watershed evaluation. Reports of the USGS scientific work are available on the internet at our AML web site at: http://amli.usgs.gov/. Please check this site for the latest releases of our scientific results.

Successful watershed-scale restoration strategies should be developed in the context of the topographic setting, climatic regime, geologic setting, hydrologic processes, and biological habitat needs of the specific environment. An understanding of the physical, chemical, hydrologic, and biological processes must be articulated to the public in a way that balances human technical capabilities, economic limitations, and realistic restoration goals and timeframes. USGS scientists are working directly with stakeholders, including local and state agencies, private landowners and industry, and other Federal agencies including BLM, NPS, USFS, and EPA to address the restoration issues at the watershed scale and preserve our nation's clean water supply. In a science-based approach to restoration, our work provides data needed to:

• Characterize metal loads from sources within the watershed,
• Define attainable in-stream water quality standards,
• Prioritize inactive mine sites for cleanup,
• Determine feasible restoration scenarios for aquatic and riparian habitats, and
• Provide predictive models for cost effective analysis of mitigation options.

Examples of these activities include:

• Characterize metal loads contributed by sources along the stream reach to determine the relative contribution from these discrete sources to the total in-stream metal load,
• Determine water quality and habitat conditions that will allow recovery of aquatic and riparian ecosystems,
• Identify mine waste piles that generate large acid and metal loads, which if removed, would result in a significant improvement in water quality in the watershed,
• Evaluate mitigation options for large, dispersed metal sources attributable to pre-1935 milling activities where industry practice resulted in impoundment or dumping of mill tailings directly into stream valleys, and
• Analysis of seismic risk, groundwater flow paths, and geologic suitability of waste repository sites.

USGS AML Science Team, 1999, Abandoned Mine lands Initiative—Providing Science for Watershed Issues, in Modreski, P.J., compiler, U.S. Geological Survey Open-File Report 99-321, p. 29-30.

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