Description

USGS scientists and their partners are studying the environmental impact of copper mining at Iron Mountain, California, in conjunction with stakeholders faced with cleaning up the site. Iron Mountain is a massive sulfide ore deposit that has been extensively mined for valuable metals, but has more recently earned the reputation as the one of the Nation's largest point sources of extremely acidic mine drainage containing large amounts of toxic metals. The total concentration of metals has been reported as high as 200 grams per liter. Iron Mountain has been a Superfund site since 1983, and is home of the world's most acidic water (negative 3.6 pH). In a word, remediating the extensive mines in a mountain of rock with almost no acid neutralization capacity is daunting. The scientists have conducted a variety of studies in direct support of the federal and state agency plans for remediation of the site. They include: Mine Shaft Plugging Assessment - Geochemical characterization of Iron Mountain and geochemical modeling studies predicted that if the mine portals were plugged, a pool of water would develop with extremely acidic pH (near 1) and high metal concentrations (grams of dissolved metals per liter). The flooded mine pool would have a volume of about 600,000 cubic meters and would present a high risk to the environment and to the local community because of the potential of catastrophic plug failures, faulty plug seals releasing mine water, and acid seeps developing in other locations. Based on this information plugging of the mine was dropped from consideration.

Characterization and Assessment Studies - USGS scientists and their partners have conducted a variety of geochemical and mineralogical characterization studies to determine the acid-generating potential of the mine sites, estimate the pre-mining water quality, and determine the loads of metals discharging from the mine and transported down the several creeks in the area as well as the Sacramento River. They also developed methods to measure the extremely low pH of the water in this harsh environment. This information has been used to design the capacity of water treatment plants and the scope of other remediation systems at the site.

Sediment Characterization Studies - Metal-contaminated sediments precipitated from the mine drainage have accumulated in the Spring Creek Reservoir and the Keswick Reservoir on the Sacramento River. USGS scientists and their partners participated in EPA's Remedial Investigation by assessing the geochemistry, toxicity, and sorption properties of the sediments in the two reservoirs. Their studies showed the unexpected result that reduced iron in pore waters caused much of the toxicity in the sediments. This new information can be used to develop better mining regulations and to help resource managers restore watersheds impacted by mining. The current focus of their research is on the geochemistry, toxicity, and sorption of metals in sediments formed by the neutralization of acid mine waters that flow into the reservoirs.