Proc. Natl. Acad. Sci. USA Vol. 96, pp. 3455–3462, March 1999 Colloquium Paper This paper was presented at the National Academy of Sciences colloquium ‘‘Geology, Mineralogy, and Human Welfare,’’ held November 8–9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA. Negative pH, efflorescent mineralogy, and consequences for environmental restoration at the Iron Mountain Superfund site, California D. KIRK NORDSTROM*† AND CHARLES N. ALPERS‡ *United States Geological Survey, 3215 Marine Street, Boulder, CO 80303-1066; and ‡United States Geological Survey, Placer Hall, 6000 J Street, Sacramento, CA 95819-6129 ABSTRACT The Richmond Mine of the Iron Mountain activities in the U.S. are detailed. Government records indicate copper deposit contains some of the most acid mine waters that many millions, perhaps billions, of fish have been killed ever reported. Values of pH have been measured as low as from mining activities in the U.S. during this century (2). 23.6, combined metal concentrations as high as 200 gyliter, Incidents of arsenic poisoning in residents of Thailand result and sulfate concentrations as high as 760 gyliter. Copious from arsenic contamination of the shallow groundwaters be- quantities of soluble metal sulfate salts such as melanterite, cause of weathering of mine wastes (3). A mine flood disaster chalcanthite, coquimbite, rhomboclase, voltaite, copiapite, in Spain occurred in April 1998 in which about 6 million m3 of and halotrichite have been identified, and some of these are acid water and sulfide tailings escaped from a breached forming from negative-pH mine waters. Geochemical calcu- impoundment and covered about 6,500 acres of farmland and lations show that, under a mine-plugging remediation sce- 3 river banks along a 70-km reach of the Guadiamar River with nario, these salts would dissolve and the resultant 600,000-m fine-grained sulfides (details at www.csic.es). Numerous rivers, mine pool would have a pH of 1 or less and contain several estuaries, and reservoirs throughout the world have been used grams of dissolved metals per liter, much like the current as dumping grounds for the large volumes of waste produced portal effluent water. In the absence of plugging or other during mineral extraction and processing. Mineral processing, at-source control, current weathering rates indicate that the in addition to fossil fuel and metal utilization, has increased the portal effluent will continue for approximately 3,000 years. concentration of selected metals and nonmetals in the atmo- Other remedial actions have greatly reduced metal loads into downstream drainages and the Sacramento River, primarily sphere. The emissions of As, Cd, Cu, Pb, Sb, and Zn from by capturing the major acidic discharges and routing them to anthropogenic sources are all greater than emissions from a lime neutralization plant. Incorporation of geochemical natural sources, sometimes several times higher (4, 5). modeling and mineralogical expertise into the decision- Acid mine drainage is produced primarily by the oxidation making process for remediation can save time, save money, of the common iron disulfide mineral pyrite. Pyrite oxidation and reduce the likelihood of deleterious consequences. is a complex process that proceeds rapidly when this mineral and other sulfides are exposed to air. A simplified represen- tation of this chemical process is given by the reaction of pyrite Mining and Water Quality with air and water, 1 7y 1 3 21 1 22 1 1 Mining of metallic sulfide ore deposits (primarily for Ag, Au, FeS2(s) 2O2(g) H2O(1) Fe(aq) 2SO4(aq) 2H(aq) Cu, Pb, and Zn) produces acid mine waters with high concen- [1] trations of metals that have harmful consequences for aquatic in which the product is a solution of ferrous sulfate and sulfuric life and the environment. Deaths of fish, rodents, livestock, and acid. The dissolved ferrous iron continues to oxidize and crops have resulted from mining activities and have been noted hydrolyze when the mine water is no longer in contact with since the days of the Greek and Roman civilizations. Mining and mineral processing have always created health risks for pyrite surfaces, 1 1 miners and other workers. In addition, mining wastes have Fe2 1 1y O 1 5y H O 3 Fe(OH) 1 2H [2] often threatened the health of nearby residents by exposure to (aq) 4 2(g) 2 2 (l) 3(S) (aq) emissions of sulfur dioxide and oxides of As, Cd, Pb, and Zn producing additional acidity. Iron- and sulfur-oxidizing bac- from smelter stacks and flues, metal-contaminated soils, and teria, especially Thiobacillus ferrooxidans, are known to cata- waters and aquatic life with high concentrations of metals. As lyze these reactions at low pH, increasing reaction rates by with most forms of resource extraction, human health risks several orders of magnitude (6). These processes occur natu- accompany mineral exploitation. rally and, indeed, natural acidic drainage is well known from In 1985, the U.S. Environmental Protection Agency (EPA) many locations (7). Mining has the overall effect of dramati- estimated that 50 billion tons (45 3 1012 kg; 1 ton 5 907 kg) cally increasing the oxidation rates by providing greater ac- of mining and mineral processing wastes had been generated cessibility of air through mine workings, waste rock, and in the United States and about 1 billion tons would continue tailings, by creating greater surface area exposure through to be generated each year (1). More recently, the EPA has described 66 ‘‘damage cases’’ at their web site (www.epa.gov, blasting, grinding, and crushing, and by concentrating sulfides search for Mining and Mineral Processing Wastes, accessed in tailings. The overall rates of sulfide oxidation and metal Sept. 9, 1998) in which environmental injuries from mining Abbreviation: EPA, Environmental Protection Agency. †To whom reprint requests should be addressed. e-mail: dkn@ PNAS is available online at www.pnas.org. usgs.gov. 3455 Downloaded by guest on September 24, 2021 3456 Colloquium Paper: Nordstrom and Alpers Proc. Natl. Acad. Sci. USA 96 (1999) release in areas affected by mining are estimated to be orders Furthermore, the town of Redding (with approximately of magnitude faster than natural rates. 100,000 residents) receives its drinking water from the Sacra- Another process, sometimes overlooked, plays an important mento River, downstream from the Iron Mountain site. Large, role in the environmental consequences of mining: the for- metal-rich sediment deposits containing toxic porewaters have mation of soluble, efflorescent salts. Acid ferrous sulfate built up in Keswick Reservoir, where the Spring Creek drain- solutions often become so enriched through rapid pyrite age from Iron Mountain empties into the Sacramento River. oxidation and evaporation that soluble salts form. These often A brief history of mining, water management, and environ- appear as white, blue-green, yellow to orange or red efflores- mental action at Iron Mountain is outlined in Table 1. cent coatings on surfaces of waste rock, tailings, and in The mineral deposits are primarily massive sulfide lenses as underground or open-pit mines. Acidity and metals, formerly much as 60 m thick containing up to 95% pyrite, variable contained in the acid mine water, are stored in the salts, which amounts of chalcopyrite and sphalerite, and averaging about can quickly be dissolved by a rising groundwater table or be 1% Cu and about 2% Zn. Some disseminated sulfides occur dissolved when exposed to rain and flowing surface waters, and along the south side of the mountain. The deposits at Iron then infiltrate to groundwaters. The Iron Mountain Mine Mountain and elsewhere in the West Shasta mining district are Superfund site is an extreme example of how the formation of Devonian in age and have been classified as Kuroko type, soluble efflorescent minerals can make certain remediation having been formed in an island arc setting in a marine alternatives much more risky and potentially disastrous than environment (9). The country rock is the Balaklala Rhyolite, might otherwise be imagined. a keratophyric rhyolite that has undergone regional metamor- phism during episodes of accretion of oceanic crust to the Iron Mountain continent. The brittle, fractured nature of the altered volcanic bedrock gives rise to a hydrologic conditions dominated by Iron Mountain is located in Shasta County, California, ap- fracture-flow at Iron Mountain. The mineral composition of proximately 14 km northwest of the town of Redding (Fig. 1), the rhyolite is albite, sericite, quartz, kaolinite, epidote, chlo- in the southern part of the Klamath Mountains. ‘‘Iron Moun- rite, and minor calcite; consequently it has little buffering tain Mine’’ is really a group of mines within Iron Mountain that capacity. Kinkel and others (10), Reed (11), and South and include Old Mine, No. 8 Mine, Confidence-Complex, Brick Taylor (12) have documented the chemical and isotopic com- Flat Open Pit Mine, Mattie Mine, Richmond and Richmond positions of ore, gangue, and country-rock minerals in the Extension Mines, and Hornet Mine. Ag, Au, Cu, Fe, Zn, and West Shasta mining district. Weathering of massive sulfide pyrite (for sulfuric acid production) were recovered at various deposits at and near the surface has given rise to large gossan times beginning in the early 1860s and ended with the termi- outcrops, enriched in Ag and Au. The 10 million tons of gossan nation of open-pit mining in 1962. Iron Mountain was once the in place prior to mining is the residue from at least 15 million largest producer of Cu in the state of California, and now it tons of massive sulfide that weathered naturally. A total of 7.5 produces some of the most acidic waters in the world. Prior to Table 1. Brief chronology of Iron Mountain mining and the late 1980s when major remediation efforts began, approx- envrionmental activities imately 2,500 tons of pyrite weathered every year from one mine alone (the Richmond Mine) and water containing about Year Activity 300 tons per year of dissolved Cd, Cu, and Zn drained from the 1860s Discovery of massive gossan outcropping site into the Sacramento River.