FEATURE
UNDERSTANDING THE
WATER QUALITY
OF PIT LAKES GLENN C. MILLER, W. BERRY LYONS, ANDY DAVIS
The increase in deep pit mining in western North America raises concerns about the environmental impact of mine closure.
Large-scale hard-rock mining in the western United States has increased substantially in the past decade. As a result, over the next several decades deep "pit lakes" are likely to form as open pit metal mines that intersect groundwater are depleted and closed. The poor water quality in several existing pit lakes suggests that the new lakes that form when these mines close may affect wildlife and the surrounding groundwater. Nevada and other states have addressed these concerns by requiring mining companies to undertake geochemical modeling of the future water quality of these lakes to help estimate the environmental risk. These geochemical and hydrologic models are still evolving as the genesis and geochemical evolution of pit lakes are better understood. Pit lakes are a relatively new phenomenon—most are less than 25 years old—and many excavations are now in progress. Because most of these large pit lakes have not been formed, models present the only way to estimate the eventual environmental risk from these lakes. Understanding how pit lakes evolve is difficult because the hydrologic and chemical inputs are qualitatively different from those of most natural lakes. The accuracy of current predictions has not been demonstrated on actual pit lakes. For trace contaminants of concern, such as arsenic and selenium, the range of factors that control input and removal from a pit lake results in uncertainty in the simulations. The recent growth in large-scale mining is the result of new methods of treating low-grade precious metal ore and stable gold prices that have allowed profitable extraction of gold and silver with values less than $8 per ton. One result of this new technology is the development of very large open pits in many areas of western North America, including British Columbia, Arizona, Montana, California and, particularly, Nevada (Figure 1). For example, the Goldstrike Mine of American Barrick Resources in the Carlin Trend of northeastern Nevada is projected to excavate more than one billion tons of rock and produce a pit up to 610 meters (m) deep and more than 2.4 kilometers (km) in diameter. Of the 43 large mines in Nevada with more than 50 employees, eight will each excavate
1 1 8 A • VOL. 30, NO. 3, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
0013-936X/96/0929-118AS12.00/0 © 1996 American Chemical Society
Current large-scale mining operations, like the Betze Pit in northeastern Nevada (top), will result in pit lakes once mining operations cease and groundwater is no longer pumped from the pit. The Betze Pit the largest gold mine in the United States, is being dewatered at the rate of more than 40,000 gallons per minute. The water quality in some pit lakes meets drinkingwater standards, but in others, such as the pit lake in the Robinson Project near Ely, Nev. (above), the water is contaminated with high concentrations of sulfate and several metals.
VOL. 30, NO. 3, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 1 1 9 A
FIGURE 1
Increase in mining to produce more pit lakes With the highest concentration of open-pit metal mines in North America, Nevada hosts more than 30 mines that intersect groundwater and are likely to form pit lakes in the future. Such mines also occur in British Columbia, Arizona, Montana, and California.
more than 500 million tons of ore and waste rock. Many of these large mines intercept groundwater, which, during mining, is pumped from interceptor wells surrounding the mines. When the mining is completed, this pumping will end. As a result, groundwater will flow into the pits and form lakes, a situation that has already occurred in some of the large open pit mines that have closed. The acid, metal-rich waters of the Berkeley Pit in Montana, a large porphyry copper excavation now part of a Superfund complex near Butte, exemplify the problems these mines can create. In 1987 the pH of the Berkely Pit lake, which began to form in 1983, ranged from 2.7 at the surface to 3.2 at depth (1). Dissolved oxygen decreased exponentially over the upper 3 m and became suboxic at depth. Metal concentrations are high, with copper and zinc levels that are toxic to aquatic life (Table 1). Although most existing pit lakes do not exhibit problems as severe as those of the Berkeley Pit, in sampled large pit lakes with volumes greater than 10,000 acre-feet, several constituents exceed drinking water and wildlife standards. Because of the immensity of many of these pits, any remediation of water quality in the pits will be very expensive and may not be realistic. The effects on water quality in the pit lakes are effectively permanent and may affect utilization of water and land resources for the foreseeable future. 1 2 0 A • VOL. 30, NO. 3, 1996/ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
In Nevada alone, more than 30 pit lakes will begin to form in the next 20 years. These lakes, when filled, will contain a total of nearly 1 million acrefeet of water. Several of these water bodies are projected to be quite large; the lake created by the Goldstrike mine is predicted to contain more than 198,000 acre-feet of water and will be 1100 feet (335 m) deep, two-thirds as deep as Lake Tahoe. Pit lakes present a sizeable challenge to regulators who have the authority and responsibility to protect water resources but who must act on modeling predictions that have not been rigorously verified (2). For example, as part of the Nevada Pollution Control Law, the state in 1989 adopted statutes aimed at preventing the release of contamination from mining. These statutes are applicable to pit lakes. "Bodies of water which are a result of mine pits penetrating the water table must not create an impoundment which: has the potential to degrade the ground waters of the state; or has the potential to affect adversely the health of human, terrestrial or avian life," according to the statute (Nevada Administrative Code 445.24352). The Nevada Division of Environmental Protection uses the results of predictive models to assist in deciding whether to grant permits for mining projects. As part of the permitting process, mining companies are required to estimate the risk that a future pit lake will violate the statute by modeling the eventual quality of pit lake water and estimating the risk that poor water quality would cause harm. If the modeling and analysis predict harm, the state can deny the mining permit or require changes in the mining plan that would preclude that problem from occurring. Thus, use of these models for regulatory purposes has generated substantial controversy.
Pit lake chemistry Existing pit lakes illustrate some of the factors that influence water quality and point to the difficulties in understanding their geochemical evolution (Table 1). In 1993, three pits containing water existed at the Robinson District porphyry copper mine in eastern Nevada, including the Ruth, Liberty, and Kimbley pits (3). The Liberty Pit had a pH of 3.2 (Table 1); the Kimbley Pit was alkaline (pH 7.6). Both the Kimbley and Liberty Pits have calcareous rocks making up part of the wall rocks, but in the Liberty Pit acid leaching during historical mining attempts to recover copper from perimeter dumps resulted in an acidic pit lake. The Getchell gold mine in northern Nevada contains arsenopyrite (AsFeS2) as one of the main mineral phases associated with the gold deposit. Three pits filled with water from 1969 through 1985 and then were dewatered when mining resumed. In 1982, filtered composite samples were taken at various depths from each of the three pit lakes. Constituents at concentrations above drinking water standards include hydrogen ion concentration and iron in the South and Center Pits; arsenic in the North Pit; and total dissolved solids (TDS), sulfate, and manganese in all three pits (Table 1). The 20-30 m deep pit lake at the oxide-hosted Cortez Gold Mine in the limestone Roberts Mountain Formation in Nevada started filling with water
TABLE 1
Geochemistry of existing pit lakesa Robinson District (3) Berkeley Pit EPA drinking Butte, MT Liberty Pit Kimbley Pit Constituent water standard 10/16/87 (f) 1993 (0.5 m) 1993 (0.5 m)
Getchell Mine (7) Yerington Pit Yerington, NV South Pit* Center Pit* North Pit* 4/28/82 4/28/92 1991 (f) 4/28/82
Cortez Pit Cortez, NV 1992-93 (M)
pH TDS6 CI F
6.5-S.5(s)** 500 (s) 250 (s) 1.4-2.4
8.07 432 24.4 2.4
N0 3 as N S0 4 As Ba
10 250 0.05 1
(s)
9
5740 0.05 1.3
Cd Cr Cu Fe
0.01 0.05 1 0.3
Pb Mn Hg Se
0.05 0.05 (s) 0.002 0.01
Ag Zn Ca Mg
0.05 5
K Na Total alk.
2.8
(s) (s)
(s)
156 386 95
280 462 201 10 72
3.21 6240 48.9 18.5
7.61 3580 286 3.01
8.45 631 36 1.4
5.96 2110 34.4 2.4
