Protect and Enhance the Nations Natural Resource Base and Environment

Precious Metals Pit Lakes: Controls on Eventual Water Quality

Issue (Who cares and why?)
      Gold-mining pit lakes in Nevada, when filled, will contain more water than all of the reservoirs within the borders of this arid state. On a statewide basis, all of the existing reservoirs within the state (excluding Lake Mead) contain approximately 600,000 acre-feet. In contrast, pit lakes in the Humbolt River Basin alone will contain 1,500,000 acre-feet.
      An important factor of pit lake water quality is the interaction of groundwater with the walls and surrounding rocks of the pit. In an open pit, oxidation reactions on the exposed walls release sulfate, acid, and metals into the lake. Additionally, when a pit is excavated below the regional water table, the aquifer in the rock is pumped out or dewatered. This typically sulfide-rich rock will then oxidize when exposed to air that is pulled into the dewatered porous rock, generating reaction products on the exposed surfaces. As the aquifer recovers following mining, those oxidation products will be flushed into the pit lake by groundwater flowing into the cone of depression. For every acre-foot of water that is replaced with air, sufficient oxygen is introduced to generate 514 mg/L of sulfuric acid.
      Thus, from a water resource perspective, pit lakes are important to Nevada, and the quality of the water in them will determine their future use, as well as their effects on the aquifer, wildlife and ecosystems.

What has been done?
      Approximately 5 years ago, the DuPont Chemical Corporation donated a Passivation technology to UNR. This patented technology was designed to prevent acid generation from pyrite ore by passivating (neutralize) the surfaces with potassium permanganate. A thin but strong manganese/iron/magnesium layer is formed on the pyrite’s surfaces preventing further oxidation, the mechanism the drives production of sulfuric acid. University of Nevada’s role has been to investigate the applications of this technology both in the laboratory and mine sites across the western US and determine the potential cost of conducting this form of remediation to the mining industry.

Impact
      “University of Nevada’s Permanganate Passivation treatment performed well, and it is cost effective compared to the other treatments”, said a spokesperson for the EPA’s Mine Waste Technology Program. The advantage of the Permanganate Passivation treatment is that, in theory, it will not degrade over time and a onetime application is all that is required. Compared against three competing technologies tested by the EPA, University of Nevada’s cost to treat 750,000 tons of waste rock came in at $3,241,408. Its next closest competitor Metals Treatment Technology cost $4,034,750 to treat the same amount of waste rock, a difference of $793,342.
      With an estimated 35 pit lakes expected to form in Nevada, containing from less than 100 acre-feet up to about 540,000 acre-feet of water, University of Nevada’s Dr. Glenn Miller said, “The mines down there are going to have a substantial impact on the groundwater system in Nevada because of the amount of water they're pumping out of the pits in order to keep them dry. Over the next ten years we figure it will create a groundwater deficit in just the Humboldt River Basin equal to 20 years of the total flow of the Humboldt River at Winnemucca. And once the cost of mining and pumping the water exceeds the value of the ore, the companies will say, That's it, and stop the pumps. That means groundwater will flow into the pits and create lakes. And if left untreated, many, if not most will be so contaminated the water will be unfit for human consumption or agriculture.”

Contact
Glenn Miller
Natural Resources and Environmental Science/186
University of Nevada
Reno, Nevada 89557
gcmiller@unr.nevada.edu