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Adit Discharge Summary for the Elkhorn and Charter Oak Mines, MT
Acid mine drainage (AMD) from abandoned mines degrades streams and riparian areas and may harm fish and wildlife. Plugging mine openings to eliminate or reduce the flow of AMD has had only limited success. Expensive water treatment plants that require power and frequent maintenance are not feasible for abandoned mine sites on remote lands managed by the U.S. Department of Agriculture, Forest Service. This report is the third that has been produced by a partnership of the Montana Bureau of Mines and Geology and the Missoula Technology and Development Center (MTDC).
In 1998 the Montana Bureau of Mines and Geology began an adit discharge-monitoring project at the Charter Oak and Elkhorn Mines in Montana. These mines are typical hard-rock mines with adit discharges similar to many mines throughout the United States. This project investigated the yearly variation in the flow and chemical composition of discharges from these mines. Typically, adits are sampled only once. Water treatment methods are chosen based on that sample. This project’s goal was to improve the ability to reduce AMD by measuring water quality and quantity throughout the year. These measurements may help explain the source of the discharge water, knowledge that could help in treating or controlling AMD from the mine.
Discharge data were collected at the two sites using conventional measuring devices and long-term recording instruments. Water-quality parameters were recorded using dedicated electronic monitoring instruments. The field parameters included: pH, specific conductivity, oxidation-reduction potential (ORP), temperature, dissolved oxygen, and turbidity. Data were retrieved and water samples were taken to determine the complete inorganic makeup (pH, Fe, SO4, Al, As, Cd, Cu, Ni, Zn, Pb) on a 6- to 8-week cycle for 18 months (November 1998 to March 2000).
The ground-water recharge and discharge system at the Charter Oak adit is localized and could easily be controlled by ground source control methods (bold italic words are defined in the glossary). Our next report will suggest how to implement a specific recharge control method using data that could be obtained with resources ordinarily available to Forest Service managers rather than the extensive water quality and quantity data we collected. The water quality at the Charter Oak Mine fluctuated throughout the year, so a treatment process based on a single measurement at this site would probably have failed.
The ground-water recharge and discharge system at the Elkhorn adit is substantial. The water quality remains constant throughout the year but the flow varies substantially. At both sites, the variation of discharge water quality and quantity were significant. Designs to improve water quality or control the adit discharge need to consider these variations. Normally, these variations are not recognized because only one measurement is taken. Ground source-control methods, such as minimizing water infiltration to the recharge area, are likely to be more successful at the Charter Oak Mine than at the Elkhorn Mine. Such methods could possibly be employed at the Elkhorn Mine, but a successful treatment program would be likely to include applications within the mine such as grouting and plugging.
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