Silver Mountain Mine covers 5 acres in Loomis, a remote area of Okanogan County, Washington. In 1980 and 1981, the mine used cyanide in its processing operations. Approximately 1,100 gallons of cyanide were poured over silver tailings, which had been placed on top of a plastic liner, in an effort to extract gold. The water running off the pile was collected in a basin, also lined with plastic. The site contains more than 2,500 tons of contaminated tailings and 20,000 gallons of contaminated liquid. At one time, the liquid contained 1,100 parts per million (ppm) cyanide, according to analyses conducted by the State. In November 1983, the State treated the liquid, reducing cyanide levels to 9 ppm. Contamination of ground water in the area is considered probable. Although the site is located in a remote area, the concentrations of cyanide present a potential health hazard.
Source: US EPA National Priority List