Superfund Record of Decision (EPA Region 10): Silver Mountain Mine, WA. (First Remedial Action), March 1990

摘要

The five-acre Silver Mountain Mine site is an abandoned mine dump in Okanogan County, north-central Washington. Land in the site vicinity is used primarily for cattle grazing, and the nearest well, two miles away, is used for cattle watering and irrigation. The nearest residence is three miles south of the site. Operations at the mine were initiated in 1902 to extract silver, gold, and copper from soil and ore. From 1980 to 1981, a cyanide leach heap of previously mined material was constructed in an attempt to extract (leach out) silver and gold. The heap consisted of 5,300 tons of ore on top of a 20 ml plastic liner. About 4,400 pounds of sodium cyanide was mixed with water and sprayed on the top of the heap. The cyanide-laden effluent was then collected in a leachate pond at the base of the heap. The heap leaching operation was abandoned in late 1981 without cleanup of contaminated material. In addition, approximately 5,200 tons of unprocessed mixed material (mine dump) lie to the west of the leach heap. In 1982, the State took action to treat the cyanide at the site using sodium hypochlorite to partially neutralize the leachate pond and heap. In 1985, the State conducted a site stabilization effort, which included removal of liquids from the leachate pond and installation of a 33 ml plastic cover over the heap and pond.