Environmental occurrence and impacts of arsenic at gold mining sites in the western United States

Arsenic is a common element in the natural environment and is frequently a significant component in gold deposits of the western United States. Such deposits contain various forms of arsenic: arsenides, sulfides, and sulfosalts. Upon weathering such minerals routinely lower the pH of nearby waters, mobilizing arsenic and other metals. Arsenic may also be mobilized in aqueous environments where cyanide has been used to leach gold ores, resulting in high water pH. Incorrect construction of monitoring wells can result in contamination by cement/grout, which may raise pH causing arsenic concentrations to appear inordinately elevated. Concentrations of arsenic toxic to humans and aquatic life have resulted from mining activities in some instances. Hence, regulatory agencies in the U.S. are requiring gold mining companies to comply with very restrictive arsenic standards. The authors of this paper have been involved on many mining projects, mostly in the Western United States, where elevated arsenic content impacted the development of mineral deposits, or created a potential for environmental problems. Several case histories of the pre-mining arsenic content in surface and ground water on gold mining projects throughout western United States and Alaska are presented. The technical aspects of an elevated arsenic content in surface and ground water, mined areas, and waste disposals are discussed. Potential for arsenic contamination of water resources during and after gold mining operations with cyanide heap leach or other gold recovery methods are also presented. Several recommendations for mine operations of how to deal with the problem of elevated arsenic content in pyritic environments are offered.