ARSENIC IN CALIFORNIA'S MOTHER LODE GOLD BELT

Soil and groundwater contamination by arsenic at some former Mother Lode (ML) Gold Belt mines in northeast-central California has recently become a concern because of encroachment of residential developments onto old mined areas. Arsenic toxicity has been known throughout historical time with arsenic oxide first commercially produced from iron and lead ore mining around 800 AD. By 1867, arsenic was commonly used in insecticides to control Colorado potato beetle infestation. Arsenic, associated with ML-type gold ore deposits, occurs as the minerals arsenian pyrite and arsenopyrite; these can contain small amounts of gold. To maximize profits, many ML gold mines attempted to recover as much gold as possible by chlorination or cyanide leaching of sulfides. Processed tailings containing considerable quantities of arsenic were generally discharged to streams and rivers. From 1912 to 1913, flooding of Central Valley agriculture by such tailings caused farmers to lobby the legislature for relief. However, agreements with mine owners to impound their mine tailings resulted in tailings being left at many mines; typical arsenic concentrations ranged to >6,000 mg/kg. Most ML mines had ceased operations by the mid to late 1950s.

Arsenic background concentrations in rock, soil, and groundwater are important for understanding arsenic geochemistry within the ML. Melones Fault Zone (MFZ) rocks typically range from <100 mg/kg to 8,000 mg/kg. Residual soil may contain arsenic that is at least 10% that of rock. Although groundwater in the mineralized and mined areas may contain arsenic, groundwater quality is highly variable and closely associated with bedrock type and MFZ mineralization. Arsenic in tailings, soil, and groundwater surrounding ML mines has caused considerable legal issues and lawsuits by State and Federal agencies, under CERCLA and other statutes. Remediation and litigation of arsenic-impacted areas from tailings has been expensive.