Health risks associated with contamination of groundwater by abandoned mines near Twisp in Okanogan County, Washington, USA.

Abandoned mines are known to contaminate private drinking water wells with toxic metals and arsenic (As). Little attention is given, however, to sites in rural areas with low population densities where natural, geogenic sources of contaminants might also occur. This study measured arsenic and trace element exposure among residents consuming water from wells adjacent to abandoned mines near Twisp, in Okanogan County, Washington, USA, estimated the risk of adverse health effects, and considered the degree of uncertainty associated with the assessed risk. Water samples were collected between October 1999 and June 2001. Average As concentrations ranged from <1 to 298 microg L(-1), lead (Pb) ranged from 0 to 94 microg L(-1), cadmium (Cd) 0-5 microg L(-1), and selenium (Se) 0-390 microg L(-1). Concentrations varied seasonally with maximum concentrations occurring in conjunction with snow-melt. The calculated risk of mortality from cancer following exposure to As at average concentrations as low as 8 microg L(-1) was greater than one in 10,000. Additional noncarcinogenic risks are associated with exposure to As, Cd, Pb and Se. A potentially affected population, estimated to be between 1000 and 1287 residents, live within a 6.5-km (4-mile) radius of the study site. This study emphasises the need to test drinking water wells in the vicinity of abandoned mines during times of maximum snow-melt to determine the extent of risk to human health. Residents drinking water from wells tested in this study who want to reduce the estimated carcinogenic risk and the noncarcinogenic hazard quotient should consider treating their water or find alternative sources.