Arsenic in groundwater is largely the result of minerals dissolving from weathered rocks and soils. Several types of cancer have been linked to arsenic in water. In 2001 the US Environmental Protection Agency lowered the maximum level of arsenic permitted in drinking water from 50 micrograms per liter (ug/L) to 10 ug/L.
The USGS has developed maps that show where and to what extent arsenic occurs in groundwater across the country. The current maps are based on samples from 31,350 wells. Widespread high concentrations were found in the West, the Midwest, parts of Texas, and the Northeast. See Ryker (2001) for more information.
See Focazio and others (2000) for the use of available data for characterizing arsenic concentrations in public-water supply systems.
See Gronberg (2011) for updated arsenic map.
In Small Doses: Arsenic: Dartmouth, A ten minute movie about the risks associated with exposure to potentially harmful amounts of arsenic in private well water. (2010)