Concentrations of naturally occurring arsenic in ground water vary regionally due to a combination of climate and geology. Although slightly less than half of 30,000 arsenic analyses of ground water in the United States were less than or equal to 1 mu g/L, about 10% exceeded 10 mu g/L. At a broad regional scale, arsenic concentrations exceeding 10 mu g/L appear to be more frequently observed in the western United States than in the eastern half. Arsenic concentrations in ground water of the Appalachian Highlands and the Atlantic Plain generally are very low (less than or equal to 1 mu g/L), Concentrations are somewhat greater in the Interior Plains and the Rocky Mountain System. Investigations of ground water in New England, Michigan, Minnesota, South Dakota, Oklahoma, and Wisconsin within the last decade suggest that arsenic concentrations exceeding 10 mu g/L are more widespread and common than previously recognized. Arsenic release from iron oxide appears to he the most common cause of widespread arsenic concentrations exceeding 10 mu g/L in ground water. This can occur in response to different geochemical conditions, including release of arsenic to ground water through reaction of iron oxide with either natural or anthropogenic (i.e., petroleum products) organic carbon. Iron oxide also can release arsenic to alkaline ground water, such as that found in some felsic volcanic rocks and alkaline aquifers of the western United States. Sulfide minerals are both a source and sink for arsenic. Geothermal water and high evaporation rates also are associated,vith arsenic concentrations greater than or equal to 10g/L in ground and surface water, particularly in the west.
