Arsenic in ground water of the United States: Occurrence and geochemistry

Concentrations of naturally occurring arsenic in ground water vary regional ly 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 re gional 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), Co ncentrations are somewhat greater in the Interior Plains and the Rocky Moun tain System. Investigations of ground water in New England, Michigan, Minne sota, South Dakota, Oklahoma, and Wisconsin within the last decade suggest that arsenic concentrations exceeding 10 mu g/L are more widespread and com mon than previously recognized. Arsenic release from iron oxide appears to he the most common cause of wide spread arsenic concentrations exceeding 10 mu g/L in ground water. This can occur in response to different geochemical conditions, including release o f arsenic to ground water through reaction of iron oxide with either natura l or anthropogenic (i.e., petroleum products) organic carbon. Iron oxide al so 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 a nd high evaporation rates also are associated,vith arsenic concentrations g reater than or equal to 10g/L in ground and surface water, particularly in the west.