The researchers evaluated wells used for irrigation, industry and the public and private water supply. The results, published in 1999, are believed to be the most accurate and comprehensive to date. Click on the map to review a graphic of arsenic concentrations in ground water in the United States . The US Geological Survey, 2000. The findings indicate that arsenic concentrations in the ground water vary greatly, but are highest in the West and parts of the Midwest and Northeast, with concentrations exceeding 10 MCL, the World Health Organization's provisional guideline for tolerable arsenic content.
Concentrations are lowest in the Southeast.
Researchers also analyzed arsenic data by county and correlated the concentrations with the public-supply systems using ground water. The small public supply systems serve between 1,000 and 10,000 people. The US Geological Survey, 2000. But as the EPA moves to change the acceptable levels of arsenic in ground water to lower concentrations, the likelihood that water resources may exceed that standard increases, study authors said. The map below depicts the counties with arsenic concentrations exceeding possible new levels based on 10 percent or more of groundwater samples.