Inorganic compounds arsenic and nitrate exceeded the U.S. Environmental Protection Agency´s drinking water standards in well water most often, detected in 11 percent and 8 percent of the sampled wells, respectively, while uranium, mercury and fluoride also exceeded standards at a smaller percentage.

Organic compounds rarely exceeded drinking water standards; however, atrazine, metolochlor, simazine, MTBE and chloroform all were detected in more than 5 percent of the wells sampled. Since the water quality of domestic wells is not federally regulated or nationally monitored, this study provides a unique perspective on the quality of the self-supplied drinking water resources used by 45 million Americans in the United States.

This national study is based on a compilation of existing data from wells sampled as part of multiple USGS programs. Ongoing research will include a broader list of contaminants from a selected set of wells to further investigate geographic patterns and the co-occurrence of multiple contaminants. The findings are expected to be released in 2007.

The newly released study on the chemical contamination of self-supplied domestic well water is featured in the August issue of the science journal Ground Water Monitoring & Remediation, a publication of the National Ground Water Association; you can access this article and data from http://health.usgs.gov/.