The chemical quality of self-supplied domestic well water in the United States

Existing water quality data collected from domestic wells were summarized to develop the first national‐scale retrospective of self‐supplied drinking water sources. The contaminants evaluated represent a range of inorganic and organic compounds, and although the data set was not originally designed to be a statistical representation of national occurrence, it encompasses large parts of the United States including at least some wells sampled in every state and Puerto Rico. Inorganic contaminants were detected in many of the wells, and concentrations exceeded the U.S. EPA maximum contaminant levels (MCLs; federal drinking water standards used to regulate public drinking water quality) more often than organic contaminants. Of the inorganic constituents evaluated, arsenic concentrations exceeded the MCL (10 μg/L) in ∼11% of the 7580 wells evaluated, nitrate exceeded the MCL (10 mg/L) in ∼8% of the 3465 wells evaluated, uranium‐238 exceeded the MCL (30 μg/L) in ∼4% of the wells, and radon‐222 exceeded 300 and 4000 pCi/L (potential drinking water standards currently under review by the U.S. EPA) in ∼75% and 9% of the wells, respectively. The MCLs for total mercury and fluoride were each exceeded in <1% of the wells evaluated. The MCL was exceeded in <1% of all wells for all anthropogenically derived organic contaminants evaluated and was not exceeded for many contaminants. In addition, 10 contaminants evaluated do not currently have an MCL. Atrazine, however, was detected in 24% of the wells evaluated and was the most frequently detected organic contaminant of the 28 organic contaminants evaluated in this study. Simazine and metolachlor each were detected in ∼9% of all wells and tied for second in frequency of detection for organic contaminants. The third and fourth most frequently detected organic contaminants were methyl tert‐butyl ether (MTBE) (6%) and chloroform (5%), respectively. Because the water quality of domestic wells is not federally regulated or nationally monitored, this study provides a unique, previously nonexistent, perspective on the quality of the self‐supplied drinking water resources used by ∼45 million Americans in the United States.