National Water-Quality Assessment (NAWQA) Program
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Concentrations of radon greater than the lower U.S. Environmental Protection Agency proposed Maximum Contaminant Level (MCL) of 300 picocuries per liter (pCi/L) were found in 65 percent of wells, which were located throughout the United States and in all 30 principal aquifers sampled. Concentrations greater than the higher proposed MCL of 4,000 pCi/L were found in 4.4 percent of wells and generally were and associated with crystalline-rock aquifers located in the Northeast, the central and southern Appalachians, and central Colorado. >, greater than; <, less than or equal to.