National Water-Quality Assessment (NAWQA) Program
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Concentrations of at least one chemical contaminant were greater than a human-health benchmark in 23 percent of 1,389 domestic wells sampled in 45 states and 25 principal aquifers. Human-health benchmarks include U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and U.S. Geological Survey Health-Based Screening Levels. Results are based on the USEPA proposed MCL for radon of 4,000 picocuries per liter. Microbial contaminants are not included.