National Water-Quality Assessment (NAWQA) Project

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Quality of Water from Public Wells in the United States

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Figure 5. Twenty-two percent of 932 source-water samples from public wells contained one or more chemical
contaminants at concentrations greater than Maximum Contaminant Levels or Health-Based Screening Levels. A
total of 80 percent of samples (206 plus 539 wells) contained one or more contaminants at concentrations greater
than one-tenth of human-health benchmarks. Public wells yielding water in which contaminant concentrations
were greater than benchmarks, as well as those in which concentrations were greater than one-tenth of
benchmarks, were distributed throughout the United States and included wells that withdraw water from every
principal aquifer rock type included in this study. (See fig. 2A for explanation of principal aquifer rock types.)

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