National Water-Quality Assessment (NAWQA) Program
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Figure 6. Ten contaminants, including seven from natural sources and three from primarily man-made sources, each were detected at concentrations greater than human-health benchmarks in at least 1 percent of source-water samples from public wells. Radium, arsenic, gross alpha-particle radioactivity, nitrate, and perchloroethene are regulated in drinking water under the Safe Drinking Water Act.