National Water-Quality Assessment (NAWQA) Program
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Figure 7. Contaminants from natural sources were detected more frequently at concentrations greater than human-health benchmarks in source-water samples from public wells than were man-made contaminants. Trace elements and radionuclides were detected at concentrations greater than benchmarks in samples from both unconfined and confined aquifers, consistent with the fact that these contaminants originate primarily from aquifer materials. By contrast, man-made contaminants were detected at concentrations greater than human-health benchmarks mainly in samples from unconfined aquifers, consistent with the fact that these contaminants originate from human activities at the land surface.