National Water-Quality Assessment (NAWQA) Program
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Figure 11. The frequent detections of man-made contaminants in source‑water samples from public wells, particularly those wells that withdraw water from unconfined aquifers, indicate the susceptibility of aquifers to contamination from human activities at the land surface. Nitrate concentrations greater than 1 milligram per liter (>1 mg/L) as nitrogen typically originate from man-made sources.