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Photo from U.S. Geological Circular 1225--The Quality of Our Nation's Waters--Nuttients and Pesticides

graph showing percentage of ground-water samples that exceed the
drinking-water standard
The percentage of ground-water samples with concentrations of nitrate exceeding the drinking-water standard of 10 mg/L decreases as depth to water increases. Mixing of shallow ground water with deeper, uncontaminated water and increased thickness of protective, impermeable geologic materials with depth may help explain this relation.


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