Groundwater contamination can occur in many ways and from many sources, both natural-and human-induced. Groundwater commonly contains one or more naturally occurring chemicals, leached from soil or rocks by percolating water, in concentrations that exceed federal or state drinking water standards or otherwise impair its use.

Dissolved Solids and Chloride. One of the most common water quality concerns is the presence of dissolved solids and chloride in concentrations that exceed the recommended maximum limits in federal secondary drinking water standards: 500 mg/L (milligrams per liter or approximately equivalent to parts per million) for dissolved solids and 250 mg/L for chloride. Such concentrations are found at the seaward ends of all coastal aquifers and are quite common in aquifers at depths greater than a few hundred feet below the land surface in many parts of the United States.

Iron and Manganese. Although not particularly toxic, iron and manganese in concentrations greater than the limits for federal secondary drinking water standards (0.3 mg/L for iron and 0.05 mg/L for manganese) can impair the taste of water, stain plumbing fixtures, glassware and laundry; and form encrustations on well screens, thereby reducing well-pumping efficiency.

Nitrate-nitrogen. Most groundwater not affected by human activity contains less than 10 mg/L nitrate-nitrogen, the maximum concentration allowed by federal primary drinking water standards. Nationwide, nitrate nitrogen concentrations of less than 0.2 mg/L generally represent natural conditions, whereas values greater than 3 mg/L may indicate the effects of human activities. Although relatively nontoxic, nitrate may be reduced by bacteria to nitrite in the intestines of newborn infants and cause the disease methemo-globinemia. Nitrate also can react with amines in the human body to form N-nitrosamines, carcinogenic chemicals known to induce tumors in laboratory animals and thought to be linked to human cancers.

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