Recent U.S. Geological Survey studies have found that while the United States has large amounts of potable water available for use, some localities have high concentrations of a variety of toxic metals, organic chemicals and petroleum products forming plumes around such point sources as leaking underground storage tanks, waste disposal sites and chemical or waste handling areas. These types of problems generally occur in urban or industrialized areas, although they are found occasionally in rural areas. Large regions have also been identified in which contaminants, derived from nonpoint sources and often at minimum detectable levels, are present in many shallow wells throughout a given area. In a small percentage of wells such contaminants as nitrate may exceed drinking water standards or health advisories. Generally, such nonpoint source contamination is associated with densely populated urban areas, agricultural land uses, and concentrations of septic systems. Such contamination commonly affects only the shallowest aquifers. About 20 percent of 124,000 wells sampled over the past 25 years contained a maximum nitrate nitrogen concentration greater than 3 mg/L, suggesting the effects of human activities. In these samples, 6 percent exceeded the federal drinking water standard for nitrate-nitrogen of 10 mg/L.

The most common sources of human-induced groundwater contamination can be grouped into four categories: waste disposal practices; storage and handling of materials and wastes; agricultural activities; and saline water intrusion.

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