NGWA Event and Education Recordings Archive

Millions of people across the United States rely on groundwater from regional and local aquifers for drinking water. Groundwater quality in these aquifers differs depending on geologic sources, human activities on the overlying landscape, and hydrogeologic and geochemical characteristics that favor or hinder contaminant transport. Recent work by the U.S. Geological Survey has synthesized data from 20 years of monitoring to provide information about baseline conditions in 40 of the Nation’s most heavily pumped, regionally extensive Principal Aquifers. About 280 chemical constituents were measured, in more than 6,000 wells. Contaminants—constituents at concentrations greater than human-health benchmarks—were present nationally in 22% of groundwater samples (mostly from private wells) from parts of aquifers used for drinking water. Constituents from geologic sources more frequently exceeded benchmarks than constituents from human sources in most aquifers. The unique characteristics of each aquifer influence which contaminants occur more—or less—frequently than national averages and where contaminants are present. In the basin-fill aquifers of the Southwest, for example, the dry climate, long flow paths, and rock types in surrounding mountains control concentrations of contaminants, such as arsenic, from geologic sources. Nationally, nitrate and other contaminants from human sources exceeded human-health benchmarks 2 to 4 times more frequently in shallow groundwater beneath agricultural or urban land than in the deeper parts of aquifers used for drinking water. Over time, this shallow groundwater can move downward, potentially affecting future drinking-water supplies. Irrigation and pumping can accelerate the downward movement of these contaminants and, in some cases, cause contaminants from geologic sources to be released into groundwater. This information, about where, why, and what kinds of contaminants are present in our Nation’s aquifers, can be used by well owners and public-health officials to help ensure the safety and quality of water provided by private wells.