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April 1, 2020

A U.S. Geological Survey study of groundwater quality across the nation now includes water-quality information for 15 of the most heavily used aquifers in the nation.

In addition to summary fact sheets previously released for 11 principal aquifers, fact sheets are now available for the Columbia-Plateau basaltic-rock aquifers (northwestern U.S.), the High Plains aquifer (western U.S.), the Ozark Plateaus aquifer system (central U.S.), and the Biscayne aquifer (southeastern U.S.).

Untreated groundwater from more than 1,100 deep public-supply wells has been sampled from the 15 aquifers and analyzed for hundreds of constituents. Concentrations of constituents detected in the samples were compared to benchmarks to place the results in a human-health context. Overall, inorganic constituents with a geologic source—related to the interaction of groundwater and aquifer rocks and sediments—most commonly exceeded human-health benchmarks.

Among the 15 aquifers, from 3 to 50 percent of samples contained at least one inorganic constituent that exceeded a benchmark. For example, at least one inorganic constituent exceeded a benchmark in 30 percent or more of samples from the Cambrian-Ordovician aquifer system, the Piedmont and Blue Ridge cyrstalline-rock aquifers, and the Rio Grande aquifer system.  In contrast, for samples from the Biscayne aquifer and the Northern Atlantic Coastal Plain aquifer system, fewer than 5 percent of samples had a constituent that exceeded a benchmark. The constituents exceeding a benchmark primarily were the trace elements arsenic, fluoride, and manganese, although in the Biscayne aquifer and the Floridan aquifer system strontium was the only trace element to exceed its human-health benchmark.

One or more radioactive constituents (such as radium and radon), which also have geologic sources, exceeded a human-health benchmark in a small percentage of samples (0 to 10 percent) in most of the 15 aquifers studied. The exceptions were the Piedmont and Blue Ridge crystalline-rock aquifers and the Cambrian-Ordovician aquifer system, where exceedances of benchmarks for radioactive constituents occurred in 30 and 45 percent of samples, respectively. Nitrate, a nutrient, was the only constituent with a manmade source that exceeded the human-health benchmark, typically in a low percentage of samples (0 to 3 percent).

About half of the nation’s population relies on groundwater for drinking water. The U.S. Geological Survey is intensively studying principal aquifers that provide most of the nation’s groundwater pumped for public supply. Water-quality information for three remaining principal aquifers is slated for publication in 2021.

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