National Water-Quality Assessment (NAWQA) Project

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Groundwater Quality in Four Principal Aquifers

Regional assessments of groundwater quality in four Principal Aquifers add to those published previously for five Principal Aquifers sampled in 2012–13. Almost 400 deep public-supply wells were sampled within the four Principal Aquifers: the Rio Grande aquifer system (southwestern U.S.), the Glacial aquifer system (northern U.S.), the Cambrian-Ordovician aquifer system (north central U.S.), and the Piedmont and Blue Ridge crystalline-rock aquifers (eastern U.S.). Groundwater samples were analyzed for hundreds of water-quality constituents.

At least one inorganic constituent exceeded a human-health benchmark in 26 to 50 percent of samples collected from the four aquifers. However, organic contaminants rarely occurred at concentrations that approached human-health benchmarks. Trace elements, such as arsenic, fluoride, and manganese, were the constituents that most frequently exceeded a human-health benchmark—these trace elements have a geologic source. Radioactive constituents, also from geologic sources, exceeded human-health benchmarks in a small percentage of samples in the Rio Grande aquifer and Glacial aquifer system, but exceedances were higher in the Piedmont and Blue Ridge crystalline-rock aquifers and the Cambrian-Ordovician aquifer system. Nitrate was the only inorganic constituent from manmade sources that exceeded its human-health benchmark, and those exceedances occurred in no more than 2 percent of samples collected from any of the four aquifers.

About 140 million people—almost one-half of the Nation’s population—rely on groundwater for drinking water. Regional assessments of groundwater quality are one component of the NAWQA Project’s ongoing efforts to assess, understand, and forecast the quality of the Nation’s groundwater.

Access a fact sheet that describes results for a Principal Aquifer by clicking on that aquifer’s pie chart.

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