More than 20% of untreated water samples from 932 public wells across the nation contained at least one contaminant at levels of potential health concern, according to a new study by the U.S. Geological Survey (USGS).
About 105 million people--or more than one-third of the nation’s population--receive their drinking water from one of the 140,000 public water systems across the United States that rely on groundwater pumped from public wells.
The USGS study focused primarily on source (untreated) water collected from public wells before treatment or blending rather than the finished (treated) drinking water that water utilities deliver to their customers.
“By focusing primarily on source water quality, and by testing for many contaminants that are not regulated in drinking water, this USGS study complements the extensive monitoring of public water systems that is routinely conducted for regulatory and compliance purposes by federal, state and local drinking water programs,” said Matthew C. Larsen, USGS associate director for water. “Findings assist water utility managers and regulators in making decisions about future monitoring needs and drinking water issues.”
Findings showed that naturally occurring contaminants, such as radon and arsenic, accounted for about three-quarters of contaminant concentrations greater than human health benchmarks in untreated source water. Naturally occurring contaminants are mostly derived from the natural geologic materials that make up the aquifers from which well water is withdrawn.
Man-made contaminants were also found in untreated water sampled from the public wells, including herbicides, insecticides, solvents, disinfection byproducts, nitrate and gasoline chemicals. Man-made contaminants accounted for about one-quarter of contaminant concentrations greater than human health benchmarks, but were detected in 64% of the samples, predominantly in samples from unconfined aquifers.
“Detections of contaminants do not necessarily indicate a concern for human health because USGS analytical methods can detect many contaminants at concentrations that are 100-fold to 1,000-fold lower than human health benchmarks,” said lead scientist Patricia Toccalino. “Assessing contaminants in these small amounts helps to track emerging issues in our water resources and to identify contaminants that may warrant inclusion in future monitoring.”
Scientists tested water samples for 337 properties and chemical contaminants, including nutrients, radionuclides, trace elements, pesticides, solvents, gasoline hydrocarbons, disinfection byproducts and manufacturing additives. This study did not assess pharmaceuticals or hormones.
Most (279) of the contaminants analyzed in this study are not federally regulated in finished drinking water under the Safe Drinking Water Act.