Contaminants found in one of five wells in recent USGS study
By Bob Berwyn
SUMMIT COUNTY — While most people assume that well water is clean and safe, a recent study by the U.S. Geological Survey shows that one out of every five wells providing public drinking water contains at least one contaminant at levels of potential health concern.
The USGS sampled 932 public wells across the country for the study. 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 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.
The contaminants included naturally occurring substances like radon and arsenic, which 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contaminants are mostly derived from the natural geologic materials that make up the aquifers from which well water is withdrawn.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 by-products, 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 percent 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.”
“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.”
Scientists tested water samples for 337 properties and chemical contaminants, including nutrients, radionuclides, trace elements, pesticides, solvents, gasoline hydrocarbons, disinfection by-products and manufacturing additives. This study did not assess pharmaceuticals or hormones.
Most of the contaminants analyzed in this study are not federally regulated in finished drinking water under the Safe Drinking Water Act.
The study also shows that contaminants found in public wells usually co-occurred with other contaminants as mixtures. Mixtures can be a concern because the total combined toxicity of contaminants in water may be greater than that of any single contaminant. Mixtures of contaminants with concentrations approaching benchmarks were found in 84 percent of wells, but mixtures of contaminants above health benchmarks were found less frequently, in 4 percent of wells.
Wells included in this study are located in 41 states and withdraw water from parts of 30 regionally extensive aquifers, which constitute about one-half of the principal aquifers used for water supply in the United States.
This study and additional information about public wells can be found on the Quality of Water from Public-Supply Wells in the United States website.
People served by public water systems can obtain information about their drinking-water quality from their water supplier. Selected water suppliers provide an annual water-quality report; some reports are available on EPA’s Consumer Confidence Reports (CRR) website.