A new study by the U.S. Geological Survey (USGS) has found that more than 20 percent of source – or untreated – water collected from public wells before being treatment contained at least one contaminant at levels of potential health concern.

The USGS collected water samples from 932 public wells in 41 states across the nation for their findings. 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.

Findings showed that naturally occurring impurities, such as radon and arsenic, accounted for about three-quarters of contaminant concentrations greater than human-health benchmarks in untreated source water.

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.

Treated drinking water from public wells is regulated under the Safe Drinking Water Act. Water utilities, however, are not required to treat water for unregulated contaminants. The EPA uses USGS information on the occurrence of unregulated contaminants to identify contaminants that may require drinking-water regulation in the future.

About 105 million people in the U.S. receive their drinking water from public water systems that rely on groundwater pumped from public wells.