Testing performed on 5,000 water wells in Pennsylvania shows that 20 percent had high or elevated levels of cancer-causing arsenic.
The U.S. Geological Survey released the findings Wednesday.
Researchers say they compiled data collected between 1969 and 2007 from industrial, public and private wells. They used the data to create a map of areas of the state most likely to have elevated levels of arsenic in groundwater.
The testing found that 8 percent had arsenic levels at or above federal drinking-water standards, while another 12 percent contained elevated levels of the carcinogen.
Geologists say the results show that private well owners should regularly test their wells. Private wells in Pennsylvania are unregulated.