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High Levels of Radon Found in Some Wells Across Pennsylvania

A new U.S. Geological Survey study has discovered high levels of radon in wells across certain areas of Pennsylvania.

A map of Pennsylvania indicating radon concentrations of water samples collected during a multi-year groundwater radon study. Ma
A map of Pennsylvania indicating radon concentrations of water samples collected during a multi-year groundwater radon study. Map by USGS. (Public domain.)

The study, which was conducted in cooperation with the Pennsylvania Departments of Health and Environmental Protection, examined 1,041 well samples and found that 14 percent had radon levels at or above the Environmental Protection Agency’s proposed alternative maximum contaminant level of 4,000 picocuries per liter. While the EPA does not currently regulate radon in drinking water, it has proposed this alternative limit for public water supplies in states like Pennsylvania, which has an EPA-approved radon indoor air quality program. For states without an approved program, the EPA has proposed a lower, more protective, maximum contaminant level of 300 picocuries per liter.

Radon – which is a colorless, odorless, radioactive gas – primarily contaminates indoor air when the gas seeps through the soil under homes and buildings, but groundwater can be a notable indoor air radon source in areas where groundwater has extreme radon concentrations. Radon dissolved in groundwater used for drinking water can escape into the air as the water leaves a faucet, which adds to any radon that enters a structure through foundation cracks. Homeowners with private wells should be aware of radon’s potential health risks because, according to the EPA, radon exposure is the second-leading cause of lung cancer in the United States.

During the study, data collected from 1986 to 2015 from wells were compiled to gain a better understanding of the radon levels in groundwater across the state.

The USGS analyzed well water sample results according to the geologic formations, also known as geologic units, they came from. Based on the Pennsylvania Geological Survey’s classifications, there are a total of 188 recognized geologic units across the state, such as the Allegheny, Brunswick and Catskill Formations.

However, due to limited data availability, only 16 of these geologic units were included in this study. The studied area accounts for about 31 percent of Pennsylvania’s total land area, which researchers noted creates some large data gaps across the state.

Five of the geologic units included in the study consist largely of shale and sandstone formations in the northern and western parts of the state. The other 11 geologic units are in southeastern Pennsylvania and consist of highly variable igneous, metamorphic and sedimentary rocks. The Peters Creek Schist, a geologic unit in southern York, Lancaster, and Chester counties, was found to have the highest potential radon exposure from both groundwater and indoor air. This area also had the highest percentage of private well users, which puts this population at greater potential of exposure to radon from groundwater and indoor air.

“This research is not intended to predict radon levels for individual wells; its purpose is to promote awareness regarding potential radon exposure in Pennsylvania and to point out data gaps that exist throughout the state,” said USGS scientist Eliza Gross, who led the study.  “The study results and associated potential radon exposure maps provide water-resource managers and health officials with useful data as they consider management actions in areas where radon levels in groundwater and indoor air have been notably high and where people rely on private wells as a water source.”

Even though only small sections of the state were analyzed for radon levels in groundwater, Gross still feels the findings are important for Pennsylvanians with private wells.

“Radon levels can vary widely, even within the same geologic unit,” Gross said. “Because of that, the only way a homeowner can know if his or her water or indoor air has high levels of radon is to get them tested, and then treat their homes if necessary.”

The study, "Evaluation of Radon Occurrence in Groundwater from 16 Geologic Units in Pennsylvania, 1986–2015, with Application to Potential Radon Exposure from Groundwater and Indoor Air," is online at

For more information about radon in Pennsylvania, visit the PADEP Radon Division Website at or call the PADEP Radon Hotline at 1-800-237-2366 or the PADEP Radon Division at 717-783-3594.

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