Evaluation of Radon in Groundwater and Indoor Air in Pennsylvania

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Existing groundwater and indoor air radon-222, hereafter referred to as "radon", concentrations were aggregated and evaluated for 16 geologic units throughout the state of Pennsylvania to provide a better understanding of potential human exposure to radon.

Purging a domestic well to be sampled for radon

A U.S. Geological Survey (USGS) scientist recording water-quality parameters (pH, temperature, and specific conductance) during the purging of a domestic well that was sampled for radon at a site north of Jersey Shore, Pennsylvania.

(Credit: Eliza L. Gross, PA Water Science Center. Public domain.)

Radon is a colorless, odorless, radioactive gas, that enters indoor air when the gas seeps through soil under homes and buildings. Radon dissolved in groundwater used for drinking water can also escape into the air, which contributes to any radon already entering a structure through foundation cracks According to the U.S. Environmental Protection Agency (USEPA), radon exposure is the second-leading cause of lung cancer in the United States.

Recognizing the need for a better understanding of potential human exposure to radon, the U.S. Geological Survey (USGS), in cooperation with the Pennsylvania Department of Environmental Protection and Pennsylvania Department of Health, aggregated and evaluated existing radon concentrations in groundwater and indoor air within 16 of the 188 geologic units in the state of Pennsylvania where data were available. Radon concentrations measured in water from domestic, public supply, irrigation, commercial, stock, or industrial wells were obtained from the USGS National Water Information System. Radon concentrations in air were obtained from the Pennsylvania Department of Environmental Protection, Bureau of Radiation Protection, Radon Division.

Maps, graphical plots, and statistical tests were used to determine and visualize variations in radon concentrations in groundwater and indoor air. Median radon concentrations in groundwater samples and median radon concentrations in indoor air samples within the 16 geologic units were classified according to proposed and recommended regulatory limits for radon concentration. These analyses are intended to refine the current understanding of potential radon exposure from groundwater and indoor air.

Fourteen percent of 1,041 wells tested across Pennsylvania from 1986-2015 contain groundwater with radon concentrations exceeding the USEPA's proposed alternative maximum contaminant level of 4,000 picocuries per liter (pCi/L). This proposed alternative limit applies to public water supplies in states, like Pennsylvania, that have an USEPA-approved radon indoor air quality program. For states without an approved program, the USEPA has proposed a lower maximum contaminant level of 300 pCi/L. Of the wells sampled for this study, 87 percent had radon concentrations exceeding 300 pCi/L.

The highest radon concentrations were measured in groundwater from the schists, gneisses, and quartzites of the Piedmont Physiographic Province. All of the geologic units, except for the Allegheny and Glenshaw Formations in the Appalachian Plateaus Physiographic Province, had median radon concentrations in groundwater greater than the proposed EPA maximum contaminant level of 300 pCi/L. The Peters Creek Schist geologic unit in southeastern Pennsylvania had the highest median radon concentrations in groundwater and indoor air, and coincidentally had the highest estimated percentage of population using domestic self-supplied water.

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 1,041 water samples collected from 1985-2015. Figure 4 from Gross, 2017, USGS Scientific Investigations Report 2017–5018.

The results of this study highlight the importance of understanding multiple exposure pathways of radon when estimating exposure. The results are useful for understanding the presence, variation, and potential radon exposure in specific geologic units. The aggregated data and maps have limitations but are useful to understand potential radon exposure and help to identify gaps in data availability throughout the state. They are not recommended for use in predicting individual concentrations at specific sites due to the limitations in spatial accuracy among data sets.

This study was done in collaboration with the Pennsylvania Environmental Public Health Tracking Program that was funded by the Centers for Disease Control and Prevention (CDC) by Cooperative Agreement Number, 5U38EH000952-05. The USGS Cooperative Water Program provided matching funding for this study.

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