Water Quality in the Lower Susquehanna River Basin, Pennsylvania and Maryland, 1992-95

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Major issues and findings --
Radon in ground water

Radon, a product of the radioactive decay of uranium, is present in ground water throughout the Lower Susquehanna River Basin (Lindsey and Ator, 1996). Airborne radon has been cited by the Surgeon General of the United States as the second leading cause of lung cancer, and the USEPA has identified ground-water supplies as possible contributing sources of indoor radon. Radon activities in 86 percent of the 165 ground-water samples tested for radon were greater than a previously proposed standard, now under review by the USEPA, of 300 pCi/L (picocuries per liter, a measurement of radioactivity). More than 30 percent of the 165 ground-water samples tested for radon contained radon at activities greater than 1,000 pCi/L.

Bar chart: Percentages of cancer deaths attributed to various factors (9188 bytes)

Many of the known cancer-causing agents cause only a relatively small percentage of cancer deaths. The risk of radon can be put in perspective by comparing the risk of radon exposure to other factors that have been identified as causes of cancer. This figure accounts for all exposures to radon, not only radon in water (not all factors are shown).

The subunit of the Study Unit underlain by crystalline rocks of the Piedmont Physiographic Province had the highest median radon activities in ground water (greater than 1,000 pCi/L), but variation in radon activities within most subunits is large. Lower median radon activities (less than 1,000 pCi/L) were measured in ground water in subunits underlain by limestone. The median activity in ground water in the subunit underlain by sandstone and shale also was less than 1,000 pCi/L; however, the maximum activity was higher than the maximum activity in any of the subunits underlain by limestone. Land use generally does not affect radon activity.

Although the ground water in some areas has higher activities of radon relative to other areas, the only way to be sure of the radon activity in water from a well is to have it tested. In homes where high indoor radon levels are measured and where water is supplied by a well, the USEPA recommends testing well water as a potential contributing source of radon. For every 10,000 pCi/L of radon in water, about 1 pCi/L of radon is released to the air, in addition to any airborne radon that may enter a home through the basement (only 1 of the 165 ground-water samples contained greater than 10,000 pCi/L of radon). If a large percentage of the radon in the house is from the water, the USEPA recommends that installation of a water-treatment system to remove radon be considered. Homes and water supplies both can be treated to reduce radon levels.

Boxplots: Radon activity in ground water, by subunit (74,695)

The activities of radon in ground water were varied but followed general patterns according to bedrock type.

U.S. Geological Survey Circular 1168

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Suggested citation:
Lindsey, B.D., Breen, K.J., Bilger, M.D., and Brightbill, R.A., 1998, Water Quality in the Lower Susquehanna River Basin, Pennsylvania and Maryland, 1992-95: U.S. Geological Survey Circular 1168, on line at <URL:>, updated June 22, 1998 .

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Last modified: Thu Jul 2 16:44:57 1998