RADON IN GROUND WATER IN CHESTER COUNTY, PENNSYLVANIA

This project was done in cooperation with the
Chester County Water Resources Authority.

NEED FOR THE STUDY

Radon-222 is a naturally occurring radioactive gas that is present in ground water. The U.S. Environmental Protection Agency has proposed a two tier maximum contaminant level (MCL) of 300 or 4,000 picocuries per liter (pCi/L) for radon-222 in drinking water to minimize health risks. The higher MCL of 4,000 pCi/L will apply if health risks from radon in indoor air are addressed. In Chester County, many residences and businesses rely on ground water for drinking water supplies that contains radon-222. Data collected by the U.S. Geological Survey in Chester County since 1985 indicate that most ground-water samples contain radon-222 in concentrations exceeding 300 pCi/L. Radon-222 concentrations in ground water appear to differ by geologic formation and are related to the uranium concentrations in the rocks. However, characterization of radon-222 concentrations in ground water in Chester County was incomplete because the data on radon-222 ground-water concentrations in some geologic formations were limited or missing, the spatial distribution of samples was uneven, and little was known about temporal variations and uranium content of geologic formations.

OBJECTIVES

Characterize radon-222 concentrations in ground water in Chester County for all geologic formations that are used for drinking water supply and assess temporal variations in radon-222 concentrations in selected wells.

APPROACH

• Collect ground-water samples for radon-222 analysis in wells in geologic formations for which data are limited or missing
• Compile existing and newly collected data on radon-222 concentrations in ground water by geologic formation
• Compute summary and comparison statistics
• Sample six or more wells in different hydrogeologic settings on a monthly or bi-monthly basis to determine magnitude of seasonal variations
• Compile and analyze existing and newly collected data on temporal variability in ground water to determine if seasonal variability is significant

STUDY CONCLUSIONS

Radon-222 concentrations in ground water in 31 geologic units in Chester County, Pa., were measured in 665 samples collected from 534 wells from 1986 to 1997. Chester County is underlain by schists, gneisses, quartzites, carbonates, sandstones, shales, and other rocks of the Piedmont Physiographic Province. On average, radon concentration was measured in water from one well per 1.4 square miles, throughout the 759 square-mile county, although the distribution of wells was not even areally or among geologic units.

The median concentration of radon-222 in ground water from the 534 wells was 1,400 pCi/L (picocuries per liter). About 89 percent of the wells sampled contained radon-222 at concentrations greater than 300 pCi/L, and about 11 percent of the wells sampled contained radon-222 at concentrations greater than 5,000 pCi/L. The highest concentration measured was 53,000 pCi/L. Of the geologic units sampled, the median radon-222 concentration in ground water was greatest (4,400 pCi/L) in the Peters Creek Schist, the second most areally extensive formation in the county. Significant differences in the radon-222 concentrations in ground water among geologic units were observed. Generally, concentrations in ground water in schists, quartzites, and gneisses were greater than in ground water in anorthosite, carbonates, and ultramafic rocks. The distribution of radon-222 in ground water is related to the distribution of uranium in aquifer materials of the various rock types.

Temporal variability in radon-222 concentrations in ground water does not appear to be greater than about a factor of two for most (75 percent) of wells sampled more than once but was observed to range up to almost a factor of three in water from one well. In water samples from this well, seasonal variations were observed; the maximum concentrations were measured in the fall and the minimum in the spring.

REPORTS RESULTING FROM THIS STUDY

Senior, L.A., 1998, Radon-222 in the ground water of Chester County, Pennsylvania: U.S. Geological Survey Water-Resources Investigations Report 98-4169, 79 p.

Senior, L.A., and Sloto, R.A., 2000, Radium-224 and its relation to gross-alpha-particle, radium-226, and radium-228 activities in ground water from rocks of the Piedmont Physiographic Province, southeastern Pennsylvania [abstract]: Geological Society of America Abstracts with Programs, Northeastern Section, v. 32, no. 1, p. A-73. [ abstract ]

Sloto, R.A, 2000, Naturally occurring radionuclides in the ground water of southeastern Pennsylvania: U.S. Geological Survey Fact Sheet 012-00, 8 p. [ online report ]    [ PDF file ]

Sloto, R.A and Senior, L.A., 1998, Radon in the ground water of Chester County, Pennsylvania: U.S. Geological Survey Fact Sheet 120-98, 4 p. [ online report ]    [ PDF file ]    [ abstract ]

Sloto, R.A., and Senior, L.A., 2000, Radon-222 activities in the ground water of the Piedmont Physiographic Province, southeastern Pennsylvania [abstract]: Geological Society of America Abstracts with Programs, Northeastern Section, v. 32, no. 1, p. A-75. [ abstract ]

MORE INFORMATION ON RADON

U.S. Geological Survey Radon Home Page

U.S. Environmental Protection Agency Radon Home Page

U.S. Environmental Protection Agency proposed MCLs for Radon in Drinking Water

Return to the USGS Chester County Water Resources Page

Return to the Water Resources of Pennsylvania Home Page
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