The 10 subunits where samples were collected were grouped into three classes - median ground-water radon activity less than 300 pCi/L, between 300 pCi/L and 1,000 pCi/L, and greater than 1,000 pCi/L. Subunits underlain by igneous and metamorphic rocks of the Piedmont Physiographic Province typically have the highest median ground-water radon activities (greater than 1,000 pCi/L); although there is a large variation in radon activities within most of the subunits. Lower median radon activities (between 300 pCi/L and 1,000 pCi/L) were found in ground water in subunits underlain by limestone and dolomite. Of three subunits underlain by sandstone and shale, one fell into each of the three radon-activity classes. The large variability within these subunits may be attributed to the fact that the uranium content of sandstone and shale is related to the uranium content of the sediments from which they formed.
The source of radon is the radioactive decay of uranium. Therefore, higher radon amounts are commonly detected in areas underlain by granites and similar rocks that usually contain more uranium than do other rock types (Faure, 1986). Radon moves from its source in rocks and soils through voids and fractures. It can enter buildings as a gas through foundation cracks or dissolve in the ground water and be carried to water-supply wells.
The amount of radon in air or water commonly is reported in terms of activity with units of picocuries per liter of air or water. An activity of 1 pCi/L (picocuries per liter) is about equal to the decay of two atoms of radon per minute in each liter of air or water (Otton, 1992). This report will refer to picocuries per liter as the radon concentration.
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Figure 1. Radon entering a home through a water system (modified from Otton, 1992).
A recent study found that cancer occurrences increase as the amount of radon in home water increases (Mose and others, 1990). Other studies have also determined that there is an increased health risk from drinking water with high concentrations of radon (Crawford-Brown, 1990); however, most research is focused on the dangers of inhaling radon gas and its decay products. The U.S. Environmental Protection Agency (USEPA) has not established a Maximum Contaminant Level (MCL) for radon in drinking water; however, the proposed MCL is 300 pCi/L (U.S. Environmental Protection Agency, 1994).
Radon gas commonly enters the air in homes through the basement. Ground water can carry additional radon into homes and other buildings, creating a health risk. Dissolved radon is easily released into the air when the water is used for showering, cleaning, and other everyday purposes. The radon, therefore, commonly is released in close proximity to those using the water (fig. 1). Also, in homes built with better insulation and better seals on windows and doors, radon has less chance to be ventilated to the outside and can become concentrated to dangerous levels in indoor air. Most radon escapes from the water at the faucet, leaving little in the water itself (Hurlburt, 1989). The radon that escapes from the water adds to the radon that enters the home through the basement, and in some cases the water contributes a large portion of the radon that is present in a home.
Water-borne radon is commonly a concern only for those who use wells for their water supply. Because of its short half-life, radon in public water supplies usually decays to low concentrations before the water is delivered to users, especially if the water has been treated. Also, public suppliers often use surface-water supplies, which generally have very low radon concentrations (Zapecza and Szabo, 1988).
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The study area includes the District of Columbia and parts of Maryland, Pennsylvania, Virginia, and West Virginia (fig. 2). Ground-water-quality assessments were conducted in subunits in the two basins that were defined on the basis of the geology (type of bedrock) and the physiographic province (an area of similar elevation, topography, and physical features). The three general types of bedrock in this area are (1) limestone and dolomite, (2) sandstone and shale, and (3) igneous or metamorphic rocks, such as granite, schist, and quartzite. The physiographic provinces studied include the Piedmont and the Ridge and Valley (table 1).
Division of the study area on the basis of bedrock type and the physiographic provinces and sections resulted in seven areas where radon in ground water was studied. Three of these areas were further subdivided on the basis of river drainage basin. Within these 10 subunits, 267 ground-water samples were collected from wells and analyzed for dissolved radon (table 1) (fig. 2). Because sampling was conducted independently in each basin, the subunits generally do not cross the basin divide. The Piedmont and Appalachian Mountain limestone subunits include those areas in both basins because both contained too little area in the Potomac River Basin to be studied separately. The areas of igneous and metamorphic bedrock are termed "crystalline" in the names of the subunits (table 1). The Piedmont sandstone and shale subunit, which also contains abundant diabase, is commonly called the "Triassic Lowlands." The Western Piedmont crystalline subunit was sampled as a part of the Piedmont subunit as defined in the Potomac NAWQA study design (Blomquist and others, in press); however, this subunit was considered separately for the radon study. The study design did not allow for sampling in every physiographic province or subunit.
The wells selected for sampling had to meet certain location and construction criteria and have similar characteristics, including type, depth, and age. All wells sampled were constructed as open boreholes, drilled and completed with casing typically set a few feet into the bedrock. Wells were to have a maximum depth of 200 feet and a maximum age of 20 years. A few of the wells sampled exceeded the age and depth criteria.
Figure 2. Location of subunits sampled and bedrock types.
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Ground-water radon concentrations are highly variable, even within individual subunits. Concentrations of radon from 9 of the 10 subunits range from less than 300 pCi/L to greater than 1,000 pCi/L (fig. 3). The central tendency of radon concentrations in the subunit, represented by the median concentration, is useful to draw general conclusions about the occurrence of radon in ground water in that subunit but should not be used to predict concentrations at specific sites.
The subunits with median concentrations of radon greater than 1,000 pCi/L (shaded in red, figs. 3 and 4) are the Piedmont crystalline subunits (not including the Western Piedmont subunit) and the Piedmont sandstone and shale subunit. The highest median concentration of all the areas sampled (3,100 pCi/L) and the highest concentration from a single sample (38,000 pCi/L) are both in the Piedmont crystalline subunit of the Lower Susquehanna River Basin.
The group of subunits with median radon concentrations between 300 and 1,000 pCi/L (shaded in orange, figs. 3 and 4) includes all the subunits underlain by limestone bedrock and the Appalachian Mountain sandstone and shale subunit in the Lower Susquehanna River Basin. The limestone subunits generally have less variability in the range of radon concentrations, even though relatively large numbers of samples were collected in these subunits. The Appalachian Mountain limestone subunit has the least amount of variability of all the subunits and also is the only subunit that did not have any samples with radon concentrations greater than 1,000 pCi/L.
The subunits with median concentrations of less than 300 pCi/L (shaded in yellow, figs. 3 and 4) include the Western Piedmont crystalline subunit and the Appalachian Mountain sandstone and shale subunit in the Potomac River Basin. The sandstone and shale subunit has the lowest median concentration of all 10 subunits (about 80 pCi/L); however, the maximum concentration detected in the subunit was 2,500 pCi/L. This again shows a high degree of variability within these physiographic and bedrock type subunits.
Of the 267 ground-water samples collected, 80 percent contained radon concentrations above the proposed MCL (300 pCi/L), 31 percent were above 1,000 pCi/L, and less than 1 percent were above 10,000 pCi/L. These percentages are similar to results reported by Swistock and others (1993) for water from 989 wells throughout Pennsylvania.
Figure 3. Distribution of concentrations of radon in ground water in the subunits sampled.
Figure 4. Sampling locations and subunits shaded to indicate median radon concentrations in ground water.
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Radon distribution in ground water in those subunits of the Lower Susquehanna and Potomac River Basins underlain by igneous and metamorphic rocks and limestone follows a general geographical pattern related to rock type. Except for the Western Piedmont subunit, median ground-water radon concentrations are higher in subunits underlain by igneous and metamorphic rocks than in those underlain by limestone (fig. 3). Most igneous and metamorphic rocks in the Piedmont are of predominantly granitic composition. These rocks contain higher concentrations of uranium, on average, than do limestones (Faure, 1986). The median ground-water radon concentration in the Western Piedmont crystalline subunit is lower than that in any of the limestone subunits. The rocks in this subunit are igneous and metamorphic, but many are not predominantly granitic like those in the rest of the Piedmont. Rocks of this type contain much less uranium, on average, than do limestones or granitic rocks (Faure, 1986).
Radon concentrations in ground water from subunits underlain by sandstone and shale are much more variable than those from subunits underlain by other rock types. Of these three subunits, one has a median radon concentration less than 300 pCi/L, one a median radon concentration between 300 pCi/L and 1,000 pCi/L, and one a median radon concentration greater than 1,000 pCi/L (fig. 3). The uranium content of sandstones and shales is commonly related to the uranium content of the sediments from which they formed. Radon concentrations in ground water from sandstones and shales can therefore be highly variable if these sediments were derived from different sources. Also, shales contain more uranium, on average, than do sandstones (Faure, 1986; Hem, 1985). Different relative amounts of sandstone and shale among these three subunits could therefore result in different median radon concentrations.
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For additional information, contact:
Director, USGS Pennsylvania Water Science Center
U.S. Geological Survey, WRD
840 Market Street
Lemoyne, Pennsylvania 17043-1586
United States Environmental Protection Agency, Region 3,
841 Chestnut St. Philadelphia, Pa. 19107, (215) 597-9800
National Radon Hotline, Box 33435, Washington, D.C., 20035-0435, (800) SOS-RADON (800-767-7236)
Maryland Department of the Environment, 2500 Broening Highway, Baltimore, Md., 21224, (800) 872-3666
Pennsylvania Department of Environmental Protection, Bureau of Radiation Protection, Harrisburg, Pa., (800) 23R-ADON (800-237-2366) or (717) 783-3594
Virginia Department of Health, 1500 East Main Street, Richmond, Va., 23219, (800) 468-0138
West Virginia Office of Environmental Health Services, 815 Quarrier Street, Suite 418, Charleston, W. Va., 25301, (800) 922-1255
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