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From figures 2.2 and 2.3, it is obvious that the Appalachian Mountains, Rocky Mountains, Colorado Plateau, and northern glaciated states (states north of the limit of glaciation) tend to have the highest radon potential and indoor radon. The principal geologic sources of radon in the United States are:
Uranium-bearing metamorphosed rocks, volcanics, and granite intrusive rocks that can be highly deformed or sheared (shear zones in these rocks cause the largest indoor-radon problems in the United States), found predominantly in the Appalachian Mountains, Rocky Mountains, and Basin and Range;
Glacial deposits derived from uranium-bearing rocks and sediments found in the northern tier of states above the limit of glaciation;
Marine black shales found in the Appalachian Plateau and Great Plains and to a smaller extent in the Coastal Plain, Colorado Plateau, and Basin and Range;
High-iron soils derived from carbonate, especially in karstic terrain found in the Appalachian Plateau, Appalachian Mountains, and Coastal Plain; and
Uranium-bearing fluvial, deltaic, marine, and lacustrine deposits and phosphatic deposits found in the Colorado Plateau, Rocky Mountains, Great Plains, Coastal Plain, Basin and Range, and Appalachian Plateau.
Radon in Groundwater and Public Water Supplies
In the 1980s, a number of national studies of radon and other radionuclides in public water supplies and groundwater in the United States were published (see (Longtin 1988; Michel and Jordana 1987; Hess and others 1985; Horton 1983). These studies examined geographic distribution, the controls of hydrogeology, and differences among private well, small public, and large public water supplies. The most common conclusions of the studies suggest that the highest radon concentrations in groundwater and public water supplies generally occur in portions of the Appalachian Mountains, Rocky Mountains, and Basin and Range. Private well sources and small public water supplies tend to be higher in radon than large public water supplies. Private well sources and small water supplies tend to be in aquifers with low capacity. When these types of aquifers are uranium bearing granite, metamorphic rocks, or fault zones (as found in the mountain states), the radon concentration in the water tends to be high. Large public water supplies tend to use high-capacity sand and gravel aquifers, which generally comprise low-uranium rocks and sediments and tend to be lower in radon.
The study of Hess and others (1985) examined 9,000 measurements of radon in water from national and state surveys. Data were compiled for all but 10 states. Public water supplies originating in surface water tended to have radon concentrations less than 4,000 Bq m-3. Private water supplies were higher in radon than public water supplies by factors of 3 to 20. States with the highest radon in private well water were Rhode Island, Florida, Maine, South Dakota, Montana, and