Natural Deposits

The majority of radioactivity in groundwater is due to seepage from natural deposits of uranium and thorium (Table 11.22.4). Strict guidelines for acceptable levels of radionuclides in drinking water exist (U.S. EPA 1986). The EPA has established maximum levels for radium (U.S. EPA 1976, 1980) to monitor for the presence of natural radionuclides (Table 11.22.5). Radon, a colorless, odorless, inert, radioactive gas that seeps out of the earth has been found at dangerously high levels in inadequately ven-

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