CONSISTENCY OF URANIUM CONCENTRATIONS IN ROCK UNITS ASSOCIATED WITH RADON RISK AREAS IN SOUTHERN CALIFORNIA

Large-scale epidemiological studies have recently substantiated the health risks of radon gas at concentrations below that of the current US Environmental Protection Agency action level of 4 pCi/l. These data now indicate an 11 – 21% increased lung cancer risk at 3 pCi/l, and radon has also been implicated as a factor in other serious diseases. Hence, the accurate identification of risk areas is an important public health service that could reduce the 21,000 deaths each year in the United States attributed to indoor radon exposure.

Radon is generated in the soil directly from the decay of uranium, and the composition and features of a geologic unit will determine its radon risk potential. Geologic maps are therefore commonly used as a first step to identify radon risk areas, with the assumption that the uranium content of a geologic unit is consistent not only throughout the unit, but from one outcropped location to another. This wide-spread assumption is critical for accurate risk mapping, but has not to date been experimentally confirmed.

Surface gamma ray spectrometry measurements of the potassium, uranium, and thorium concentrations were recorded from two distant locations of a uranium-rich Miocene subunit that has been shown to support high indoor radon levels in Los Angeles County, California. Care was taken to selectively collect data at positions that appeared to represent native soils and were well within the subunit mapped boundaries. Results from these two locations are compared and discussed in terms of the accuracy of risk assignments based on geologic maps, and alternate strategies are proposed to improve the reliability of radon risk maps.