DISCOVERY OF A NEW RADON-PRONE METROPOLITAN AREA IN SOUTHERN CALIFORNIA

California as a whole is rated to have low to moderate radon potential, although select communities located on uranium-rich subunits of the Miocene Monterey Formation have been recently found to exhibit significant radon levels. Because uranium is the source of radon gas, other communities underlain by related sediments may also contain high risk neighborhoods, but their locations are currently unknown due to a lack of sufficient data. Since radon is responsible for 21,000 cancer deaths each year and constitutes a serious health hazard, the identification of specific radon-prone neighborhoods can be an effective means to reduce public exposure and direct county and state health efforts.

Mission Viejo is an affluent community of approximately 130,000 residents, located near the Southern California coast 48 miles southeast of Los Angeles. The major underlying rock units are identified as marine shales from the middle Miocene to early Pliocene Epochs that outcrop throughout the area in roughly equal proportions. Past studies in Woodland Hills and Palos Verdes, California, both located on the Miocene Monterey Formation, have shown strong correlations between surface uranium concentrations and indoor radon levels. Equivalent uranium (eU) concentrations of 1.6 - 4.4 ppm were shown to correspond to 2 - 29% of homes with radon levels that exceed the US Environmental Protection Agency (EPA) action level of 4 pCi/l.

Preliminary surface gamma ray spectrometry measurements from the Mission Viejo area were taken using a California Division of Mines and Geology report as a guide. Sampling positions were carefully chosen at sites that appeared to represent native soils, and measurements were categorized according to the mapped subunits. Averages of the potassium, uranium, and thorium concentrations from the predominant subunits are compared with those from Woodland Hills and Palos Verdes, in order to estimate the indoor radon levels that may be present in the Mission Viejo area. The results indicate that in certain sub-areas, a large fraction of the homes may exceed the US EPA action level, indicating the need for community involvement in radon awareness and remediation efforts.