Radiometric Survey of the Rincon Shale in Santa Barbara County, California: Uranium Concentration Mapping of a High Radon Potential Rock Unit

Santa Barbara County, located along the Southern California coastline, is one of only two counties in California designated as a high radon potential region by the US EPA. The county-wide average of homes exceeding the 4 pCi/l action limit is approximately 15%. However within the county the Rincon Shale, a marine-derived unconsolidated gray shale from the Miocene Epoch, is responsible for a disproportionately large fraction of the high reading homes, with up to 70% of homes exhibiting excess radon levels. In contrast, the predicted state-wide average is only 1%, and this low average value has resulted in a correspondingly low level of awareness of radon health risks among Californians.

Our past studies have shown that residences that exhibit high indoor radon tend to be concentrated in relatively small areas, but within these areas a high percentage of homes can be affected. For example, the densely populated San Fernando Valley, located northwest of Los Angeles, may support up to 35% of homes exceeding the action level. Hence, for these affected neighborhoods, specific knowledge of the extent of high radon soils is important for encouraging radon testing and prevention efforts.

Localized gamma ray spectrometry measurements that quantify surface uranium concentrations can rapidly delineate high radon potential neighborhoods, since soil uranium is the source of radon gas. Although several studies have documented elevated indoor radon levels on the Rincon Shale, only very sparse aerial radiometric data exists regarding the actual uranium content of this unit, and these data are often convoluted by flight line averaging over adjacent formations. In the present study, results from surface gamma ray spectrometry measurements are reported from well-defined areas of the Rincon Shale, in order to clearly characterize the uranium content of this unit, and to directly compare mapped boundaries with measured uranium concentrations for radon potential mapping.