DELINEATION OF HIGH- AND LOW-RISK RADON AREAS BASED ON GROUND GAMMA-RAY SPECTROMETRY MEASUREMENTS ON THE PALOS VERDES PENINSULA, CALIFORNIA

Previous studies conducted on the Palos Verdes Peninsula have shown that school sites located on the upper Altamira Shale subunit of the Miocene Monterey Formation display elevated surface equivalent uranium (eU) levels of 4.6 ± 1.1 ppm, with correspondingly high classroom radon levels. In contrast, schools situated on the lower Altamira Shale subunit exhibit more modest eU concentrations of 1.6 ± 0.6 ppm, with considerably lower indoor radon levels. Since together these subunits comprise the majority of the peninsula and present substantially different radon risks, the accurate determination of the subunit boundaries is a significant community health concern for the 70,000 residents of the peninsula.

Two recent geologic maps of Palos Verdes (Dibblee 1999, and Conrad and Ehlig 1987) delineate somewhat different boundaries for the upper and lower subunits, and discrepancies are also noted in the existence and location of other minor subunits in both maps. However, because the eU content of the Altamira Shale subunits appears to be radiometrically distinct, the mapped location of the boundaries can be verified by ground gamma-ray spectrometry measurements along appropriate transections.

Tests of the subunit boundaries were conducted at various locations throughout the Palos Verdes Peninsula using the recent Dibblee geologic map (Dibblee 1999) as a guide. Ground measurements were taken only in areas clearly marked as upper-lower Altamira Shale boundaries, and care was used to avoid measurements from non-native soils. Boundaries defined by the changes in eU concentration are compared with those of the mapped boundaries in order to determine the accuracy of the Dibblee map. These experimentally determined boundaries based on eU content are the most relevant to the assessment of indoor radon risk, and can serve to accurately define local high and low risk radon areas within the community.