Discovery of radon potential in the Rincon Shale, California

Geological exploration has identified an unsuspected radonprone belt in southern California. Detailed analysis of aeroradiometric (NARR) data, soil-gas radon, soil permeability, and finally indoor radon has identified the Rincon Shale and Rincon-derived soils in Santa Barbara County as anomalous in uranium and radon. Roughly 74% of houses on the Rincon Shale exceed 4 pCi/l and 26% exceed 20 pCi/l in standardized screening tests (3 to 7 day AC, ''closed-house'' lived-in rooms). Fifty three percent exceed 4 pCi/l in linger-term measurements 1 to 6 mo, AT) under normal ventilation conditions. Unusually strong correlations between aeroradiometric data, soil-gas radon adjusted for soil permeability, geology, and indoor radon reflect the unaltered character of sedimentary host rocks and a consequent tendency for anomalous uranium to occur throughout given rock units rather than in epigenetic mineralized zones. Under these circumstances, deliberate geological exploration may be a more efficient approach to indoor radon risk identification than simple random sampling or non-random testing of houses. By the same token, geological parameters can facilitate radon risk assessment on undeveloped lands. Attention is drawn to multiple populations within radon test samples and the consequent problems in estimating regional parameters. (au)