Select communities in California that are located on uranium-rich subunits of the Miocene Monterey Formation have been found to exhibit significant indoor radon levels. Since uranium is the source of radon gas, other areas underlain by related sediments may also contain high-risk neighborhoods that may be overlooked due to a lack of data or inaccurate extrapolations. Because radon is a serious health hazard that accounts for approximately 21,000 cancer deaths each year, identification of specific radon-prone neighborhoods can help to reduce public exposure to this known carcinogen.

Current methods for identifying radon-prone areas include the use of indoor radon measurements, geological subunit composition, and aerial radiometric data collected for the National Uranium Resource Evaluation (NURE) Program during the early 1970s. Unfortunately, due to air space restrictions, many densely populated areas where radiometric measurements would be the most beneficial were not surveyed by the NURE effort.

Though present methods depict generalized estimates for widespread areas, our recent studies demonstrate that surficial equivalent uranium (eU) concentrations can change significantly in as little as 30 feet, which can cause misleading assignments of uranium content to important geological units, and thus lead to inaccuracies in radon potential mapping. In addition, because aerial radiometric data was based on flight line grid cells of approximately 3 x 12 miles, these data lack the spatial resolution to reflect the radon potential for specific sites and densely populated neighborhoods.

The present study suggests that site-specific ground gamma-ray spectrometry measurements can be an accurate and cost-efficient method for assessing the radon potential of specific locations on the Monterey Formation. Furthermore, with appropriate supporting data, this method could be extended to other uranium-rich geologic subunits to obtain more precise risk assesments.