MEASUREMENTS OF SURFICIAL URANIUM CONCENTRATIONS FROM A HIGH RADON POTENTIAL DISTRICT IN LOS ANGELES, CALIFORNIA

Radon is the second leading cause of lung cancer in the United States, resulting in approximately 21,000 deaths each year. It is a colorless, odorless gas that is generated directly from the decay of naturally occurring uranium present in most soils. In order to help the public address this health hazard, the US EPA and USGS have published state and overall county radon risk maps. High, moderate, and low radon risk levels are assigned to specific areas by combining results from indoor radon measurements, geologic data, and aerial radiometric measurements.

The available risk maps, however, may have limitations. In many areas, there is only sparse indoor radon data due to the lack of homeowner response to large-scale radon measurement projects. Furthermore, through past studies, we have found that the surface uranium concentrations of identically labeled rock units can vary widely, even within a county, and hence their radon potential may be inconsistent. Finally, when aerial radiometric flight line data is available, it often lacks the spatial resolution to detect rapid changes in surface uranium concentrations that can occur. Many metropolitan areas have no flight line data whatsoever due to airspace restrictions.

Because of these uncertainties, direct surficial uranium concentration measurements are invaluable in determining the radon source potential for any given area. Although laborious, ground gamma ray spectrometry measurements can resolve rapidly changing uranium concentrations on very small scales, and do not rely on assumptions of the average uranium content of the underlying rock units.

Pacific Palisades is a small, affluent district within the city of Los Angeles, and was recently designated as having high indoor radon potential by the 2005 Radon Potential Zone Map of Southern Los Angeles County, California. In order to confirm the accuracy of this radon risk assignment, surficial gamma ray spectrometry measurements of uranium, potassium, and thorium were recorded at over forty locations within the district. This data was then compared to other surface spectrometry data in well-documented, high radon potential areas in Los Angeles County, to determine the validity of its high radon potential designation and to assess the accuracy of the mapped boundary.