GROUND GAMMA-RAY SPECTROMETRY MEASUREMENTS AND RADON RISK ASSESSMENT OF SUBUNITS OF THE MIOCENE MONTEREY FORMATION IN WOODLAND HILLS, CALIFORNIA

The San Fernando Valley is a densely populated area in northwestern Los Angeles County that supports approximately two million residents. Recent geologic maps have identified the Miocence Monterey Formation as a major geologic unit in the valley [Dibblee 1999], and based on the potential uranium content of this formation [Churchill 1991], it has been estimated that over 700,000 residents could be at risk from elevated indoor radon [Fukumoto 2004].

Since different subunits of the Monterey Formation can display widely varying uranium concentrations [Duval 2004], the risk of indoor radon may not be homogenously distributed throughout the valley, and can be concentrated in specific sub-areas. Unfortunately, the absence of surface uranium data in this region prevents the identification of neighborhoods that may be at higher risk due to the underlying geology. In fact, relative to the population, there have been relatively few indoor radon measurements, and residents are apparently unaware that radon can be a serious health issue.

We present the results of ground gamma-ray spectrometry measurements from three prominent subunits of the Monterey Formation in the Woodland Hills area of the San Fernando Valley. A prioritization of relative indoor radon risk is assigned to each subunit, based on its average uranium content and emanation fraction. Indoor radon data from the California Radon Database [Blood 2002] is compared with the ground measurements, in order to estimate the radon risk in other sections of the San Fernando Valley and other areas of California.