GEOLOGICAL FACTORS AFFECTING CLASSROOM RADON CONCENTRATIONS IN SCHOOLS OF THE PALOS VERDES PENINSULA UNIFIED SCHOOL DISTRICT, PALOS VERDES, CALIFORNIA

Measurements of indoor radon in classrooms of the Palos Verdes Peninsula School District revealed that approximately 10% of the rooms had radon levels exceeding the U.S. Environmental Protection Agency action level of 4 pCi/L. The geologic units that underlie most of the peninsula are marine sediments of the Tertiary Monterey Formation, and parts of the formation are known to be rich in organic carbon derived from marine organisms. Past analyses have shown that the organic rich sediments contain elevated amounts of trace elements that include selenium, cadmium, zinc, and uranium. Our ground measurements of the potassium, uranium, and thorium concentrations at various locations on the Palos Verdes Peninsula indicate that areas mapped as organic-rich shale have uranium concentrations greater than 10 parts per million eU (equivalent uranium). These measurements were made using a gamma-ray spectrometer and suggest that available geologic maps can provide a general indication of areas more likely to have elevated indoor radon. However, factors such as soil permeability and characteristics of building construction complicate the development of more predictive measures. The present gamma-ray spectrometer and indoor radon data indicate that sediments of the Altamira Shale member are more likely to be a significant source of radon than other parts of the Monterey Formation. The Monterey Formation outcrops in other populated regions of California and we believe that elevated levels of indoor radon are likely to be found in those areas.