A CASE STUDY IN MEDICAL GEOLOGY: INVESTIGATION OF THE GEOLOGIC CONTROLS ON BLACK HILLS RADON RATES

The presence of excessive indoor radon in some of the Black Hills homes is a well-established fact and high radon levels in the Black Hills have worried homeowners and public health experts for decades. With exposure to excessive radon determined as the second leading cause of lung cancer in the US according to the EPA (2012), it is imperative that the residents of the Black hills should have information on radon in their homes to limit their exposure. Through the process of GIS interpolation and field work, this study shows high-risk radon zones in the Black hills region including densely-populated Rapid City, SD. The locations exceeding the safe limit of indoor radon levels (4 pCi/L) suggested by EPA have been mapped and high-risk zones have been correlated with the geologic units. It appears that the main cause of extremely high radon (~10 to 25 times higher than 4 pCi/L) can be related to uranium-bearing Archean-Precambrian basement rocks in the region. However, there are homes built on non-uranium bearing formations and still fall in high-risk zones. GIS interpolation of major structures indicates that fault/fracture sets can be controlling seepage of radon through younger sedimentary units. Maps of the Black Hills area showing radon potential is a main product of this research, and these maps can provide residents of the area with information about their individual potential exposure.