INVESTIGATION OF THE SOURCE OF HIGH RADON LEVELS IN CLEVELAND COUNTY, NORTH CAROLINA

The Cherryville Granite is a metamorphosed intrusion that occurs as a northeast trending body west of the Kings Mountain shear zone in North Carolina. The granite is about 350 ma, and pre-dates regional deformation at the Inner Piedmont-Kings Mountain belt boundary. High uranium-bearing monazite is reported as an accessory mineral in the Cherryville and possibly other associated granites in Cleveland County near Shelby. The distribution of Cherryville Granite bodies is coincident in part with: (1) the elevated concentrations of uranium in alluvial deposits, assuming that many of these deposits are locally derived from the Cherryville Granite and that uranium is carried by monazite, and (2) the relatively high levels of radon in some homes in the Moss Lake area, that covers a large part of the main Cherryville Granite body and its contacts with several country rock units. This progress report evaluates relationships among local geology, soils, home radon test data, and soil radon gas readings to better understand the generative factors for radon distribution in the area. Approximately 100 radon soil gas readings and 85 home radon tests have been collected. Radon concentrations in homes underlain by Cherryville Granite average 6.53 pCi/L, but within the data values range from 1.3 to 23.7 pCi/L. Construction style has a strong effect on these measurements, although in general houses placed on Cherryville Granite show the highest values. Radon concentrations measured from in situ soil gas samples within the Cherryville Granite are consistently over 1000 pCi/L at an average depth of 32 cm in contrast with a range of 40 to 560 pCi/L in rocks around the granite. There is thus a coincidental relationship between variations in soil gas radon, living space radon, and the spatial distribution of Cherryville Granite. We are presently conducting heavy mineral analyses and petrographic investigations of bedrock, saprolite, and soil samples to determine the extent of monazite and zircon occurrence throughout the Moss Lake area as a means of mapping radon generation gradients within the Cherryville Granite.