SPATIAL ANALYSIS OF FACTORS AFFECTING HOME RADON LEVELS AROUND MOSS LAKE, NC

Moss Lake is a reservoir in the Inner Piedmont of North Carolina west of the Kings Mountain shear zone. The underlying geology comprises white-mica schist intruded by the uranium-bearing Cherryville granite (350 Ma). According to NURE data, the central and eastern Inner Piedmont are known to have high (7.5-237.8 ppm) uranium concentrations. The area is a positive gamma ray anomaly on the Charlotte 1º x 2º aeroradioactivity map. The study area was chosen because previous home radon tests revealed high radon concentrations (5-39 pCi/L) above the EPA indoor action level of 4 pCi/L. The present study examines the influence of underlying bedrock and foundation types on home radon concentrations in the vicinity of Moss Lake in an effort to develop a spatial model to predict radon levels in untested homes.

Approximately 65 homes within 1.5 km of Moss Lake were tested using 72-hour charcoal test kits following EPA protocol. The test results were entered into a GIS database and intersected with maps of land ownership, geology and color imagery for the area around the lake. Analysis of variance (ANOVA) showed that there is a significant difference between radon concentrations on homes underlain by Cherryville granite (average 6.53 pCi/L) and those underlain by white-mica schist (average 4.27 pCi/L). When foundation type and radon concentration were examined, homes with basements (average 7.01 pCi/L) or slabs (average 8.8 pCi/L) had higher radon concentrations than homes with crawl-spaces (average 2.6 pCi/L). When rock type and foundation were examined together, the preliminary results show that rock type is not significant.

The higher average radon concentrations associated with the Cherryville granite are due to elevated levels of uranium within the bedrock. We believe that the basements have higher concentrations because they are underground and have more surface area in contact with the bedrock. Slabs may have higher concentrations because, unlike crawl-spaces, there is no fresh air moving beneath the home, diluting the radon. In an effort to improve our confidence in the results and facilitate spatial modeling of radon risk, more homes within the Cherryville granite and white-mica schist are being tested and incorporated in the GIS. The effect of seasonal variations in home radon levels presents an issue that must be addressed.