Radon in homes, soils and caves of north central Tennessee and implications forthe home construction industry

A survey of radon concentrations in 59 basements and crawl spaces within houses built on karst in the Highland Rim and Central Basin provinces of central Tennessee had a measurement range of 0.1 to 37.6 pCi/L with twenty-two percent of the measurements exceeding EPA’s 4 pCi/L recommended maximum. Excepting the highest measurement, the population had an arithmetic mean of 2.6, a median of 2.2 and a standard deviation of 1.95. Although the Chattanooga Shale and lower rocks were poorly represented within the survey compared to Fort Payne and Warsaw strata, the ranking of the means according to rock type was as follows: Bigby-Cannon (4.4 pCi/L), Fort Payne (3.4 pCi/L), Chattanooga (3.3 pCi/L), and Warsaw (1.6 pCi/L). The similarities between the means suggests that no single lithology other than perhaps the Warsaw was found to have a significantly different radon occurrence potential. The degree of home weatherization proved to be a poor indicator of the magnitude of likely indoor radon levels but was significant in its effect on the distribution of radon progeny within 11 individual homes. Highly weatherized homes demonstrated a lower average reduction factor in radon progeny from basements to first floors than did average and poorly insulated homes. Homes with concrete foundations and basements had a slightly higher average radon concentration than did homes with crawl spaces. The depth of home excavation correlated positively with radon concentration at a 99.9% confidence level. A case study of a house overlying a small near-surface cave system indicated the likelihood that radon-bearing cave air was entering the soil via enlarged fractures visible in outcrop. Anomalous indoor concentrations, ranging from 3.5 to 37.6 pCi/L, were thought to result from the advective transport of radon through the foundation via small fractures in the concrete foundation and loose utility fittings. A soil radon progeny traverse perpendicular to a near surface cave passage of known location and radon content surprisingly revealed the lowest progeny concentration immediately overlying the cave passage of interest. Anomalously high radon progeny concentrations were unexpectedly found 220 feet normal to the passage within a soil of unusually high porosity and permeability. Radon’s abundance and transport are controlled by a large number of different factors, and radon occurrence is not accurately predictable based on geological and home construction characteristics and should be measured using a time-integrated device to accurately assess the radon content of an existing home. Where development is planned over- lying karst or other terrains implicated as radon sources, soil radon surveys are recommended to evaluate the potential need for preventive construction techniques.