Karst Geology, Radon Fluctuations. and Implications for Measurement and Mitigation

Starting in December, 1996 and continuing through April, 1998, the Tennessee Radon Program, USEPA Region 4 Office, the Southern Regional Radon Training Center, and USEPA Headquarters conducted an investigation of several houses in Livingston, TN in which homeowners had discovered indoor radon concentrations far in excess of their original postmitigation test levels. The investigation team conducted intensive diagnostic procedures and deployed equipment to provide long-term data on indoor radon concentrations and several other building and environmental parameters. Analyses of these data indicated extraordinary fluctuations in soil gas and indoor radon concentrations primarily related to outdoor temperature. Other investigators had previously described the same phenomena in geologically similar areas characterized by extreme karst development. It is probable that the extensive network of solution cavities provides pathways for significant bulk flow of soil gas over great horizontal and vertical distances, and in areas with elevation differences, enables pressure-driven flow through these pathways caused by stack effect developed due to temperature differences between the ground and outdoor air. Limitations of standard measurement and mitigation procedures in light of the magnitude of the radon concentration fluctuations are discussed, along with recommendations for measurement, diagnostic and mitigation procedures based on analysis of the long-term data and the efficacy of various system configurations.