Southeastern Section–55th Annual Meeting (23–24 March 2006)

Paper No. 7
Presentation Time: 4:00 PM


CAMPBELL, Ted R., Aquifer Protection Section, NCDENR - DWQ, 2090 U.S. 70 Highway, Swannanoa, NC 28778,

High levels of carcinogenic radionuclides – most notably radon - naturally occur in ground water drinking supplies in the Blue Ridge and Piedmont Provinces of Western North Carolina. This is problematic because about half of the residents in the region rely on ground water as their principle potable supply. Further, eight counties in NC - all in Western NC – are classified as EPA Zone 1 counties with predicted average indoor radon concentrations above the EPA action level of 4 picocuries per liter (pCi/L). The main source of these radionuclides is uranium rich rock – including granites and gneisses – prevalent across the region.

Ground water samples collected by the NC Division of Water Quality from 103 private drinking wells in Buncombe, Henderson, and Transylvania Counties contained ubiquitously high levels of radon (109 to 45,600 pCi/L; median = 6060 pCi/L). About 98 percent of the wells exceeded EPA's recommended maximum contaminant level (MCL) of 300 pCi/L, and 64 percent exceeded the EPA alternate MCL of 4000 pCi/L. Radon was significantly higher in wells in igneous rock (avg = 9800 pCi/L; max = 45,600 pCi/L) than in metamorphosed metasedimentary and metavolcanic rock (avg = 4300 pCi/L; max = 10,700 pCi/L). Radon and uranium were significantly higher in oxidizing ground waters (avg = 8018 pCi/L and 1.78 micrograms per liter (ug/L), respectively) than in reducing ground waters (avg = 1930 pCi/L and near zero, respectively). Radium-226 levels countered this finding. Uranium (max = 63 ug/L) and gross alpha (max = 56 pCi/L) exceeded the EPA MCL in about 3 percent of wells. All isotopes of radium were found at low levels. Radon was 3 to 5 orders of magnitude higher than radium-226, implying that the vast majority of radium-226 - the parent source of radon - was sorbed on fractures near the well.

The risk from exposure to water high in radon is still under debate, and the recommended MCL has yet to be finalized. Our study suggests that a large percentage of private well owners in the region may be using water with radon over 20 times the recommended MCL. It is believed that most are unaware of the issue. This investigation was conducted in cooperation with the NC Radiation Protection Section and the USEPA in an effort to evaluate the scope and implications of naturally occurring carcinogenic radionuclides in water supplies in the region.