REVISED DISSOLVED RADON SUSCEPTIBILITY MAP OF NORTH CAROLINA UTILIZING THE LITHOTECTONIC MAP OF THE SOUTHERN APPALACHIANS – A PRELIMINARY MAP

Radon-222, a naturally occurring radioactive gas and human carcinogen, is elevated in groundwater drinking supplies in some areas of NC. We present a preliminary revised dissolved radon (dis-Rn) susceptibility map of NC based on the Hibbard et al. (2006) lithotectonic map of the southern Appalachians. Existing dis-Rn susceptibility mapping utilizes the 1985 geologic map of NC. The Hibbard map incorporates new geologic research and geochronologic data not available for the 1985 map, thus allowing a better understanding of the geologic provenance of various terranes in NC. Importantly, the lithotectonic map separates plutonic rocks in NC into over 10 magmatic events, events which show characteristic dis-Rn susceptibility “signatures” (e.g. Paleozoic and Middle Paleozoic felsic plutons tend to be associated with much higher dis-Rn than other events and rock types).

To allow areas of similar lithologic origin to be grouped and their Rn susceptibility assessed, 1,958 dis-Rn results were overlain on the Hibbard map. Four color-coded categories of susceptibility are mapped: high (Rn > 10,000 pCi/L dis-Rn), moderate (10,000 > Rn > 4000), low to moderate (4000 > Rn > 300), and relatively low susceptibility to dissolved Rn (Rn < 300). Because outliers (low or high) may occur in any rock type, these four areas are overlain with symbols that show dis-Rn data that do not conform to the mapped susceptibilities; these symbols show the location of and extent to which outliers occur in each category. This same approach will later be applied using detailed 1:24,000-scale geologic maps in various locations in NC.

The reliability of this preliminary susceptibility map is dependent upon the scale and accuracy of the base map and location of unit contacts, as well as the dis-Rn dataset that is inherently limited in scope. The role of radium-226-enriched fracture coatings (half-life = 1622 years and parent of radon) on “outlier” dis-Rn observations has not been studied but may explain the transport of a radon source from one area or geology to another that is downgradient. This presented map will be refined as additional data and geologic mapping become available.