LITHOLOGIC CONTROLS ON DISSOLVED RADON IN NORTH CAROLINA: PRELIMINARY FINDINGS BASED ON UPDATED GEOSPATIAL DATASETS

Radon-222 (Rn), a naturally occurring radioactive gas and human carcinogen, is elevated in groundwater drinking supplies in some areas of NC. The current conceptual model holds that dissolved Rn (dis-Rn) is controlled by lithology of proximate rocks and (or) the presence of sorbed radium-226 (Rn’s parent) on hydraulically connected fractures or sediments. Here, lithology is understood in terms of rock type, age, and terrane/location. Dis-Rn also may be influenced by shear zones that expose more rock surface (containing radium (Ra)) and allow greater mobility of Rn-rich water from outlying units (Rn half life = 3.8 d).

Preliminary findings from an updated geodatabase of 1958 wells link elevated dis-Rn to felsic intrusive (FI) rocks (med = 3180 pCi/L) and lower dis-Rn to mafic rocks and felsic volcanic (FV) rocks (med = 630 and 430, respectively). FI > ultramafic (med = 1460 pCi/L) > shear zone (1990) > mélange (1390) > orthogneiss (1000) > mafic intrusive (630) > FV (430). Paleozoic rocks are linked to higher dis-Rn (med = 3550 pCi/L) while Mesozoic & Cenozoics (Triassic basin & Coastal Plain) and most Proterozoics are linked to lower dis-Rn (820, 240, and 1160, respectively). Exceptions to these are presented.

Dis-Rn results also show that outliers occur in most rock units (almost half of the 55 mapped units had a dis-Rn mode of at least 5000 pCi/L). That is, a high dis-Rn value may occur in a unit associated with lower dis-Rn and vice versa. This is believed to be due to 1) mapping limitations (e.g. unmapped FI bodies may “infest” low Rn potential rocks; contacts may not be mapped accurately at the scales being investigated; and the subsurface orientation of high dis-Rn bodies may influence whether or not a well penetrates the unit at depth), and (or) 2) current or historic flowpath geochemistry that can move radium from its original source to distant locations (Ra-226 half life = 1622 y). Inconsistencies also may be due to factors that have not been properly understood or measured (construction details such as depth, casing depth, and yield often are unavailable).

Current research is focused on using updated, larger scale digital geologic maps, assessing the influence of individual magmatic events, conducting multiple linear regression and geospatial modeling at varying scales, and developing a reliable statewide dis-Rn potential map.