Southeastern Section - 57th Annual Meeting (10–11 April 2008)

Paper No. 7
Presentation Time: 10:20 AM


VINSON, David S.1, CAMPBELL, Ted R.2, HIRSCHFELD, Daniella1, DWYER, Gary S.1 and VENGOSH, Avner1, (1)Division of Earth & Ocean Sciences, Nicholas School of the Environment and Earth Sciences, Duke University, Box 90227, Durham, NC 27708, (2)NC Dept of Environment and Natural Resources, Division of Water Quality - Aquifer Protection Section, 2090 U.S. 70 Highway, Swannanoa, NC 28778,

Results are presented for about 150 water samples from rock units in the Piedmont of North Carolina: (1) Henderson Gneiss (HG); (2) Slate Belt (metavolcanic/metasedimentary and related intrusions); (3) Raleigh Belt metamorphic rocks; and (4) Rolesville Granite (RG). Orders-of-magnitude variation in radon (Rn), radium (Ra), and uranium (U) is evident between rock types, with Ra and U highest in RG and Rn highest in RG and HG. In RG, the daughter/parent ratio 222Rn/226Ra varies within a large range (~ 102-105, median 9,052). In HG, 222Rn/226Ra is higher (range ~ 104-106, median 185,576). The other rock types demonstrate overall lower radionuclide activities, but Ra and/or Rn exceed drinking water standards in localized spots.

In RG, a continuum is seen between oxic, low-TDS waters with low bicarbonate and high Rn, and lower-DO, higher-TDS waters containing higher bicarbonate and elevated Ra. The high 222Rn/226Ra in HG is consistent with the relatively oxic, low-TDS conditions in most of the wells sampled. In RG, ln 222Rn is positively correlated with casing depth (r= 0.58). In HG, 222Rn is also somewhat associated with casing depth. Thus, thicker saprolite (more chemically-weathered rock) is associated with higher 222Rn, possibly indicating long-term downward U transport into the bedrock fracture network during weathering, where its decay products remain. In addition to fracture concentrations of U and Th and their decay products, redox state appears to be a major influence on 222Rn/226Ra, as ln 222Rn/226Ra is correlated with dissolved oxygen (DO) in RG (r= 0.50). Abiotic reductants (biotite and/or pyrite) are present in the bedrock fracture network; however, correlation of bicarbonate with DO (r= -0.68) and preliminary δ13CDIC (r= -0.68) suggests that low DO is associated with organic carbon oxidation. Thus, water acquires its redox chemistry primarily during recharge through the soil and saprolite, where organic carbon may be present, and this redox chemistry influences the adsorption of Ra in the bedrock fracture network.

In fresh waters at near-neutral pH, reductive dissolution of redox-sensitive adsorption sites appears to enhance the mobility of Ra relative to Rn. In contrast to Ra, on which both rock type and water chemistry appear to be important influences, Rn occurrence in excess of drinking water standards is closely linked to the presence of U-rich rocks including granite.