REDOX AND WEATHERING INFLUENCES ON NATURALLY-OCCURRING RADIONUCLIDES IN THE SAPROLITE-FRACTURED CRYSTALLINE ROCK GROUNDWATER SYSTEM OF THE PIEDMONT

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 δ¹³C_DIC (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.