LABORATORY EXPERIMENTS OF CARBON-14 UPTAKE BY CARBONATE ROCKS, CALCITE, AND DOLOMITE

Carbon-14 is a radionuclide byproduct of underground nuclear testing. Current numerical flow and radionuclide transport models for the Nevada Test Site in southern Nevada treat carbon-14 as a conservative constituent. However, previous laboratory sorption and flow-through column experiments conducted with NTS rocks have shown some retardation of carbon-14. Carbon-14 occurs as an anion (predominantly as bicarbonate or carbonate above pH 6.4) in most NTS groundwater, where the pH ranges from about 6.7 to 9. Unlike cations, which sorb readily onto aquifer mineral surfaces at these pH values, anions tend not to sorb unless the pH is significantly below the point of zero charge of mineral surfaces (typically below a pH of 5 or 6). Thus, carbon-14 is potentially mobile in NTS groundwater. Despite this potential mobility, there are several processes by which carbon-14 may be effectively retarded. These processes include matrix diffusion, secondary mineral precipitation/dissolution, and isotopic exchange. A series of batch sorption experiments using carbon-14 and NTS carbonate aquifer rocks, high purity calcite, and dolomite were conducted to evaluate the importance of these processes. The experimental variables included different mineral saturation states, carbon-14 activity, and particle size. Initial results demonstrated a non-equilibrium up-take process that cannot be described by a simple isotherm. A conceptual model of carbon-14 uptake that captures the non-equilibrium retardation behavior has been developed based on the results of the laboratory investigation.