LABORATORY EXPERIMENTS OF CARBON-14 UPTAKE AND RELEASE FROM 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 carbonate rocks have shown some retardation of carbon-14 as well as a slow release of carbon-14 at later times. A series of uptake/release experiments (analogous to sorption/desorption experiments) using carbon-14 with high purity calcite and dolomite, the two primary constituents in NTS carbonate rocks, were conducted in small columns to evaluate the degree of release relative to uptake. Here, the terms “uptake” and “release” refer to isotopic exchange and precipitation/dissolution processes as well as sorption and desorption. The experimental variables included different mineral saturation states of the solutions, solution pH, particle size, and particle pre-treatment. Initial results demonstrated a non-equilibrium release process that occurs at a much slower rate than uptake. A conceptual model of carbon-14 uptake and release that captures these non-equilibrium processes has been developed based on the results of the laboratory investigation.