TRANSPORT OF CARBON-14 IN A LARGE UNSATURATED SOIL COLUMN

Wastes buried at the Radioactive Waste Management Complex (RWMC) of the Idaho National Engineering and Environmental Laboratory (INEEL) include activated metals that release radioactive ¹⁴C as they corrode. To test and refine transport predictions that describe the relative upward and downward releases of these contaminants over time, we conducted a series of transport experiments, with ¹⁴C and non-reactive gas-phase and aqueous-phase tracers, in a large unsaturated soil column filled with sediment representative of that at the RWMC. Results confirm that ¹⁴C behaves largely as predicted in these sediments, with transport effectively controlled by diffusion in the gas phase retarded by partitioning into the aqueous and solid phases. Based on the combined tracer and ¹⁴C transport tests, we found that a K_D of ~0.4 mL/g provided the best fit for the ¹⁴C transport experiment, with gas-aqueous partitioning and gas-phase tortuosity well-described by, respectively, carbonate equilibria expressions and a slightly modified form of the Millington equation. These parameters are consistent with those derived from smaller-scale experiments with the same sediments, demonstrating that laboratory-based measurements provide a valid means of estimating transport behavior at much larger scales. Assuming that CO₂transport in the RWMC is also dominated by diffusion implies that most ¹⁴C released via corrosion of activated metals would be discharged to the atmosphere, rather than to the underlying Snake River Plain aquifer.