Transport of Carbon-14 in a Large Unsaturated Soil Column

Abstract

Wastes buried at the Radioactive Waste Management Complex (RWMC) of the Idaho National Engineering and Environmental Laboratory (INEEL) include activated metals that release radioactive 14 C as they corrode. To test and refine transport predictions that describe releases to the environment with time, we conducted a series of transport experiments with nonreactive gas- and aqueous-phase tracers and inorganic 14 C species in a large unsaturated soil column filled with sediment representative of that at the RWMC. The tracer tests, hydraulic measurements, and chemical monitoring provided constraints on physical transport parameters, water content, and aqueous–gas partitioning behavior. With those constraints, we estimated a solid–aqueous distribution coefficient for the sediment through inverse modeling of the 14 C transport data, using both a simple gas-diffusion model and a multiphase flow and transport simulator (STOMP). Results indicate that 14 C transport in this system is well described by a reactive gas diffusion model, with a pH-dependent retardation factor. Fitting transport simulations to the early-time transport data yielded K d ≈ 0.5 ± 0.1 mL g −1 , while soil samples removed approximately 1 yr later yielded K d values of 0.8 to 2.4 mL g −1 . These values are consistent with those derived from smaller-scale experiments, demonstrating that laboratory-based measurements provide a valid means of estimating transport behavior at much larger spatial and temporal scales. Assuming that 14 CO 2 migration in the RWMC is dominated by gas transport, our results suggest that most 14 C released from the RWMC would discharge to the atmosphere rather than to the underlying Snake River Plain aquifer.