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 releases to the environment with time, we conducted a series of transport experiments with nonreactive gas- and aqueous-phase tracers and inorganic 14C 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 14C transport data, using both a simple gas-diffusion model and a multiphase flow and transport simulator (STOMP). Results indicate that 14C 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 Kd ≈ 0.5 ± 0.1 mL g−1, while soil samples removed approximately 1 yr later yielded Kd 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 14CO2 migration in the RWMC is dominated by gas transport, our results suggest that most 14C released from the RWMC would discharge to the atmosphere rather than to the underlying Snake River Plain aquifer.