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Transport of Carbon-14 in a Large Unsaturated Soil Column

Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, MS 2107, Idaho Falls, ID 83415-2107

Correspondence: ^* Corresponding author (plumma{at}inel.gov).

Received for publication 10 July 2003. 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 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 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.

Abbreviations: DIC, dissolved inorganic carbon • INEEL, Idaho National Engineering and Environmental Laboratory • LLW, Low-Level Waste • RWMC, Radioactive Waste Management Complex • SDA, Subsurface Disposal Area • STOMP, Subsurface Transport Over Multiple Phases [model]

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