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Title: Multiphase Carbon-14 Transport in a Near-Field-Scale Unsaturated Column of Natural Sediments

Wastes buried at the Subsurface Disposal Area (SDA) of the Idaho National Engineering and Environmental Laboratory include activated metals that release radioactive carbon-14 (14C) as they corrode. To better understand 14C phase partitioning and transport in the SDA sediments, we conducted a series of transport experiments using 14C (radio-labeled sodium carbonate) and nonreactive gas (sulfur hexafluoride) and aqueous (bromide and tritiated water) tracers in a large (2.6-m high by 0.9-m diameter) column of sediments similar to those used as cover material at the SDA. We established steady-state unsaturated flow prior to injecting tracers into the column. Tracer migration was monitored using pore-water and pore-gas samples taken from co-located suction lysimeters and gas ports inserted at ~0.3-m intervals along the column’s length. Measurements of 14C discharged from the sediment to the atmosphere (i.e., 14CO2 flux) indicate a positive correlation between CO2 partial pressure (pCO2) in the column and changes in 14CO2 flux. Though 14CO2 diffusion is expected to be independent of pCO2, changes of pCO2 affect pore water chemistry sufficiently to affect aqueous/gas phase 14C partitioning and consequently 14C2 flux. Pore-water and -gas 14C activity measurements provide an average aqueous/gas partitioning ratio, Kag, of 4.5 (±0.3). This value is consistent withmore » that calculated using standard carbonate equilibrium expressions with measured pH, suggesting the ability to estimate Kag from carbonate equilibrium. One year after the 14C injection, the column was cored and solid-phase 14C activity was measured. The average aqueous/solid partition coefficient, Kd, (1.6 L kg-1) was consistent with those derived from small-scale and short-term batch and column experiments using SDA sediments, suggesting that bench-scale measurements are a valid means of estimating aqueous/solid partitioning at the much larger spatial scale considered in these meso-scale experiments. However, limitations at the bench scale prevent observation of spatially- and temporally-varying parameters that affect contaminant transport in the natural environment. In addition to a temporally-variable 14CO2 flux, in response to changes of pCO2, we observed non-uniformities in Kag and Kd that were not observed in bench-scale studies. Our results suggest that 14C transport is effectively controlled by gas diffusion with minimal retardation by partitioning onto the solid phase, and little long-term retention. The implication for the SDA is that 14C released via corrosion of activated metals is primarily transported by gas-phase diffusion rather than by liquid-phase advection. Calculations show that, because the atmospheric boundary is so much closer than the aquifer boundary at the SDA, most of the 14C will diffuse upward to the atmosphere.« less
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Publication Date:
OSTI Identifier:
Report Number(s):
TRN: US200724%%342
DOE Contract Number:
Resource Type:
Technical Report
Research Org:
Idaho National Laboratory (INL)
Sponsoring Org:
Country of Publication:
United States
54 - ENVIRONMENTAL SCIENCES; ADVECTION; AQUIFERS; CARBON 14; CARBONATES; CORROSION; DIFFUSION; LYSIMETERS; PARTIAL PRESSURE; RETENTION; SEDIMENTS; SODIUM CARBONATES; SULFUR; TRANSPORT; TRITIUM OXIDES; WASTES; WATER CHEMISTRY Bromide; Buried Radioactive Waste; Carbon-14; Meso-Scale Column Experiments; Phase Partitioning; Sediments; Snake River Plane Aquifer; Tracer; Tracer Retardation; Transport; Tritium; Vadose Zone