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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

View larger version (24K): [in a new window]   Fig. 1. Location of the Subsurface Disposal Area (SDA) of the Radioactive Waste Management Complex (RWMC) at the Idaho National Engineering and Environmental Laboratory (INEEL).

View larger version (46K): [in a new window]   Fig. 2. Schematic diagram of the mesoscale unsaturated flow column showing vertical placement of sampling and monitoring devices.

View larger version (56K): [in a new window]   Fig. 3. Schematic diagram (plan view) of instrumentation at each of the eight sampling and monitoring levels and at Level 0, where water is removed via suction lysimeters.

View larger version (51K): [in a new window]   Fig. 4. Water content and moisture potential histories since start of water application, 11 July 2001. (A) Reflectometer data; (B) the moisture potential record, as measured with in-situ tensiometers; (C) the average water content calculated from the column water balance data. Note that the reflectometer at Level 1 malfunctioned and was used only as a relative measure of water content. Lettered vertical lines indicate the date of (a) the Br– injection, (b) soil sampling, (c and d) sulfur hexafluoride gas injections, and (e) the 14C-labeled bicarbonate injection.

View larger version (22K): [in a new window]   Fig. 5. Comparison of modeled and observed SF6 breakthrough curves for (A) the initial, prewetting, SF6 test and (B) the first test conducted after reaching near steady-state flow. The modeled curves were computed from an analytical solution (Luikov, 1968) to the diffusion equation.

View larger version (18K): [in a new window]   Fig. 6. Bromide concentration data (symbols) from the six sampling levels below the injection plane and CXTFIT2-fit curves (lines).

View larger version (33K): [in a new window]   Fig. 7. Measured 14C gas-phase breakthrough curves (symbols) and simulated responses (lines) at each sampling level. Simulated curves were calculated from an analytical solution to a conceptual model that considers reactive diffusive transport in the gas phase, but neglects aqueous transport. Average column water content was set to 28%; Kd = 0.45 mg L–1 was determined via least squares fit to the observations.

View larger version (26K): [in a new window]   Fig. 8. Calculated aqueous/gas partitioning ratio profile (solid line) for the 14C transport experiment, based on pH measurements made just before the 14C injection and measured aqueous-gas partitioning ratios (symbols) following the 14C injection, based on aqueous- and gas-phase 14C measurements from each sampling level.

View larger version (33K): [in a new window]   Fig. 9. Measured 14C gas-phase breakthrough curves (symbols) and simulated responses (lines) at each sampling level. Simulated curves were calculated using the multiphase flow and transport model STOMP, using the nonuniform, pH-based, aqueous/gas partitioning ratio profile shown in Fig. 8. Average water content for the flow simulation was 28%; Kd 0.5 mg L–1.

View larger version (32K): [in a new window]   Fig. 10. Measured 14C aqueous-phase breakthrough curves (symbols) and simulated responses (lines) at each sampling level. Simulated curves were calculated using the multiphase flow and transport model STOMP, using the nonuniform, pH-based, aqueous/gas partitioning ratio profile shown in Fig. 8. Average water content for the flow simulation was about 28%; Kd 0.5 mg L–1.

View larger version (20K): [in a new window]   Fig. 11. Mass balance on effluent from the column and 14C remaining in the column, from effluent monitoring and samples collected approximately 1 yr after injection.