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Tritium Plume Dynamics in the Shallow Unsaturated Zone Adjacent to an Arid Waste-Disposal Facility, Amargosa Desert Research Site, Nevada
AdvisorAndraski, Brian J
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Previous studies at the U.S. Geological Survey's Amargosa Desert Research Site (ADRS) in southern Nevada have documented two plumes of tritiated water-vapor (<super>3</super>HHO<sub>g</sub>) adjacent to a closed, commercial low-level radioactive waste disposal facility. Wastes were disposed at the facility from 1962-92. Tritium has moved long distances (> 400 m) through a shallow (1-2-m depth) dry gravelly layer--orders of magnitude further than predicted by standard transport models. Geostatistical methods, spatial moment analyses and tritium flux calculations were applied to assess shallow plume dynamics. A grid-based plant-water sampling method was utilized to infer detailed, field-scale <super>3</super>HHO<sub>g</sub> concentrations at 5-yr intervals during 2001-11. Results indicate that the total gravel-layer <super>3</super>HHO<sub>g</sub> mass diminished faster than would be expected from radioactive decay (~70% in 10 yr; 43% expected from decay). Both plumes exhibited center-of-mass stability, suggesting that bulk-plume movement was minimal during the period of study. Nonetheless, evidence of localized lateral advancement along some margins, combined with increases in the spatial covariance of concentration distribution, indicates that intra-plume mass redistribution is ongoing. Previous studies have recognized that vertical movement of tritiated water from sub-root-zone gravel into the root-zone contributes to atmospheric release via evapotranspiration. Estimates of lateral and vertical tritium fluxes during the study period indicate (1) vertical tritiated water fluxes were dominated by diffusive-vapor fluxes (> 90%), and (2) vertical diffusive-vapor fluxes were roughly an order of magnitude greater than lateral diffusive-vapor fluxes. This behavior highlights the importance of the atmosphere as a tritium sink. Estimates of cumulative vertical diffusive-vapor flux and radioactive decay with time were comparable to observed declines in total shallow plume mass with time. This suggests observed changes in plume mass may (1) be attributed, in considerable part, to these removal mechanisms, and (2) appreciable input from the adjacent disposal facility is not occurring at this time.