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Transport of Tritium Contamination to the Atmosphere in an Arid Environment

^a U.S. Geological Survey, 2730 N. Deer Run Rd., Carson City, NV 89701
^b U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025
^c Desert Research Institute, 2215 Raggio Pkwy., Reno, NV 89512
^d Dep. of Geological Sciences, Univ. of Nevada, Reno, NV 89557

Correspondence: ^* Corresponding author (cgarcia{at}usgs.gov).

Received for publication 4 February 2008. Soil–plant–atmosphere interactions strongly influence water movement in desert unsaturated zones, but little is known about how such interactions affect atmospheric release of subsurface water-borne contaminants. This 2-yr study, performed at the U.S. Geological Survey's Amargosa Desert Research Site in southern Nevada, quantified the magnitude and spatiotemporal variability of tritium (³H) transport from the shallow unsaturated zone to the atmosphere adjacent to a low-level radioactive waste (LLRW) facility. Tritium fluxes were calculated as the product of ³H concentrations in water vapor and respective evaporation and transpiration water-vapor fluxes. Quarterly measured ³H concentrations in soil water vapor and in leaf water of the dominant creosote-bush [Larrea tridentata (DC.) Coville] were spatially extrapolated and temporally interpolated to develop daily maps of contamination across the 0.76-km² study area. Maximum plant and root-zone soil concentrations (4200 and 8700 Bq L^–1, respectively) were measured 25 m from the LLRW facility boundary. Continuous evaporation was estimated using a Priestley–Taylor model and transpiration was computed as the difference between measured eddy-covariance evapotranspiration and estimated evaporation. The mean evaporation/transpiration ratio was 3:1. Tritium released from the study area ranged from 0.12 to 12 µg d^–1 and totaled 1.5 mg (8.2 x 10¹⁰ Bq) over 2 yr. Tritium flux variability was driven spatially by proximity to ³H source areas and temporally by changes in ³H concentrations and in the partitioning between evaporation and transpiration. Evapotranspiration removed and limited penetration of precipitation beneath native vegetation and fostered upward movement and release of ³H from below the root zone.

Abbreviations: ADRS, Amargosa Desert Research Site • ET, evapotranspiration • ³H, tritium • ³HHO, tritiated water • LLRW, low-level radioactive waste

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