Transport of Tritium Contamination to the Atmosphere in an Arid Environment

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.