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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 ((3)H) 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 (3)H concentrations in water vapor and respective evaporation and transpiration water-vapor fluxes. Quarterly measured (3)H 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-km(2) 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 mu g d(-1) and totaled 1.5 mg (8.2 x 10(10) Bq) over 2 yr. Tritium flux variability was driven spatially by proximity to (3)H source areas and temporally by changes in (3)H 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 (3)H from below the root zone.