Transport of Tritium Contamination from the Shallow Unsaturated Zone 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 water borne contaminant fluxes from the subsurface to the atmosphere.  The objective of this field study was to quantify the magnitude and spatio-temporal variability of tritiated-water-vapor (HTO_v) transport from the shallow unsaturated zone to the atmosphere adjacent to a low-level radioactive waste (LLRW) facility.  The HTO_v fluxes were quantified using a one-layer, two-component model which combined evaporation and transpiration fluxes with HTO_v concentrations in shallow soil and plants, respectively.  Continuous evaporation was estimated using a Priestley-Taylor model, calibrated with periodic bare-soil evaporation and micrometeorological measurements.  Continuous transpiration was computed as the difference between continuously measured eddy-covariance evapotranspiration and estimated evaporation.  The 2-yr mean daily E-to-T ratio was 75:25 percent.  Concentrations of HTO_v in plants (creosote bush [Larrea tridentata (Sessé & Moc. Ex DC.) Coville]) and root-zone soil, measured quarterly during a 2-yr period, were spatially extrapolated and temporally interpolated to develop daily maps of contamination across the 0.75 km² study area.  Maximum plant- and root-zone soil HTO_v concentrations of 4,200 Bq L^–1 and 8,700 Bq L^–1, respectively, were measured 25 m from the LLRW facility boundary.  Daily HTO_v fluxes ranged from 10^–5 to 10^–3 mg study_area^–1 d^–1 and released a cumulative mass of 1.5 mg over the 2-yr study.  Variability in the HTO_v flux was spatially driven by proximity to HTO_v source areas and temporally controlled by evapotranspiration fluxes, concentration dilution from precipitation, and root-water extraction depth.  Evapotranspiration was shown to not only remove and limit percolation of precipitation beneath native vegetation, but also foster upward movement and release of HTO_v from below the root zone.