2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 11
Presentation Time: 4:15 PM

Transport of Tritium Contamination from the Shallow Unsaturated Zone to the Atmosphere in An Arid Environment

GARCIA, C. Amanda1, ANDRASKI, Brian J.1, COOPER, Clay A.2, STONESTROM, David A.3, JOHNSON, Michael J.1, MICHEL, Robert L.4 and WHEATCRAFT, Stephen W.5, (1)U.S. Geological Survey, 2730 N. Deer Run Road, Carson City, NV 89701, (2)2215 Raggio Parkway, Reno, NV 89512, (3)US Geological Survey, 345 Middlefield Road MS-420, Menlo Park, CA 94025, (4)US Geological Survey, 345 Middlefield Dr, MS 434, Menlo Park, CA 94025, (5)Graduate Program of Hydrologic Sciences, University of Nevada Reno, Reno, NV 89557, cgarcia@usgs.gov

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 (HTOv) transport from the shallow unsaturated zone to the atmosphere adjacent to a low-level radioactive waste (LLRW) facility.  The HTOv fluxes were quantified using a one-layer, two-component model which combined evaporation and transpiration fluxes with HTOv 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 HTOv 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 HTOv concentrations of 4,200 Bq L–1 and 8,700 Bq L–1, respectively, were measured 25 m from the LLRW facility boundary.  Daily HTOv 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 HTOv flux was spatially driven by proximity to HTOv 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 HTOv from below the root zone.