David Stonestrom

ABSTRACT 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.

ABSTRACT Effective isolation of tritium (H-3) and other contaminants at waste-burial facilities requires improved understanding of transport processes and pathways. Previous studies documented an anomalously widespread (i.e., theoretically unexpected) distribution of H-3 (>400 m from burial trenches) in a dry, sub-root-zone gravelly layer (1-2-m depth) adjacent to a low-level radioactive waste (LLRW) burial facility in the Amargosa Desert, Nevada, that closed in 1992. The objectives of this study were to: (i) characterize long-term, spatiotemporal variability of H-3 plumes; and (ii) quantify the processes controlling H-3 behavior in the sub-root-zone gravelly layer beneath native vegetation adjacent to the facility. Geostatistical methods, spatial moment analyses, and mass flux calculations were applied to a spatiotemporally comprehensive, 10-yr data set (2001-2011). Results showed minimal bulk-plume advancement during the study period and limited Fickian spreading of mass. Observed spreading rates were generally consistent with theoretical vapor-phase dispersion. The plume mass diminished more rapidly than would be expected from radioactive decay alone, indicating net efflux from the plume. Estimates of upward H-3 efflux via diffusive-vapor movement were >10x greater than by dispersive-vapor or total-liquid movement. Total vertical fluxes were >20x greater than lateral diffusive-vapor fluxes, highlighting the importance of upward migration toward the land surface. Mass-balance calculations showed that radioactive decay and upward diffusive-vapor fluxes contributed the majority of plume loss. Results indicate that plume losses substantially exceeded any continuing H-3 contribution to the plume from the LLRW facility during 2001 to 2011 and suggest that the widespread H-3 distribution resulted from transport before 2001.

Thermally driven water-vapor flow can be an important component of total water movement in bare soil and in deep unsaturated zones, but this process is often neglected when considering the effects of soil–plant–atmosphere interactions on shallow water movement. ...

ABSTRACT Vegetative controls on the water balance in shallow desert soils near the Amargosa Desert Research Site, Nevada were investigated with transient, one-dimensional models. Movement of liquid water, water vapor, and heat were simulated in sparsely vegetated and non-vegetated soils. Dominant transport mechanisms simulated within the root zone (upper 100 cm) were liquid water flow subsequent to precipitation and thermal-water-vapor flow during dry periods. Liquid water fluxes were greater beneath non-vegetated soils. Thermal-water-vapor fluxes beneath vegetated soils were as much as 10 times greater than non-vegetated soils and up to 1000 times greater than liquid water fluxes during warm dry periods. Total head gradients beneath non-vegetated soils were predominantly downward while those below vegetated soils were seasonally trending with upward gradients during warm dry periods and downward gradients during cool wet periods. Moisture content beneath non- vegetated soils greatly exceeded residual moisture content observed and simulated beneath vegetated soils. Sustained root-zone moisture accumulation, which does not exist beneath vegetated soils, increases the potential for downward migration of water and contaminants.

behave nearly identically in the subsurface and can move in both the liquid and vapor phases (Phillips, Cost-effective methods are needed to identify the presence and 1994). distribution of tritium near radioactive waste disposal and other con- Ground water and soil water monitoring can provide taminated sites. The objectives of this study were to (i) develop a simplified sample preparation

ABSTRACT Vegetative controls on the water balance in shallow desert soils near the Amargosa Desert Research Site, Nevada were investigated with transient, one-dimensional models. Movement of liquid water, water vapor, and heat were simulated in sparsely vegetated and non-vegetated soils. Dominant transport mechanisms simulated within the root zone (upper 100 cm) were liquid water flow subsequent to precipitation and thermal-water-vapor flow during dry periods. Liquid water fluxes were greater beneath non-vegetated soils. Thermal-water-vapor fluxes beneath vegetated soils were as much as 10 times greater than non-vegetated soils and up to 1000 times greater than liquid water fluxes during warm dry periods. Total head gradients beneath non-vegetated soils were predominantly downward while those below vegetated soils were seasonally trending with upward gradients during warm dry periods and downward gradients during cool wet periods. Moisture content beneath non- vegetated soils greatly exceeded residual moisture content observed and simulated beneath vegetated soils. Sustained root-zone moisture accumulation, which does not exist beneath vegetated soils, increases the potential for downward migration of water and contaminants.

Naturally occurring perchlorate is known to be associated with nitrate deposits of the hyperarid Atacama Desert in Chile, and recent large-scale sampling has identified a substantial reservoir (up to 1 kg/ha) of natural perchlorate in diverse unsaturated zones of the arid and semiarid Southwestern United States (Rao et al., 2007, ES&T, DOI: 10.1021/es062853i). The objective of the Amargosa Desert work is to develop a better understanding of the deposition, accumulation, and biological cycling of perchlorate in arid environments. Occurrence of perchlorate was evaluated by sampling shallow soil profiles up to 3 m in depth at four different locations and at two different time periods, and by sampling dominant plant species growing near the subsurface profiles. Deposition of perchlorate was evaluated by analyzing both bulk deposition (precipitation plus dry fall, collected under oil) collected on site and wet deposition samples collected by the National Atmospheric Deposition program at a nearby site. Soil samples and atmospheric-deposition samples were tested for both perchlorate (ClO4- ) and major anions. Perchlorate concentrations (0.2-20 µg/kg) were variable with depth in soil profiles and generally correlated most highly with chloride (Cl-) and nitrate (NO3-), although the intensity of these relations differed among profiles. Plant concentrations were generally above 1 mg/kg, suggesting ClO4- accumulation. Concentrations of ClO4- were generally much greater in total deposition than wet deposition samples, indicating a substantial dryfall component of meteoric deposition. This presentation will present the mass distribution and variability of perchlorate in bulk deposition, soils, and plants. Reasons for observed relations between subsurface concentrations of perchlorate and other anions will be explored.

ABSTRACT Questions concerning radionuclide migration include the spatial extent of contamination and the rate at which contamination spreads. Previous work at the Amargosa Desert Research Site, Nevada used a plant-based method to map large-scale, tritium plumes in a 72-ha area adjacent to a closed, commercial low-level radioactive waste facility. The objectives of this study were to (i) determine if the plant-mapped contaminant distribution has changed with time and (ii) use soil water-vapor samples to gain insight into subsurface transport. Mapping of plant-water tritium concentrations was repeated 5 yr after the initial mapping. During this 5-yr period, annual soil water-vapor samples from the root zone (~0 to 1-m depth) and sub-root-zone gravelly sand (~1 to 2-m depth) were collected along transects that passed through two tritium hot spots. Mapped plant-water tritium concentrations within the main body of both hot spots decreased ~30-40% over 5 yr. In contrast, the plant data along the periphery of the south hot spot showed little change with time, while those along the west hot spot increased and indicated continued lateral advancement of the western plume. The peak sub-root-zone water-vapor concentration within the south hot spot (2,150 Bq/L) was measured 100 m from the facility and that for the west hot spot (11,970 Bq/L) was immediately adjacent to the facility. Soil water-vapor transect data supported the plant-mapping results. For example, over 5 yr, sub-root-zone concentrations at south-transect locations within 200 m of the facility and west-transect locations within 25 m of the facility decreased by ~40%. In addition, the west-transect 200-m location showed a relatively steady increase in annual sub-root-zone water-vapor concentrations that confirmed lateral advancement of the western plume. Data collected in the south hot spot show that long-distance lateral migration of the shallow vapor-phase tritium plume occurs preferentially through the gravelly layer directly beneath the root zone. On the west site of the facility, a drainage-diversion ditch cuts through this near-surface gravel layer, but the gravel-layer discontinuity created by the ditch is not reflected in the plant and soil water-vapor tritium distributions. Thus, an alternative conceptual model is needed for the western plume. Recognizing the importance of upward flow processes in arid unsaturated zones, it is likely that the western plume represents tritium moving laterally through a deeper gravel layer and then upward into near-surface soils with subsequent release to the atmosphere. This evaluation of tritium distributions in relation to site features provides insight into field-scale transport in an arid environment.