Field Studies Provide Insight into Tritium Migration in an Arid Environment

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.