Importance and Use of Plants in Evaluating Water Flow and Contaminant Transport in Arid Environments
by B.J. Andraski, M.W. Sandstrom, R.L. Michel, J.C. Radyk, D.A. Stonestrom, M.J. Johnson, and C.J. Mayers
Based on a poster presented at the American Geophysical Union's Fall 2002 Meeting, December 6-10, 2002
Improved understanding of soil-plant-atmosphere interactions is critical to water-resource and waste management decisions. Multiple-year field studies of soil-water movement at the Amargosa Desert Research Site (ADRS) identified plants as the primary control on the near-surface water balance. The boundary conditions imposed by plant activity in the uppermost soil layer also result in episodic, deep drying below the root zone during periods of below-average precipitation. The findings help to explain evidence for negligible recharge and upward flow that has been inferred from environmental-tracer and soil-physics-based studies of deep unsaturated zones at undisturbed, arid sites.
Studies at the ADRS also are using plants to investigate tritium transport away from a low-level radioactive waste disposal area. Soil-gas sampling results indicated that tritium has moved as much as 300 m from the disposal area, and that transport primarily occurs in the gas phase with preferential transport through coarse-textured sediment layers. The need for an efficient means of gathering plume-scale data led to the development of a method that uses plant water to identify tritium contamination. Tritium concentrations in plant water determined with the new method did not differ significantly from those determined with the standard (and more laborious) toluene-extraction method or from concentrations in root-zone soil-water vapor. The new method provides a simple and cost-effective way to identify plant and soil contamination. Although work to date has focused on one desert plant, the approach may be transferable to other species and environments.
Figure 1. Location of Amargosa Desert Research Site, Nevada .
Arid environments often are considered ideal for waste isolation because the natural environment has features that can minimize the risk of waste migration to the underlying water table (e.g., low precipitation, high evapotranspiration, thick unsaturated zone). The processes influencing the transport of water and contaminants in deserts, however, are not well understood and can be affected in dramatic ways by temporal and spatial changes in precipitation, vegetation, and soils.
The objective of research at the Amargosa Desert Research Site (ADRS) is to develop a fundamental understanding of hydrologic conditions and contaminant-transport processes in arid environments. The ADRS is located about 17 km south of Beatty, Nevada (fig. 1) and is adjacent to a disposal facility for low-level radioactive and hazardous waste. Precipitation during 1981-2001 averaged 108 mm/yr. The surface soil layer was formed by eolian deposition and cumulative soil development beneath a desert pavement. The underlying sediments are fluvial deposits. Depth to the water table is about 110 m. Vegetation is sparse; Larrea tridentata (creosote bush), an evergreen shrub, is the dominant species. The rooting depth of Larrea at the ADRS is about 0.75-1 m.
WATER BALANCE AND FLOW
Water-balance data show that plants typically contribute to the annual depletion of water that accumulates in the root zone (fig. 2). The small net increase in water storage for vegetated soil in December 1998 occurred in response to increased precipitation during the 1997-98 El Niño cycle. Storage decreases for vegetated soil are due to evapotranspiration. Storage decreases for devegetated soil are due to bare-soil evaporation and percolation. Water-potential data show that plants also contribute to episodic, deep drying of sediments well below the root zone during years with below-average precipitation (e.g., 1989-90; fig. 3).
Figure 2. Cumulative changes in soil-water storage for the 0- to 1-m depth interval relative to initial (fall 1987) values (Data from Andraski, 1997; Johnson and others, 2002).
Figure 3. Sub-root-zone soil-water potentials and precipitation. (Data from Andraski, 1997)
These findings help to explain evidence for negligible recharge and upward flow that has been inferred from studies of the deep unsaturated zone at undisturbed, arid sites. For example, chloride-concentration profiles at the ADRS indicate that percolation past the 10-m depth has been negligible for the past 16,000 yr (fig. 4A). In addition, water-potential data indicate upward driving forces for water movement in the upper 60 m (fig. 4B). As a result, new conceptual models have been developed to incorporate the influence of desert vegetation in analyses of paleo- to present-day water fluxes in deep unsaturated zones (Walvoord and others, 2002a, 2002b; Scanlon and others, 2003).
Figure 4. (A) Chloride mass-balance age and (B) soil-water potential profiles. (Data from Prudic, 1994 (A); Stonestrom and others, 1999 (B))
DETECTORS OF CONTAMINATION
A simplified method was developed to identify tritium contamination in plants and soil. The method entails sample collection and solar distillation (8 hours) of plant water from foliage; distillate is collected by pipet (fig. 5). Plant water then is filtered and passed through a graphite-based, solid-phase-extraction (SPE) column to adsorb scintillation-interfering constituents (fig. 6). A 2-g-carbon-SPE column was found to be necessary and sufficient for accurate determinations of known tritium concentrations in Larrea water.
Figure 6. (A) Batch filtration and SPE-column apparatus showing (a) syringe-less filters, (b) SPE columns, and (c) 15-ml sample bottles. (B) Bottles of untreated and treated solar-distilled Larrea water.
Tritium concentrations in plant water determined with the new method did not differ significantly from those determined with the standard, and more laborious, toluene-extraction method or from concentrations in root-zone soil-water vapor (fig. 7). Although work to date has focused on one desert plant, the approach may be transferable to other species and environments after site-specific investigations establish its efficacy elsewhere. Two main sources of uncertainty that can affect the accuracy of the solar distillation-SPE method and warrant further study are: (1) the exact mechanisms that interfere with liquid-scintillation counting and (2) the effects of isotopic fractionation on solar-distilled tritium concentrations.
Figure 7. Relation between tritium concentrations in (A) solar-distilled, SPE Larrea water and toluene-extracted Larrea water; (B) solar-distilled, SPE Larrea water and root-zone soil-water vapor (0.5-m sampling depth).
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