Importance and Use of Plants in Evaluating Water Flow and Contaminant Transport in Arid Environments

Brian J. Andraski¹, Mark W. Sandstrom², Robert L. Michel³, John C. Radyk³, David A. Stonestrom³, Michael J. Johnson¹, and Charles J. Mayers¹

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; http://nevada.usgs.gov/adrs/) near Beatty, Nevada identified plants as the primary control on the near-surface water balance and showed that the boundary conditions imposed by plant activity in the uppermost soil layer results in episodic, deep drying well below the root zone during periods of below-average precipitation. The results help to explain the evidence for negligible recharge and upward flow that has been inferred from environmental-tracer and soil-physics based studies of undisturbed, arid sites. The findings have contributed to the development of new conceptual models that incorporate the influence of desert vegetation in analyses of paleo- to present-day water fluxes in deep unsaturated zones.

Studies at the ADRS are also using plants to investigate the transport of contaminants away from a closed 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 the presence and distribution of tritium. The method entails field sampling and solar distillation of foliage to collect plant water, followed by laboratory filtration and adsorption of scintillation-interfering constituents on a graphite-based, solid-phase-extraction (SPE) column. The method was evaluated using an evergreen shrub (creosote bush; Larrea tridentata (DC.) Cov.). Tritium concentrations in plant water determined with the distillation-SPE method did not differ significantly from those determined with the standard (and more laborious) toluene-extraction method or from concentrations in soil-water vapor collected using gas-sampling methods. Thus, the solar distillation-SPE method provides a simple, cost-effective, and accurate alternative approach to identify areas of plant and soil contamination. Although work to date has focused on one plant, the approach may be transferable to other species and environments. Work at the ADRS has confirmed the importance of vegetation in arid-site hydrodynamics. Future studies will attempt to better quantify and understand the flux of tritium from the subsurface to the atmosphere through a combination of soil, plant, and evapotranspiration measurements.

¹U.S. Geological Survey, 333 West Nye Lane, Room 203, Carson City, NV 89706, USA (andraski@usgs.gov, johnsonm@usgs.gov, cjmayers@usgs.gov)

²U.S. Geological Survey, Denver Federal Center, P.O. Box 25046, Lakewood, CO (sandstro@usgs.gov)

³U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA (rlmichel@usgs.gov, jradyk@usgs.gov, dastones@usgs.gov)

This abstract was published in Eos, Transactions, American Geophysical Union, v. 83, no. 47, Fall. Meet. Suppl., Abstract H52A-0836, CD-ROM computer file.