Soil, plant, and terrain effects on natural perchlorate distribution in a desert landscape.

Perchlorate (ClO) is a contaminant that occurs naturally throughout the world, but little is known about its distribution and interactions in terrestrial ecosystems. The objectives of this Amargosa Desert, Nevada study were to determine (i) the local-scale distribution of shallow-soil (0-30 cm) ClO with respect to shrub proximity (far and near) in three geomorphic settings (shoulder slope, footslope, and valley floor); (ii) the importance of soil, plant, and terrain variables on the hillslope-distribution of shallow-soil and creosote bush [ (Sessé & Moc. ex DC.) Coville] ClO; and (iii) atmospheric (wet plus dry, including dust) deposition of ClO in relation to soil and plant reservoirs and cycling. Soil ClO ranged from 0.3 to 5.0 µg kg. Within settings, valley floor ClO was 17× less near shrubs due in part to enhanced leaching, whereas shoulder and footslope values were ∼2× greater near shrubs. Hillslope regression models (soil, = 0.42; leaf, = 0.74) identified topographic and soil effects on ClO deposition, transport, and cycling. Selective plant uptake, bioaccumulation, and soil enrichment were evidenced by leaf ClO concentrations and Cl/ClO molar ratios that were ∼8000× greater and 40× less, respectively, than soil values. Atmospheric deposition ClO flux was 343 mg ha yr, ∼10× that for published southwestern wet-deposition fluxes. Creosote bush canopy ClO (1310 mg ha) was identified as a previously unrecognized but important and active reservoir. Nitrate δO analyses of atmospheric deposition and soil supported the leaf-cycled-ClO input hypothesis. This study provides basic data on ClO distribution and cycling that are pertinent to the assessment of environmental impacts in desert ecosystems and broadly transferable to anthropogenically contaminated systems.

Simplified method for detecting tritium contamination in plants and soil.

Cost-effective methods are needed to identify the presence and distribution of tritium near radioactive waste disposal and other contaminated sites. The objectives of this study were to (i) develop a simplified sample preparation method for determining tritium contamination in plants and (ii) determine if plant data could be used as an indicator of soil contamination. The method entailed collection and solar distillation of plant water from foliage, followed by filtration and adsorption of scintillation-interfering constituents on a graphite-based solid phase extraction (SPE) column. The method was evaluated using samples of creosote bush [Larrea tridentata (Sessé & Moc. ex DC.) Coville], an evergreen shrub, near a radioactive disposal area in the Mojave Desert. Laboratory tests showed that a 2-g SPE column was necessary and sufficient for accurate determination of known tritium concentrations in plant water. Comparisons of tritium concentrations in plant water determined with the solar distillation-SPE method and the standard (and more laborious) toluene-extraction method showed no significant difference between methods. Tritium concentrations in plant water and in water vapor of root-zone soil also showed no significant difference between methods. Thus, the solar distillation-SPE method provides a simple and cost-effective way to identify plant and soil contamination. The method is of sufficient accuracy to facilitate collection of plume-scale data and optimize placement of more sophisticated (and costly) monitoring equipment at contaminated sites. Although work to date has focused on one desert plant, the approach may be transferable to other species and environments after site-specific experiments.