SOIL, PLANT, AND TERRAIN CONTROLS ON NATURAL PERCHLORATE IN A DESERT LANDSCAPE—AMARGOSA DESERT RESEARCH SITE, NEVADA

Growing concern about perchlorate (ClO₄⁻) contamination of drinking water and food has heightened the need for improved understanding of its natural occurrence. Natural ClO₄⁻ is being studied at the USGS Amargosa Desert Research Site to determine factors and processes that control its distribution in a desert-shrub landscape. Total atmospheric deposition measured approximately quarterly over 6 yr had an average ClO₄⁻ concentration of 608 ng L⁻¹ and the flux was 300 mg ha⁻¹ yr⁻¹. Comparisons between total deposition results and previous work on wet deposition indicated that dry deposition could be an important contributor to the occurrence of ClO₄⁻ in arid environments. Feature-based sampling was used to evaluate the effects of landscape setting (shoulder slope, footslope, and valley floor) and plant proximity (far versus near) on shallow (0-30 cm) soil ClO₄⁻. Soil-ClO₄⁻ concentrations and Cl⁻/ClO₄⁻ molar ratios ranged from 0.26–5.00 μg kg⁻¹ and 3300–84,300 mol mol⁻¹, respectively. Valley floor concentrations showed a large (~20 fold) decrease near plants, but those for the shoulder and footslope increased (~2 fold) near plants. These increases were attributed to a reduction in near-plant soil leaching and increased accumulation of ClO₄⁻ from a bioconcentrated-leaf source. Bioaccumulation was indicated by high leaf-ClO₄⁻ concentrations (14,000–60,000 μg kg⁻¹) and low leaf-Cl⁻/ClO₄⁻ molar ratios (60–180 mol mol⁻¹). Grid-based sampling was used to assess soil, plant, and terrain influences on soil and leaf ClO₄⁻ concentrations across a 9-ha hillslope. Soil and leaf concentrations ranged from 0.82–11.81 μg kg⁻¹ and 9700–59,000 μg kg⁻¹, respectively. Regression results for soil ClO₄⁻ identified elevation and slope gradient as significant variables that explained 42% of the variation; both variables are suggestive of processes affecting deposition and transport of ClO₄⁻ by wind or water. For leaf ClO₄⁻, 74% of the variation was explained by elevation, soil CO₃-C, soil total-organic C, and aspect. The soil variables indicated caliche and organic matter influences on ClO₄⁻ uptake, and aspect was related to shrub growth and ClO₄⁻ bioaccumulation. This study provides a framework for improved understanding of the interactive factors and processes that control the distribution of ClO₄⁻ in a desert landscape.