Paper No. 16
Presentation Time: 9:00 AM-6:30 PM
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.