HR: 16:50h
AN: H42K-04    [PDF]
TI: Selenium and Arsenic - Nitrate-facilitated Pyrite Oxidation in an Urban Watershed
AU: * Sjolin, J
EM: jenann@citrus.ucr.edu
AF: University of California, Riverside, Department of Environmental Sciences, Riverside, CA 92521
AU: Meixner, T
EM: tmeixner@ucr.edu
AF: University of California, Riverside, Department of Environmental Sciences, Riverside, CA 92521
AU: Hibbs, B
AF: California State University, Los Angeles, Department of Geological Sciences, Los Angeles, CA 90032
AU: Amrhein, C
AF: University of California, Riverside, Department of Environmental Sciences, Riverside, CA 92521
AU: Walker, J
AF: California State University, Los Angeles, Department of Geological Sciences, Los Angeles, CA 90032
AB: A century of Orange County urbanization on the Newport Bay / San Diego Creek Watershed has had dramatic impacts on surface geomorphology. Draining of historic wetlands and subsequent channel incision has caused oxidation and mobilization of toxic trace elements accumulated in the former wetlands. Levels of selenium (maximum 270 �g/L, median 28 �g/L) and arsenic (maximum 40 �g/L, median 5 �g/L) released to surface waters are as much as 50 times the standard for wildlife toxicity. Additionally field evidence indicates that high levels of nitrate present in the soil (6 mg/kg) and ground water (maximum 55 mg/L, median 7 mg/L), originating in past agricultural activity in the watershed, enhances trace element mobilization via microbially mediated denitrification and pyrite oxidation. The spatial pattern of groundwater concentrations of nitrate, selenium and arsenic suggest an inverse relationship that is indicative of this redox reaction. Preliminary results from a batch study using local vadose zone soils demonstrates a trend for greater mobilization of arsenic by a 200 ppm nitrate solution than by control solution. A positive correlation between sulfate, selenium and arsenic in groundwater samples supports concurrent oxidation and mobilization; it also defines pyrite and related sulfur compounds as candidates for original sequestration in the historic wetland. Ratios of chloride to sulfate as well as sulfur isotopes in groundwater samples show sulfate enrichment indicative of oxidative dissolution processes, possibly of pyrite, as opposed to evaporative concentration or external loading. Elevated bicarbonate concentrations in regions of high selenium and arsenic also support this hypothesis, since acidity generated by pyrite oxidation facilitates calcite dissolution. A positive correlation between arsenic and molybdenum suggests a significant role for surface adsorption of oxidized arsenic and reduced selenium species in controlling trace element mobility. While the processes controlling mobilization of selenium and arsenic in the basin are complex, field evidence supports an increase in trace element mobility due to wetland draining and subsequent nitrogen enrichment of groundwater aquifers in the basin.
DE: 1065 Trace elements (3670)
DE: 1803 Anthropogenic effects
DE: 1831 Groundwater quality
DE: 1871 Surface water quality
DE: 1890 Wetlands
SC: Hydrology [H]
MN: 2003 Fall Meeting