HR: 0800h
AN: H31C-0514    [Abstracts]
TI: Stable Isotopes of Selenium and Common Elements (N, O, S) Reveal Redox Controls on the Mobility of a Naturally Occurring Source of Selenium
AU: * Andrus, R E
EM: rellenan@yahoo.com
AF: California State University- Los Angeles, Department of Geological Sciences, 5151 State University Dr. Physical Sciecnes 216, Los Angeles, CA 90032, United States
AU: Hibbs, B
EM: barryhibbs@yahoo.com
AF: California State University- Los Angeles, Department of Geological Sciences, 5151 State University Dr. Physical Sciecnes 216, Los Angeles, CA 90032, United States
AU: Ellis, A
EM: aellis@utep.edu
AF: University of Texas- El Paso, Geological Sciences, 500 W. University Ave., El Paso, TX 79968-0555, United States
AB: Historically, an extensive wetland known as the "Swamp of the Frogs" occupied the central, low-lying part of the San Diego Creek Watershed in Orange County, California. The swamp acted as a sink for the redox sensitive trace element selenium. Toward the end of the nineteenth century the swamp was drained to make way for the expansion of ranching and agriculture, resulting in a shift in prevailing redox conditions. High levels selenium have been reported in the shallow groundwaters associated with the historic swamp often causing surface water concentrations to exceed the EPA chronic criterion of 5 ug/L. Nitrate in groundwater often exceeds 10 mg/L NO3-N due to leaching of fertilizer used on citrus crops. Published research has shown that nitrate can act to oxidize reduced forms of sulfur and selenium. Stable isotopes of selenium, N and O in nitrate and S in sulfate are used to investigate the link between denitrification and the oxidation of reduced forms of sulfur and selenium along two groundwater flowpaths within the historic swamp. The "El Modena" flowpath begins roughly at the northern boundary of the historic swamp. Along this flowpath selenium increases from an upgradient value of 12 ug/L to a downgradient value of 80 ug/L while nitrate varies from 16 to 28 mg/L NO3-N. Isotopes of N and O in nitrate indicate a denitrification trend along the flowpath as delta 15N [NO3-] increases from 6.8 per mille to 19.0 per mille and delta 18O [NO3-] increases from 6.8 per mille to 14.0 per mille. The behavior of sulfur isotopes varies inversely with that of nitrogen with delta 34S [SO4 2-] decreasing from 0.1 per mille to -17.2 per mille. This, coupled with a slight enrichment in delta 13C-DIC, suggests that denitrification is coupled to the oxidation of metal sulfides disseminated in soils. Selenium isotopes (del 82/76Se [SeO4 2-]) did not grow isotopically depleted as expected, but varied from about 1.0 per mille at the outer boundary of the historic marsh, decreasing to about -1.0 per mille as groundwater moves slightly inside the marsh interior, increasing to about 2.3 per mille toward the end of the flowpath, deep within the historic marsh. The relative availability of sulfur and selenium may explain the observed pattern in selenium isotopes. The "Valencia" flowpath begins roughly at the southeast boundary of the historic swamp. Selenium increases from an upgradient value of 42 ug/L to a downgradient value of 141 ug/L while nitrate varies from 13 to 18 mg/L NO3-N. Groundwaters flowing along this flowpath did not show a change in isotope ratios. Neither delta 34S [SO4 2-] nor delta 15N [NO3-] and delta 18O [NO3-] vary by more than 1 per mille, which is small enough to discount a role of redox processes. Anomalously high concentrations of silica along this flowpath may point to a possible explanation for this result.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
SC: Hydrology [H]
MN: 2007 Fall Meeting