HR: 1340h
AN: H13G-1664    [Abstracts]
TI: Denitrification-Coupled Iron(II) Oxidation: A Key Process Regulating the Fate and Transport of Nitrate, Phosphate, and Arsenic in a Wastewater-Contaminated Aquifer
AU: * Smith, R L
EM: rlsmith@usgs.gov
AF: U.S. Geological Survey, 3215 Marine St., Boulder, CO 80303, United States
AU: Kent, D B
EM: dbkent@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menol Park, CA 94025, United States
AU: Repert, D A
EM: darepert@usgs.gov
AF: U.S. Geological Survey, 3215 Marine St., Boulder, CO 80303, United States
AU: Hart, C P
EM: cphart@usgs.gov
AF: U.S. Geological Survey, 3215 Marine St., Boulder, CO 80303, United States
AB: Denitrification in the subsurface is often viewed as a heterotrophic process. However, some denitrifiers can also utilize inorganic electron donors. In particular, Fe(II), which is common in many aquifers, could be an important reductant for contaminant nitrate. Anoxic iron oxidation would have additional consequences, including decreased mobility for species like arsenic and phosphate, which bind strongly to hydrous Fe(III) oxide. A study was conducted in a wastewater contaminant plume on Cape Cod to assess the potential for denitrification- coupled Fe(II) oxidation. Previous changes in wastewater disposal upgradient of the study area had resulted in nitrate being transported into a portion of the anoxic zone of the plume and decreased concentrations of Fe(II), phosphate, and arsenic. A series of anoxic tracers (groundwater + nitrate + bromide) were injected into the unaffected, Fe(II)-containing zone under natural gradient conditions. Denitrification was stimulated within 1 m of transport (4 days) for both low and high (100 & 1000 μM) nitrate additions, initially producing stiochiometric quantities of nitrous oxide (>300 μM N) and trace amounts of nitrite. Subsequent injections at the same site reduced nitrate even more rapidly and produced less nitrous oxide, especially over longer transport distances. Fe(II) and nitrate concentrations decreased together and this was accompanied by an increase in colloidal Fe(III) and decreases in pH, total arsenic, and phosphate concentrations. All plume constituents returned to background levels several weeks after the tracer tests were completed. Groundwater microorganisms collected on filters during the tracer test rapidly and immediately reduced nitrite and oxidized Fe(II) in 3-hr laboratory incubations. Several pure cultures of Fe(II)-oxidizing denitrifying bacteria were isolated from core material and subsequently characterized. All of the isolates were mixotrophic, simultaneously oxidizing organic carbon and Fe(II). These results demonstrate that denitrification-coupled Fe(II) oxidation can readily occur within anoxic groundwater; that it could be key to controlling iron speciation in certain situations and that the outcome of the process can impact the mobility of other chemical species that are not directly involved in the oxidation-reduction reaction.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0470 Nutrients and nutrient cycling (4845, 4850)
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: 2007 Fall Meeting