HR: 17:15h
AN: B24B-06    [Abstracts]
TI: Using a Gel Probe Equilibrium Sampler to Measure Arsenic Mobilization and Sorption Gradients in Haiwee Reservoir Sediments
AU: * Campbell, K M
EM: katec@caltech.edu
AF: California Institute of Technology, 1200 E California Blvd, MC 138-78, Pasadena, CA 91125 United States
AU: Root, R
EM: rroot@ucmerced.edu
AF: University of California, Merced, 4225 N. Hospital Rd, Bldg 1200, Atwater, CA 95301 United States
AU: O'Day, P A
EM: poday@ucmerced.udu
AF: University of California, Merced, 4225 N. Hospital Rd, Bldg 1200, Atwater, CA 95301 United States
AU: Hering, J G
EM: jhering@caltech.edu
AF: California Institute of Technology, 1200 E California Blvd, MC 138-78, Pasadena, CA 91125 United States
AB: Sediment deposited at Haiwee Reservoir (Olancha, CA, USA) is enriched in arsenic and iron as a result of a large-scale water treatment process to remove naturally occurring arsenic from the Los Angeles Aqueduct (LAA). A poorly crystalline iron oxyhydroxide floc is precipitated in the aqueduct to adsorb dissolved arsenic, removing it from the water column. The iron/arsenic floc then settles in the inlet channel into Haiwee Reservoir. Previous studies at this site indicate that a fraction of the total arsenic is released into the sediment porewaters at depth, most likely as a result of bacterial reductive dissolution of the iron oxyhydroxide. A gel probe equilibrium sampler was used to measure gradients in porewater concentrations and arsenic sorption chemistry in situ in Haiwee Reservoir sediments. The gels were composed of an inert polyacrylamide matrix (92% water). They were inserted into a ladder-like Plexiglas holder, and covered with a 0.45æm membrane filter. The probe was allowed to equilibrate for 24 hours with the sediment porewaters. Two types of gels were deployed simultaneously in the probe: undoped (clear) gels to measure porewater composition, and hydrous ferric oxide (HFO)-doped gels to determine arsenic sorption behavior as a function of porewater composition. Upon removal from the sediments, the gels were re-equilibrated in acid, and a suite of metals and metalloids were measured using ICP-MS. In addition, arsenic speciation was measured in the clear gels using LC-ICP-MS. X-ray absorption spectroscopy was used to determine the oxidation state of arsenic sorbed onto HFO in the doped gels as well as arsenic in sediment cores collected when the gel probes were deployed. Arsenic in the sediment cores was reduced from As(V) to As(III) by a depth of 10 cm, but mixed oxidation states were observed in the porewaters as arsenic was mobilized into the dissolved phase, typically at depths greater than 10 cm. Dissolved arsenic was correlated to iron in the porewaters, indicating that As mobilization was linked to the reductive dissolution of Fe oxyhydroxides in the sediment. Iron reduction occurs deeper in the sediment column than arsenic reduction. Using the gel probe in combination with sediment cores, regions of iron reduction, arsenic mobilization, and arsenic sorption were observed and correlated to other metals such as Sr, Mo, Ba, and W as well as organic carbon. It is likely that sediment porewater composition affects arsenic resorption onto the solid phase. A complex structure of porewater and sorption gradients was observed within these sediments, driven by a combination of bacterial arsenic and iron reduction and porewater chemistry.
DE: 0463 Microbe/mineral interactions
DE: 0489 Trace element cycling (4875)
SC: Biogeosciences [B]
MN: Fall Meeting 2005