HR: 11:05h
AN: H32C-04    [Abstracts]
TI: The Effect of Reductive Mineral Dissolution and Porewater Chemistry on Arsenic Mobilization
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: Dixit, S
EM: dixit2@llnl.gov
AF: Lawrence Livermore National Laboratory, L-206 7000 East Avenue, Livermore, Ca 94550 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: The mobilization of As from sediments into porewater has been observed at Haiwee Reservoir (Olancha, CA, USA), where a large-scale treatment of drinking water supply to Los Angeles has resulted in an accumulation of iron- and arsenic-rich sediments. Arsenic is associated with amorphous iron oxide phases and is partially mobilized into the porewater at depth. In order to study the mechanisms of arsenic mobilization {\it in situ}, a gel probe has been constructed based on previous work. Porewater samples are collected using polyacrylamide gel slabs held in a Plexiglas ladder probe. The probe is inserted into the sediments and allowed to equilibrate with the porewater. Upon removal, the gels are re-equilibrated in acid, and As, P, Mn, and Fe are measured. Several probes were deployed in the field in October 2003 and cores were collected at the same time and locations for sequential extraction. Substantial variability was observed among the five sets of probes and cores. However, there were some general observations common to all sample sets. Dissolved concentrations of As, Fe, Mn, and P were found to increase at depth, with a strong correlation between As and Fe. Although Mn and P were also mobilized into the porewater, their concentrations with depth did not follow the same pattern as those of As. In the solid phases, As and Fe concentrations were strongly correlated and 55-95% of the As was released during extraction with 1M NaH$_{2}$PO$_{4}$, suggesting that As is primarily associated with the solid phase by adsorption. The amount of As, Fe, and Mn in the sediments decrease with depth, in contrast to their concentrations in porewaters, which increase with depth. These results are indicative of reductive dissolution of iron oxides in the sediment. It is likely that porewater composition influences the fate of As released into the porewater by reductive dissolution of Fe oxides. These effects are examined using modified gel probes in which hydrous ferric oxide (HFO), an amorphous Fe oxyhyroxide, is embedded in the gel slabs. By loading the probe with both clear and HFO gels, it is possible to simultaneously measure porewater composition and sorption behavior. Preliminary studies indicate that 24 hours is necessary to reach equilibrium with the sediment porewater when HFO gels are present, possibly due to sediment re-supply and/or sorption kinetics. Based on laboratory sediment microcosms, porewater composition may limit arsenic resorption onto fresh sorption sites.
DE: 1806 Chemistry of fresh water
DE: 1831 Groundwater quality
DE: 1030 Geochemical cycles (0330)
DE: 1065 Trace elements (3670)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting