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Naturally-occurring arsenic (As) in the Los Angeles Aqueduct, a major drinking water source for the City of Los Angeles, is removed by precipitating an amorphous iron (Fe) oxyhydroxide floc in the aqueduct waters. The floc is removed via sedimentation at Haiwee Reservoir, where the Fe- and As-rich sediments provide a unique field site for studying the mechanisms of As mobilization to sediment porewater.
A gel probe equilibrium sampler was developed to measure the porewater concentrations and As sorption behavior in Haiwee Reservoir sediments. The gels consisted of a polyacrylamide polymer matrix and were 92% water. Undoped gels (clear gels) were used to determine porewater composition, and hydrous ferric oxide (HFO)-doped gels were used to measure in situ As adsorption chemistry. Gels were placed in a plastic holder, covered with a membrane filter, and allowed to equilibrate with the sediment porewaters. This study combined data from the gel probe samplers, gravity cores, and laboratory studies, to elucidate the biogeochemical processes governing As partitioning between the solid and aqueous phases. The gel probe device allowed for in situ observation of the effect of porewater chemistry on As adsorption.
Arsenic was reduced from As(V) to As(III) in the upper layers of the sediment, but the change in redox state did not cause As to be mobilized into the porewaters. Arsenic mobilization occurred during reductive dissolution of Fe(III) oxides. Arsenate and Fe(III) reduction were probably microbially mediated. Arsenic sorption onto the HFO-doped gels was inhibited at intermediate depths, probably due to dissolved carbonate produced from organic carbon mineralization. The partitioning of As onto the sediment in this region may be primarily controlled by porewater chemistry, rather than sorption site availability. Deeper in the sediment column, the Fe(III) phase was partially transformed to carbonate green rust, possibly sequestering dissolved carbonate. In this region, As adsorption onto HFO-doped gels was controlled by dissolved phosphate. The accumulation of As in the porewater in this region may be due to lack of available surface adsorption sites on the sediment. Arsenic partitioning between solid and aqueous phases depends on microbially driven diagenetic processes, as well as porewater composition.