The effect of reductive dissolution and porewater composition on arsenic mobilization in Haiwee Reservoir sediments

摘要

Sediments in Haiwee Reservoir (Olancha, CA, USA) have elevated concentrations of arsenic (As) and iron (Fe) due to a largescale water treatment process. The sediments were studied using a gel probe equilibrium sampler and analyzing core samples. The gel probe equilibrium sampler was developed to measure porewater concentrations of As and other elements as well as As sorption behavior by using two types of polyacrylamide gels. Undoped (clear) gels were used to measure porewater composition and hydrous ferric oxide (HFO) doped gels were used to measure in situ As adsorption chemistry. Arsenic was reduced from As (Ⅴ) to As(Ⅲ) in the upper layers of the sediment and was mobilized into the porewaters at depth due to reductive dissolution of Fe(Ⅲ) oxides. Eoth As and Fe reduction were microbially mediated. Arsenic adsorption onto the HFO-doped gels was inhibited most likely due to the effects of elevated dissolved carbonate concentrations at intermediate depths. Deeper in the sediment column, the amorphous Fe(Ⅲ) oxide solid phase transformed into poorly crystalline carbonate green rust, possibly sequestering dissolved carbonate. Arsenic adsorption at depth onto the HFO- doped gels was controlled primarily by the presence of dissolved phosphate.