Arsenic Diagenesis at the Sediment-Water Interface of a Recently Flooded Freshwater Sediment

摘要

In a previous study, elevated dissolved arsenic concentrations were detected in the Chattahoochee River (GA) and decreased in concentration with increasing distance from a suspected point-source, suggesting that arsenic could be scavenged by particles and settle to the sediment-water interface (SWI). In this study, the distribution of arsenic in sediments was determined to assess its biogeochemical cycling close to the SWI following a high deposition event. A combination of analytical techniques -ICP-MS and voltammetry - was used to measure the speciation of dissolved arsenic in two sediment cores. In addition, the porewater distribution of O_2, Fe(Ⅱ), Mn(Ⅱ), and ΣH_2S was measured with Au/Hg voltammetric microelectrodes to obtain high resolution profiles of the redox species involved in arsenic cycling. Our data show that dissolved arsenic is in the form of As(Ⅴ) only and accumulates directly below the SWI. The significant correlation between Fe(Ⅱ) and As(Ⅴ) indicates that As(Ⅴ) is released from iron oxides during their microbial reduction. These results suggests that sedimentation of As(Ⅴ) occurring after storm events is folLowed by remobilization of As(Ⅴ) in porewaters and upward diffusion, thus recycling arsenic into the overlying water.