Non-steady state modeling of arsenic diagenesis in lake sediments

A one-dimensional reactive transport model describing the coupled biogeochemical cycling of As, C, O, Fe, and S was used to interpret an extensive geochemical sediment (As, Fe, S, (210)Pb, (137)Cs, C(org)) and pore water (As, Fe, SO(4)(2-), SigmaS(-II) and pH) data set collected in the perennially oxygenated basin of an oligotrophic lake. Historical variations in atmospheric deposition of As and SO(4)(2-) were explicitly included as upper boundary conditions in the model calculations. The results show that the depth profile of sediment-bound As reflects both the past changes in As deposition and the diagenetic redistribution of As among the Fe(III) oxyhydroxide and Fe(II) sulfide pools. The model-predicted benthic release of dissolved As to the water column peaks 26 years after the maximum anthropogenic As input to the lake, which occurred around 1950. Two major environmental forcings of the benthic recycling of As are the organic matter degradation in the sediment and the atmospheric sulfate deposition to the lake. More oxidizing conditions associated with lower organic matter degradation rates yield a greater abundance of Fe(III) oxyhydroxides in the topmost sediment, which act as a barrier to pore water As. Variations in sulfate availability have more complex effects on benthic As remobilization, since sulfide produced by sulfate reduction may enhance both the uptake of dissolved As through the precipitation of Fe(II) sulfides and the release of dissolved As through the reductive dissolution of Fe(III) oxyhydroxides.