Mobilisation of arsenic following sea-water inundation of acid sulfate soils

摘要

Tidal seawater inundation of coastal acid sulfate soils can generate Fe- and SCvreducing conditions in previously oxic sediments. This creates potential for mobilisation of As during the redox transition. We investigate the hydrology, porewater geochemistry, solid-phase speciation and mineralogical partitioning of As across two tidal fringe toposequences and explore the consequences for As. Seawater inundation induced a tidally-controlled redox gradient. Maximum porewater As (-400 ug/L) occurred in the intertidal, redox transition zone between Fe-oxidizing and SO_4-reducing conditions. Mechanisms of As mobilisation include the reduction of solid-phase As(V) to As(III), reductive dissolution of As(V)-bearing secondary Fe(III) minerals and competitiveanion desorption. There was attenuation of porewater As in the zone of contemporary pyrite reformation. Oscillating hydraulic gradients caused by tidal pumping promote upward advection of As and Fe~2+)-enriched porewater in the intertidal zone, leadingto accumulation of As(V)-enriched Fe(III) (hydr)oxides at the oxic sediment-water interface. This creates a natural reactive-Fe barrier, but does not completely retard the flux of porewater As to overtopping surface waters. The accumulated Fe minerals may be prone to future reductive dissolution. A conceptual model of As hydro-geochemical coupling is presented.