Arsenic mobilization during seawater inundation of acid sulfate soils - hydro-geochemical coupling at the tidal fringe

Coastal acid sulfate soils (CASS) are rich in meta-stable Fe(III) minerals that are important sorbents for arsenic under oxic conditions. Tidal seawater inundation to remediate CASS has recently been trailed on a large scale and has potential to moblise arsenic during the redox transition. Tidal seawater inundation caused reductive dissolution of As(V)-bearing Fe(III) minerals, resulting in elevated concentrations of Fe2+ (2000 mg L-1) and As (~400 μg L-1) in upper-intertidal zone groundwater. Oscillating vertical and horizontal hydraulic gradients caused by tidal pumping promoted upward advection of As and Fe2+-enriched groundwater within the intertidal zone. This led to flux of Asaq and Fe2+ aq to surface waters and the accumulation of As(V)-enriched Fe(III) (hydr)oxides at the oxic sediment-water interface. Fe(III) (hydr)oxides at the sediment-water interface act as a natural reactive-barrier, retarding As flux to overlying surface waters. However, they also represent a highly transient phase that is prone to reductive dissolution during future redox boundary migration. A conceptual model is presented to explain landscape-scale patterns of As and Fe hydrogeochemical zonation.