A large-scale field trial indicates that tidal seawater inundation of coastal acid sulfate soils (CASS) stimulates iron and sulfate reducing conditions, leading to the generation of internal alkalinity and greatly decreasing soil / groundwater acidity. This remediation technique can be considered highly effective from the relatively narrow perspective of decreasing acidity and raising pH. However, a broader perspective reveals that tidal seawater inundation initiates complex and tightly coupled hydrological and geochemical processes within sediments and porewaters. These processes have had a profound effect on the mobilisation, redistribution and transformation of Fe minerals in the landscape (Johnston et al., 2011a) – in turn influencing both the mobilisation of arsenic (Johnston et al., 2010) and the biogeochemical behaviour of reduced inorganic sulfur species (Burton et al., 2011). These processes have longer-term implications for landscape management, which we examine here. Tidal inundation initiated reductive dissolution of As(V)-bearing Fe(III) minerals, including jarosite (KFe3(SO4)2(OH)6). This resulted in highly elevated porewater concentrations of Fe2+ (2000 mg L-1) and As (~400 g L-1) in the former sulfuric horizons of the upper-intertidal zone (Johnston et al., 2010). Groundwater in former sulfuric horizons is subject to oscillating vertical and horizontal hydraulic gradients caused by tidal pumping. This promotes upward advection of As and Fe2+-enriched groundwater within the intertidal zone and leads to the accumulation of As(V)-enriched Fe(III) (hydr)oxides at the oxic sediment-water interface (Johnston et al., 2011b).
This coupling of a redox transition triggering Fe and As mobilisation, with a physical forcing process that redistributes those mobilised products, is an important feature of tidal seawater inundation of CASS. Oscillating hydraulic gradients create potential for dynamic exchange of aqueous species with overlying surface waters. There is some flux of Fe2+ aq and Asaq to overtopping tidal surface waters – a process which is enhanced by porewater advection through preferential flow pathways in the soil (Johnston et al., 2011c).
There is clear evidence demonstrating that the magnitude and spatial heterogeneity (at a cm-scale) of enrichment in Fe and As in oxic-interface sediments is influenced by the occurrence of surface connected macropores. Fe(III) minerals that form at the sediment-water interface are poorly crystalline and display a diverse mineralisation sequence related to tidal zonation. These Fe(III) phases act as a natural reactive-Fe barrier and help retard As flux from groundwater to overlying surface waters. However, they also represent a highly transient phase that is prone to reductive dissolution during future redox boundary migration (Johnston et al., 2011b). This has uncertain consequences regarding the potential future release of co-associated trace elements. The extreme enrichment of poorly crystalline Fe(III) minerals (~40% Fe w/w) near the sediment surface is a function of the interplay between tidally influenced hydrology, topography, geochemistry and microbiology. It has important consequences for reduced inorganic sulfur cycling – favouring the formation of elemental sulfur and AVS species in the short term. Over the medium to longer term this enrichment in reactive Fe phases creates potential for large (>10%) accumulations of pyrite near the surface (Keene et al., 2011). A landscape-scale conceptual model is presented which explains the observed hydro-geochemical cycling of Fe, As and S.
Johnston, SG, Keene, AF, Burton, ED, Bush, RT & Sullivan, LA 2012, 'Hydro-geochemical coupling in seawater inundation acid sulfate soils: mobilisation of arsenic and hysteresis in iron and sulfur cycling', in C Clay (ed.), 3rd National Conference on Acid Sulfate Soils, Melbourne, Vic., 5-7 March, pp. 50-51, Southern Cross University, Lismore, NSW.