Over 17 million ha of coastal lowlands contain acid sulfate soils (CASS). These soils are rich in meta-stable, redoxsensitive Fe (III)-minerals. Large areas of CASS are at risk of increased saline tidal inundation due to sea-level rise. Fieldbased CASS remediation trials reveal that tidal seawater inundation initiates radical changes in sediment hydrogeochemistry, stimulating Fe and SO4 reducing conditions, generating alkalinity and greatly decreasing the acidity hazard. However, these changes also have profound consequences for the fate, mobilisation, redistribution and transformation of Fe minerals and co-associated trace elements. Here, we examine the consequences for iron and arsenic by investigating the hydrology, in situ porewater geochemistry, solid-phase Fe and As fractions and Fe mineralogy across a tidally inundated CASS toposequence.
Reductive dissolution of As (V)-bearing Fe (III) minerals, including jarosite (KFe3 (SO4)2 (OH)6), resulted in elevated concentrations of porewater Fe2+ (2000 mg L-1) and As (~400 ìg L-1) in former sulfuric horizons in the upper-intertidal zone. Oscillating hydraulic gradients caused by tidal pumping promoted upward advection of this As and Fe2+-enriched porewater. This led to accumulation of As (V)-enriched Fe (III) (hydr)oxides at the oxic sediment-water interface and some flux of Asaq and Fe2+ aq to overtopping tidal surface waters. Fe (III) (hydr)oxides at the surface-water interface were poorly crystalline and displayed a diverse mineralisation sequence related to tidal zonation. Whilst these Fe (III) (hydr)oxides act as a natural reactive-Fe barrier, they represent a transient phase that is prone to future reductive dissolution. This has uncertain consequences regarding the potential release of co-associated trace elements.
The extreme enrichment of poorly crystalline Fe (III) (hydr)oxides (~40% Fe w/w) near the surface is a function of the interplay between tidally influenced hydrology, topography and geochemistry. This enrichment not only affects As partitioning, it strongly influences contemporary SO4 reduction products and pathways. It also represents a substantive hysteresis in the geochemical trajectory of the CASS landscape as it undergoes remediation. Conceptual models are presented to explain the observed landscape-scale patterns of Fe and As hydro-geochemical zonation. These findings have broad geochemical relevance to the saline tidal inundation of low lying Fe-rich landscapes.