Iron geochemical zonation in a tidally inundated acid sulfate soil wetland

Abstract

Tidal inundation is a new technique for remediating coastal acid sulfate soils (CASS). Here, we examine the effects of this technique on the geochemical zonation and cycling of Fe across a tidally inundated CASS toposequence, by investigating toposequence hydrology, in situ porewater geochemistry, solid-phase Fe fractions and Fe mineralogy. Interactions between topography and tides exerted a fundamental hydrological control on the geochemical zonation, redistribution and subsequent mineralogical transformations of Fe within the landscape. Reductive dissolution of Fe(III) minerals, including jarosite (KFe 3(SO 4) 2(OH) 6), resulted in elevated concentrations of porewater Fe 2+ (> 30 mmol L −1) in former sulfuric horizons in the upper-intertidal zone. Tidal forcing generated oscillating hydraulic gradients, driving upward advection of this Fe 2+-enriched porewater along the intertidal slope. Subsequent oxidation of Fe 2+ led to substantial accumulation of reactive Fe(III) fractions (up to 8000 μmol g −1) in redox-interfacial, tidal zone sediments. These Fe(III)-precipitates were poorly crystalline and displayed a distinct mineralisation sequence related to tidal zonation. Schwertmannite (Fe 8O 8(OH) 6SO 4) was the dominant Fe mineral phase in the upper-intertidal zone at mainly low pH (3–4). This was followed by increasing lepidocrocite (γ-FeOOH) and goethite (α-FeOOH) at circumneutral pH within lower-intertidal and subtidal zones. Relationships were evident between Fe fractions and topography. There was increasing precipitation of Fe-sulfide minerals and non-sulfidic solid-phase Fe(II) in the lower intertidal and subtidal zones. Precipitation of Fe-sulfide minerals was spatially co-incident with decreases in porewater Fe 2+. A conceptual model is presented to explain the observed landscape-scale patterns of Fe mineralisation and hydro-geochemical zonation. This study provides valuable insights into the hydro-geochemical processes caused by saline tidal inundation of low lying CASS landscapes, regardless of whether inundation is an intentional strategy or due to sea-level rise.