Journal article
Iron geochemical zonation in a tidally inundated acid sulfate soil wetland
Chemical Geology, Vol.280(3-4), pp.257-270
2011
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Abstract
<p>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<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub>), resulted in elevated concentrations of porewater Fe<sup>2+</sup> (> 30 mmol L<sup>−1</sup>) in former sulfuric horizons in the upper-intertidal zone. Tidal forcing generated oscillating hydraulic gradients, driving upward advection of this Fe<sup>2+</sup>-enriched porewater along the intertidal slope. Subsequent oxidation of Fe<sup>2+</sup> led to substantial accumulation of reactive Fe(III) fractions (up to 8000 μmol g<sup>−1</sup>) 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<sub>8</sub>O<sub>8</sub>(OH)<sub>6</sub>SO<sub>4</sub>) 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<sup>2+</sup>. 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.</p>
Details
- Title
- Iron geochemical zonation in a tidally inundated acid sulfate soil wetland
- Creators
- Scott G Johnston - Southern Cross UniversityAnnabelle F Keene - Southern Cross UniversityRichard T Bush - Southern Cross UniversityEdward D Burton - Southern Cross UniversityLeigh A Sullivan - Southern Cross UniversityLloyd S Isaacson - Southern Cross UniversityAngus E McElnea - Department of Environment and Resource Management, QldColin R Ahern - Department of Environment and Resource Management, QldC Douglas Smith - Department of Environment and Resource Management, QldBernard Powell - Department of Environment and Resource Management, Qld
- Publication Details
- Chemical Geology, Vol.280(3-4), pp.257-270
- Identifiers
- 2229; 991012820351402368
- Academic Unit
- Science; School of Environment, Science and Engineering; Faculty of Science and Engineering
- Resource Type
- Journal article