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 (KFe₃(SO₄)₂(OH)₆), resulted in elevated concentrations of porewater Fe²⁺ (> 30 mmol L⁻¹) in former sulfuric horizons in the upper-intertidal zone. Tidal forcing generated oscillating hydraulic gradients, driving upward advection of this Fe²⁺-enriched porewater along the intertidal slope. Subsequent oxidation of Fe²⁺ 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 (Fe₈O₈(OH)₆SO₄) 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 Fe²⁺. 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.

The accumulation and behavior of arsenic at the redox interface of Fe-rich sediments is strongly influenced by Fe(III) precipitate mineralogy, As speciation, and pH. In this study, we examined the behavior of Fe and As during aeration of natural groundwater from the intertidal fringe of a wetland being remediated by tidal inundation. The groundwater was initially rich in Fe²⁺ (32 mmol L⁻¹) and As (1.81 μmol L⁻¹) with a circum-neutral pH (6.05). We explore changes in the solid/solution partitioning, speciation and mineralogy of Fe and As during long-term continuous groundwater aeration using a combination of chemical extractions, SEM, XRD, and synchrotron XAS. Initial rapid Fe²⁺ oxidation led to the formation of As(III)-bearing ferrihydrite and sorption of >95% of the As(aq) within the first 4 h of aeration. Ferrihydrite transformed to schwertmannite within 23 days, although sorbed/coprecipitated As(III) remained unoxidized during this period. Schwertmannite subsequently transformed to jarosite at low pH (2−3), accompanied by oxidation of remaining Fe²⁺. This coincided with a repartitioning of some sorbed As back into the aqueous phase as well as oxidation of sorbed/coprecipitated As(III) to As(V). Fe(III) precipitates formed via groundwater aeration were highly prone to reductive dissolution, thereby posing a high risk of mobilizing sorbed/coprecipitated As during any future upward migration of redox boundaries. Longer-term investigations are warranted to examine the potential pathways and magnitude of arsenic mobilization into surface waters in tidally reflooded wetlands.

Solid phase Fe and S fractions were examined in an acid sulfate soil (ASS) wetland undergoing remediation via tidal inundation. Considerable diagenetic enrichment of reactive Fe(III) oxides (HCl- and dithionite-extractable) occurred near the soil surface (0–0.05 m depth), where extremely large concentrations up to 3534 μmol/g accounted for ~90% of the total Fe pool. This major source of reactive Fe exerts a substantial influence on S cycling and the formation, speciation and transformation of reduced inorganic S (RIS) in tidally inundated ASS. Under these geochemical conditions, acid volatile sulfide (AVS; up to 57 μmol/g) and elemental sulfur (S0; up to 41 μmol/g) were the dominant fractions of RIS in near surface soils. AVS–S to pyrite–S ratios exceeded 2.9 near the surface, indicating that abundant reactive Fe favoured the accumulation of AVS minerals and S0 over pyrite. This is supported by the significant correlation of poorly crystalline Fe with AVS–S and S0–S contents (r = 0.83 and r = 0.85, respectively, P < 0.01). XANES spectroscopy provided direct evidence for the presence of a greigite-like phase in AVS–S measured by chemical extraction. While the abundant reactive Fe may limit the transformation of AVS minerals and S0 to pyrite during early diagenesis (~5 years), continued sulfidisation over longer time scales is likely to eventually lead to enhanced sequestration of S within pyrite (with a predicted 8% pyrite by mass). These findings provide an important understanding of sulfidisation processes occurring in reactive Fe-enriched, tidally inundated ASS landscapes.

The drainage-induced oxidation of iron-sulfide minerals in acid-sulfate soils has adversely affected large areas of coastal floodplains. Re-flooding of these soils, via the re-establishment of more natural drainage regimes, is a potential remediation approach. Here we describe the mobility of Al, As, Fe, Mn, Ni and Zn during controlled re-flooding of an Fe- and organic-rich acid-sulfate soil material. Soil re-flooding caused the onset of microbially-mediated Fe(III)-reduction, which raised the pH of the initially acidic (pH 3.4) soil to pH 6.0 to 6.5, thereby immobilizing Al. The process of Fe(III)-reduction released high concentrations of FeII and was associated with significant mobilization of As. During the early stages of re-flooding, FeII mobility was controlled by dissolution of schwertmannite (Fe8O8(OH)6SO4) with an ion activity product (IAP) of 1019 ± 2. The mobility of FeII was subsequently controlled by the precipitation of siderite (FeCO3) with an IAP spanning 10− 10 to 10− 7.5. The formation of acid-volatile sulfide (AVS), as a product of SO4-reduction, further retarded the mobility of FeII. Interactions with AVS also strongly immobilized Mn, Ni and Zn, yet had little effect on As which remained relatively mobile in the re-flooded soil. This study shows that the mobilization of As and Fe during soil re-flooding should be considered when planning remediation approaches for acid-sulfate soils.

Marine tidal inundation was partially restored to a severely degraded tropical acid sulfate soil landscape after having been excluded for over 30 years. The effects on soil acidity and iron-sulfide mineral reformation were investigated by comparing the geochemistry of soils before and after five years of regular tidal inundation. The soil pH increased by 2–3 units and titratable actual acidity (TAA) decreased by 40–50 μmol H+ g− 1 within former sulfuric horizons. Relict acidity remained at depth (> 1 m) in the underlying sulfidic horizons. δ34S data indicate that tidal inundation caused exchange of marine solutes within former sulfuric horizons, but not within underlying sulfidic material. There was considerable reformation of pyrite within former sulfuric horizons after tidal inundation with reduced inorganic sulfur increasing by 60 µmol g− 1. Acid-volatile sulfide also accumulated, but mainly near the soil surface (up to 16 µmol g− 1). Reduction of Fe(III) minerals strongly influences the geochemistry of the tidally inundated soils. After tidal inundation the soil pH and Eh closely followed the iron redox couple and there was non-sulfidic solid-phase Fe(II) up to 600 µmol g− 1. There was also substantial diagenetic enrichment of poorly crystalline Fe-oxides near the soil surface following tidal inundation, with reactive Fe spanning 400–1800 µmol g− 1. While the decreases in soil acidity documented here are likely due to a combination of marine alkalinity inputs and reduction of both Fe and SO42−, the relative importance of each process remains to be determined. This study demonstrates that marine tidal inundation can be an effective landscape-scale strategy for ameliorating severe acidity associated with drained acid sulfate soils.

The formation of iron sulfides in wetland soils is an acidconsuming process, which can sequester a range of trace metals and metalloids. In acid-sulfate soil wetlands, iron sulfide formation can therefore aid in neutralisation of acidity and immobilization of contaminants. For this reason, iron sulfide formation as a result of reductive S biomineralisation processes is an attractive site remediation objective. In this study, we quantified the in situ rates and products of dissimilatory SO4 2- reduction across a landscape-scale spatio-temporal hydrogeochemical gradient in an acid-sulfate soil wetland. The study site is an 800 ha tidal wetland that was extensively drained in the 1970’s. Drainage triggered in situ pyrite (FeS2) oxidation and acidification of surface-waters and shallow groundwaters. In 2001-2002, a remediation program was initiated which involved progressively re-flooding the site via controlled tidal inundation. We examined spatio-temporal dynamics in S biogeochemistry at the current fringe of tidal inundation about 5 years after commencement of remediation activities. In situ SO4 2- reduction was confined to near-surface soil layers (to ~60 cm below ground surface) and occurred at rates up to ~300 nmol cm-3 day-1. Elemental S was the main short-term product of SO4 2- reduction, as a result of (1) reaction between S (-II) and abundant jarosite-derived Fe (III), and (2) shortterm redox oscillations near the soil surface. Sulfur K-edge XANES spectroscopy showed that S (0)(s) was abundant in near-surface soils, which corroborated selective extraction data showing S (0)(s) up to ~40 μmol g-1. The iron sulfide thiospinel, greigite (Fe3S4), was also an important biomineralisation product, as evident from XRD, XANES spectroscopy and analytical electron microscopy. The results are discussed in terms of thermodynamic and kinetic constraints on the spatio-temporal behaviour of S biomineralisation products, especially S (0)(s) and greigite. Dynamic tide-induced redox oscillations in near-surface soil have a central role in the formation and fate of these observed S biomineralisation products.

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.

The anticipated impacts of climate change are warmer conditions, an increasing proportion of rainfall to occur from heavy falls, increasing occurrence of drought in many regions, increasing frequency of intense tropical cyclones, rising sea levels and frequency of extreme high seas (e.g. storm surges). All of these predicted impacts have direct relevance to coastal acid sulfate soils landscapes, through either exacerbating sulfide oxidation by drought, re-instating reductive geochemical processes or changing the export and mobilisation of contaminants. The interaction of specific land management factors such as man-made drainage will also have a significant role in how the predicted impacts of climate change affect these landscapes.

Understanding the potential impacts of climate change for coastal lowland acid sulfate soils is particularly important, given the utility of these areas for agriculture and urban communities, their unique capacity to cause extreme environmental degradation, and their sensitivity to climatic factors such as temperature and hydrology and susceptibility to sea-level inundation.

There is a strong and expanding fundamental knowledge of processes in coastal acid sulfate soils, but limited studies to date that consider the impacts of climate change. Using data from our research group this paper examines some of the key issues of climate change of relevance to acid sulfate soil. We investigate the hydrogeochemical consequences of seawater inundation of an 800 Ha acid sulfate soil wetland and study of current drought triggered broad-scale oxidation (i.e. 20, 000 Ha of exposed soils) of lake bed sediments in the lower Murray-Darling River Basin, South Australia.

The effects of restoring marine tidal inundation to a severely degraded acid sulfate soil landscape were investigated. Five years of regular tidal inundation led to substantial improvements in a range of key parameters used to assess soil and water quality. The pH of estuarine creeks improved dramatically following reintroduction of tidal inundation. Time series water quality and climatic data indicate a substantial decrease in the magnitude of creek acidification per given quantity of antecedent rainfall. The soil pH also increased by 2–3 units and titratable actual acidity (TAA) decreased by ~40–50 μmol H+ g-1 within former sulfuric horizons. Tidal inundation stimulated Fe and SO4 2- reduction within the landscape, leading to internal alkalinity generation and the reformation of considerable quantities of pyrite within former sulfuric horizons. In addition, there were large decreases in water-soluble and exchangeable Al fractions within former sulfuric horizons, which is an important finding from an eco-toxicology perspective.

However, the radical change in hydrology and geochemistry initiated by tidal inundation has had profound consequences for the fate, mobilisation, redistribution and transformation of Fe minerals and co-associated trace elements. There was substantial diagenetic enrichment of poorly crystalline Fe-oxides near the soil surface following tidal inundation. This was also associated with enrichment of some trace metals. High concentrations of arsenic were observed in porewaters (~300 μg L-1) and were associated with reductive dissolution of secondary iron minerals, including jarosite, which had formed during the previous oxic / acidic phase. This study demonstrates that marine tidal inundation can be an effective method for remediating acid sulfate soils at a landscape-scale. However, there are a range of potential geochemical complexities which need to be considered prior to implementing this technique.