Edward D Burton

Quantifying retained acidity in acid sulfate soil materials: an evaluation of routine methods

Chamindra Vithana et al. — Tue, 07 May 2013 19:38:25 PDT

Retained Acidity is an important aspect of acid sulfate soil (ASS) but techniques for the quantification of this property have not yet been systematically assessed. In this study, the utility of routine techniques for quantifying the Retained Acidity fraction will be examined. The relatively insoluble minerals such as jarosite and schwertmannite are major contributors to Retained Acidity in ASS. Known quantities of synthetic and natural jarosites and schwertmannite will be added to two non-acid sulfate soil samples plus to one quartz sand sample. By using SNAS in the chromium suite and SRAS in the SPOCAS suite (Ahern et al. 2004), the Retained Acidity fraction of the spiked samples will be assessed. The method introduced by Li et al. (2007) and the acidified ammonium oxalate method (Regenspurg et al. 2004) will be undertaken to recover the added jarosite and schwertmannite, respectively. The mineralogy of the samples will be characterized by X-ray diffraction (XRD). In the second part of this study, the potential interference from

A critical evaluation of Retained Acidity (AR) estimating methods in Acid Base Account (ABA) approach for Acid Sulfate Soils (ASS)

Chamindra Vithana et al. — Thu, 02 May 2013 22:47:43 PDT

Understanding the behaviour of schwertmannite and jarosite in acid sulfate soils

Chamindra Vithana et al. — Thu, 02 May 2013 22:47:42 PDT

Readily available acidity in schwertmannite

Chamindra Vithana et al. — Thu, 02 May 2013 22:47:41 PDT

Schwertmannite and jarosite are considered as less soluble ironhydroxy sulfate minerals which are present in highly acidic environments (pH < 3). These minerals release acidity in the long run as they weather by hydrolysis [1]. However, 1M KCl extraction of soil samples (Clarence and Quartz) spiked with those two minerals showed that schwertmannite has some acidity that may be readily available.

Underestimation of retained acidity measured using chromium suite in acid sulfate soils containing schwermannite and jarosite

Chamindra Vithana et al. — Wed, 01 May 2013 18:42:43 PDT

Arsenic mobilization during seawater inundation of acid sulfate soils - hydro-geochemical coupling at the tidal fringe

Scott G. Johnston et al. — Wed, 01 May 2013 18:13:04 PDT

Hydro-geochemical coupling in seawater inundation acid sulfate soils: mobilisation of arsenic and hysteresis in iron and sulfur cycling

Scott G. Johnston et al. — Wed, 01 May 2013 18:13:03 PDT

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.

Seawater-induced mobilization of trace metals from mackinawite-rich estuarine sediments

Vanessa NL Wong et al. — Wed, 01 May 2013 18:13:02 PDT

Benthic sediments in coastal acid sulfate soil (CASS) drains can contain high concentrations (∼1–5%) of acid volatile sulfide (AVS) as nano-particulate mackinawite. These sediments can sequester substantial quantities of trace metals. Because of their low elevation and the connectivity of drains to estuarine channels, these benthic sediments are vulnerable to rapid increases in ionic strength from seawater incursion by floodgate opening, floodgate failure, storm surge and seasonal migration of the estuarine salt wedge. This study examines the effect of increasing seawater concentration on trace metal mobilization from mackinawite-rich drain sediments (210–550 μmol g⁻¹ AVS) collected along an estuarine salinity gradient. Linear combination fitting of S K-edge XANES indicated mackinawite comprised 88–96% of sediment-bound S. Anoxic sediment suspensions were conducted with seawater concentrations ranging from 0% to 100%. We found that mobilization of some metals increased markedly with increasing ionic strength (Cu, Fe, Mn, Ni) whereas Al mobilization decreased. The largest proportion of metals mobilized from the labile metal pool, operationally defined as ∑exchangeable + acid-extractable + organically-bound metals, occurred in sediments from relatively fresh upstream sites (up to 39% mobilized) compared to sediments sourced from brackish downstream sites (0–11% mobilized). The extent of relative trace metal desorption generally followed the sequence Mn > Ni ≈ Cu > Zn > Fe > Al. Trace metal mobilization from these mackinawite-rich sediments was attributed primarily to desorption of weakly-bound metals via competitive exchange with marine-derived cations and enhanced complexation with Cl⁻ and dissolved organic ligands. These results have important implications for trace metal mobilization from these sediments at near-neutral pH under current predicted sea-level rise and climate change scenarios.

Arsenic mobilization and iron transformations during sulﬁdization of As(V)-bearing jarosite

Scott G. Johnston et al. — Sun, 28 Apr 2013 21:13:08 PDT

Jarosite (KFe3(SO4) 2(OH)6) is an important host-phase for As in acid mine drainage (AMD) environments and coastal acid sulfate soils (CASS). In AMD and CASS wetlands, jarosite may encounter S(−II) produced by sulfate reducing bacteria. Here, we examine abiotic sulﬁdization of As(V)-bearing K-jarosite at pH 4.0, 5.0, 6.5 and 8.0. We quantify the mobilization and speciation of As and identify corresponding Fe mineral transformations. Sulﬁde-promoted dissolution of jarosite caused release of co-precipitated As and the majority of mobilized As was re-partitioned to a readily exchangeable surface complex (AsEx). In general, maximum As mobilization occurred in the highly sulﬁdized end-members of all treatments and was greatest at low pH, following the order pH 5.0≈4.0>8.5> 6.5. X-ray absorption spectroscopy revealed that most solid-phase As remained as oxygen-coordinated As(V) when pH values were >5.0 — even during latter stages of sulﬁdization and the presence of ≥100 μM dissolved S(−II). In contrast at pH 4.0, As transitioned from oxygen-coordinated As(V) to a sulfur-coordinated orpiment-like phase. This transition coincided with a marked decrease in AsEx, attenuation of As(aq) and TEM-EDX spectra indicate concurrent formation of nano-scale zones variably enriched in As (~1–15%). Although discordant with geochemical modeling, the formation of an orpiment-like precipitate appears to be a primary control on As mobility during the late stages of complete jarosite sulﬁdization under acidic conditions (pH 4.0). Mackinawite was the main Fe-mineral end product in all pH treatments. However, at pH 8.0, jarosite rapidly (b1 h) transformed to a lepidocrocite intermediary. Although lepidocrocite efﬁciently adsorbed As(aq), the transformation process itself was incongruent with electron transfer to Fe(III). Further investigation is required to determine whether the electron donor triggering this transformation was direct via S(−II), or indirect via surface complexed Fe(II) and hence akin to the widely-known Fe(II)-catalyzed transformation of Fe(III) minerals. The results demonstrate that abiotic sulﬁdization of As(V)-co-precipitated jarosite can mobilize substantial As and that pH exerts a major control on the subsequent As solid-phase speciation, electron transfer kinetics and Fe mineralization pathways and products. The ﬁndings are particularly relevant to heterogeneous sediments in which As-bearing jarosite encounters dissolved sulﬁde under a range of pH conditions.

Decoupling between water column oxygenation and benthic phosphate dynamics in a shallow eutrophic estuary

Peter Kraal et al. — Sun, 21 Apr 2013 19:51:07 PDT

Estuaries are crucial biogeochemical filters at the land–ocean interface that are strongly impacted by anthropogenic nutrient inputs. Here, we investigate benthic nitrogen (N) and phosphorus (P) dynamics in relation to physicochemical surface sediment properties and bottom water mixing in the shallow, eutrophic Peel-Harvey Estuary. Our results show the strong dependence of sedimentary P release on Fe and S redox cycling. The estuary contains surface sediments that are strongly reducing and act as net P source, despite physical sediment mixing under an oxygenated water column. This decoupling between water column oxygenation and benthic P dynamics is of great importance to understand the evolution of nutrient dynamics in marine systems in response to increasing nutrient loadings. In addition, the findings show that the relationship between P burial efficiency and bottom water oxygenation depends on local conditions; sediment properties rather than oxygen availability may control benthic P recycling. Overall, our results illustrate the complex response of an estuary to environmental change because of interacting physical and biogeochemical processes.

An acid sulfate soil precursor: iron, phosphorus and sulfur dynamics in sediments from the Peel-Harvey estuary, WA

Peter Kraal et al. — Tue, 16 Apr 2013 19:42:48 PDT

The Peel-Harvey Estuary in south-western Australia is faced with a dire combination of man-made challenges. Agricultural activity in the adjacent area has for decades caused the influx of large quantities of nutrients (most notably phosphorus) into the estuary, fuelling algal blooms and the formation of organic-rich, sulfidic sediments. The abundance of sulfide in this restricted system has had detrimental effects on both the water quality and marine biology. Free sulfide in the water column is highly toxic to marine life and the disturbance and oxidation of sulfidic sediments can cause severe deoxygenation in the water column. At the same time, sulfide-rich sediments on the edges of the estuary are being drained for urban development, causing problems associated with sulfide oxidation and soil acidification. As such, the sulfidic sediments that form in the estuary today are tomorrow’s acid sulfate soils. Besides management challenges, the reducing sediments in the estuary also provide opportunities to study geochemical processes under extreme conditions. These investigations may not only help to combat the environmental threats to the Peel-Harvey estuary, but also to better understand the links between geochemical signatures in ancient sediments and the palaeo-environmental conditions during deposition. The present as key to understanding the past, however ugly both may be. In this study, we investigate in detail the dynamics of iron (Fe), phosphorus (P) and sulfur (S) in sediments from different sites in the Peel-Harvey Estuary. At several sites, there is an abundance of iron monosulfide minerals (FeS) up to ~ 40 cm sediment depth. These minerals are commonly highly reactive and rapidly transform into more stable pyrite (FeS2). Grain-size distribution seems to play an important role in sedimentary FeS preservation: the oxygen required for partial oxidation of FeS and subsequent formation of FeS2 diffuses relatively slowly into fine-grained sediments, extending the lifetime of labile FeS. The fine-grained, FeS-rich sediments are easily disturbed and oxidised, which may negatively affect water oxygenation and ecoystem health. The abundant pore-water sulfide effectively scavenges dissolved Fe to form FeS, resulting in low pore-water Fe concentrations and a very high fraction of total Fe in reduced reactive minerals. The sulfide-rich pore-waters also record very high concentrations of nutrient N and P. The sequestration of P through calcium phosphate mineral formation in these reducing sediments is strongly limited. The sediments thus act as a source of nutrients back to the water column, adding to any fluvial input. As such, the rapid internal recycling of nutrients in the estuary may drive the system into a eutrophied state which is more or less independent of external inputs. The coupled Fe, P and S biogeochemical cycles can create an environment which is optimal for the continued formation of organic-rich, sulfidic sediments that may be the precursors of acid sulfate soils in this heavily anthropogenically altered ecosystem.

Impact of silica on the reductive transformation of schwertmannite and the mobilization of arsenic

Edward D. Burton et al. — Tue, 19 Mar 2013 21:47:25 PDT

Schwertmannite is an important Fe(III) mineral in acid–sulfate soil and acid–mine drainage environments because it is both widespread and highly reactive towards trace elements, such as As. Transformation of schwertmannite to more crystalline phases, such as goethite, may strongly influence As mobility. However, previous research suggests that the rate and extent of schwertmannite transformation can be strongly retarded by the presence of Si – a ubiquitous species in natural waters. The present study examines the impact of Si on reductive transformation of schwertmannite and the associated behavior of Fe and coprecipitated As. Synthetic As(V)-coprecipitated schwertmannite (Fe₈O₈(OH)_4.2(SO₄)_1.9(AsO₄)_0.0005) was subjected to microbially-mediated reducing conditions for 126 days in the presence of three environmentally-relevant Si concentrations (0, 1.9 and 9.5 mM Si). In addition, complementary sorption experiments and short-term abiotic mineral transformation experiments were conducted to examine the interactive impacts of Si and Fe²⁺ on schwertmannite stability. Sorption experiments revealed negligible Si sorption to schwertmannite under acidic conditions, with Si sorption only being important towards near-neutral pH. In the 126 day biotic incubations, the onset of reducing conditions in the initially acidic schwertmannite suspensions stimulated dissimilatory Fe(III) reduction, producing Fe²⁺ and simultaneously causing pH to increase to ∼6.5. Sorption of Si to the schwertmannite surface at this near-neutral pH partially retarded the rate of Fe²⁺-catalyzed transformation of schwertmannite to goethite. However, the effect of Si was minor under microbially-reducing conditions, with Fe²⁺ catalyzing rapid schwertmannite transformation even in the presence of abundant Si. Our short-term abiotic experiments demonstrate that the limited effect of Si is a consequence of Fe²⁺ being produced concurrently with increases in pH. This allows Fe²⁺-schwertmannite interactions to proceed prior to the formation of surface-passivating Si species. The Fe²⁺ catalyzed transformation of As(V)-coprecipitated schwertmannite to goethite caused a major increase in -extractable As, but had little effect on aqueous As concentrations. The reduction of Fe(III) and the subsequent onset of dissimilatory reduction led to formation of siderite (FeCO₃) and mackinawite (FeS), respectively. The reduction of As(V) to As(III) was associated with the Si-dependent mobilization of As into the aqueous-phase. There was a concurrent decrease over time in the concentrations of -extractable As, which occurred independent of Si concentrations and appeared to be related to formation of siderite and mackinawite. The findings from this study provide new insights into the evolution of iron mineralogy and associated arsenic mobility following the establishment of reducing conditions in schwertmannite- and Si-rich environments.

Coupling of arsenic mobility to sulfur transformations during microbial sulfate reduction in the presence and absence of humic acid

Edward D. Burton et al. — Tue, 19 Mar 2013 21:47:24 PDT

Microbial sulfate reduction is an important terminal electron accepting process in arsenic-contaminated subsurface environments. Humic acids are ubiquitous in such environments, yet their impact on arsenic mobility under sulfate-reducing conditions is poorly understood. In this study, we examined the effects of microbial sulfate reduction and humic acid on arsenic mobilization via a series of advective-flow column experiments. The initial solid-phase in these experiments comprised quartz sand that was coated with As(III)-sorbed goethite (α-FeOOH). The effect of humic acid was assessed by comparing columns that received artificial groundwater in which humic acid was either absent or present at 100 mg L^{− 1}, whilst the effect of microbial sulfate reduction was investigated by comparing columns that were inoculated with the sulfate-reducer Desulfovibrio vulgaris (ATCC strain 7757) versus abiotic control columns. The presence of high concentrations of humic acid alone did not enhance the overall extent of arsenic release from either the abiotic or the inoculated (sulfate reducing) columns. This is consistent with similar arsenic concentrations in porewaters filtered to both < 0.45 μm and < 3 kDa, demonstrating that aqueous arsenic did not form mobile colloidal humic acid complexes. In contrast, microbial sulfate reduction was found to mobilize substantial levels of arsenic relative to those observed in the corresponding abiotic control columns. Iron and sulfur K-edge X-ray absorption spectroscopy (XAS) showed that reaction between goethite and microbially-produced sulfide lead to accumulation of mackinawite (FeS) and elemental S. Microbial sulfate reduction also caused important changes in arsenic speciation, especially the formation of aqueous dithioarsenate and monothioarsenate. However, arsenic K-edge XAS showed that arsenic sulfide mineral phases (orpiment and realgar) did not form during the 60 day advective-flow experiment. The formation of poorly-sorbing thioarsenate species appeared to contribute to the observed enhancement of arsenic mobilization from the inoculated columns. Dithioarsenate and monothioarsenate were relatively stable, and were found to make up > 40% of aqueous arsenic even at very low porewater sulfide concentrations (i.e. < 10 μmol L^{− 1}). Accordingly, the formation, stability and sorption–desorption of thioarsenate species need to be considered when evaluating and predicting subsurface arsenic mobility.

Sulfate availability drives divergent evolution of arsenic speciation during microbially mediated reductive transformation of schwertmannite

Edward D. Burton et al. — Tue, 19 Mar 2013 21:47:23 PDT

The effect of SO₄^2– availability on the microbially mediated reductive transformation of As(V)-coprecipitated schwertmannite (Fe₈O₈(OH)_3.2(SO₄)_2.4(AsO₄)_0.004) was examined in long-term (up to 400 days) incubation experiments. Iron EXAFS spectroscopy showed siderite (FeCO₃) and mackinawite (FeS) were the dominant secondary Fe(II) minerals produced via reductive schwertmannite transformation. In addition, 25% to 65% of the initial schwertmannite was also transformed relatively rapidly to goethite (αFeOOH), with the extent of this transformation being dependent on SO₄^2– concentrations. More specifically, the presence of high SO₄^2– concentrations acted to stabilize schwertmannite, retarding its transformation to goethite and allowing its partial persistence over the 400 day experiment duration. Elevated SO₄^2– also decreased the extent of dissimilatory reduction of Fe(III) and As(V), instead favoring dissimilatory SO₄^2– reduction. In contrast, where SO₄^2– was less available, there was near-complete reduction of schwertmannite- and goethite-derived Fe(III) as well as solid-phase As(V). As a result, under low SO₄^2– conditions, almost no Fe(III) or As(V) remained toward the end of the experiment and arsenic solid-phase partitioning was controlled mainly by sorptive interactions between As(III) and mackinawite. These As(III)–mackinawite interactions led to the formation of an orpiment (As₂S₃)-like species. Interestingly, this orpiment-like arsenic species did not form under SO₄^2–-rich conditions, despite the prevalence of dissimilatory SO₄^2– reduction. The absence of an arsenic sulfide species under SO₄^2–-rich conditions appears to have been a consequence of schwertmannite persistence, combined with the preferential retention of arsenic oxyanions by schwertmannite. The results highlight the critical role that SO₄^2– availability can play in controlling solid-phase arsenic speciation, particularly arsenic–sulfur interactions, under reducing conditions in soils, sediments, and shallow groundwater systems.

Iron and arsenic cycling in intertidal surface sediments during wetland remediation

Scott G. Johnston et al. — Wed, 30 Jan 2013 17:52:22 PST

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.

Quantifying alkalinity generating processes in a tidally remediating acidic wetland

Scott G. Johnston et al. — Mon, 10 Dec 2012 20:53:57 PST

Lime-assisted tidal exchange (LATE) is a new remediation technique that is demonstrably effective at decreasing acidity in coastal acid sulfate soils (CASS). However, the relative magnitude of the major in situ alkalinity generating processes and external alkalinity inputs that dominate neutralization of acidity during LATE have not been quantified. Here, we combine investigations of porewater and solid-phase geochemistry from a remediating CASS wetland to derive first-order estimates of alkalinity generating processes and inputs after 6 years of LATE. Quantified inputs include: marine derived HCO3− from tidal exchange; hydrated lime additions; and in situ alkalinity from anaerobic metabolism of organic carbon coupled with reduction of iron and sulfate. A progressive increase in tidal inundation led to the development of significant relationships (

Sulfur biogeochemical cycling and novel Fe-S mineralization pathways in a tidally re-flooded wetland

Edward D. Burton et al. — Mon, 10 Dec 2012 20:53:52 PST

Sulfur biogeochemical cycling and associated Fe–S mineralization processes exert a major influence over acidity dynamics, electron flow and contaminant mobility in wetlands, benthic sediments and groundwater systems. While S biogeochemical cycling has been studied intensively in many environmental settings, relatively little direct information exists on S cycling in formerly drained wetlands that have been remediated via tidal re-flooding. This study focuses on a tidal wetland that was drained in the 1970s (causing severe soil and water acidification), and subsequently remediated by controlled re-flooding in 2002. We examine View the MathML source reduction rates and Fe–S mineralization at the tidal fringe, 7 years after the commencement of re-flooding. The initial drainage of the wetland examined here caused in-situ pyrite (FeS2) oxidation, resulting in the drained soil layers being highly acidic and rich in View the MathML source-bearing Fe(III) minerals, including jarosite (KFe3(SO4)2(OH)6). Tidal re-flooding has neutralized much of the previous acidity, with the pore-water pH now mostly spanning pH 5–7. The fastest rates of in-situ View the MathML source reduction (up to ∼300 nmol cm−3 day−1) occur within the inter-tidal zone in the near-surface soil layers (to ∼60 cm below ground surface). The View the MathML source reduction rates correlate with pore-water dissolved organic C concentrations, thereby suggesting that electron donor supply was the predominant rate determining factor. Elemental S was a major short-term product of View the MathML source reduction, comprising up to 69% of reduced inorganic S in the near-surface soil layers. This enrichment in elemental S can be partly attributed to interactions between biogenic H2S and jarosite – a process that also contributed to enrichment in pore-water Fe2+ (up to 55 mM) and View the MathML source (up to 50 mM). The iron sulfide thiospinel, greigite (Fe3S4), was abundant in near-surface soil layers within the inter- to sub-tidal zone where tidal water level fluctuations created oscillatory redox conditions. There was evidence for relatively rapid pyrite re-formation within the re-flooded soil layers. However, the results indicate that pyrite re-formation has occurred mainly in the lower formerly drained soil layers, whereas the accumulation of elemental S and greigite has been confined towards the soil surface. The discovery that pyrite formation was spatially decoupled from that of elemental S and greigite challenges the concept that greigite is an essential precursor required for sedimentary pyrite formation. In fact, the results suggest that greigite and pyrite may represent distinct end-points of divergent Fe–S mineralization pathways. Overall, this study highlights novel aspects of Fe–S mineralization within tidal wetlands that have been drained and re-flooded, in contrast to normal, undisturbed tidal wetlands. As such, the long-term biogeochemical trajectory of drained and acidified wetlands that are remediated by tidal re-flooding cannot be predicted from the well-studied behaviour of normal tidal wetlands.

Water chemistry and nutrient release during the resuspension of FeS-rich sediments in a eutrophic estuarine system

Bree Morgan et al. — Mon, 10 Dec 2012 20:53:50 PST

The objective of this study was to investigate the impact of resuspending FeS-rich benthic sediment on estuarine water chemistry. To address this objective, we conducted (1) a series of laboratory-based sediment resuspension experiments and (2) also monitored changes in surface water composition during field-based sediment resuspension events that were caused by dredging activities in the Peel–Harvey Estuary, Western Australia. Our laboratory resuspension experiments showed that the resuspension of FeS-rich sediments rapidly deoxygenated estuarine water. In contrast, dredging activities in the field did not noticeably lower O2 concentrations in adjacent surface water. Additionally, while FeS oxidation in the laboratory resuspensions caused measurable decreases in pH, the field pH was unaffected by the dredging event and dissolved trace metal concentrations remained very low throughout the monitoring period. Dissolved ammonium (NH4+) and inorganic phosphorus (PO4–P) were released into the water column during the resuspension of sediments in both the field and laboratory. Following its initial release, PO4–P was rapidly removed from solution in the laboratory-based (< 1 h) and field-based (< 100 m from sediment disposal point) investigations. In comparison to PO4–P, NH4+ release was observed to be more prolonged over the 2-week period of the laboratory resuspension experiments. However, our field-based observations revealed that elevated NH4+ concentrations were localised to < 100 m from the sediment disposal point. This study demonstrates that alongside the emphasis on acidification, deoxygenation and metal release during FeS resuspension, it is important to consider the possibility of nutrient release from disturbed sediments in eutrophic estuaries.

Anthropogenic forcing of estuarine hypoxic events in sub-tropical catchments: landscape drivers and biogeochemical processes

Vanessa NL Wong et al. — Mon, 10 Dec 2012 20:53:49 PST

Episodic hypoxic events can occur following summer floods in sub-tropical estuaries of eastern Australia. These events can cause deoxygenation of waterways and extensive fish mortality. Here, we present a conceptual model that links key landscape drivers and biogeochemical processes which contribute to post-flood hypoxic events. The model provides a framework for examining the nature of anthropogenic forcing. Modification of estuarine floodplain surface hydrology through the construction of extensive drainage networks emerges as a major contributing factor to increasing the frequency, magnitude and duration of hypoxic events. Forcing occurs in two main ways. Firstly, artificial drainage of backswamp wetlands initiates drier conditions which cause a shift in vegetation assemblages from wetland-dominant species to dryland-dominant species. These species, which currently dominate the floodplain, are largely intolerant of inundation and provide abundant labile substrate for decomposition following flood events. Decomposition of this labile carbon pool consumes oxygen in the overlying floodwaters, and results in anoxic conditions and waters with excess deoxygenation potential (DOP). Carbon metabolism can be strongly coupled with microbially-mediated reduction of accumulated Fe and Mn oxides, phases which are common on these coastal floodplain landscapes. Secondly, artificial drainage enhances discharge rates during the flood recession phase. Drains transport deoxygenated high DOP floodwaters rapidly from backswamp wetlands to the main river channel to further consume oxygen. This process effectively displaces the natural carbon metabolism processes from floodplain wetlands to the main channel. Management options to reduce the impacts of post-flood hypoxia include i) remodifying drainage on the floodplain to promote wetter conditions, thereby shifting vegetation assemblages towards inundation-tolerant species, and ii) strategic retention of floodwaters in the backswamp wetlands to reduce the volume and rate during the critical post-flood recession phase.

Microbial sulfidogenesis in ferrihydrite-rich environments: effects on iron mineralogy and arsenic mobility

Edward D. Burton et al. — Mon, 10 Dec 2012 20:53:47 PST

Microbial sulfidogenesis plays a potentially important role in Fe and As biogeochemistry within wetland soils, sediments and aquifers. This study investigates the specific effects of microbial sulfidogenesis on Fe mineralogy and associated As mobility in mildly acidic (pH 6) and mildly basic (pH 8) advective-flow environments. A series of experiments were conducted using advective-flow columns, with an initial solid-phase comprising As(III)-bearing ferrihydrite-coated quartz sand. Columns for each pH treatment were inoculated with the sulfate-reducing bacteria Desulfovibrio vulgaris, and were compared to additional abiotic control columns. Over a period of 28 days, microbial sulfidogenesis (as coupled to the incomplete oxidation of lactate) caused major changes in Fe mineralogy, including replacement of ferrihydrite by mackinawite and magnetite at the in-flow end of the inoculated columns. At pH 8, the Fe2+ produced by electron transfer between sulfide and ferrihydrite was mainly retained near its zone of formation. In contrast, at pH 6, much of the produced Fe2+ was transported with advecting groundwater, facilitating the downstream Fe2+-catalyzed transformation of ferrihydrite to goethite. At both pH 6 and pH 8, the sulfide-driven reductive dissolution of ferrihydrite and its replacement by mackinawite at the in-flow end of the inoculated columns resulted in substantial mobilization of As into the pore-water. At pH 8, this caused the downstream As concentrations within the inoculated columns to be greater than the corresponding abiotic column. However, the opposite occurred under pH 6 conditions, with the Fe2+-catalyzed transformation of ferrihydrite to goethite in the inoculated columns causing a decrease in downstream As concentrations compared to the abiotic column. Although thermodynamically favorable at intermediate times and depth intervals within the inoculated columns, solid As sulfide phases were undetectable by As XANES spectroscopy. Our findings show that microbial sulfidogenesis can trigger significant As mobilization in subsurface environments with advective groundwater flow. The results also demonstrate that formation of mackinawite by sulfidization of ferric (hydr)oxides is not effective for the immobilization of As, whereas the Fe2+-catalyzed transformation of ferrihydrite to goethite under mildly acidic conditions may mitigate As mobility.