Warren S. Hicks

Sulfidic materials in dryland river wetlands

Due to a combination of river regulation, dryland salinity and irrigation return, lower River Murray floodplains (Australia) and associated wetlands are undergoing salinisation. It was hypothesised that salinisation would provide suitable conditions for the accumulation of sulfidic materials (soils and sediments enriched in sulfides, such as pyrite) in these wetlands. A survey of nine floodplain wetlands representing a salinity gradient from fresh to hypersaline determined that surface sediment sulfide concentrations varied from <0.05% to ~1%. Saline and permanently flooded wetlands tended to have greater sulfide concentrations than freshwater ones or those with more regular wetting–drying regimes. The acidification risk associated with the sulfidic materials was evaluated using field peroxide oxidations tests and laboratory measurements of net acid generation potential. Although sulfide concentration was elevated in many wetlands, the acidification risk was low because of elevated carbonate concentration (up to 30% as CaCO3) in the sediments. One exception was Bottle Bend Lagoon (New South Wales), which had acidified during a draw-down event in 2002 and was found to have both actual and potential acid sulfate soils at the time of the survey (2003). Potential acid sulfate soils also occurred locally in the hypersaline Loveday Disposal Basin. The other environmental risks associated with sulfidic materials could not be reliably evaluated because no guideline exists to assess them. These include the deoxygenation risk following sediment resuspension and the generation of foul odours during drying events. The remediation of wetland salinity in the Murray–Darling Basin will require that the risks associated with disturbing sulfidic materials during management actions be evaluated.

Effect of season and landscape position on the aluminium geochemistry of tropical acid sulfate soil leachate

Acid sulfate soils (ASS) occupy an estimated 5.8 × 106 ha of coastal Australia. In tropical Australia, the processes operating in these soils, and their environmental hazards, are poorly understood. Drainage of a tropical estuarine wetland containing extensive ASS deposits left the area in a highly degraded condition. Surface and soil water pH values from the site were consistently <5 and commonly <3.5. Aluminium activity was several orders of magnitude greater than the level set for the protection of aquatic ecosystems, with a seasonal variation of 3 orders of magnitude. Aluminium behaved conservatively at the discharge point to receiving waters. In drainage lines and soil solution, aluminium activity was limited by elevated sulfate activity. Aluminium was commonly supersaturated with respect to alunite and behaved as though an aluminium species with the stoichiometry Al:OH:SO4 regulated its activity, which was 2–5 orders of magnitude lower than if gibbsite or amorphous aluminium hydroxide solubility was the control. While jurbanite (AlOHSO4.5H2O) is no longer considered a potential mineral in ASS, these data again raise the question of a satisfactory explanation of aluminium activity. Sulfate activity was influenced by seasonal factors. Wet season conditions were reducing and favoured the dissolution of acid iron oxidation products. The dry season oxidising and drying conditions favoured their precipitation, resulting in seasonal cycling. Based on our findings we developed a landscape geochemical process model for the site.

Porewater geochemistry of inland Acid sulfate soils with sulfuric horizons following postdrought reflooding with freshwater.

Following the break of a severe drought in the Murray-Darling Basin, rising water levels restored subaqueous conditions to dried inland acid sulfate soils with sulfuric horizons (pH <3.5). Equilibrium dialysis membrane samplers were used to investigate in situ changes to soil acidity and abundance of metals and metalloids following the first 24 mo of restored subaqueous conditions. The rewetted sulfuric horizons remained severely acidified (pH ∼4) or had retained acidity with jarosite visibly present after 5 mo of continuous subaqueous conditions. A further 19 mo of subaqueous conditions resulted in only small additional increases in pH (∼0.5-1 pH units), with the largest increases occurring within the uppermost 10 cm of the soil profile. Substantial decreases in concentrations of some metal(loid)s were observed with time most likely owing to lower solubility and sorption as a consequence of the increase in pH. In deeper parts of the profiles, porewater remained strongly buffered at low pH values (pH <4.5) and experienced little progression toward anoxic circumneutral pH conditions over the 24 mo of subaqueous conditions. It is proposed that low pH conditions inhibited the activity of SO-reducing bacteria and, in turn, the in situ generation of alkalinity through pyrite production. The limited supply of alkalinity in freshwater systems and the initial highly buffered low pH conditions were also thought to be slowing recovery. The timescales involved for a sulfuric horizon rewetted by a freshwater body to recover from acidic conditions could therefore be in the order of several years.