Assessment of acid sulfate soil materials (Phase 2) Younghusband West wetland, South Australia

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Shand, Paul; Baker, Andrew; Grocke, Sonia Grocke; Smith, Lester; Cozens, Gillian; Fiebiger, Catherine




An initial Phase 1 acid sulfate soil investigation of the Younghusband West wetland during February 2008 showed acid sulfate soils to be a priority concern within this wetland complex. Based on Phase 1 recommendations, a Phase 2 investigation was undertaken for the Younghusband West wetland to determine the nature, severity and the specific risks associated with acid sulfate soil materials. The 24 hour reactive metals tests were undertaken to determine those metals and metalloids extractable with a moderately strong acid i.e. potentially available from binding sites on soil minerals such as iron (Fe), manganese (Mn) and aluminium (Al) oxides. Although comparisons can be made with soil and sediment quality guidelines, these are defined for total concentrations and not partial extractions. The results showed that concentrations were below the sediment quality guideline and soil ecological investigation level for those elements where guidelines are available. Although concentrations of other elements did not breach sediment quality guidelines or soil ecological investigation levels trigger values, the concentrations of some elements were high enough that they may impact water quality if mobilised. The contaminant and metalloid dynamics tests were undertaken to assess the release of metals during a water extraction, and to assess dynamic response to saturation over time by incubating soil materials for periods of 1, 7, 14 and 35 days. The degree to which metal and metalloid concentrations exceed ANZECC/ARMCANZ guideline values was used to characterise the degree of hazard. For Younghusband West wetland, no elements were assigned a high hazard with no concentrations exceeding ANZECC/ARMCANZ guidelines by more than 100 times. Aluminium (Al), cobalt (Co) and zinc (Zn) were assigned a moderate hazard with concentrations exceeding ANZECC/ARMCANZ guidelines by more than 10 times. The dominant control on metal solubility is the pH of the extractions. One sample was sulfuric and typically contained the highest concentrations of trace metals and arsenic (As). Vanadium (V) was high in the least acidic sample. Three of the four soils analysed showed an increase in pH which limits the solubility of many metals. Only one sampled showed little change in pH, remaining acidic over the 35 day duration of the tests. High aluminium (Al) was found in the acidic layer, and also in one sample with circumneutral pH, significantly above equilibrium concentrations. It is concluded that much of this aluminium (Al) exists as colloidal particles, thus less toxic than Al3+ the dominant form at lower pH. All samples showed a decrease in Eh, but the extent of decrease was small in the acidic (sulfuric) material studied. The Eh decrease in the more acidic samples may have been insufficient for iron oxide/oxyhydroxide dissolution, hence iron (Fe) and associated metal release may occur over longer timescales. The Younghusband West wetland has been classified as medium conservation status by the SA Murray–Darling Basin Natural Resources Management Board (Miles et al. 2010). The main hazards considered in this study that may impact on wetland values are acidification, contaminant mobilisation, and deoxygenation. The wetland has been allocated a high risk rating for acidification and a high contaminant risk rating for soils. For surface waters, the risk is largely dependent on surface and sub-surface hydrology and thus scenario dependent. Taking into account the range of likely scenarios, from very low flows (highest risk) to very high flows (lowest risk), the risk to surface waters in the wetland has been allocated a medium risk rating for acidification and medium to high risk rating for contaminant mobilisation respectively. A risk assessment for deoxygenation was not undertaken for this wetland as it was determined that there was no identified hazard associated with monosulfide formation. In designing a management strategy for dealing with acid sulf

Murray-Darling Basin Authority


Acid sulfate soil; wetland; metal; metalloid

Environmental Monitoring

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Water for a Healthy Country Flagship Report series ISSN: 1835-095X


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Shand, Paul; Baker, Andrew; Grocke, Sonia Grocke; Smith, Lester; Cozens, Gillian; Fiebiger, Catherine. Assessment of acid sulfate soil materials (Phase 2) Younghusband West wetland, South Australia. Canberra: Murray-Darling Basin Authority; 2011.

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