An initial Phase 1 acid sulfate soil investigation of the Paiwalla wetland in March 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 Paiwalla wetland to determine the nature, severity and the specific risks associated with acid sulfate soil materials. The wetland had dried during previous drought conditions, but had partially rewet at the time of sampling.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 manganese (Mn) was the only element above the soil ecological investigation Level (EIL). Although concentrations of other elements did not breach sediment quality guideline (SQG) and soil ecological investigation level trigger values, the concentrations of many elements were high enough that they may impact water quality if mobilised, particularly for aluminium (Al), and iron (Fe). The contaminant and metalloid dynamics tests were undertaken to assess the release of metals during a water extraction, and to assess dynamics in response to saturation overtime by incubating soil materials for periods of 1, 7, 14 and 35 days. The degree to which metal and metalloid concentrations exceed guideline values (ANZECC/ARMCANZ 2000) was used to characterise the degree of hazard. For Paiwalla wetland, aluminium (Al), arsenic (As), copper (Cu), iron (Fe) and vanadium (V) were assigned a low hazard with concentrations exceeding ANZECC/ARMCANZ guidelines by less than 10 times. The surface soil layer from the profile studied showed the biggest change towards reducing conditions and the release of iron (Fe) from this layer correlated with increases in arsenic (As) and a number of other trace metals and metalloids. The dominant control on metal solubility is the pH and Eh of the sediments at the time of the extractions, with the reductive dissolution of iron (Fe) and manganese (Mn) oxides/oxyhydroxides probably controlling the high concentrations of metalloids at high pH. Although metal concentrations were generally low, increasing arsenic (As) and vanadium (V) suggest that these elements form the greatest hazard to soil and surface waters over time.The Paiwalla 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 medium risk due to acidification and contaminant mobilisation of soils. For surface waters, the risk is largely dependent on surface and sub-surface hydrology and is thus scenario dependent. Taking into account the range of likely scenarios, and that the wetland is separated from the river by a control structure, the risk to surface waters in the wetland has been allocated low (minor consequence) risk rating due to acidification and a medium risk rating for contaminant mobilisation. The risk rating for deoxygenation is high due to high concentrations of monosulfides present in the surface soil layer.In designing a management strategy for dealing with acid sulfate soils in Paiwalla wetland, other values and uses of the wetland need to be taken into account to ensure that any intervention is compatible with other management plans and objectives for the wetland. The wetland had been refilled at the time of sampling following a dry period and since the wetland has a control structure, management options can focus on preventing oxidation. Due to the risks to the wetland valu
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