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Open File Report 165

Survey and description of sulfidic materials in wetlands of the Lower River Murray Floodplains: Implications for floodplain salinity management

Lamontagne S, Hicks WS, Ritzpatrick RW, Rogers S


This study was undertaken to determine whether environmentally significant deposits of sulfidic materials are present in Lower River Murray floodplains. Sulfidic materials are soils and sediments enriched in sulfide minerals, such as pyrite (FeS2) and monosulfides (FeS). These materials tend to accumulate in environments where elevated SO42– concentrations, a high availability of labile carbon and anoxic conditions favour high rates of sulfate reduction. It was suspected that wetlands of Lower River Murray floodplains would be at risk of accumulating sulfidic materials because current conditions (that is river regulation and salinisation) should have promoted the conditions favourable to high rates of sulfate reduction. Sulfidic materials are usually stable as long as they remain undisturbed. However, when they are exposed to oxygen (through drainage or resuspension in the water column) they pose a number of environmental risks, including deoxygenation of the water column, acidification and the generation of noxious smells. Whether or not sulfidic materials occur in Lower River Murray floodplains is important on a management point of view because many recently proposed floodplain salinity remediation initiatives could result in exposing sulfidic materials to the atmosphere.

Eight wetlands in the Riverland region of South Australia and one wetland near Buronga in New South Wales were surveyed for the presence of sulfidic materials. These wetlands were selected to represent a range in salinity and water regime manipulation, from freshwater wetlands with near natural wetting and drying cycles to hypersaline evaporation basins. The survey was exploratory with limited sampling (one to three sites) within each wetland. Within a wetland, specific sampling locations were chosen based on observed site conditions such as different phases of the wetting and drying cycle or changes in the wetland morphology. The presence and the characterisation of the sulfidic materials at each site was achieved through a range in chemical, mineralogical and microbiological analyses.

The survey showed that sulfidic materials are widespread in Lower River Murray floodplains and that the conditions for their formation are ubiquitous, with sufficient sulfate, iron and carbon available. The limiting factor in their formation appeared to be labile carbon. Although the conditions for formation existed, significant accumulation seemed to occur only when flooded conditions are maintained for significant periods (years to decades). Seasonal wetting and drying may prevent accumulation by destroying the sulfides as the wetland dries and conditions become oxidising.

A preliminary assessment of the environmental risks associated with sulfidic materials was also made. In general, acidification did not appear to be a major risk because wetlands with a high sulfide content also tended to have significant acid neutralising capacities (i.e., had high carbonate concentrations in their sediments). However, two wetlands had potential acid sulfate soil conditions (i.e., are at risk of acidification) and one (Bottle Bend Lagoon, NSW) had severely acidified (pH < 3) during a recent draw down event. The aesthetic risk (noxious smells) was widespread in Riverland disposal basins (including the Loveday, Berri and Ramco basins) as assessed by the response of the local communities to the recent drying of some of these basins. We could not define the deoxygenation risk because there is presently no agreed method to assess it. However, anecdotal evidence suggest that deoxygenation events have occurred in River Murray wetlands when sulfidic sediments have been disturbed during managed wetland wetting/drying operations. The factors that could contribute to the deoxygenation risk would include the suspended sediment load, sediment sulfide concentration, the form of sulfide present, water column residence time, the reaction rate of the sulfides, and the critical dissolved oxygen levels for the target organisms. It is important to note that a good acid neutralising potential (i.e., low acidification risk) has no bearing on the deoxygenation or aesthetic risks.

The issue of sulfidic materials in the Lower River Murray has some similarities and differences relative to the problem of acid sulfate soils (ASS) in coastal environments. We found the field measurements and tests used in coastal ASS to be directly transferable, as were the laboratory methods for sulfur species determination. However, the routine manometric method for soil carbonate has a detection limit that is too high in comparison with the trigger value for reduced sulfur. The major difference between the two environments may be that acidification is the main risk in coastal environments whereas it is not in the floodplain context. Thus, the guidelines used to trigger management action in the coastal ASS context may not be suitable for the floodplain one. This would be especially true for the deoxygenation risk, which currently does not have proper assessment guidelines.

Survey and description of sulfidic materials in wetlands of the Lower River Murray floodplains vi

The recommendations arising from this study include:

  • Complete a survey of the habitats suspected to have accumulated significant sulfidic material deposits in the Lower Murray;
  • Further define the regional-scale factors contributing to the acidification risk;
  • Determine the rates at which sulfidic materials are formed or are oxidised under different salinity and water level management conditions;
  • Assess the spatial variability in the distribution of sulfidic materials in representative wetlands;
  • Identify the compounds responsible for the noxious smell problems and the optimal conditions under which these are produced and, conversely, minimised;
  • Define the mass-balance for S and alkalinity during wetting-drying cycles in wetlands;
  • Understand the role of sulfidic materials and of anoxic groundwater in causing wetland acidification and deoxygenation;
  • Determine if monosulfides form a significant component of the reduced S pool in Riverland wetlands;
  • Educate the management groups whose actions may impact the hydrology of River Murray wetlands about the risks associated with disturbing sulfidic materials.


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