CRC LEME
Open File Report 165
ABSTRACT
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
Summary
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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