The water storage dam supplying Great Palm Island’s community of 5000 people has fallen to critically low levels. After an extensive review of options for alternative sources of potable water, the Palm Island Aboriginal Shire Council (PIASC), supported by the Queensland Department of Infrastructure, Local Government and Planning (QDILGP), resolved that installation of a desalination plant was the most viable and achievable option.
Commissioning of the proposed plant will require the positioning of permanent marine infrastructure, specifically the installation of pipelines for raw water intake and brine discharge, within the Great Barrier Reef Marine Park. The Great Barrier Reef Marine Park Authority (GBRMPA) and Queensland Parks and Wildlife Service (QPWS), as the managing and permitting agencies, will be responsible for ensuring that the development of this infrastructure and its operation will not pose a threat to the values of the GBR. AIMS has been asked to provide a preliminary scientific assessment to support this process.
Marine science to support immediate and ongoing action
The Australian Institute of Marine Science (AIMS) is providing scientific support for Queensland Government efforts to address the water shortage. During December 2015, AIMS undertook a range of investigations, including water circulation modelling, habitat mapping and experiments aimed at assessing the impact of increased salinity on survival and settlement of coral larvae. A review of scientific literature focussing on environmental impacts of desalination discharge was also conducted. The results of this work, along with an interpretation of the results in the context of pipeline and discharge plume associated with the proposed desalination plant, were provided to the Queensland Department of Infrastructure, Local Government and Planning (QDILGP).
While most of the existing scientific literature on marine environmental impacts of desalination focusses on operations having more than ten times the capacity of the plant proposed for Great Palm Island, there is consistency of evidence that discharge plumes dilute rapidly and salinity returns to background levels relatively quickly.
Previously published work shows that, while seagrasses are able to survive elevated salinities over a range of timeframes, studies of coral species and genera that occur in waters around Great Palm Island reveal only limited tolerance for increased salinity - elevations of just 2ppt above ambient levels (around 35ppt) can lead to reduced photosynthetic rates and/or mortality. It is presumed that larval and juvenile corals may be more vulnerable than adult corals.
Other organisms showed mixed tolerance for increased salinity - soft sediment in-fauna, fish and clams show significant tolerance across a range of salinities while even moderately elevated salinities can cause significant shifts in microbial community composition.
Other potential environmental concerns include the potential introduction of contaminants and for entrainment (marine organisms entering the pipe) and impingement (collision of marine organisms with the intake screen) effects on local biological communities. It is understood that anti-scalants will be the only contaminant likely to be discharged by the proposed plant and that, if are detected at all in the discharge water, levels are expected to be well below toxic levels. The impacts of entrainment and impingement will need to be considered given the biodiverse communities of fish and other mobile species that occur in the area.
AIMS science activities to date
Hypersaline tolerance of early coral life stages
A series of experiments, conducted in AIMS’ National Sea Simulator (SeaSim), assessed the capacity of coral larvae and newly settled recruits of the common staghorn coral Acropora tenuis to cope with exposure to hypersaline seawater. Researchers found that:
- New recruits (coral larval that have settled and become part of the adult population) showed no abnormal effects after a 24 hour exposure to salinities up to 38.5 ppt – higher than projected levels expected to be associated with the proposed Great Palm Island desalination plant.
- Settlement of young larvae (9-11 days old) in salinities of up to 48 ppt was similar to control treatments in ambient salinity; however the relative size of recruits decreased as salinity increased, especially at 44 ppt and above.
Previous research on salinity tolerance of coral larvae has generally focused on low salinity scenarios – the new results presented here are possibly the first hypersaline experiments with early life stages of coral, and AIMS has established experimental protocols which may aid further research in this area.
Hydrodynamic plume modelling
Two-dimensional hydrodynamic models of near-field (where speed of discharge and differences in water density determine plume movement) and far-field (where currents and waves have greater impact) plume movement were established for the proposed discharge locations. The predicted trajectory indicated that the plume’s centreline would reach the seabed after horizontal distances of 16 m (low current scenario) and 27 m (average current scenario) and that these distances are probably a worst case scenario. Modelling also revealed that the vertical limits of plume distribution (9 m in low current scenarios and 8 m in average scenarios) might allow the plume to break through the sea-water surface. All far-field modelling scenarios showed an immediate and significant dilution of the plume equating to a salinity elevation of just 0.01 ppt above ambient levels.
Marine habitat mapping
A habitat map showing bottom dwelling ecological communities, derived from satellite images of the fringing reef and surrounding areas, was ground-truthed against field inspections conducted by the AIMS Long-term Monitoring Team. The area is dominated by intertidal sand, seagrass, macroalgae and fringing reef. An extensive photographic record of marine communities at each surveyed site has been compiled and is available for future analysis and monitoring purposes.
Pipeline location and route
Two alternative locations were considered: Frances Creek and Challenger Bay. Plume dilution and benthic community sites were similar at both locations. Analyses identified three constraints that should be considered during site selection:
- Areas of high sand mobility should be avoided due to the risk of sand movements uncovering the pipeline.
- Discharge outlets should be located at least 47 m from key habitat types (seagrasses and coral reefs).
- Discharge outlets should be located at least 10 m beneath the water surface.
Science to support medium-term detailed advice (6 months)
- Once the final precise location and timing of the discharge is known, develop a 3D ocean circulation model to include a near-field plume sub-model, to more accurately predict the fate of the hypersaline plume in a range of climatological and extreme event scenarios.
- Deploy a suit of oceanographic observing instruments to measure physical parameters in the receiving environment of the hypersaline discharge, to ensure the validity of modelling.
- Establish a Marine Monitoring Program (MMP)-style benthic monitoring location, to establish a basis for long-term monitoring of adjacent reef environments; and conduct benthic surveys on 2 occasions (prior to discharge; and 6 months later).
- Produce a report presenting outputs of above, and interpret results in the context of the pipeline infrastructure and discharge plume, report on actual measured changes (if any), and suggest management or operational interventions in future.
Monitoring activities (12 months)
- Survey the MMP sites (1 year post-commencement of desalination operations)
- Compile ocean observations compared with model predictions.
- Repeat hypersaline exposure experiments with early life stages of coral, to validate preliminary results.
- Produce a report on the above, including long-term observed changes in the context of the desalination operation.
Accurate prediction of the impact of desalination discharge into receiving waters at Great Palm Island is difficult in the absence of precedents in tropical receiving waters that are similar to the inshore GBR. Therefore, a monitoring program for marine water quality and ecosystem condition is recommended.
While hypersaline exposure experiments indicated adequate tolerance of coral larvae and new recruits to the expected salinity elevations, it is recommended that further experiments occur during the next spawning season, involving a number of different species that occur at Great Palm Island.
Near-field modelling already undertaken predicts that the desired water quality standard was reached well within expected limits. However, there remains a need to re-visit both the near-field modelling once the actual discharge location is known, and the far-field modelling to both verify the model predictions, and improve the models themselves.
It will be necessary to deploy instruments around the Great Palm Island site, to make salinity, temperature and current observations, preferably over a 12 month period.
For further information, please contact:
AIMS Research Manager David Souter – 0413 734 467