Update on Investigations at Aldinga Washpool

Since early 2021 NGT has been investigating the ecological history and restoration potential of the Aldinga Washpool/Wakondilla, one of the last coastal lagoons of its type in the Adelaide region. Our 2022 Report proposed several actions that could be taken to undo some of the damage that has occurred to this wetland since European colonisation. Options included removing sediment that has been deposited into the lagoon, restoring the original water level by constructing a levee and spillway, and removing the road (Button Rd) that passes through the lagoon dividing it in two.

As reported in our September 2023 newsletter, last year we commenced further investigations to inform the restoration of the Washpool, supported by a Green Adelaide Grassroots Grant. This second grant is to implement the “High Priority Next Steps” identified in the 2022 Report. Specifically to:

• update Washpool catchment modelling to determine if future inflows will be sufficient to justify the increased volume of water that a restored Washpool would hold. We predict it would increase from 2.8 to 128 ML when full;

• determine accurately, and at a fine scale, the depth of sediment that has been deposited on the bed of the Washpool post-colonisation. This will provide guidance on the removal of this material to restore the original ‘floor’ (bathymetry) of the lagoon;

• assess the suitability of accumulated sediment within the Washpool for use as levee construction material. We proposed the construction of a levee to enable water levels in the Washpool to be restored without compromising stormwater drainage from Silver Sands housing estate. The use of material already within the Washpool would be more efficient, and presumably more culturally appropriate, than bringing in material from elsewhere to construct the levee;

• undertake an acid sulfate soil risk assessment of the proposed excavation footprint, which is a standard requirement when undertaking earthworks in a saltmarsh environment such as the Washpool.

One year in, we have made good progress on all four objectives. Updated catchment modelling has been completed by Tonkin Consulting and results are shown below. The modelling shows that inflows have exceeded the total volume of the Washpool if it was restored (128 ML) in about 90% of the last 103 years. We think this provides confidence that increasing the volume of the Washpool through restoration is worth doing; it will continue to fill regularly.

We have been working with Associate Professor John Tibby and his team at the University of Adelaide to determine the depth of the sediments that have been deposited within the Washpool after European colonisation. To do this we have taken 21 sediment cores, mostly from within the proposed excavation footprint. The presence of pine pollen in sediment indicates that it has been deposited after the establishment of pine plantations in the vicinity, which in the case of the Washpool means after the 1880s. We have detected pine pollen as deeply as 65 cm below the current surface. John has also been using other techniques such as radiocarbon dating to examine the sediments. The work is ongoing but at this stage looks promising. It appears likely that the sediment proposed for excavation in our 2022 report has all been deposited post-colonisation. Lachie and Ben recently obtained accurate elevations for all the core locations and various other features around the Washpool, meaning we can now express all our levels at the Washpool in mAHD (metres above mean sea level).

CMW Geosciences have been helping us determine if the sediment proposed for excavation would be suitable to construct the levee required to separate the Washpool from the stormwater drains that service Silver Sands housing estate. Their work is complete and the short answer is yes, the sediment has the right geotechnical properties and could be used for that purpose, provided a few standard procedures are followed. This is good news as it appears the excavated material won’t need to be carted offsite and the levee material won’t need to brought in from elsewhere.

Associate Professor Luke Mosely and his team at the University of Adelaide have been helping with the acid sulfate soil risk assessment of the proposed excavation footprint. This work is ongoing but the sediment samples analysed to date suggest there is a very low risk of exposing acid sulphate soils at the Washpool.

Our investigations at the Washpool will help inform what is required should restoration proceed, but there is more to the story. The perspectives of the Traditional Owners, provided through the Kaurna Parks Advisory Committee and other forums, are obviously central to any decision. Additionally, given the Washpool is part of Aldinga Conservation Park, the proponent of restoration would actually be the National Parks and Wildlife Service of South Australia.

If you’d like to see this site in person, this is one of the stops on our upcoming Nature Festival Bus Tour! Join us on Friday October 11th, departing from Unley, you’ll visit three sites, native fish release site near Breakout Creek on the River Torrens / Karrawirra Pari, Aldinga Washpool / Wakondilla and a Fleurieu Peninsula Swamp at Mount Compass.