New study finds very low risk of acid sulfate soils at the Aldinga Washpool
The Aldinga Washpool is one of Adelaide’s last remaining coastal lagoons. Those familiar with NGT’s ongoing work at the Aldinga Washpool, from past articles and our 2022 report, will know that one of our proposed restoration options for this significant lagoon involves sediment excavation. This is because our analysis of the history of the Washpool and crucially its catchment, indicated that the wetland was partially filled by sediment washed in from its catchment due to vegetation clearance and erosion dating back to the early days of European settlement. This has significantly reduced the area, depth and volume of the wetland.
To reinstate something approximating the original Washpool bathymetry (depth profile and shape of its bed) we proposed undertaking excavation work across an area of 9 ha to remove accumulated sediment. Our 2022 report describes this option in some detail, but noted that excavation in a saltmarsh environment such as the Washpool risks exposing sulfidic sediments, which can generate sulfuric acid when exposed to the air, causing ecological harm. Therefore, a pre-requisite to any earthworks at the Washpool is the completion of an acid sulfate soil (ASS) assessment, to assess the risks and propose a risk management approach.
Funded by a Green Adelaide Grassroots Grant, we collaborated with Dr Brett Thomas and Associate Professor Luke Mosely from The University of Adelaide’s Acid Sulfate Soil Centre to undertake this assessment. The investigation commenced in December 2023, with the initial collection of soil cores, supervised by Kaurna cultural heritage monitors, from 20 locations within the proposed excavation footprint at the Washpool (see below).
We are pleased to announce this study is now complete and the good news, to quote the report, is that “all soils within the excavation footprint and design level are expected to classify as non-ASS material”. This means that excavation, with caution and some standard risk management procedures in place, is highly unlikely to expose sulfidic materials. There was some hyposulfidic material present in Core 20, presenting a low acidification hazard, but the material was 150-165 cm below the surface, much lower than our proposed excavation depth of around 60 cm maximum.
Interestingly, the report found that the original (pre-colonisation) bed of the Washpool is located 170-200 cm below the existing surface at Core 20, illustrating the huge volume of sediment that has accumulated and just how significantly the bathymetry of the Washpool has been altered in the last 190 years post-European colonisation. A parallel study that is almost complete has investigated this question in more detail – so stay tuned for more detail in future articles.
