Example of oxidation of acid sulfate soils,
Bottle Bend Lagoon, NSW (Kerry
Whitworth, Murray–Darling Freshwater
Accumulations of monosulfidic material in
River near Donald, Victoria (Melanie Watts,
Catchment Management Authority)
Acid sulfate soils are those soils containing iron sulfide minerals. These soils may either contain sulfuric acid (sulfuric material), or have the potential to form sulfuric acid (sulfidic material), or cause de-oxygenation (monosulfidic material often known as monosulfidic black ooze), or release contaminants when the sulfide minerals are exposed to air (oxygen).
Acid sulfate soils form naturally when sulfate-rich water (e.g. saline groundwater, sea water) mixes with sediments containing iron oxides and organic matter. Under waterlogged, anaerobic (oxygen-free) conditions, bacteria convert sulfates to sulfides. These sulfides react with metals to form sulfidic sediments.
While this process occurs naturally, changes to the hydrology of inland rivers and wetlands have contributed to the accumulation of these sediments in some areas. If left undisturbed and covered with water, sulfidic sediments pose little threat. However, when exposed to oxygen, such as under drought conditions, chemical reactions may lead to the generation of sulfuric acid. When these sulfuric sediments become wet again and the amount of sulfuric acid produced exceeds the buffering capacity of the system (the ability to keep the pH or acid/alkaline level stable), sulfuric acid may be released into the water, impacting on the health of rivers and wetlands.
Other risks associated with acid sulfate soils include mobilisation of heavy metals, decrease in oxygen in the water column, and production of noxious gases. These processes can lead to substantial environmental damage and serious impacts on water supplies and human health.
In 2008, the Murray-Darling Basin Ministerial Council requested a Basin-wide assessment of the threat posed by acid sulfate soils. The resultant Acid Sulfate Soils Risk Assessment Project was established to assess the occurrence of, and risk posed by, acid sulfate soils at priority wetlands in the River Murray system, at Ramsar wetlands and other key environmental sites in the Murray-Darling Basin.
The record low inflows and river levels of recent drought years led to the drying of many permanent wetlands in the Murray–Darling Basin, resulting in the exposure of acid sulfate soils. The impacts of these soils had previously been an issue only at localised sites, but it became clear that acid sulfate soils may present a significantly larger scale issue in some parts of the Basin.
A panel of experts and wetland managers identified which wetlands should be included in the assessment project from the tens of thousands in the Murray-Darling Basin. Selected wetlands included those of ecological significance as well as those that may pose a threat to the surrounding environment if they are affected by acid sulfate soils.
A tiered assessment program was utilised, whereby priority wetlands were screened through a desktop assessment stage, followed by a rapid on-ground appraisal, and then detailed on-ground assessment if results of previous stages indicated an increased likelihood of occurrence of acid sulfate soils.
Black monosulfide accumulations in Burnt
Creek near Dunolly, Victoria (Rob Fitzpatrick,
A thick salt crust overlies acid sulfate soils
in a Little Toolunka Flat complex wetland,
lower Murray River, South Australia
(Gerard Grealish, CSIRO)
The Murray–Darling Basin Acid Sulfate Soils Risk Assessment project substantially increased knowledge of the occurrence of acid sulfate soils throughout the Basin, and the hazards and risks associated with these materials. In all, over 19,000 wetlands received desktop assessment; 1,385 wetlands received rapid on-ground assessment, and nearly 200 wetlands and river reaches received detailed assessment, including 14 of the 16 Ramsar wetlands in the Murray–Darling Basin.
The summary project report, Acid Sulfate Soils in the Murray–Darling Basin, was released by the Murray–Darling Basin Ministerial Council in May 2011. This report is supported by more than 70 detailed reports produced throughout the project that document the following outcomes:
Many of the affected wetlands were re-flooded during the extensive floods of 2010–11. The impacts on water quality through re-flooding of acid sulfate soils were diluted by the size of the flood. However, despite the floods, acid sulfate soil material is still likely to be present in many of the wetlands previously affected. Furthermore, the underlying conditions, which caused the formation of acid sulfate soils in the first place, have not changed. Therefore, the ongoing risk posed by acid sulfate soils is unlikely to have been mitigated and hence will need to be managed.