Acid sulfate soil is a common name given to soils containing iron sulfides. These soils form naturally in coastal regions, such as the south-west of Western Australia, with the iron sulfides (e.g. pyrite) contained in a layer of waterlogged soil.
When disturbed by draining or clearing, oxidation of sulfide creates sulfuric acid. The acid produced may damage buildings and infrastructure (terrestrial impacts) and may also leach metals including aluminium, iron, manganese and arsenic from the soil. This acidic, metal-rich water may pose a threat to the ecological health of rivers, estuaries and wetlands. Acidic drainage from acid sulfate soils can manifest in a number of potential aquatic impacts.
In addition to acidity (hydrogen ions) and metal ions, the drainage water also contains high concentrations of sulfate. Under anoxic conditions, in the presence of sulfate-reducing bacteria and organic material (for example the floor of a waterway), gelatinous black organic sludges known as monosulfidic black ooze (MBO) may form. When disturbed (e.g. through dredging) this MBO may release previously bound contaminants into waterways, as well as rapidly reducing the concentration of dissolved oxygen, potentially resulting in fish deaths.
There are a number of ways to remediate disturbed acid sulfate soils (ASS), however the most effective management strategy is to recognise when ASS is present and to avoid disturbing it. While the Department of Environment and Conservation is the lead agency in identifying and managing acid sulfate soils, the role of the Department of Water is to assess the aquatic impacts of acid sulfate soils and MBO. To date, this assessment has been through investigations in waterways and groundwater in coastal catchments of south-western Australia. These include a surveillance program assessing water and sediment quality within coastal catchments and estuaries, metal leachability from various soil types, bioaccumulation of metals and ecological impacts of acidifying wetlands. From these investigations, chemical indicators have been proposed to distinguish between background acidity and acidification through disturbance of acid sulfate soils to enable a risk assessment of potential impacts on aquatic ecosystems.
Click here to download the brochure 'Acid sulfate soils & acidic drainage - impacts on coastal waterways of south west Western Australia'.