Acid sulfate soil is a common name given to soils containing iron sulfides. These soils form naturally in coastal regions, 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 and may also leach metals including aluminium, iron, manganese and arsenic from the soils. This acidic, metal-rich water may pose a threat to the ecological health of rivers and estuaries.
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) these MBOs 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 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 MBOs. To date, this assessment has been through investigations in waterways and groundwater in coastal catchments of south-western Australia. These include a surveillance programme 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.
A brochure introducing acid sulfate soil in south-western Australia has been published. There are also a number of Department of Water publications relevant to ASS, accessible through the relevant links toolbar on the right.