Acid sulfate soil is a name given to soils or sediments containing iron sulfides.
Iron sulfides are micro-crystalline minerals such as pyrite that have formed naturally in soils where long-term water-logged conditions occur such as estuaries, wetlands and shallow groundwater in deep sands.
When exposed to air by drainage, lowering of water-tables or excavation, oxidation of the sulfides creates sulfuric acid which can trigger a range of flow-on effects, including:
- acidification of groundwater, wetlands and waterways
- damage to building footings and underground infrastructure from acid and sulfate attack
- leaching of aluminium, iron and arsenic from the soils deteriorating groundwater, wetlands, rivers and estuaries
- formation of black muds known as monosulphidic black ooze (MBO) that are highly reactive and prone to rapidly deoxygenating waters if disturbed (e.g. by dredging) potentially resulting in fish deaths.
There are extensive areas of coastal Western Australia where acid sulfate soils have been mapped which has implications for the management of groundwater abstraction, planning of urban drainage and management of risks to groundwater resources, water-ways and estuaries.
Acidity coupled with saline groundwater also occurs in extensive areas of the Wheatbelt that affects numerous lakes and hundreds of kilometres of waterways. View a map of the affected areas of the Wheatbelt.
This acidity is naturally occurring in the groundwater, but has been brought to the surface by secondary salinisation processes.
Increased seepage of the acidity with the saline groundwater has resulted in the formation of acidic conditions in lakes and waterways similar to those of acid sulfate soils, but more saline.
This acidification imposes additional stresses on the ecology of these waterways in addition to the impacts of secondary salinisation. The acidity also has implications for managing saline lands using drainage, or use of saline groundwater for aquaculture or industry.
Management of acid sulphate soils
Activities with the potential to disturb acid sulfate soils must be managed carefully to avoid serious environmental harm.
There are a number of ways to remediate disturbed acid sulfate soils and acid drainage, however we adopt the most effective management strategy of recognising when acid sulfate soils are present and avoiding generating acidity through disturbance, drainage or changes in groundwater levels.