Acid sulphate soils

Acid sulphate soil is a name given to soils or sediments containing iron sulphides. Iron sulphides 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 disturbed by drainage, lowering of water-tables or excavation, oxidation of the sulphides creates sulphuric 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 sulphate attack
Leaching of aluminium, iron, manganese 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 sulphate soils have been mapped which has implications for the management of groundwater abstraction, planning of urban drainage and management of risks in 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 (see map). This acidity is naturally occurring in the groundwater, but has been brought to the surface by secondary salinisation processes. Increased discharge of the acidity with the saline groundwater has resulted in the formation of acidic conditions in lakes and waterways similar to those of acid sulphate 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.

There are a number of ways to remediate disturbed acid sulphate soils (ASS) and acid drainage, however the department adopts the most effective management strategy of recognising when ASS is present and avoiding generating acidity through drainage or changes in groundwater levels.

The Department of Water understands and manages for impacts on acid sulphate soils and groundwater acidity in planning and allocating groundwater, as well as regional urban and wheatbelt drainage.

This is achieved through:

investigations of acid sulfate soils in wetlands and regional groundwater systems
assessing the aquatic impacts of acid sulfate soils, MBOs and acidified groundwater
developing aquatic indicators and assessment tools including guideline limits for acidity in Wheatbelt waterways
trialling and identifying practical options to neutralise saline acidic drainage.

These activities complement the impacts of land development on acid sulphate soils regulated by the Department of Environmental Regulation.

Further information:

Understanding acid sulphate soils and acidic groundwater:

ASS risk and acidity characterisation mapping:

Aquatic impacts of ASS:

Treatment and management:

Journal articles:

Miller et al. 2009. Relationship between Metals Leached and Soil Type from Potential Acid Sulphate Soils under Acidic and Neutral Conditions in Western Australia. Water Air & Soil Pollution.
Kilminster, K & Cartwright, I 2011. 'A sulfur-stable-isotope-based screening tool for assessing impact of acid sulphate soils on waterways', Marine and Freshwater Research, Vol. 62 (2) pp. 152–61.
Degens 2011. Performance of Pilot-Scale Sulphate-Reducing Bioreactors Treating Acidic Saline Water Under Semi-Arid Conditions. Water Air & Soil Pollution.
Degens, BP, Muirden, PD, Kelly, B and Allen, M (2012). Acidification of salinised waterways by saline groundwater discharge in southwestern Australia. Journal of Hydrology. 470-471:111-123.