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.
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.
Understanding acid sulphate soils and acidic groundwater:
- Introduction to acidic saline groundwater in the WA wheatbelt
- Using composting beds to treat acidity in saline drain water
- Treating acidity in saline water using a hydrated lime dosing unit
ASS risk and acidity characterisation mapping:
- ASS survey of the Scott Coastal Plain
- ASS survey of the Peel-Harvey inlet
- Interaction of groundwater with ASS – Torbay case study
- Avon Catchment acidic groundwater assessment
- Wheatbelt acidic groundwater assessment
Aquatic impacts of ASS:
- Acid sulphate soils impacts on sediments in southern WA estuaries
- Acid sulphate soils impacts on coastal waterways of southern WA
Treatment and management:
- 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.