Acid sulfate soils (ASS) are naturally occurring soils and sediments containing iron sulfides, most commonly pyrite.

When ASS are exposed to air the iron sulfides in the soil react with oxygen and water to produce a variety of iron compounds and sulfuric acid. Initially a chemical reaction, the process is accelerated by soil bacteria. The resulting acid can release other substances, including heavy metals, from the soil and into the surrounding environment.

If at all possible, do not disturb. ASS are benign when left in a waterlogged, undisturbed environment. Avoiding disturbance is often the most environmentally sustainable and economic option.

The impact of disturbing ASS

Without proper management, disturbing ASS can have serious environmental, economic, engineering, and health impacts, and can constrain development, construction and agriculture in affected areas. Activities that might disturb ASS in high risk areas include:

• major earthworks - large scale excavations for canal developments and estates
• infrastructure earthworks - digging for clearways, roads and railways and excavating for sewage pipes, power lines and drainage channels
• maintenance dredging - boating channels and canal estates
• drainage channels - digging channels to manage water logging in agricultural areas, and
• lowering of the groundwater table - from low rainfall and over use of groundwater.

Our Role

The Department's Contaminated Sites Branch deals with ASS issues in Western Australia. The branch provides advice on the management of ASS and develops relevant policies and guidelines, including:

• how to identify ASS - the branch has carried out mapping to identify high risk ASS areas under development pressure on the Swan Coastal Plain
• communicating ASS issues and problems
• establishing guidelines for investigation, treatment, and management of ASS
• providing advice to community groups, local government, consultants and developers on investigating and managing ASS
• facilitating remediation of disturbed ASS.

Acid sulfate soils in Western Australia

• dark organic rich soils and muds
• peaty wetland soils
• some pale grey sands (Bassendean sands and Spearwood sands)
• 'coffee rock' (cemented iron and/or organic rich sands) found below the watertable.

ASS in Western Australia commonly occur in low lying wetlands, back-swamps, estuaries, salt marshes and tidal flats but are not constrained to coastal areas.

ASS risk mapping

DEC has compiled maps of ASS risk areas for several coastal regions of WA. These provide a broad-scale indication of the areas where ASS are most likely to exist. The ASS risk maps are not intended to provide site specific ASS information but rather a broad scale identification of where the ASS layers are present. Limited features such as roads and major towns have been included to assist locating a particular area.

The data derived from the maps cannot be used to determine whether a specific property is affected by ASS but should be used to trigger site-specific investigations and management strategies for ASS soil disturbance and/or lowering of the water table.

Note: The ASS risk maps should only be used at the scale at which they are published. Enlarging the maps will produce distortions so that boundaries on the map bear little resemblance to the occurrence of ASS material on the ground.

Maps of ASS risk areas in Western Australia are published within the Western Australian Planning Commission's (WAPC) Planning Bulletin 64, Acid Sulfate Soils.

The maps can also be viewed on DEC's Geographic Data Atlas. The ASS datasets are listed under the folder "Geoscientific Information". (Note - the map can only be viewed at large scales and will not be visible at scales below 1:50 000 (as displayed next to the distance scale at the bottom of the spatial data viewer).)

The GIS datasets can be directly downloaded (with metadata) from the Geographic Data Atlas. These datasets can only be viewed using GIS software (e.g. Arc GIS) and do not include roads, LGA boundaries or cadastre. These data downloads cannot be viewed without using a GIS application. Users of the maps should ensure that they read the metadata statements for the data.

National maps of ASS risk areas within Australia can be viewed at http://www.asris.csiro.au/index_ie.html.