What are acid sulfate soils?
Acid sulfate soils is the common name given to naturally occurring soils and sediments that contain iron sulfide (pyrite). As sea levels slowly rose (between 6,000 and 10,000 years ago), substantial deposits of pyritic sediments formed in estuarine mud, where tidal seawater (containing sulfur) met and mixed with freshwater outflows (containing iron). Acid sulfate soils are defined as either:
- Actual acid sulfate soils - where the soils have already been exposed to oxygen and have a pH < 4, or
- Potential acid sulfate soils - where the soils have not been exposed but have the potential to generate sulfuric acid if exposed.
Left undisturbed, acid sulfate soils do not pose any harm. However, if they are disturbed and exposed to oxygen (air) through activities such as excavation or the lowering of the water table, sulfuric acid may be produced in large quantities. A tonne of acid sulfate soils has the capacity to generate 1.6 tonnes of pure sulfuric acid. Depending on the particular sediment’s ability to buffer acid generation, sulfuric acid can continue to be generated for many hundreds of years if not treated correctly.
Impacts of acid sulfate soils
Sulfuric acid can have detrimental effects on the natural or built environment by:
- Corroding and weakening concrete, iron, steel and certain aluminium alloys.
- Dissolving certain essential elements from the soil, making it toxic and adverse to plant growth.
- Affecting biodiversity and ecological integrity by degrading habitat, soil and water quality, potentially resulting in the death or disease of fish and other organisms.
Where are acid sulfate soils found?
Acid sulfate soils are typically found in low lying areas near the coast, such as mangrove and salt marsh areas, tidal areas, at the bottom of coastal lakes, estuaries and under sand dunes. They usually occur below 5 metres AHD1 and beneath the water table but occasionally have been found above the water table. The LEP contains an acid sulfate soils map which gives an indication of the extent of acid sulfate soils in the Shire.
Acid sulfate soil classes and impact triggers
Acid sulfate soils have been classified based on the likelihood of the acid sulfate soils being present in particular areas and at certain depths. There are five classifications:
Acid sulfate soils in a class 1 area are likely to be found on and below the natural ground surface. Any works2 will trigger the requirement for assessment and may require management.
Acid sulfate soils in a class 2 area are likely to be found below the natural ground surface. Any works2 beneath the natural ground surface, or works2 which are likely to lower the water table, will trigger the requirement for assessment and may require management.
Acid sulfate soils in a class 3 area are likely to be found beyond 1 metre below the natural ground surface. Any works2 that extend beyond 1 metre below the natural ground surface, or works2 which are likely to lower water table beyond 1 metre below the natural ground surface, will trigger the requirement for assessment and may require management.
Acid sulfate soils in a class 4 area are likely to be found beyond 2 metres below the natural ground surface. Any works2 that extend beyond 2 metres below the natural ground surface, or works2 which are likely to lower the water table beyond 2 metres below the natural ground surface, will trigger the requirement for assessment and may require management.
Acid sulfate soils are not typically found in Class 5 areas. Areas classified as Class 5 are located within 500 metres on adjacent class 1,2,3 or 4 land. Works2 in a class 5 area that are likely to lower the water table below 1 metre AHD1 on adjacent class 1, 2, 3 or 4 land will trigger the requirement for assessment and may require management.
Note: 1 Australian Height Datum, and 2 'work' is defined as any works that disturb more than one (1) tonne of soil, or lower the water table.
Developing in an acid sulfate soil area
- Is your development situated in a Class 1, 2, 3, 4 or 5 acid sulfate soils area?
- Is your proposed development likely to impact on acid sulfate soils and trigger clause 6.1 of SSLEP 2015?
Acid sulfate soils - Shire Maps
If your answer to both the questions above is yes, then you will need to appoint a suitably qualified environmental scientist to test your soils in the area of potential disturbance in accordance with the Acid Sulfate Soils Manual (ASSMAC). The following is essential and must be submitted with your development application:
- Results of a minimum of four sampling holes taken in the area of disturbance, that extend to at least one (1) metre beyond the proposed depth of excavation or the estimated drop in water table, or to a minimum two (2) metres below the land surface, whichever is the greatest.
- A report prepared in accordance with ASSMAC stating whether acid sulfate soil management is or is not required and the reasons for the decision.
- An acid sulfate soils management plan if results from the samples indicate that management of acid sulfate soils is required.
Any deviation from these requirements must be approved by council prior to submission of your development application.