Acid sulfate soils are the common name given to soils containing iron sulfides. When exposed to air, these sulfides oxidise to produce sulfuric acid, hence the name acid sulfate soils. They were formed under tidal conditions and are generally found in low-lying areas near the coast where the surface elevation is less than five metres above mean sea level. Acid sulfate soil risk maps are available to identify potential acid sulfate areas. One tonne of iron sulfides can produce about 1.5 tonnes of sulfuric acid when oxidised.
Drainage and excavation of these soils exposes the iron sulfide layers to air so that large slugs of acid groundwater are subsequently released rapidly into estuarine streams. The concentrated acid can overwhelm the stream’s capacity to neutralise it. The acid can then affect the health of fish and other organisms.
As sulfuric acid moves through the soil, it strips iron, aluminium and sometimes manganese from the soil. In some cases it also dissolves heavy metals such as cadmium. In the soil this mixture can make the soil so acid and toxic that few plants can survive. Significant fish kills can occur when sulfuric acid is washed into waterways.
The problems associated with drainage or exposure of acid sulfate soils mean that drainage works need to be undertaken with extreme caution and in consultation with relevant authorities. Reference can be made to LEP 2000
where any landform alteration may result in the disturbance of acid sulfate soil. Written approvals are required in most cases before these materials are disturbed.
Disturbance of these materials is often required for activities such as construction works and agricultural practices. Various management techniques exist for dealing with acid sulfate soil disturbances to ensure that acidic discharges are not produced. Generally, soil samples are taken, the acid generating potential of the soil is calculated and the soil neutralised with agricultural lime to prevent low pH discharges.
Tweed Shire Council's website contains mapping information (see under Our Services>Mapping>Planning and Landuse) which identifies land affected by acid sulfate soils. Council’s Environmental Health Services can provide advice regarding acid sulfate soil assessment and management. The following website links provide useful information on this topic: