When exposed to air as a result of drainage or disturbance, acid sulfate soils (ASS) produce sulfuric acid, and often release toxic quantities of iron, aluminium and heavy metals. This can have major environmental, health, engineering, and economic effects.
Acid-tolerant water lillies, which can dominate aquatic ecosystems affected by acidic discharge
In most cases involving ASS, high intensity rainfall after long dry periods triggers localised mobilisation of acid. In aquatic environments (especially estuarine) this can:
- kill fish, crustaceans, annelid worms, shellfish and oysters
- cause fish diseases
- change aquatic plant communities
In 1995, a massive fish kill occurred in the Pimpama River and estuary in Queensland after the breaking of an extended drought.
In this state, any such incident should be reported using the protocol for reporting fish kills.
The main ecological effects of acid sulfate soils are habitat degradation and poor plant productivity.
In waterway habitats, drainage from oxidised acid sulfate soils:
- destroys food resources
- displaces biota
- precipitates iron that smothers vegetation and microhabitat
- alters the chemical and physical properties of the water
- degrades spawning and nursery grounds
Poor plant productivity
Poor plant productivity and stunted growth at low soil pH can be caused by:
- toxic effects of aluminium, iron and manganese
- deficiency in plant base minerals such as calcium, magnesium and potassium
- low availability of nutrients
- increased attacks by plant pathogens
- decrease in soil microbes, particularly those responsible for nitrogen fixation
- stunting of roots producing water stress
A pond in South-East Queensland, typical of acid sulfate soils disturbance. The clarity of the water and its blue-green colour indicate the presence of aluminium
Though not yet fully documented, the possible health impacts of acid sulfate soils are under investigation. Indications are that they could include:
- stunted growth, poor health and mental impairment caused by drinking or bathing in aluminium-rich waters
- dermatitis as a result of skin contact with acid soil materials
Heavy metals in acid sulfate soils can also become soluble when sulfuric acid is produced, ending up in toxic quantities in leachate and nearby waterbodies.
Economic and engineering effects
A concrete bridge pylon showing considerable degradation from sulfuric acid attack. The concrete matrix has broken down, leaving exposed aggregate
A pump filter totally clogged with iron precipitate
A road which has slumped because of the low load-bearing capacity of the underlying acid sulfate soils material
In ASS areas, developments such as large scale drainage and flood mitigation schemes can lead to widespread acidification of land, lakes and streams and subsequent economic losses to other industries.
In some cases, multi-million dollar coastal developments have been stalled or abandoned; millions of dollars worth of oysters, prawns and fish have been destroyed; fish-breeding areas have been decimated, and millions of dollars worth of infrastructure have had to be replaced due to acid attack.
Attack on steel, aluminium and concrete
Acidified waters weaken concrete and steel infrastructure such as culverts, pipes and bridges. This accelerates maintenance and replacement costs.
As many ASS are unconsolidated estuarine muds and clays with gel-like properties and low load-bearing capacity, foundations or earthworks built on these materials may settle or subside unevenly and slowly. Many surface structures thus deep piles or an extensive load-spreading membrane. This construction must be carried out in conjunction with extensive drainage, even though this such usually results in production of more acid.
Last reviewed 16 December 2011
Last updated 24 June 2008