Coastal development often throws up the term ‘acid sulfate soils’. What are acid sulfate soils and why are they associated with environmental damage?
Acid sulfate soils (ASS) are typically formed by bacteria interacting with vegetable matter and iron, as sea levels rose along the coast at the end of the last ice age. In an undisturbed state below the water table, these soils are harmless (and known as potential acid sulfate soils or PASS). However, when exposed to air, either by excavation or through lowering of the water table, acid sulfate soils release sulfuric acid.
Rainfall following dry weather typically triggers the release of sulfuric acid, turning downstream waters acidic and releasing metals like iron and aluminium. At sufficient concentration, the acid strips the protective mucous coating from fish and the dissolved metals are toxic. ‘Acidification events’ have been associated with the mass mortality of fish and shellfish, and reduce the resilience of these creatures to other diseases. Chronic low-level acidification can also lead to changes in a waterway’s plant and animal communities. Acidified water also attacks concrete and steel structures like bridge pylons and pipes.
In areas like the northern Gold Coast, draining coastal land for agriculture in the early 20th century left a legacy of acidification. Dredging coastal sediments can also expose acid sulfate soils, whilst seawater (rich in carbonates) acts to neutralize the acid.
The process of acidification is now well understood by scientists, and a range of management practices are available that prevent or neutralize the release of acid. All assessable development in Queensland must consider the potential for disturbance of acid sulfate soils, and if relevant prepare an ‘acid sulfate management plan’ for government approval.
The State government offers a series of informative articles relating to acid sulfate soils at https://www.qld.gov.au/environment/land/soil/acid-sulfate/