What are the effects of acid sulfate soils?

Loss of vegetation

The drainage and oxidation of acid sulfate soils causes acidity to be generated, with soil pH falling below 4.0. This can result in large areas becoming scalded and completely devoid of vegetation.

Kills of aquatic life

When discharged into streams, often following rainfall, acidity and aluminium toxicity can cause massive kills of aquatic life, particularly sediment-dwelling organisms or those which are immobile, such as oysters.

Fish are generally highly mobile, and kills only occur in certain circumstances, such as when they become trapped in a branching drainage network.

Impact on aquatic systems

In addition to the acute effects of acid sulfate soils, chronic effects on aquatic systems are also common and widespread. These include:

  • habitat degradation
  • changes to communities of water plants, including invasion by acid-tolerant weeds
  • reduced hatching, survival and growth rates
  • outbreaks of disease, especially red spot disease in fish which has been linked to exposure to acid water.

Economic impact

Testing, treating and monitoring acid sulfate soils can add substantially to the cost of urban development in coastal areas. These costs associated with damage to public and private infrastructure, such as corrosion of pipes, bridge footings and floodgates, can be significant.

Acid sulfate soils have economic impacts on most industries on the NSW coast, including recreational fishing, commercial fishing, oyster growing and other aquaculture industries, cropping, grazing and dairying. The impacts of acid sulfate soil oxidation constitute the most acute water-based environmental problem in coastal areas of NSW, comparable in environmental and economic terms to the effects of salinity on inland waters.

It is now recognised that certain environmental effects of the oxidation of acid sulfate soils can last for hundreds or even thousands of years.

Other impacts

Acid sulfate oxidation can also contribute to low dissolved oxygen levels in streams. This occurs through the secondary oxidation of Fe2+ which consumes oxygen and lowers dissolved oxygen levels, together with the decomposition of pasture plants killed by waterlogging.

Other potential impacts may include animal ill-health caused by polluted water. For humans, physical contact with ground and water containing toxic concentrations of acid and metal contaminants can cause skin irritation and dermatitis, while dust from disturbed acid sulfate soils may also irritate eyes. Exposure to hydrogen sulfide gas should be avoided as it is highly toxic.

Recent research

Much recent research has been directed towards metal contamination. Many metals are more soluble and mobile in low pH conditions. Iron and aluminium can become soluble in toxic quantities, with their precipitates affecting water quality and coating stream banks and benthic (sediment-dwelling) organisms.

A range of other metals including arsenic, cadmium, chromium, copper, lead, nickel, selenium and zinc may also be mobilised.

Oxidation and subsequent reactions also change the soil fabric, leading to irreversible shrinkage and lowering of ground surfaces.

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Page last updated: 11 October 2013