Acid sulfate soils occur in many parts of coastal Australia, but particularly in the tropics and sub- tropics. Very rarely do they occur at heights more than one metre above sea level but large areas can occur related to the extent of coastal low lands and floodplains.

Acid sulfate soils are formed when organic matter, such as leaves, falls into the mud in an estuary. These muds are commonly devoid of oxygen just below the surface. Next time you are in an estuary, dig down in the mud and notice the colour change from brown on the surface to black at a depth of 5 cm or so.

The bacteria that break down the leaves are anaerobic - they cannot use oxygen and instead use sulfates dissolved in the seawater. A by-product from the breakdown of the leaves is iron sulfide compounds (amongst others). These compounds lie in the mud in a chemical state that will react to oxygen.

This process has been occurring for thousands of years and thus the acid sulfate soils may occur in areas that were once mangroves but are now dry land, as a result of the gradual (and natural) infilling of the estuaries over long periods of time. Acid sulfate soils commonly occur underneath floodplain wetlands, even freshwater ones.

Normally these compounds will never be exposed to oxygen and are thus quite safe. However, the drainage of wetlands for agriculture and flood mitigation has exposed huge areas of acid sulfate soils to the air. Coastal development has also played a role but in a far more limited way.

Once the soil is exposed to the air the sulfides become oxidised. Add fresh water and sulfuric acid is produced. Not surprisingly sulfuric acid and waterways do not mix. After rain the acid is washed out of drains and into nearby waterways. If the acid water goes into an area where there is a lot of seawater, then the large amount of dissolved carbonate in seawater may counter the acid. However, most acid-soil problems are located in areas where there is little salt water and thus the acid causes dramatic reductions in the pH of the receiving water.

The pH of the water can even drop to less than 3. This causes clay particles to disaggregate and release soluble iron and aluminium into the water column. Dissolved aluminium is toxic to plants and animals at very low concentrations. When the pH of the water rises (due to mixing with non-acidified water), iron forms hydroxide compounds which coat the surface of sediments and plants, smothering benthic animals and preventing photosynthesis in plants.

The results of acid ‘slugs’ in waterways can be spectacular. Depending on how water moves in the affected waterway, fish can become trapped and massive fish kills can result. For those fish that do escape, the chance of getting an ulcerative fungal disease - called red spot - is almost certain. Research work on prawns shows that growth rates decline as the pH drops below 6.0. In the case of oysters the settling success of the young oysters, or spat, is negatively affected by acid and oyster growth and survival is also affected.

From information gathered from overseas experiences in acid-rain- affected lakes it is also likely that the spawning success of species that spawn in estuaries is reduced. For the Australian bass it is known that hatching rates are reduced, a significant result given that bass spawn in areas of low salinity.

Acid water also has ecosystem- level effects. Although many estuarine organisms are tolerant of occasional changes in water quality (including acid), it is the regularity of the acid inputs which can result in long-term changes to fauna. Such changes include the loss of species diversity and decline in productivity.

Action by government to date has focused on changing planning laws for new developments along with providing guidelines and technical information/training for landholders. While there has been some valuable progress, the fact remains that there is still a major, multi-million dollar problem that the government is failing to address. There is insufficient public pressure to loosen up government purse strings – serious money is needed if the poison is to be stopped. If a factory disposed of billions of litres of sulfuric acid into a waterway it would be shut down; just because the source is a farm does not mean the pollution is acceptable.

* Duncan Leadbitter is Executive Officer of Ocean Watch.