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Acid sulfate soils

Acid sulfate soils can pose a serious risk to the health of the Murray–Darling Basin and the people and wildlife who live there. The risk is highest during droughts when there are low flows and low water levels in the lower Murray region in South Australia.

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Why a build-up of sulfides in the soil is a problem

When rivers and wetlands dry out, sulfides that have built up in the previously submerged soil can be exposed to oxygen and may release sulfuric acid into the water. This puts the health of people, plants and animals at risk.

This happened during the Millennium Drought. In the southern Basin, the Coorong and Lower Lakes had some of the lowest water levels on record. As a result, 20,000 hectares of acid sulfate soils were exposed.

Soils and water around the edges of these lakes became acidic enough to damage steel. To stop further damage, large barriers were put in place to keep some of the acidified soils covered with water, and crushed limestone was dropped into affected areas from the air to decrease the acidity of the water.

One of the aims of the Basin Plan is to lower the risk of acid forming when parts of the river dry out. This is done by releasing water to:

  • keep the water levels in the Lower Lakes above sea level
  • dilute the amount of salt in the water

Acid sulfate soil and acidification

Over time, chemicals known as ‘sulfides’ naturally build up in soil. In inland Australia, the largest source of sulfate is saline groundwater. When rising groundwater brings salt to the surface, acid sulfate soils can form.

Acid sulfate soils form through a series of chemical reactions that occur when there is a combination of:

  • sediments that are covered by water
  • sources of sulfate such as salt in groundwater
  • organic matter from wetland or fringing vegetations
  • iron at levels commonly found in soil
  • bacteria that convert sulfate to sulfide.

Sulfides are harmless when they remain covered in water or mud. However, when water levels fall and wetlands and riverbeds dry out, the sulfides can react with oxygen in the air to produce sulfuric acid. When the soils acidify they may also release toxic metals such as manganese, aluminum and arsenic. When water begins to flow again, the acid and metals move from the soil into the water and spread through the system. Not all sulfidic materials acidify and become sulfuric.

The environment can absorb and neutralise some acid. However, if there is too much, the soil and water in the ground remain acidic. In extreme cases, some acid sulfate soils can remove all the oxygen from the water, killing fish and other aquatic organisms. 

Problems caused by acid in soils

Parts of the southern Murray–Darling Basin, especially in the lower River Murray including the Lower Lakes in South Australia, can be damaged by acid sulfate soils. Acidification harms the Basin’s plant and animal life in a number of ways:

  • Acid that is released can kill plants, which damages animal habitats.
  • When some acid sulfate soils are disturbed, this can remove oxygen from the water, killing fish and other aquatic organisms.
  • Rainfall can wash acid and toxic metals into waterways, poisoning fish and other aquatic organisms.

People and communities are also affected:

  • Exposure to acidic water can irritate eyes and skin.
  • Drinking acidic water can make humans and animals sick.
  • Acid and toxic metals in the water make outbreaks of disease more likely for fish.
  • Sulfuric acid can corrode concrete, iron and steel, eventually damaging infrastructure like buildings, roads and bridges.
  • When people can’t safely swim or fish, tourism is affected. Tourists spend $11 billion in the Basin every year. Communities suffer when those tourist dollars disappear.

The effects of acidification can last a long time, leaving high amounts of acid in soils and groundwater for many years.

What can be done to minimise the impact of acid sulfate soils

In 2014, the MDBA introduced an Emergency Framework for the Lower Lakes, which addresses both salinity and acidity. The framework aims to prevent irreversible damage from acidification and to help farmers and local communities to make decisions during droughts.

The most important way to limit the damage from acid sulfate soils is to prevent them from being exposed in the first place. Ways to do this include:

  • monitoring water quality along the River Murray to know when acid is forming
  • introducing a wetting and drying regime that limits the build-up of sulfides to levels that cause problems
  • using environmental water to keep water above sea level in high-risk areas, such as the Lower Lakes
  • installing temporary barriers called ‘bunds’ to keep acid sulfate soils underwater. This is a last resort, and only done after extensive consultation with the states and communities.

Once acid sulfate soils have been exposed, diluting or removing acid from the Basin can help a small amount. In the Millennium Drought, the states:

  • replanted vegetation over exposed acid sulfate soils to help reduce sulfate levels
  • neutralised acidic water with limestone.

Diluting or removing acid from the Basin only has a small impact. It is better to keep water at the right level so that acid sulfate soils are not exposed in the first place.

The best strategy is to implement a natural wetting and drying regime in wetlands. This helps prevent sulfidic sediments from building up to levels where they become a problem. When the wetlands are wetted again, any acid that has been produced by the oxidation of wetland sediments can be flushed out of the wetlands and neutralised by alkalinity in the river water.

Reducing salinity can also help to lower the risk of acid sulfate soils developing.

Updated: 10 Jun 2022