Acid sulfate soils

​​​​​​​Acid sulfate soil is the common term for soil which contains chemical compounds known as metal sulfides.

Soil containing metal sulfides is usually not a concern when it remains undisturbed or covered by water, but if it is exposed to air it can pose a risk to water quality.

Use this guidance to help you understand and manage the effect of acid sulfate soils on water quality in Australia.

Conditions that lead to acid sulfate soils

Acid sulfate soil forms when there is a combination of:

  • waterlogged and/or oxygen-free conditions
  • a source of sulfate from seawater or saline groundwater
  • the presence of organic matter and metals such as iron.

In these conditions, naturally-occurring bacteria obtain energy from carbon in organic matter to convert sulfate to sulfide. Sulfide in the soil then reacts with metals to form metal sulfides that release acid when exposed to air.

If exposed to air, the metal sulfides react with oxygen to produce sulfuric acid, which can seriously affect water and soil quality. Heavy metals and other toxicants can also be released and dissolved oxygen concentration in water is likely to be low in affected areas.

Dry, cracked and exposed acid sulfate soils along a creek bed in the Coorong National Park, South Australia.
Coorong National Park, South Australia. Nerida Sloane, Commonwealth Department of Environment and Energy

Effects of acid sulfate soils

Acid sulfate soil can lead to reduced pH, decreased oxygen concentration in water and the release of heavy metals such as cadmium and lead, and metalloids such as arsenic.

Acid and other contaminants can enter waterways and wetlands when soils are rewetted.

Decline in water and soil quality poses a risk to:

  • aquatic ecosystems
  • human health
  • infrastructure
  • primary industries
  • social amenity of waterways.

Human activities can be affected through poor drinking water quality and limiting recreation when foul odours are released by the chemical reactions occurring in acid sulfate soils.

Infrastructure damage can include corrosion of metal and weakening of concrete structures such as weirs, bridge pylons and fencing.

These effects can be very expensive to treat. While many ecosystems have the capacity to absorb and neutralise acid, some aquatic organisms may be killed by the lower pH, exposure to heavy metals or a lack of dissolved oxygen in the water column.

Exposure and oxidation of acid sulfate soil in a drying scenario (not to scale)
In a dry scenario characterised by a lowered water table, low or no surface water and water stressed vegetation, soils may be exposed to oxygen resulting in oxidation. Once exposed, acid and metals accumulate in the soil and oxidsied (actual) acid sulfate soil and anoxic acid sulfate soil may form above the native sediments.

Managing acid sulfate soil

The best way to manage acid sulfate soil is to determine where it might occur and avoid exposing affected soils to oxygen.

Avoiding exposure of affected soils is not always possible, in which case an adaptive management approach should be implemented.

Activities involved in the adaptive management of acid sulfate soil:

  • describe current condition of soils
  • identify questions to be answered (e.g. what are the threats and consequences)
  • identify management objectives and options
  • predict response to management options
  • implement chosen options
  • monitor results
  • evaluate response
  • refine management options by evaluating and fine tuning predictions and management objectives.
Adaptive management of acid sulfate soils

Long-term management requires regular monitoring and reduction of additional inputs of sulfate. Regular wetting and drying in some systems can also help prevent the build-up of large quantities of acid.

Depending on the risk level and local conditions, acidification may be neutralised by:

  • applying alkaline products such as lime
  • planting vegetation or increasing organic matter inputs to encourage micro-organisms to metabolise acidity and metals
  • diverting saline groundwater to disposal basins
  • maintaining water levels with temporary regulators
  • reinstating wetting and drying patterns to wet soils and prevent the build-up of sulfidic sediments through dilution with freshwater flows.

National strategy and guidance

The National Water Quality Management Strategy (NWQMS) provides guidance on monitoring and managing water to protect various environmental values.

In addition, the Commonwealth Government and the National Committee for Acid Sulfate Soils has developed a suite of resources under a National Acid Sulfate Soils project.

This new guidance material:

  • provides clear, non-prescriptive advice for managing acid sulfate soils based on current scientific knowledge
  • complements existing national guidance material.

Selecting the right guidance

Find the right acid sulfate soils guidance appropriate to your context, including your location and the activities you are planning to undertake.

In all cases, also refer to relevant state and territory acid sulfate soils guidelines and requirements.

Overview of past and present guidance

Have you encountered or is there potential to encounter Acid Sulfate Soils (ASS)?

Read:

  • National Acid Sulfate Soils Guidance: A synthesis – review of current and past primary sources of acid sulfate soils guidance, including examination of the coverage of the current main issues of concern surrounding assessment and management.

Geographical setting

Choose guidance specific to your geographical context.

Coastal:

Inland:

Field and laboratory methods

Use these current good practice methods in the field and in the lab.

Field methods:

Laboratory methods:

Soil and contamination management

Use this guidance to appropriately manage acid sulfate soils you encounter during different soil management activities.

Dewatering:

Dredging:

Disturbance of monosulfidic black ooze (MBO):

Download this advice in our interactive Decision s​upport tool​ (PDF 878 KB) Use the tool or the advice on this page to help decide which national guidance document is most relevant to your needs.

Additional resources

Resources for identifying and managing potential acid sulfate soil include:

Murray-Darling Basin

The Murray-Darling river system crosses multiple jurisdictions and poses unique risks.

Research into acid-sulfate soils by the Murray Darling Basin Authority (MDBA) showed that in recent years, low inflows and river levels have led to the drying of many wetlands, the exposure of acid sulfate soil materials and the risk of acidification in some wetlands.

The MDBA is working closely with the relevant state and territory governments to implement the Basin Plan, which aims to return more natural flow regimes to the system.

Water reform in the Murray-Darling Basin is supported by:

  • investing in new water use efficient infrastructure
  • purchasing water entitlements with the objective of returning more water to the environment.

These entitlements become part of the Commonwealth’s environmental water holdings and are managed to restore the health of rivers, floodplains and wetlands.

Environmental watering also helps to achieve more natural wetting and drying cycles, flushing out toxicants, improving water quality, and minimising exposure of potential acid sulfate soil to oxygen.

Glossary

Acid sulfate soil: material or sediments containing sulfides which have oxidised and become severely acidic.

Environmental water: water released from a reservoir to maintain downstream water levels, or water retained in a system to satisfy ecological requirements.

Potential acid sulfate soil (PASS): soils or sediments that contain sulfides and have the potential to oxidise and become severely acidic.

​​​​​​