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.
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
- 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)
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)?
- 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.
Choose guidance specific to your geographical context.
- National Strategy for the Management of Coastal Acid Sulfate Soils — a holistic and comprehensive approach to define the Acid Sulfate Soil problem, prevent it from increasing and encourage remedial actions to reduce existing acid water runoff.
- National Guidance for the Management of Acid Sulfate Soils in Inland Aquatic Ecosystems — for the identification and management of inland acid sulfate soils, to reduce or eliminate the risks they pose to the Australian environment and its economy.
Field and laboratory methods
Use these current good practice methods in the field and in the lab.
- National acid sulfate soils sampling and identification methods manual — technical and practical advice on the identification and sampling of acid sulfate soil materials prior to field investigations. Also provides sampling requirements necessary to define the extent of acid sulfate soil materials in the landscape.
- National acid sulfate soils identification and laboratory methods manual — sets out the current good practice acid sulfate soils laboratory analytical methods for soil samples. Used to conclusively identify the presence or absence of acid sulfate soils and to quantitatively assess associated hazards.
Soil and contamination management
Use this guidance to appropriately manage acid sulfate soils you encounter during different soil management activities.
- Guidance for the dewatering of acid sulfate soils in shallow groundwater environments — technical and practical advice on managing acid sulfate soils to help prevent or minimise harm to the environment.
- Guidelines for the dredging of acid sulfate soil sediments and associated dredge spoil management — technical and procedural advice to avoid environmental harm from acid sulfate soils encountered during dredging projects.
Disturbance of monosulfidic black ooze (MBO):
- Overview and management of monosulfidic black ooze (MBO) accumulations in waterways and wetlands — technical and procedural advice to assess and manage MBOs encountered in waterways and wetlands.
Download this advice in our interactive Decision support 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.
Resources for identifying and managing potential acid sulfate soil include:
- Australian Soil Resource Information System (ASRIS) — provides access to the best available soil and land resource information (spatial and temporal databases) in a consistent format across Australia. It provides a scientific information infrastructure for assessing and monitoring the condition of Australia’s soil and land resources.
- National Water Commission Sulfidic Sediments project — co-funded research, tools and guidance on ways to reduce the impact of sulfidic sediments on inland wetlands.
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.
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.
Murray-Darling Basin Authority
Southern Cross University – Geoscience
Western Australian guidelines for managing acid sulfate soil in their regions
Tasmanian Acid Sulfate Soil Management Guidelines
Queensland Government’s information on acid sulfate soils
Victorian Government’s information on acid sulfate soils
South Australian Acid Sulfate Soils Research Program
New South Wales Government’s information on Acid Sulfate Soils