Inland acid sulfate soil and water quality fact sheet

Department of Sustainability, Environment, Water, Population and Communities, 2012

'Acid sulfate soil' is the common term for soil which contains chemical compounds known as metal sulfides. This soil is usually not a concern when it remains undisturbed or covered by water.

Cracking acid sulfate soils in the Coorong National Park. Nerida Sloane, DSEWPaC.

If exposed to air, water and soil quality may be seriously affected as the metal sulfides react with oxygen to produce sulfuric acid. Heavy metals and other toxicants can also be released and dissolved oxygen concentration in water is likely to be low in affected areas. This combination of factors can result in environmental, economic and social impacts.

How does acid sulfate soil form?

Acid sulfate soil forms when there is a combination of several factors: waterlogged and oxygen-free conditions, a source of sulfate from seawater or saline groundwater, and 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. The sulfide then reacts with metals to form metal sulfides that release acid if subsequently exposed to air.

Potential acid sulfate soil (PASS) is soil or sediment containing metal sulfides which have potential to oxidise and become acidic if exposed, whereas actual acid sulfate soil (AASS) has already produced acid from oxidisation. The formation of acid sulfate soil may occur naturally or be influenced by human activities which alter water regimes and mobilise salt, such as dams, irrigation, earthworks, low flow events, dredging, mining, land-clearing, and any activity which lowers the water table.

Acid sulfate soil is most common in coastal regions but may also occur in inland waterways, wetlands, drainage channels and saline seepage areas. Some of the signs of acid sulfate soil are visual indicators such as orange-brown water and soil, oil-like slicks and subsurface 'monosulfidic black oozes', the presence of salinity or salt crusts, and vegetation dieback or shifts to acid-tolerant species (for example, smartweed, water lilies and spike rushes).

A three-year study by the Murray-Darling Basin Authority found that acid sulfate soils were extensive throughout many wetlands in the lower River Murray in South Australia, the western part of the Edward-Wakool River in New South Wales, and around Mildura in Victoria. Acid sulfate soils are also widespread in Western Australia.

Diagram showing the acid sulfate soil adaptive management framework.

Acid sulfate soil adaptive management framework

Text alternative for diagram

  • Describe current condition, leads to
  • Identify questions, leads to
  • Identify management objectives and options, leads to
  • Predict response, leads to
  • Implement, leads to
  • Monitor, leads to
  • Evaluate response, leads to
  • Evaluate predictions and objectives, leads to
  • Refine management options, circles back to predict response

Why is acid sulfate soil a problem?

Acid sulfate soil can reduce pH or result in decreased oxygen concentration in water and the release of heavy metals such as cadmium and lead, and metalloids such as arsenic. The acid and other contaminants can enter waterways and wetlands when soils are rewetted. The decline in water and soil quality poses a risk to aquatic ecosystems, human health, primary industries and the built environment. 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.

Acid sulfate soil can impact on human activities by causing poor drinking water quality and limiting recreation when foul odours are released. Infrastructure damage can include corrosion of metal and weakening of concrete structures such as weirs, bridge pylons and fencing.

Diagram showing the exposure and oxidation of ASS in a drying scenario.

Exposure and oxidation of ASS in a drying scenario

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A lowered water table, low/no surface water and water stressed vegetation can lead to exposure to oxygen and oxidation; acid and metal accumulation, oxidsied (actual) acid sulfate soil and anoxic acid sulfate soil above the native sediments.

How can acid sulfate soil be managed?

The best way to manage acid sulfate soil is to determine where it may occur and avoid exposing it to oxygen. Resources for identifying and managing potential acid sulfate soil include the Australian Soil Resource Information System (ASRIS) and an interactive decision support tool.

Avoiding exposure of affected soils is not always possible, in which case an adaptive management approach should be implemented, as outlined in the figure above. For long-term management, it is vital to undertake regular monitoring and reduce 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; or
  • re-instating wetting and drying patterns to wet soils and prevent the build-up of sulfidic sediments through dilution with freshwater flows.

Western Australia has produced a number of guidance documents for managing acid sulfate soil in their regions: www.dec.wa.gov.au/management-and-protection/land/acid-sulfate-soils/guidelines.html.

What is the Australian Government doing about acid sulfate soil?

The Australian Government works collaboratively with state and territory agencies on the management of acid sulfate soils including, for example, the development of relevant guidance material for managing inland acid sulfate soil.

In previous years, low inflows and river levels have led to the drying of many wetlands. This has resulted in the exposure of acid sulfate soil materials and the risk of acidification in some wetlands. In the Murray-Darling river system, the Murray-Darling Basin Authority is working closely with the relevant states and territories to develop a Basin Plan which will return more natural flow regimes to the system. Further information about the Basin Plan can be found at: www.mdba.gov.au.

As part of Murray-Darling Basin reforms the Australian Government is acquiring water entitlements with the objective of returning more water to the environment. These entitlements become part of the Commonwealth environmental water holdings and are managed so that increased flows are provided to rivers and wetlands. Environmental watering also helps to achieve more natural wetting and drying cycles, flushing out toxicants, improving water quality, and minimising exposure of soil to oxygen.

Further information about the Commonwealth Environmental Water Office can be found at: www.environment.gov.au/ewater.

Furthermore, the National Water Quality Management Strategy provides guidance on monitoring and managing water to protect various environmental values.

The National Water Commission has co-funded research on ways to reduce the impact of sulfidic sediments on inland wetlands: www.mwwg.org.au/sulfidic.php.

Glossary

Actual acid sulfate soil (AASS):
soils 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.
Monosulfidic material or 'black ooze' (MBOs):
a gel-like material in soil which is highly reactive.
Potential acid sulfate soil (PASS):
soils or sediments that contain sulfides and have the potential to oxidise and become severely acidic.