Managing acid sulfate soils

The Managing Acid Sulfate Soils Project will manage exposed acid sulfate soils through the application of neutralising limestone, assisting in prevention of potentially severe environmental degradation.

Managing acid sulfate soils

The Managing Acid Sulfate Soils Project will only be triggered if water level and quality drop to certain levels. If a trigger is met, the project will aim to prevent potentially severe environmental degradation in the Coorong, Lower Lakes and Murray Mouth (CLLMM) region from the exposure and rewetting of acid sulfate soils, by neutralising acidified soil and water by the application of limestone.

Limestone application methods can include large-scale aerial dosing, limestones barriers, and the localised application of limestone slurry to acidified waters.

As environmental (water level and quality) conditions in the CLLMM region are currently stable, the Managing Acid Sulfate Soils project is currently on standby until triggers for limestone dosing are met.

A quick snapshot

• Acid sulfate soils occur naturally and are not a problem in natural drying and wetting cycles or if they are kept underwater.

• Water levels were so low (as low as -1 m AHD) in the CLLMM region during the 2006-2010 drought that up to a total of 20 000 ha of acid sulfate soils were exposed.

• Exposed acid sulfate soils can form sulfuric acid and release metals from the soil which can then be transferred to waterways.

• When the soils are re-wet, they can cause waterways to become acidic if there is not enough buffering capacity in the water.

• This can impact on native plants and animals, agriculture and livestock.

Aerial limestone dosing at Currency Creek © 2009

The Coorong, Lower Lakes and Murray Mouth (CLLMM) Recovery Project is a key component of South Australia’s $610 million Murray Futures program, funded by the Australian Government’s Water for the Future initiative.

The CLLMM Recovery Project, is comprised of a suite of management actions that collectively aim to improve the ecological features of the CLLMM site to deliver a healthy, productive and resilient wetland of international importance, as well as to increase capacity, knowledge and understanding across communities. It is being delivered in collaboration with the community and Ngarrindjeri, the areas traditional owners.

What are acid sulfate soils?

Acid sulfate soils naturally occur in coastal and freshwater areas where there are large amounts of sulfate and organic material in the water.

As long as the soils are covered by water they are harmless to the environment. But if water levels drop and the soils are exposed to air, they react with oxygen to form sulfuric acid. This is the same acid as is found in car batteries and it can release metals from the soils.

Why are acid sulfate soils a problem in the Lower Lakes?

Natural cycles of flooding and drying once flushed the small amounts of acid formed by the soils from the system.

However, controlling the River Murray’s flow has resulted in a build-up of acid sulfate soils in some areas such as the Lower Lakes.

When water levels in the Lower Lakes reached unprecedented lows during the drought of 2006-2010, large areas of acid sulfate soils were exposed.

Exposed acid sulfate soils can cause the soil and water to become very acidic. The acid can also release toxic metals such as manganese, aluminium and arsenic from the soil.

When the soil is re-wet, through rainfall or increased river flow, the acid and metals can be transported and affect large areas.

Acidification and high amounts of metals in the water can cause native plants and animals to die, as well as affect agriculture and livestock.

Keeping soils submerged

The main way to prevent acidification is to ensure built up acid sulfate soils are kept wet and not exposed to air.

Acid sulfate soils can be managed in the Lower Lakes by maintaining water levels at between 0.4 m and 0.8 m AHD.

Bioremediation

Bacteria in the soil can reverse the process of acid sulfate soils forming sulfuric acid. This is called bioremediation.

The bacteria use iron and organic matter, as well as sulfate in the acid, to do this, so making sure these are bioavailable is very important.

Growing plants can increase organic matter, but it is only one part of the longer term bioremediation process. If the soil is too acidic then the bacteria will struggle. Adding finely ground limestone can neutralise acid released from exposed soils and help natural bioremediation take place.

Acid sulfate soils occur naturally and are not a risk while underwater. Exposed acid sulfate soils can form sulfuric acid, resulting in the release of acid and dangerous metals into the environment.

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Limestone barrier Currency Creek © 2009

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What has been done to manage acid sulfate soils so far?

During the drought of 2006-2010, the South Australian Government took emergency actions to manage the lake levels to reduce the severe environmental degradation associated with exposed acid sulfate soils.

Actions included the construction of the Narrung Bund and the Clayton and Currency Creek temporary regulators, as well as various methods of limestone treatment, including aerial dosing, limestone barriers, and limestone slurry application.

As the water returned, the risks associated with acid sulfate soils slowly reduced. As a result, limestone treatment is no longer required, and the bund and temporary regulators have been removed.

Why do triggers need to be met?

Improved inflows since 2010-2011 have ensured that water levels have been maintained sufficiently to keep acid sulfate soils submerged. The triggers for limestone dosing are based on a fall in water level to below 0m AHD, alkalinity levels, and/or the presence of acidity in the water body.

Ongoing, water quality and soil monitoring is being undertaken by the CLLMM Recovery Project. This monitoring will identify if any significant changes to water quality have occurred, and if the triggers for limestone dosing have been met.

Improving understanding

The CLLMM Recovery Project includes research to better understand acid sulfate soils and their impacts to inform management of the CLLMM site for variable lake levels and if low inflows return.

Research questions include:

• What are the major processes related to the formation of acid sulfate soils in the CLLMM region?

• How does acidification affect key aquatic organisms?

• What are the rates of recovery of acidic sediments and what is driving recovery?

• What are the minimum water levels required to protect key species?

Several long term research projects that include field work, laboratory work and computer-driven modeling have been instigated to address these and other related questions.

What are we learning?

Key findings from the research so far include:

• Soils that acidified during the drought continue to be remediated by a number of processes including bioremediation (the process of promoting naturally occurring bacteria to return contaminated environments to a healthy state).

• Sulfate is being reduced to reform sulfidic sediment in the surface soil layers where organic matter is provided by vegetation (such as Phragmites) which survived the return of water in 2010-2011.

• Acidity, ammonia and metals are still a concern in sediments below 50 mm in some areas that acidified during the drought.

• The drought has left a legacy of acidified sediments which could take many years to recover.

• Continued soil and water monitoring indicates recovery is still taking place and is likely to take decades.

Installing ASS reseach equipmentEPA 2013

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Sharing knowledge

The knowledge being gained at the CLLMM site can be used for management of other sites where acid sulfate soils are present.

Workshops provide an opportunity for scientists and environmental managers from around Australia and internationally to exchange information and share their experiences in managing acid sulfate soils.

The National Committee for Acid Sulfate Soils (NatCASS) includes representatives from each state and the Northern Territory and from the research industry and farming sectors. It shares knowledge and advises government, industry and other relevant bodies on planning and management for acid sulfate soils.

Such information sharing opportunities are vital to increasing our knowledge and understanding of acid sulfate soils, and how to manage them.

Case study – Limestone dosing

• Aerial limestone dosing was one of the methods used to manage the risk of acidification in the CLLMM region. Finely ground limestone was used as a buffer to neutralise acid in the water, thereby stopping acid from being transported in waterways. Adding limestone to acidic sediment can also help natural bioremediation to take place by keeping the pH within a range that bacteria can naturally remove acid from the soil.

• Trials to test the effectiveness of limestone dosing and application methods began in 2009. These trials focussed on Currency Creek and Finniss River, where tributary and lake margins had been exposed, putting the Goolwa Channel at risk of acidification.

• Water quality monitoring showed that adding finely-ground limestone addressed much of the acid formed in the waterways and transported by autumn and winter flows.

• In 2010, finely ground limestone was aerially dosed successfully to manage acidification of Boggy Lake in Lake Alexandrina and reduce the extent and duration of ecological impacts that were occurring.

• A number of acid sulfate soil hot spots around the Lower Lakes and waterways continue to be monitored to determine if, where, and how much limestone may need to be added in the future.

Exposed ASS at Boggy Lake © 2010

Futher information

Department of Environment, Water and Natural Resources T: (08) 8204 1910

www.naturalresources.sa.gov.au

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