Managing Acid Sulfate Soils

Managing acid sulfate soils

The Managing Acid Sulfate Soils Project will manage exposed • Water levels were so low (as low as -1 m AHD) in the CLLMM acid sulfate soils through the application of neutralising region during the 2006-2010 drought that up to a total of limestone, assisting in prevention of potentially severe 20 000 ha of acid sulfate soils were exposed. environmental degradation. • Exposed acid sulfate soils can form sulfuric acid and Managing acid sulfate soils release metals from the soil which can then be transferred to waterways. The Managing Acid Sulfate Soils Project will only be triggered if • When the soils are re-wet, they can cause waterways to water level and quality drop to certain levels. If a trigger is met, become acidic if there is not enough buffering capacity in the project will aim to prevent potentially severe environmental the water. degradation in the Coorong, Lower Lakes and Murray Mouth (CLLMM) region from the exposure and rewetting of acid sulfate • This can impact on native plants and animals, agriculture soils, by neutralising acidified soil and water by the application of and livestock. limestone.

Limestone application methods can include large-scale aerial The Coorong, Lower Lakes and Murray Mouth (CLLMM) dosing, limestones barriers, and the localised application of Recovery Project is a key component of South Australia’s limestone slurry to acidified waters. $610 million Murray Futures program, funded by the As environmental (water level and quality) conditions in the Australian Government’s Water for the Future initiative. CLLMM region are currently stable, the Managing Acid Sulfate The CLLMM Recovery Project, is comprised of a suite of Soils project is currently on standby until triggers for limestone management actions that collectively aim to improve the dosing are met. ecological features of the CLLMM site to deliver a healthy, A quick snapshot productive and resilient wetland of international importance, as well as to increase capacity, knowledge and understanding • Acid sulfate soils occur naturally and are not a problem across communities. It is being delivered in collaboration with in natural drying and wetting cycles or if they are the community and Ngarrindjeri, the areas traditional owners. kept underwater. 2 | Managing acid sulfate soils

What are acid sulfate soils? Keeping soils submerged

Acid sulfate soils naturally occur in coastal and freshwater areas The main way to prevent acidification is to ensure built up where there are large amounts of sulfate and organic material in acid sulfate soils are kept wet and not exposed to air. the water. Acid sulfate soils can be managed in the Lower Lakes by As long as the soils are covered by water they are harmless to the maintaining water levels at between 0.4 m and 0.8 m AHD. environment. But if water levels drop and the soils are exposed to Bioremediation 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 Bacteria in the soil can reverse the process of acid sulfate soils the soils. forming sulfuric acid. This is called bioremediation.

Why are acid sulfate soils a problem in the Lower Lakes? 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. Natural cycles of flooding and drying once flushed the small amounts of acid formed by the soils from the system. Growing plants can increase organic matter, but it is only one part of the longer term bioremediation process. If the soil is too However, controlling the River Murray’s flow has resulted in a acidic then the bacteria will struggle. Adding finely ground build-up of acid sulfate soils in some areas such as the Lower Lakes. limestone can neutralise acid released from exposed soils and When water levels in the Lower Lakes reached unprecedented help natural bioremediation take place. lows during the drought of 2006-2010, large areas of acid sulfate soils were exposed. Acid sulfate soils occur naturally and are not Exposed acid sulfate soils can cause the soil and water to become a risk while underwater. Exposed acid sulfate very acidic. The acid can also release toxic metals such as soils can form sulfuric acid, resulting in the manganese, aluminium and arsenic from the soil. release of acid and dangerous metals into When the soil is re-wet, through rainfall or increased river flow, the acid and metals can be transported and affect large areas. the environment. Acidification and high amounts of metals in the water can cause native plants and animals to die, as well as affect agriculture and livestock.

What has been done to manage acid sulfate Research questions include: soils so far? • What are the major processes related to the formation of During the drought of 2006-2010, the South Australian acid sulfate soils in the CLLMM region? Government took emergency actions to manage the lake levels to • How does acidification affect key aquatic organisms? reduce the severe environmental degradation associated with • What are the rates of recovery of acidic sediments and what exposed acid sulfate soils. is driving recovery? Actions included the construction of the Narrung Bund and the • What are the minimum water levels required to protect Clayton and Currency Creek temporary regulators, as well as key species? various methods of limestone treatment, including aerial dosing, limestone barriers, and limestone slurry application. Several long term research projects that include field work, laboratory work and computer-driven modeling have been As the water returned, the risks associated with acid sulfate soils instigated to address these and other related questions. slowly reduced. As a result, limestone treatment is no longer required, and the bund and temporary regulators have been removed. What are we learning?

Why do triggers need to be met? Key findings from the research so far include:

Improved inflows since 2010-2011 have ensured that water levels • Soils that acidified during the drought continue to be have been maintained sufficiently to keep acid sulfate soils remediated by a number of processes including submerged. The triggers for limestone dosing are based on a fall bioremediation (the process of promoting naturally occurring in water level to below 0m AHD, alkalinity levels, and/or the bacteria to return contaminated environments to a healthy presence of acidity in the water body. state). Ongoing, water quality and soil monitoring is being undertaken • Sulfate is being reduced to reform sulfidic sediment in the by the CLLMM Recovery Project. This monitoring will identify if surface soil layers where organic matter is provided by any significant changes to water quality have occurred, and if the vegetation (such as Phragmites) which survived the return of triggers for limestone dosing have been met. water in 2010-2011. • Acidity, ammonia and metals are still a concern in sediments Improving understanding below 50 mm in some areas that acidified during the drought. The CLLMM Recovery Project includes research to better • The drought has left a legacy of acidified sediments which understand acid sulfate soils and their impacts to inform could take many years to recover. management of the CLLMM site for variable lake levels and if low inflows return. • Continued soil and water monitoring indicates recovery is still taking place and is likely to take decades.

EPA 2013 Installing ASS reseach equipment 4 | Managing acid sulfate soils

Sharing knowledge Case study – Limestone dosing The knowledge being gained at the CLLMM site can be used for • Aerial limestone dosing was one of the methods used to management of other sites where acid sulfate soils are present. manage the risk of acidification in the CLLMM region. Workshops provide an opportunity for scientists and Finely ground limestone was used as a buffer to neutralise acid in the water, thereby stopping acid from environmental managers from around Australia and being transported in waterways. Adding limestone to internationally to exchange information and share their acidic sediment can also help natural bioremediation to experiences in managing acid sulfate soils. take place by keeping the pH within a range that bacteria The National Committee for Acid Sulfate Soils (NatCASS) includes can naturally remove acid from the soil. representatives from each state and the Northern Territory and • Trials to test the effectiveness of limestone dosing and from the research industry and farming sectors. It shares application methods began in 2009. These trials knowledge and advises government, industry and other relevant focussed on Currency Creek and Finniss River, where bodies on planning and management for acid sulfate soils. tributary and lake margins had been exposed, putting Such information sharing opportunities are vital to increasing our the Goolwa Channel at risk of acidification. knowledge and understanding of acid sulfate soils, and how to • Water quality monitoring showed that adding finely- manage them. ground limestone addressed much of the acid formed in the waterways and transported by autumn and winter flows. Futher information • In 2010, finely ground limestone was aerially dosed Department of Environment, Water and Natural Resources successfully to manage acidification of Boggy Lake in T: (08) 8204 1910 Lake Alexandrina and reduce the extent and duration of ecological impacts that were occurring. www.naturalresources.sa.gov.au • 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.