Anglesea Futures

Coastal Acid Sulfate Soils around the lower Anglesea River

A study was undertaken to determine the potential for soil acidification in Anglesea River estuary and the mobilisation of metals within the soils and sediments of areas surrounding the river, in particular around Coogoorah Park.

What are coastal acid sulfate soils?Coastal acid sulfate soils (CASS) are soils that contain excessive sulfuric acid or have the potential to form excessive sulfuric acid when exposed to oxygen.

Most CASS found in Victoria were formed within the last 10,000 years after the last major sea level rise, when levels were about 1.5m higher than today.

When sea water flooded the land, the sulfate it contained mixed with land sediments containing iron oxides and organic matter.

Under these waterlogged, anaerobic (oxygen-free) conditions, bacteria assist in converting sulfates into sulfides. These sulfides react with metals to form metal sulfides, typically pyrite.

If left undisturbed and covered in water, sulfidic materials (or potential acid sulfate soil) pose little or no threat. However, when exposed to oxygen through drying under drought conditions, changes in the water table or from excavating or dredging, chemical reactions may lead to the formation of sulfuric materials (pH < 4) (or actual acid sulfate soil) and the generation of sulfuric acid.

Sulfuric materials have the potential to release significant amounts of sulfuric acid which can cause the acidification of wetlands and estuaries. Metals and metalloids (such as arsenic) can also be mobilised from sulfuric materials.

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How was the study done?The study was conducted by experts from Federation University of Australia in accordance with the Victorian Best Practice Guidelines for Assessing and Managing Coastal Acid Sulfate Soils (2010).

Samples were taken at 59 sampling sites at 50cm depth increments (or at least 1 sample per soil horizon) to a depth of 1.25m below current water table levels.

The samples were analysed for:

Net Acidity

metal and metalloid mobilisation

rates of acidification.

Sampling locations for this study: Coogoorah Park and the eastern side of the estuary south of the Great Ocean Road bridge at Anglesea.

What the study foundSoils which have the potential for acidification are almost entirely contained within Coogoorah Park. The south-east side of the Great Ocean Road bridge location has negligible potential for acidification from acid sulfate soils.

Field investigations determined that there were no acid sulfate soil materials in the vegetated areas near the eastern and western margins of Coogoorah Park (see areas in green).

The sulfidic soils in Coogoorah Park contain appreciable levels of potential acidity and are prone to rapid acidification upon oxidation.

The soils from the study locations have a generally low potential to adversely affect the surrounding environment via metal mobilisation (mainly arsenic and nickel).

The levels of metals in the soils analysed from both locations were all well below the required levels for contaminants in soils for use as recreational areas, including parks, playgrounds and footpaths according to the National Environment Protection (Assessment of Site Contamination) Measure, 2011.

What this meansIf water levels decrease for long periods, there would be an appreciable potential for the release of acidity from areas within Coogoorah Park. For example, when excavated, the sulfidic soils generally acidify within a few weeks under worst case conditions. However, if the water levels around Coogoorah Park are managed appropriately, there will be little potential for the release of acidity or metals from acid sulfate soil materials.

Alternatively, management actions, such as adding neutralising materials, can also limit the release of acidity or metals from acid sulfate soil materials.

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Map of Coogoorah sampling sites. The green areas indicate where acid sulfate soils are absent.

Where to from here?The Department of Environment, Land, Water and Planning is investigating short and long term options for the future management of the Anglesea River estuary to manage the water levels around Coogoorah Park.