Rehabilitation of Acid Sulfate Soils


Acid sulfate soils are natural sediments that contain iron sulfides. When disturbed or exposed to air these soils can release acid and other heavy metals, which can have severely damaging effects on aquatic ecosystems.

Council continues to address the remediation of a State recognised acid sulfate soil (ASS) hotspot through the staged implementation of the Big Swamp project.

The Big Swamp is a 2000 hectare coastal floodplain at Coralville which has been extensively cleared and drained for agriculture.

This has resulted in the generation and discharge of ASS pollution into the Manning River Estuary, which has adverse impacts on water quality, aquatic ecology, oyster production and commercial and recreational fishing.

With funding provided through the NSW Estuary Management Program and Council’s Environmental Levy, Council has recently purchased an additional 170 hectares of ASS affected land, building on the 700 hectares of land already acquired and remediated through the project to date.

Remediation activities have included extensive drain modification works to reinstate the natural hydrology of the landscape and introduce tidal flows to reduce the amount of acid runoff entering the River.

As a result of the works, both saltwater and freshwater wetlands are now re-establishing at the site and improvements in water quality are already evident.

The newly acquired land is also high in biodiversity value with a recent vegetation survey discovering a significant population of the endangered plant Noah’s False Chickweed growing on the site.

The acquisition of this land complements work undertaken at Cattai Wetlands, where an additional 500 hectares of land have been remediated and has also been instrumental in securing into public ownership a regional wildlife corridor which links habitat within the adjoining Crowdy Bay National Park to the forests of the Lansdowne escarpment.

The Big Swamp is between Crowdy Bay National Park and Coopernook State Forest. It is north-east of the Cattai Wetlands and east of the Pacific Highway and Moorland Village.  Currently a mix of farmland and remnant coastal wetland, it was once a large wetland that fed into Cattai Creek and supported an extensive array of wildlife, particularly birdlife.

The Big Swamp project area is around 2000 hectares and includes all land below 2 metres sea level. This does not mean whole properties are involved – only the low lying areas. The land is privately owned, involving over 15 landowners.

During the early 1900’s a large portion of the Big Swamp was drained under a public works program to ‘reclaim’ the land for agriculture, but by 1912 the Pipeclay Canal project was declared a failure. An article published in the Sydney Morning Herald reported on its failings and the associated environmental effects, which we now know as acid sulfate soils.

In 1999 the State Government identified twenty-six acid sulfate soil hot spots in NSW, four of which are in the Greater Taree local government area. Leading experts in the field recognise the Cattai Creek-Pipeclay Canal area as one of the worst hot spots in NSW.

The impacts of these soils are often experienced on-site with poor agricultural production (limited crop production, water unsuitable for stock). Downstream water quality is the biggest issue which can result in a reduction in oyster production and fish kills. Acidity in this area has had pH readings as low as 2.4 (in comparison, saltwater has a pH of 8). Given the acid is generated when these soils are exposed to air, re-wetting the soils is often the answer.

The $2 million grant funded component of the project is now complete and has achieved the following results:

  • Preparation of a Hydrological Study to identify, prioritise and guide the remediation of acid sulfate soils within the Big Swamp project area.
  • Purchase and remediation of around 700 hectares of private land affected by acid sulfate soils.
  • Filling of over 14kms of paddock drains, removal of floodgates and levees, and the creation of two new tidal swales.
  • The establishment of a long-term water quality and vegetation monitoring program.
  • Secured around $250,000 in additional funding from the State Government to continue the project.
  • The review of the 2004 Cattai Wetlands Plan of Management in consultation with the community and the revised actions documented in the Cattai Wetlands Future Directions Strategy(PDF, 16MB).
  • Construction of additional visitor facilities at the Cattai Wetlands including an outdoor educational facility.

We will keep on pursuing funding so the project can be continued as around 1200 hectares of land within the Big Swamp project area is still producing acid sulfate runoff into the Manning River. 

We will also continue to monitor a range of environmental indicators on site to determine how successful the project has been and to guide future remediation efforts.

The Big Swamp hydrological study(PDF, 18MB) is a technical document which provides a comprehensive scientific analysis of the on-ground acid sulfate soil remediation activities proposed for the Big Swamp project.  The Executive summary(PDF, 3MB)  provides an overview of the aims and outcomes of the Study including the development of potential restoration strategies, the prioritisation of remediation areas, and the provision of information to show that the on-ground works will not negatively influence local flooding patterns. 

The Water Research Laboratory (WRL) of the University of New South Wales, who undertook the Study, delivered a presentation(PDF, 9MB)  to local residents on the findings of the Hydrological Study. A full copy of the Study can be obtained by contacting Council’s Environmental Services Section.