Acid Sulphate Soils Outline

The Denmark Environment Centre has worked closely with state government regarding the issue of Acid Sulphate Soils. This finally resulted in the adoption of the Acid Sulphate Soils Management Guidelines and the formation of the WA Acid Sulphate Soils Management Committee.

Acid Sulphate Soil (ASS) is the name given to sediment and soil containing iron sulfides (principally contain iron pyrite or iron di-sulfide). The relatively restricted conditions under which ASS are formed limit their formation to low-lying parts of coastal flood plains, rivers and creeks. This will include areas with saline or brackish water such as deltas, coastal flats, back swamps and seasonal or permanent freshwater swamps which were formerly brackish. Pyrite sediment may be found at any depth in the soil layer in suitable coastal sediments usually beneath the water table.

The exposure of pyrite in these soils to oxygen by drainage or excavation, leads to the generation of sulfuric acid. Acidic leachate can dissolve clay and release toxic concentrations of aluminium, iron and other metals into water bodies. Drainage waters from areas of ASS will affect water quality and can lead to the death or disease of aquatic organisms.

Acid Sulphate threatens wetlands in National Parks

As surface water pollutants, the ASS products released into surface waters can degrade water supplies for stock, birds and humans, severely modify and change vegetation, clog aquifers and degrade potable water. Marked impacts on estuarine receiving habitats have included major fish kills, probable displacement of fish population, degradation and destruction of benthic food chains, and changes in riverine and estuarine planktonic structures. Such degradation of estuary ecosystems has far-reaching implications directly on food chains, for such biota as estuarine and coastal sea birds, and indirectly on coastal habitats and fisheries resources. Concrete and steel pipes and other engineering infrastructure are susceptible to acid attack.

The DEP recently confirmed that potential acid Sulphate soils exist at the proposed mine site adjacent to Lake Jasper.

What will this mean for the Lake Jasper wetlands if mining proceeds?

BHP closed their Beenup mineral sands mine near Augusta after experiencing enormous problems with ASS, which is close to Scott National Park, Scott River, Blackwood River & Hardy Inlet. BHP has not been able to stop acid leachate from entering the groundwater adjacent to the mine!

Further exploration permits exist for the Scott National Park & the Gingilup Conservation Reserve, which adjoin the D’Entrecasteaux National Park.

Lake Jasper too Magnificent to mine.

Attended by Geoff Evans

What are acid sulphate soils?

Acid sulphate soils are the common name given to naturally occurring soil and sediment containing iron sulfides (commonly known as iron pyrites).

In Australia, the acid sulphate soils of most concern are those that formed in the Holocene geological period (the last 10,000 years) after the last major sea level rise.

During the sea level rise new coastal landscapes were created as a result of rapid sedimentation, and acid sulphate soils were created when bacteria in these organically rich waterlogged sediments converted the sulphate from the seawater and iron from the sediments into iron sulfides.

These naturally occurring sulfides are generally found in a layer of waterlogged soil or sediment, and are benign in their natural state. When disturbed and exposed to air, however, they undergo a series of chemical reactions and ultimately produce sulfuric acid. While the soil itself can neutralise some of the sulfuric acid, the remaining acid moves through it and acidifies the soil water, groundwater and surface water it comes into contact with. This, in turn, dissolves metals such as aluminium, iron and arsenic from the soil.

Why are acid sulphate soils a planning issue?

Release of acid and metals as a result of the disturbance of acid sulphate soils can cause significant harm to the environment and infrastructure.

The principal environmental, social and economic impacts of acid
sulphate soils have been documented as follows:

  • adverse changes to soils and water quality;
  • deterioration of ecosystems and the ecosystem services associated with soils, groundwater, wetlands, watercourses and estuarine environments;
  • local and regional loss of biodiversity in areas affected by acid sulphate soils leachate;
  • loss of groundwater and surface water resources used for irrigation and other purposes;
  • reduction in opportunities for agriculture and aquaculture;
  • human health concerns particularly from arsenic contamination of groundwater in areas affected by acid sulphate soils;
  • corrosion of engineering works and infrastructure such as bridges, culverts, floodgates, weirs, drainage pipes and sewerage lines;
  • conflict between activities that depend on healthy surface and groundwater regimes (e.g. commercial fishing, recreation and tourism) and activities that may have resulted in disturbance to acid sulphate soils (mining, agriculture and urban development);
  • loss of visual amenity from plant deaths, weed growth and invasion by acid tolerant water plants and algae; and
  • costs to the community in terms of financial outlays and the communities and government’s time and effort in minimising impacts and rehabilitating disturbed areas.

In Western Australia the main impacts associated with acid sulphate soils to date have been:

  • wetlands degradation;
  • localised reduction in habitat and biodiversity;
  • deterioration of surface and groundwater quality;
    loss of groundwater for irrigation;
  • increased health risks associated with arsenic and heavy metals contamination in surface and groundwater, and acid dust;
  • risk of long-term infrastructure damage through corrosion of sub-surface pipes and foundations by acid water; and
  • invasion by acid tolerant water-plants and dominance of acid tolerant plankton species.

The presence of acid sulphate soils is, therefore, a planning issue that should be taken into account in planning decision-making.

Types of development that may cause acid sulphate soil problems

The types of development that may disturb acid sulphate soils typically involve large-scale drainage and excavation works, which expose these, soils to air, and may include:

  • coastal developments such as residential estates (including canal estates), marinas, tourist developments and golf courses;
    large-scale dewatering and drainage works associated with the development of residential estates;
  • developments involving disturbance to wetlands, mangrove swamps, salt marshes, lakes and waterways;
  • infrastructure projects such as bridges, port facilities, flood gates, dames, dredging, railways and roads;
  • mining and quarrying operations;
  • rural drainage, which lowers the water table; and
  • flood mitigation works including construction of levees.

Location of acid sulphate soils

Acid sulphate soils occur throughout Australia, but have only recently come to prominence in land use planning as a result of the detrimental environmental consequences that can arise from disturbing them.

The Australia-wide distribution of potential acid sulphate soils is a work in progress, although extensive mapping programs have been undertaken in Queensland, New South Wales, South Australia and Western Australia.

In Western Australia acid sulphate soils are found in, though
not limited to, the following locations:

  • the south west of the State, between Perth and Busselton, in estuarine, floodplain and wetland areas;
  • the Scott River Plain on the south coast and extending to Albany;
  • some parts of the Wheatbelt where in-land salinisation has occurred;
  • the northern parts of the State’s coastline including the Pilbara and Kimberley coastlines.

At these locations, there could be a risk of disturbing acid sulphate soils in the following areas:

  • Areas identified as acid sulphate soil areas or acid sulphate soil risk areas on government agency mapping, or on mapping from any other reputable source.
  • Areas depicted in the Environmental Geology maps published by the Department of Mineral and Petroleum Resources as Holocene Swamp, Tidal and Estuarine Deposits, or Marshes and Floodplains.

Areas depicted in:

  • the Land System and Soil-Landscape System mapping by Agriculture WA; or
  • soil, geology or geomorphological mapping, which indicates geologically recent shallow tidal, estuarine, marine, wetland, floodplain or waterlogged areas where deposition of fine sediments may have occurred or may be occurring.

Areas depicted in vegetation mapping as mangroves, or wetland dependent vegetation such as reeds and paperbarks.

Areas identified in geological descriptions or in maps as bearing acid sulfide minerals or former marine or estuarine shales and sediments, or mineral sand deposits.

Coastal areas where the following pre-disposing factors exist:

  • areas known to contain peat or a build up of organic material;
  • areas near bores in which peat or other organic deposits have been recorded as part of the stratigraphy;
  • permanently inundated wetlands;
  • seasonally or occasionally saturated or inundated floodplains and sumplands;
  • shallow estuarine areas receiving alluvium;
  • mangrove areas;
  • tidal swamps, wetlands and shallow estuarine areas receiving alluvium;
  • artificial lakes excavated in peaty material;
  • sites known or believed to contain carbonaceous or pyritic material, such as:
  • sites containing fill;
  • existing or former municipal waste disposal sites;
  • industrial sites;
  • food industry waste disposal areas;
  • animal-based waste disposal areas;
  • areas where the highest known water table level is within three (3) metres of the surface; and
  • areas where the pH of the soil or water is less than 5.

Any areas in Western Australia (including inland areas) where a combination of all the following pre-disposing factors exist:

  • organic matter;
  • iron minerals;
  • waterlogged conditions or a high water table;

Any areas where field tests, visual signs and other methodologies indicate that there is a likelihood of acid sulphate soils being present.

Increasing community awareness about the problems associated with acid sulphate soils, together with the detailed knowledge as to the location of these soils, after a previous State Government allocated funding for a detailed State-wide acid sulphate soils risk mapping program.

Planning Guidelines for Acid Sulphate Soils

While the presence or possible presence of acid sulphate soils is a development constraint that should be subject to an appropriately rigorous risk assessment. The current detailed knowledge as to the location of acid sulphate soils in Western Australia provides a comprehensive planning response to the issue.

Nevertheless developers, planners and the community need to be aware of the implications of the presence of acid sulphate soils and, perhaps more importantly, the management processes required avoiding their potential adverse effects on the environment and infrastructure.

The following general guidelines should be used in the assessment of town planning scheme amendments, subdivision and strata applications and applications for planning approval where there is evidence of a significant risk of disturbing acid sulphate soils.