1 Overview of acid sulfate soils in Western Australia

1.1 The purpose of this guidance

The purpose of the document is to provide:

• An overview on how, where, when and why acid sulfate soil problems may arise; and
  
  
• Guidance on managing acid sulfate soils and activities that may disturb acid sulfate soils.

A summary of the management advice is the flow-chart in Appendix 1.

The information in this guidance should be read in conjunction with the Western Australian Planning Commission's Planning Bulletin No. X: Acid Sulfate Soils.

1 It should be noted, however, that the removal of peat and other materials may be subject to the provisions of the Mining Act 1978. It is advisable to check with the Department of Mineral and Petroleum Resources.

This guidance is not intended to address the management of acid sulfate soils or acid drainage associated with mining proposals¹, as mining can raise complex issues that require site specific management techniques. Soil acidity problems other than caused by acid sulfate soils are not addressed in this guidance.

1.2 Introduction - What are acid sulfate soils?

When acid sulfate soils are exposed to air, the iron sulfides (commonly iron pyrite) oxidise and produce sulphuric acid, iron precipitates, and concentrations of dissolved metals such as aluminium, iron and arsenic. This leachate has been responsible for environmental damage, damage to infrastructure and buildings, and human health problems.

While the eastern coastline of Australia has had to deal with acid sulfate soils problems for some time, Western Australia has, until recently, only infrequently encountered the issue. While, it is thought that acid sulfate soils problems in Western Australia are on a more localised and less severe scale, there is still much to be learnt about the issue.

For the purposes of this guidance, acid sulfate soils include both actual acid sulfate soils and potential acid sulfate soils.

Actual acid sulfate soils are soils or sediments that contain iron sulfides and/or other sulfidic material that have been exposed to the air and have oxidised, producing highly acidic soil horizons or layers. Queensland and New South Wales guidelines recognise a pH criterion of 4 or less for actual acid sulfate soils.

Upon exposure of acid sulfate soils to air, acidity problems may persist for a long time. In some areas of Australia, acid sulfate soils drained 100 years ago are still releasing acid (Sammut 2000).

Potential acid sulfate soils (PASS) are soils or sediments which contain iron sulfides and/or other sulfidic material that have not oxidised by exposure to air. Based on criteria in the Queensland and New South Wales guidelines for PASS, the field pH of these soils in their undisturbed state is pH 4 or more and may be neutral or slightly alkaline. These soils or sediments are invariably saturated with water in their natural state. This waterlogged layer may be peat, clay, loam or sand and is usually dark grey and soft.

PASS are not associated with environmental problems unless they are exposed to air. While the natural exposure of these soils or sediments to air (eg during severe droughts), is associated with the generation of acid, the acidity tends to occur in low frequency, low magnitude, short duration events after drought breaking rains (NHT 2001).

1.3 Where are acid sulfate soils found?

• In estuarine areas and coastal lowland areas such as mangroves, tidal flats, salt marshes and swamps, in deposits laid down within the past 10,000 years;
  
  
• In many wetland areas, particularly in the coastal region;
  
  
• In saline inland areas; and
  
  
• Near some mining operations.

Particular areas of concern in Western Australia include (though are not limited to):

• The south west of the State between Perth and Busselton, in estuarine, floodplain and wetland areas eg Peel-Harvey estuarine system and the Vasse River area;
  
  
• The northern parts of the State's coastline including the Pilbara and Kimberly coastlines;
  
  
• The Scott River Plain on the South Coast; and
  
  
• Some parts of the Wheatbelt where land salinisation has occurred.

In the Perth area, specific examples of disturbances of acid sulfate soils include sediments disturbed during bridge construction at the Garrett Road and Guildford bridges on the Swan River; disturbances in Stirling, Bassendean, Guildford and Bayswater; and exposure of acid sulfate soils in wetlands eg Lake Gnangara.

The identification of areas in Western Australia where there may be a risk of disturbing acid sulfate soils is further addressed in Section 2.3 and Appendix 2.

1.4 What types of development may cause acid sulfate soil problems?

Examples include:

• Coastal developments eg canal estates, marinas, golf courses and general urban development;
  
  
• Dewatering and drainage works associated with urban development, including permanent or temporary drainage or pumping of groundwater;
  
  
• Developments involving disturbance of wetlands and waterways eg artificially deepened lakes in public open space, and removal of peat;
  
  
• Infrastructure projects eg bridges, roads, dredging, port facilities and flood mitigation works;
  
  
• Rural drainage which lowers the water table;
  
  
• Mining and quarrying operations;
  
  
• Aquaculture eg prawn farms in mangrove communities; and
  
  
• Filling (filling has been identified as a problem in Queensland, since, in places, filling can compact saturated soils or sediments and/or laterally displace previously saturated sediments, resulting in groundwater extrusion and aeration of acid sulfate soils).

1.5 What are the potential impacts from disturbing acid sulfate soils?

The potential environmental, social and economic impacts that may be experienced in Western Australia include:

• Adverse changes to the water quality of the soil, groundwater (including shallow aquifers), wetlands, watercourses and estuaries;
  
  
• The deterioration of ecosystems and the ecosystem services associated with soils, groundwater, wetlands, watercourses and estuarine environments;
  
  
• Local and regional loss of biodiversity;
  
  
• The deterioration of significant conservation areas eg Ramsar wetlands;
  
  
• Increased risk of algal blooms and larger algal blooms (Weber);
  
  
• Land subsidence due to soil shrinkage when acid sulfate soils are drained (this can make farmland for example more prone to flooding and waterlogging);
  
  
• Conflict with activities that depend on healthy surface or ground water regimes, including commercial fishing, recreation and tourism activities;
  
  
• Loss of quality of groundwater and surface water sources 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 sulfate soils;
  
  
• Corrosion of concrete structures (concrete cancer) such as bridges, piles, pylons, floodgates, drainage pipes and sewerage lines;
  
  
• Corrosion of iron, steel and certain aluminium structures;
  
  
• Blockage of perforated plastic pipe drainage systems by iron precipitates;
  
  
• Loss of visual amenity from rust coloured stains, scums and slimes from iron precipitates, plant deaths, weed growth and increase in algae; and
  
  
• Costs to the community, in terms of financial outlays and the community's and government's time and effort in minimising impacts and repairing disturbed areas.

In the Perth region, acid sulfate soils problem have been triggered by the disturbance of wetlands, peat and the lower parts of waterways. The main concerns identified to date in the Perth region are as follows:

• Degradation of wetlands;
  
  
• Localised reduction in habitat, biodiversity and surface water quality of estuarine waterways;
  
  
• Reduction in the usability of groundwater for irrigation;
  
  
• Human health risks associated with arsenic, aluminium and other heavy metals contamination in surface and groundwater, and acid dust;
  
  
• Risk of long term infrastructure damage through the corrosion of sub-surface pipes and foundations by acidic water; and
  
  
• Invasion by acid tolerant water plants and dominance of acid tolerant plankton species.

1.6 National Strategy for the Management of Coastal Acid Sulfate Soils

The National Strategy identifies four principle objectives:

• Identify and define coastal acid sulfate soils in Australia. To assist landowners, coastal resources managers and planners to identify the areas of risk, an accurate environmental hazard assessment process at a catchment level and a reliable property assessment method are needed to help establish whether particular properties are at risk, and if so, the extent and severity of acid sulfate soils;
  
  
• Avoid disturbance of coastal acid sulfate soils. Undisturbed acid sulfate soils pose little problem for the environment. Avoidance of exposure of acid sulfate soils to air prevents environmental damage and obviates the need for expensive remedial works. Avoidance of problems is most likely to be achieved through a combination of education programs, well considered development and planning controls and promotion of best management practices;
  
  
• Mitigate (and manage) impacts when acid sulfate soils disturbance is unavoidable. If development has to occur on these soils it should be undertaken in a manner which ensures that there is no resultant acid water discharge into streams and waterways. Management presents difficulties and some risk. However expensive treatment technologies have enabled some major developments to occur; and
  
  
• Rehabilitate disturbed acid sulfate soils and acid drainage. Where past land use practices such as excavation and drainage have disturbed areas of acid sulfate soils, works to rehabilitate areas will be necessary to improve water quality and mitigate on-going adverse effects.

1.7 Statutory controls, and agencies that may provide advice

The land use planning process is arguably best positioned to ensure that new developments in areas prone to acid sulfate soils are appropriately managed to meet the community's planning objectives. The WAPC has prepared Planing Bulletin No. X: Acid Sulfate Soils to provide advice and guidance on matters that should be taken into account in the rezoning, subdivision and development of land that contains acid sulfate soils.

Proposed developments that may disturb acid sulfate soils may also be subject to the provisions of the Environmental Protection Act 1986 - the licensing provisions, the environmental impact assessment process, and/or the provisions of various environmental protection policies. Where pollution arising from the disturbance of acid sulfate soils has occurred, the general pollution prevention provisions of the Act may, depending on the circumstances, be applicable.

Groundwater extraction and dewatering in declared public drinking water source areas including underground water pollution control areas, water reserves and public water supply catchment areas declared under the Metropolitan Water Supply, Sewerage and Drainage Act 1909 and the Country Areas Water Supply Act 1947 are subject to licensing by DEWCP.

Development within and abutting the Swan River Trust management area generally requires the approval of the Swan River Trust. Similarly, other catchment management authorities eg the authorities for the Peel Inlet, Leschenault Inlet, Wilson Inlet and Albany Harbour areas determine some approvals in their area of jurisdiction.

At this stage, the Western Australian government department leading action on the issue is DEWCP. Advice on technical aspects of acid sulfate soils issues is available from DEWCP's Land and Water Quality Branch.

Other agencies that may be able to provide advice in relation to specific categories of developments include:

• Department of Agriculture Western Australia - for advice on agricultural land developments;
  
  
• Department of Fisheries - for advice on aquaculture projects; and
  
  
• Department of Mineral and Petroleum Resources - for advice on mining projects and the statutory controls applying to the removal of peat and other materials that are subject to the Mining Act 1978.
  
  
• Department of Conservation and Land Management - for areas that affect on conservation areas including wetlands.