Summary & Recommendations



Response to Problems









6.1 Stirling acidity issue

The contamination is most extensive down-gradient in the direction of groundwater flow (southwest) from the Roselea development and south-west of excavated lakes on Spoonbill Reserve. Contamination caused by activities on the proposed Stirling Lakes residential estate is much more localised.

Without adequate management, acidic groundwater and drainage from the site has the potential to continue to affect groundwater usage from shallow bores, affect the environmental values of the conservation-category Herdsman Lake, and pose a threat to the long-term integrity of subsurface urban infrastructure in the area (concrete and steel footings, pipelines etc.). The inappropriate use of bores contaminated by arsenic (use for drinking, children playing under sprinklers) could also continue to pose a risk to public health. The following measures are recommended to ameliorate these potential impacts:

6.1.1 Development of Acid Sulfate Soil Management Plans.

It is recommended that no further dewatering or peat excavation takes place on the Roselea or Stirling Lakes estates until the proponents have developed an acid sulfate management plan and have demonstrated to the satisfaction of planning and environmental regulatory authorities that the distribution of Potential Acid Sulfate Soil (PASS) has been identified and that management measures are in place to prevent further groundwater quality impacts. Western Australia does not currently have guidelines for the assessment and management of acid sulfate soils, so it is recommended that guidelines developed by the Queensland Department of Natural Resources are used as these incorporate best management practices and, together with similar New South Wales guidelines, are de facto national standards. The Queensland assessment and management guidelines form part of a State Planning Policy (SPP) for acid sulfate management for that State, and are available as PDF files at the following web site:

6.1.2 Remediation of acidic surface water bodies

Without ongoing treatment with lime or other management measures, the acidity and metal contamination within the Spoonbill lakes and excavations to the water table on the Roselea estate will continue to present a public health risk, particularly to young children. They will also continue to act as a source of groundwater acidity and arsenic contamination. These water bodies currently have limited environmental values, and it is recommended that options for remedying acidic water bodies on Spoonbill Reserve be investigated immediately and include measures to retard any further pyrite oxidation. A practical option is to backfill the affected lakes and impacted water bodies with alkaline materials (such as crushed limestone). Soil around the lakes in the Spoonbill Reserve may be contaminated with high concentrations of heavy metals, and the islands in the centre of the lakes may have been formed from excavated soil that could contain pyrite. It is recommended that the extent and severity of metal contamination in this material is assessed using contaminated soil guidelines developed by the Department of Environmental Protection, and that contaminated soil is excavated and disposed of at a suitable landfill site or in an acceptable manner approved by the regulatory agency.

6.1.3 Removal of peat stockpiles

The peat stockpiles on the Stirling Lakes and Roselea Estates contain material with a moderate to high acid generation capacity and pose an ongoing risk to groundwater quality downgradient (south-west) of the stockpiles. It is recommended that this material is removed from the area as soon as is practicable to mitigate this risk. This action has been initiated by the Department of Environmental Protection.

Without adequate lime dosing (at least 1.5 times the acid generation capacity) the peat will be unsuitable for use in agriculture due to the risk of generating excessive soil acidity. Much of material will also be unsuitable for use in gardens due to its high heavy metal content and so disposal at a suitable landfill site may be the only disposal option. The New South Wales EPA recommends the following measures are adopted when disposing of acid sulfate soils in landfill sites:

  • The status of the waste ASS (potential or actual ASS) should be determined using the assessment techniques outlined in acid sulfate soil assessment guidelines before disposal is considered.

  • Potential and actual ASS must be treated by the generator before acceptance by a landfill occupier for disposal. Treatment should be undertaken in accordance with the neutralising techniques outlined in acid sulfate soil assessment guidelines.

Landfill operators should consider the following points when accepting ASS for disposal in a landfill:

  • Significant amounts of waste ASS should be managed within a discrete cell (that is, a lined monocell) of a landfill. This will ensure that any potential acidic leachate generated by waste ASS that may not be fully neutralised by the above treatment can be controlled to reduce the likelihood of such leachate coming into contact with other types of waste.

  • Special care should be taken to ensure that contaminated, hazardous or industrial wastes are not in the vicinity of the ASS.

  • ASS must not be used as a cover material, as it may oxidise and produce highly acidic leachate.

6.1.4 Water table management

Groundwater acidity problems may continue to occur in residential areas adjacent to the Roselea and Stirling Lakes estates unless management measures are implemented to control the extent to which the watertable is lowered by groundwater pumping and seasonal factors in the area. Management measures that could be implemented include:

  • Determine the vertical distribution of potential acid sulfate soils in the area to allow maximum extent that the water table can be drawn down by pumping before additional acidity may be caused by the exposure of sulfide minerals to air. This work is proposed as part of a proposed project to be undertaken by a masters student in the area.

  • Install monitoring and aquifer testing bores to determine how the water table in the area responds to different rates of groundwater pumping and to variations in winter rainfall. Ongoing monitoring of water levels in monitoring bores would indicate when the water table has declined to the point that further acid generation is possible.

  • Develop a flexible and adaptive water allocation policy for domestic and council bores in the area.

  • Ensure that any drains in the area are not excavated into potential acid sulfate soils below the water table. Broad, shallow drains are preferable to deep, narrow drains.

It is recommended that the Water and Rivers Commission instigate the development of a groundwater management strategy for the area in consultation with the City of Stirling, adjoining developers and local residents to prevent further groundwater acidification occurring. Any further development would need to be consistent with the groundwater management strategy.

6.1.5 Rehabilitation and monitoring of affected domestic bores

A combination of climatic variation and peat disturbance has contributed to the acidification of groundwater. Domestic bores in the immediate area that are affected by acidity should be remediated such as by bore deepening. It is recommended that the adjoining developers and the City of Stirling consult with the affected bore owners in this regard. It is also recommended that the City of Stirling facilitate ongoing monitoring of pH to ensure that new bores are not affected by groundwater acidity. Bores with pH values less than 5.5 should be sampled by the City of Stirling for arsenic and heavy metal content with advice from the Water and Rivers Commission.

6.1.6 Assessment of the effects of acidity on urban infrastructure

The presence of acid sulfate soils in the vicinity of the Roselea and Stirling Lakes estates may present a long term threat to sub-surface infrastructure. It is recommended that the owners of sub-surface infrastructure carry out an assessment of any current and potential impacts of acidity on infrastructure in the area.

6.1.7 Protecting Herdsman Lake from acidic drainage

Drainage from the vicinity of the Roselea and Stirling Lakes is carried in drains that discharge into Herdsman Lake and has the potential to affect this conservation-category wetland. The acidity of drainage downstream of the Roselea and Hamilton Lake estates is currently being moderated by the use of limestone in drains, but this is likely to only be a short-term strategy. Additionally, the neutralisation of the acid drainage is generating highly turbid water containing flocs of aluminium and iron hydroxides that may also cause environmental effects in Herdsman Lake.

It is recommended that the Water and Rivers Commission, in consultation with the City of Stirling and the Water Corporation, undertake a risk assessment of the effects of drainage from the vicinity of the Roselea and Hamilton Lake estates on Herdsman Lake and implement appropriate management measures to protect this wetland.

6.2 State planning and environmental approvals issues

The groundwater acidity and arsenic contamination issue in Stirling has resulted from the inadequate consideration of acid sulfate soil issues in current State planning and environmental approval processes. This does not absolve developers from ensuring that developments are environmentally acceptable or suggest that informal reviews are not the appropriate mechanism for dealing with land developments. However, unlike many other states, Western Australia does not have a State Planning Policy that specifically addresses acid sulfate soils, and consequently there is an ongoing risk that sites with acid sulfate soils will be developed inappropriately within WA. Issues like the groundwater acidity problem in Stirling could be largely eliminated if WA were to develop and adopt an acid sulfate SPP modelled on Queensland and New South Wales policies. This would ensure that acid sulfate soils are considered at a very early stage in a development program before soils are disturbed and acidity problems are created. It is also recommended that Western Australia adopts and implements measures set out in the National Strategy for the Management of Coastal Acid Sulfate Soils (available at web site ).

The principal objectives of the National Strategy are:

  1. to identify and define the extent of acid sulfate soils in coastal areas of Australia
  2. to avoid disturbance of coastal acid sulfate soils wherever possible
  3. to mitigate impacts when disturbance of these soils is unavoidable
  4. to rehabilitate environmental impacts caused by the disturbance of acid sulfate soils.

In Western Australia, the first objective is particularly important as there is currently only a very general understanding of the distribution of acid sulfate soils within the State. Queensland and New South Wales have produced detailed acid-sulfate risk maps for coastal areas, and a similar mapping exercise in Western Australia could identify areas where inappropriate development may cause acidity problems.

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