Summary & Recommendations



Response to Problems









Identification of Problem and Immediate Response

Concern about acidic groundwater in Stirling was raised in mid-December 2001 by a concerned resident whose garden was dying. The resident contacted the City of Stirling, and preliminary testing by the City of Stirling indicated that the groundwater was extremely acidic and contained high metal concentrations. The City of Stirling contacted the Swan Regional Office of the Water and Rivers Commission on 20 December 2001, and the Commission provided technical advice to Stirling on 21 December 2001 that the problem was likely to be due to the disturbance of acid sulfate soils.

The City of Stirling invited officers from the Commission to a meeting and site inspection about the issue on 7 January 2002 and this indicated the scale of the problem. On 10 January 2002 the Department of Health (DOH) released a press release about acidity and the City of Stirling delivered letters warning of the issue to householders in an area that could be affected by groundwater acidity. The letter also indicated that the Water and Rivers Commission would undertake groundwater testing to determine the extent of the acidity problem.

Further testing was carried out on the initial affected bore for arsenic on 14 January 2002 as this contaminant was not originally sampled and commonly occurs in groundwater in areas affected by acid sulfate soils. The results of chemical analysis indicating a high arsenic concentration were received by the Commission on 29 January 2002. This was immediately referred to DOH. This agency put out a media release about the detection of arsenic on 1 February 2002.


Since January 2002 a major program of investigation and testing has been undertaken. Preliminary groundwater sampling indicated that arsenic and heavy metal concentrations of possible health concern were only found where groundwater was acidic (less than pH 5.5), and therefore measurement of pH was used a screening tool to select bores that required chemical analysis for arsenic and heavy metals.

A total of 802 domestic bores were screened for pH, and 49 bores were sampled for arsenic and heavy metals. Of these bores, 22 were found to have arsenic concentrations that exceeded the Australian drinking water guideline value (7µg/L).

Between 14 and 22 February, the Commission installed 13 investigation bores to determine possible sources of acidity and arsenic contamination. The drilling indicated that the major sources of contamination were new residential developments on the Roselea and Hamilton Lake estates and excavated wetlands in public open space on Spoonbill Reserve. Drilling and groundwater sampling downgradient of peat stockpiles and excavated lakes indicated that groundwater was acidic to 3-5 metres below the watertable but that pH values returned to background values of 6 to 7 at greater depths.

Vegetable, fruit and soil samples were collected from 11 premises to determine whether crops were accumulating heavy metals and arsenic from soil. None of the plant samples exceeded the Australian food standard for arsenic, but 14 samples exceeded the standard for lead. Lead in food is unlikely to be associated with groundwater acidification but the Department of Health is continuing to investigate this issue.

Of a total of 20 peat samples taken from peat stockpiles on development sites, nine indicated that the material had a moderate to high acid generation potential, suggesting that the peat is the major source of acidity in the area.

Causes of the Acidity and Arsenic in the Groundwater

There are several factors that have contributed to groundwater acidity in the area. These include:

  • Dewatering - the water table in the development areas was lowered by as much as 6 metres to allow the excavation and removal of peat which was unsuitable for housing foundations. Additional dewatering was carried out in the area to construct sewerage infrastructure.

  • Peat stockpiles - peat excavated from the development areas was stockpiled before removal. Some of this material had a high acid generation potential, and visual inspection of the stockpiles indicated the presence of large amounts of jarosite and other sulfate salts.

  • Peat fire - a fire burning within peat in the development area for about three weeks is also likely to have contributed to oxidation of sulfide minerals.

  • Excavated wetlands - lakes in public parks had been created by excavating soils to the water table and stockpiling the spoil in the centre of the lakes to create islands. Many shallow bores downgradient in the direction of groundwater flow from these lakes had low pH values and high arsenic concentrations.

  • Climatic factors - Perth has had a long succession of dry winters, and the winter of 2001 was exceptionally dry. The water table during the summer months of 2001/2002 was lower than average, and many shallow bores ran dry due to the combined effects of the dry weather and dewatering.

Extent of Acidity at Stirling

Investigations to date show that groundwater acidity issue is restricted to 49 domestic bores immediately downgradient in the direction of groundwater flow from the Roselea and Stirling Lakes residential estates, and the Spoonbill Reserve.

Public Health and Environmental Implications - Test results discussion

Arsenic and heavy metal concentrations in groundwater in the area are unlikely to have a significant effect on public health as groundwater in the area is mostly used for irrigation and not for drinking. The Department of Health recommends that water from domestic bores in the metropolitan area is not used for drinking, mainly because of the risk of bacterial contamination.

Although some Water Corporation water supply bores occur in the area, these pump water from deep in the aquifer, and ongoing monitoring indicates that these are unaffected by contamination.

Immediate Corrective Action

The stockpiling of peat on the Stirling Lakes estate is in breach of the dewatering conditions for the site, and the proponent has been directed to remove the material. Under Section 73 of the Environmental Protection Act, proponents on the Roselea estate have been served with notices to remove the stockpiled peat from the area. It is proposed that proponents on the Stirling Lakes estate be served with a Section 73 notice shortly.

History of Regulatory actions

Roselea development was subject to Water and Rivers Commission groundwater abstraction permit and licence to cover the company's dewatering operations that included the submission of a Sampling and Analysis Plan for approval. Monitoring results between December 1999 to April 2001 did not indicate adverse changes in the groundwater condition during the active dewatering periods. Although some metals such as aluminium, zinc and cadmium were occasionally detected in the groundwater, it did not however, impact on the receiving surface waters. Since that time no further monitoring results have been received. Dewatering operations ceased in August 2001. Roselea's groundwater permit expired on 17 December 2001. An application has been received for renewal and is currently being reviewed.

In May 2001 several complaints were received by the Department of Environmental Protection about the peat stockpile fire at the Roselea Estate. Investigation revealed that the peat was smouldering for about three weeks impacting on the local residents with offensive odour and smoke.

Stirling Lakes was granted a groundwater permit on 26 May 2000 with commitment from the developer (MM Development) to maintain water quality standards (ANZECC, 1992) including the removal of peat as soon as practicable following extraction. Monitoring results indicated little variation in pH values between June 2000 and May 2001.

Departmental fax on 11 September 2000 to Welltech (Dewatering Operator) indicated that the monitoring results showed marginal decline in the groundwater pH and slight elevation of zinc and lead contamination. Regular lime dosing was undertaken to overcome the problem.

Statutory Approvals for Developments

The assessment of new urban developments is the joint responsibility of the Department of Environmental Protection (DEP), the Environmental Protection Authority (EPA), the Western Australian Planning Commission (WAPC), and local government. The bulk of land development proposals are informally assessed by the EPA (DEP under delegation) on the basis of competent and responsible developers ensuring developments are environmentally acceptable. The system recognises the degree of management that should be done by the government agencies while not subsidising from the public purse the development industry which reaps the benefits of projects. Currently there are about 965 informal assessments per year out of 1000 proposals referred to the EPA (ie EPA formally assesses 35 proposals a year). The EPA/DEP receives about 160 subdivision referrals per year and 350 rezoning proposals. Currently urban development approvals are not referred to WRC, although this system will change as part of the formation of the Department of Environment, Water and Catchment Protection.

Future Management of Acid Sulfate Soil Exposures

The significance of acid sulfate soils is widely understood in the literature and is relevant on the eastern seaboard. Historically, however, this issue has not been seen as critical in urban Perth. As noted above, responsibility for environmental management of urban development rests across state and local governments and developers. The Stirling groundwater acidity and arsenic contamination issue highlights the need for state planning and environmental approvals processes and the management actions of developers to better identify and respond to the potential for acid sulfate soils exposure.

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 in WA. The soil characteristics in the Stirling area are not typical of acid sulfate soils found in other states. Further research is required to properly identify the characteristics for acid forming soils in WA. Issues such as the groundwater acidity problem in Stirling could be effectively managed 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.

In February 2002, the Water and Rivers Commission conducted a desktop study of areas with potential for acid sulfate soils (PASS) in the Perth metropolitan area. This indicative study is a first pass only to guide future detailed mapping of PASS and ASS. DEWCP will now write to each metropolitan local government authority advising them of the potential for sulfate soils presence in their areas asking that the issue be taken into consideration for any future residential developments. DEWCP will also write to the WA Planning Commission, Urban Development Institute of Australia (WA) and Association of Consulting Engineers (WA) ensuring they are aware of the issue and that it is considered as part of their deliberations.

Conclusion and Recommendations

The acidification of groundwater in Stirling presents a considerable management challenge for the State. It is manageable, provided State and local governments and developers act on recommendations contained in this report. Until recently, acid sulfate soils have not been a significant issue in Western Australia but the episode in Stirling provides a serious wake-up call to lift the profile of the issue and our capacity to respond to it.

The following measures are recommended to ameliorate the groundwater acidity and arsenic contamination issue in Stirling:

  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 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.

  2. Backfilling of acid surface water bodies - It is recommended that options for remedying the conditions of the acidic water bodies on Spoonbill Reserve and the excavated surface water bodies on the Roselea estate be investigated immediately. A practical option is to backfill the affected lakes and water bodies with alkaline material (such as crushed limestone).

  3. Removal of peat stockpiles - Peat stockpiles should be removed from the site to prevent further acid generation on the Roselea and Stirling Lakes estates. The material should be treated to prevent formation of acid conditions and safely disposed of.

  4. Water table management - Groundwater pumping in the area should be managed to minimise water table declines to prevent ongoing acidification. 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.

  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. 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.

  7. Assessment of drainage impacts on 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.

    Further general recommendations for future management

  8. The State should develop and implement a Statement of Planning Policy on Acid Sulfate Soils modelled on those developed in other states.

  9. It is recommended that Western Australia adopts and implements measures set out in the National Strategy for the Management of Coastal Acid Sulfate Soils.

  10. The State should support the National Coastal Mapping project for ASS proposed by the National Coastal Acid Sulfate Soil Committee (NatCASS).

  11. It is recommended that the EPA should review its informal review process to ensure issues such as acid sulfate soils are highlighted as part of management requirements for developers.

NOTE: Recommendations 8 through 10 are unfunded and cannot be implemented with current departmental resources.

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