INVESTIGATION OF SOIL AND GROUNDWATER ACIDITY, STIRLING
Summary & Recommendations
Response to Problems
INVESTIGATION OF SOIL AND GROUNDWATER ACIDITY, STIRLING
6.0 CONCLUSIONS AND RECOMMENDATIONS
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
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
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
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:
- to identify and define the extent of acid sulfate soils in coastal areas of Australia
- to avoid disturbance of coastal acid sulfate soils wherever possible
- to mitigate impacts when disturbance of these soils is unavoidable
- 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.