Decision Report
8. Risk assessment
8.6 Risk Assessment – Acid Sulfate Soils discharges to the environment (applicable to construction only)
Based upon the sensitivity of the environment, the Delegated Officer has determined that the impact of dust on surface water quality during construction may have mid-level off-site impacts with the relevant assessment criteria at risk of not being met. Therefore, the Delegated Officer considers the consequence to be major.
Likelihood of consequence Public health
Based upon the very close proximity to receptors and considering the proposed short duration of the works and use of water for dust suppression purposes, the Delegated Officer has determined that dust impacts on residences may occur in some circumstances. Therefore, the Delegated Officer considers the likelihood to be possible.
Environment
Based upon the close proximity to surface water, the proposed short duration of the works and the use of water for dust suppression purposes, the Delegated Officer has determined that major dust impacts on surface water quality would probably not occur in most circumstances.
Therefore, the Delegated Officer considers the likelihood to be unlikely.
Overall rating Public health
The Delegated Officer has compared the consequence and likelihood ratings described above for the Risk Criteria (Table 11) and determined that the overall rating for the risk of dust
impacts on sensitive receptors during operation is medium.
Environment
The Delegated Officer has compared the consequence and likelihood ratings described above for the Risk Criteria (Table 11) and determined that the overall rating for the risk of dust
impacts on sensitive receptors during operation is medium.
8.6 Risk Assessment – Acid Sulfate Soils discharges to the environment (applicable to construction only)
General hazard characterisation and impact
Construction of the proposed pumping station will require a total of just under 1,000m3 of soil to be excavated. ASS have been identified on the site at levels exceeding DER ASS Guideline action criteria. Based on this, construction activities have the potential to expose and disturb ASS during excavation and dewatering.
ASS and PASS are naturally occurring soils and sediments containing iron sulfides, most commonly pyrite. When ASS are exposed to air, the iron sulfides in the soil react with oxygen
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20 and water to produce a variety of iron compounds and sulfuric acid, and hydrogen sulfide gas release that could affect the health and amenity of residents in the area. The resulting acidic conditions can modify the solubility of other substances, including heavy metals, which have the potential to impact on water quality within the surrounding environment.
Exposure of ASS during construction of the proposed pumping station has the potential to impact groundwater, resulting in disruption to existing bore users, vegetation stress, and reduction in water quality. Acidification caused by excavation or dewatering may result in elevated concentrations of acidity and dissolved metals, which may impact the quality of surface water in Helena and Swan River systems. This may affect the ecology of the river systems including the potential to impact on riparian vegetation and other flora or fauna dependent on the river ecosystems.
Criteria for assessment
The Delegated Officer has considered the following Department published guidance in assessing the risk of ASS at the Premises and the appropriateness of controls for ASS.
Identification and investigation of acid sulfate soils and acidic landscapes, June 2015
Treatment and management of soil and water in acid sulfate soil landscapes, June 2015
Acid Sulfate Soils Fact Sheet: Managing urban development in acid sulfate soil areas, July 2015
Applicant controls
A Management Plan was submitted by the Applicant as part of the works approval application.
Further to the completion of the Management Plan, Preliminary Construction Plans were produced by Wood & Grieve Engineers (Water Corporation drawings MM06-3-1 and MM06- 71-2) which detail the proposed construction approach. The final construction method is yet to be decided, and will modify the expected volumes of ASS to be disturbed by excavation and dewatering activities.
The Management Plan and Preliminary Construction Plans specify that excavated soils are to be treated prior to disposal to an offsite facility. However, DWER is of the understanding that the chosen construction approach may consider reuse of treated ASS to backfill excavations if soil properties are favourable. It is noted that a CEMP will be developed to detail execution of the Management Plan and construction requirements under the chosen construction method.
On this basis DWER will only assess risks based on the application of controls outlined in the Management Plan, with consideration to the preferred construction method.
The controls that have been proposed to minimise impacts from excavation and treatment of ASS during construction are outlined below in Table 14.
Table 9: Applicant controls for ASS Control Description
Limestone pad Soil excavated from the PASS/ASS area will be identified and separately stockpiled away from the non-ASS material.
Stockpiled ASS/PASS will be limited to approximately 300m3 and will be stored on a 300mm compacted limestone base constructed with 150mm limestone perimeter bunds.
Lime treatment Soil excavated from ASS/PASS areas will be treated with aglime or an equivalent
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neutralising agent within 18 hours of excavation and removed offsite to an appropriate disposal facility within this timeframe.
Treatment stockpiles will be limited to 200m3 in volume and will be segregated according to ASS risk.
The treatment pad will be tested and decommissioned post-construction.
Removal or Re- use
The treated soils will be taken to an appropriate landfill facility.
Validation Monitoring
The Applicant has committed to sampling treated soil at four times the rate required in the Department’s Landfill Waste Classification and Waste Definitions 1996 which sets a minimum of four samples to be taken per 100 - 200m3.
If test results for pHFIELD and pHFOX are >6.5, the ASS is assumed to be effectively neutralized. In the event that results are <6.5, additional treatment followed by validation testing will occur.
Monitoring Program
Groundwater and surface water will be monitored during and post-construction.
Details of the proposed monitoring program are outlined in section 8.7.3.
Key findings
The Delegated Officer has reviewed the information regarding ASS impacts from the Premises and has found:
1. Field testing and laboratory testing results are indicative of ASS being present at the Premises which supports the risk identified in the Department’s mapping software.
2. There is a high likelihood that ASS or potential ASS will be encountered during the construction of the WWPS.
3. ASS impacts can be appropriately managed if treated in accordance with DER’s Guideline: Treatment and management of soil and water in acid sulfate soil landscapes (June 2015) and as per the above proposed controls with the exception of re use of treated soil to backfill excavation. It is expected that management of ASS for reuse will be detailed in the CEMP.
Consequence
Based upon results from the site investigation and the nature of sensitive receptors, the Delegated Officer has determined that mid-level onsite impacts and low level local offsite impacts may be associated with ASS. Therefore, the Delegated Officer considers the consequence to be moderate.
Likelihood of consequence
Based upon the proposed management controls and the duration of the works, the Delegated Officer has determined that ASS impacts will probably not occur in most circumstances.
Therefore, the Delegated Officer considers the likelihood to be unlikely.
Overall rating
The Delegated Officer has compared the consequence and likelihood ratings described above
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22 for the Risk Criteria and determined that the overall rating for the risk of ASS impacts on sensitive receptors during construction is medium.