Decision Report
8. Risk assessment
8.7 Risk Assessment – Dewatering (applicable to construction only)
Works Approval: W6115/2018/1
IR-T04 Decision Report Template v2.0 (July 2017)
22 for the Risk Criteria and determined that the overall rating for the risk of ASS impacts on sensitive receptors during construction is medium.
8.7 Risk Assessment – Dewatering (applicable to construction
Works Approval: W6115/2018/1
IR-T04 Decision Report Template v2.0 (July 2017)
23 On review of the outcomes of the ASS Investigation and Management Plan, the preferred construction method was revised to consist of secant-pile, sheet pile, or similar water-tight construction to manage the extent of groundwater drawdown during excavation works. The extent of the extension of the cone of depression is estimated to be approximately 10% of the calculated cone of depression presented in Figure 7. Further construction details were
provided in the Preliminary Construction Plans (Water Corporation drawings MM06-3-1 and MM06-71-2). Detail on expected abstraction volumes and design requirements for the construction of sedimentation and infiltration basins to manage dewatering effluent are expected to be developed following determination of final construction methods.
Criteria for assessment
The Delegated Officer has considered the DER Guideline: Treatment and management of soil and water in acid sulfate soil landscapes (June 2015) (DER ASS Guideline) in considering the impacts of dewatering and ASS.
Applicant controls
It is noted that a CEMP will be developed to detail execution of the Management Plan and dewatering management measures required under the chosen construction method. It is noted that design requirements are not currently outlined for the construction of the
impermeable sedimentation basin, or the permeable infiltration basin. Therefore, DWER have assumed worst-case scenarios in the assessment of risk pertaining to dewatering effluent management. It is understood that infrastructure will be appropriately designed prior to construction.
On this basis DWER will only assess risks based on the application of controls outlined in the Management Plan, with consideration to preferred construction method.
As specified in the Management Plan, and Construction Plans, the controls proposed to manage impacts from dewatering operations are detailed in Table 15 below.
Table 10: Applicant controls for dewatering
Control Description Groundwater
drawdown management
The wet well construction will consist of secant piling and the
construction of all other excavations occurring below groundwater table (including valve pit, emergency storage tanks, thrust pit and inlet chamber and receival pit) will consist of sheet piling.
Construction methodology utilising secant piling and sheet piling, or use of alternative water-tight construction methodologies proposed by the contractor require approval by the Superintendent and Water Corporation.
Excavation and dewatering operations will be developed to minimise the vertical and lateral extent of groundwater drawdown.
Works Approval: W6115/2018/1
IR-T04 Decision Report Template v2.0 (July 2017)
24 Control Description
Sedimentation and Infiltration basins
All dewatering effluent will be aerated and discharged to an impermeable sedimentation basin for treatment and to capture aluminum and iron oxide precipitation post-abstraction. The impermeable basin will be lined with lime to neutralize pH. Water exiting the sedimentation basin will undergo treatment and testing and be subject to action criteria and contingency measures prior to
discharge.
Treated effluent will be discharged to a permeable basin to allow infiltration of treated effluent into groundwater.
Dewatering effluent treatment
Effluent in the infiltration basin will be lime dosed and monitored. In the event that total acidity or pH are exceeded, the water will be treated through automated lime dosing for additional water treatment.
In the event that parameters for pH (6 or 8.5) or total acidity (40 mg/L as CaCo3) are triggered, corrective action will be undertaken, likely by way of using a lime dosing plant. In the event that water quality cannot be brought back into acceptable levels, dewatering will cease until such a time that treatment can achieved the relevant criteria.
Dewatering Monitoring Program
Dewatering effluent
Dewatering effluent will have daily field recordings of pH, electrical conductivity, total titratable acidity and total alkalinity.
Weekly analysis will be undertaken for total titratable acidity, total alkalinity and pH for effluent prior to being infiltrated.
Broz Lake and Helena River monitoring
Baseline surface water data, including water levels, will be obtained prior to construction works commencing.
Daily field monitoring of Broz Lake and the Helena River for pH, electrical conductivity, redox potential, dissolved oxygen, total dissolved solids, total titratable acidity and total alkalinity will be undertaken during dewatering activities.
Weekly analysis of the following parameters will also be undertaken: total acidity, total alkalinity, sulfate, chloride, total aluminum, total iron,
ammoniacal nitrogen, total nitrogen, total phosphorus, filterable reactive phosphorus and dissolved (filtered) metals (aluminum, arsenic, cadmium, chromium, iron, manganese, nickel, selenium and zinc)
Bi-monthly monitoring for six months after completion of dewatering will be undertaken.
Groundwater monitoring bores
Three groundwater monitoring bores will be installed at the Premises.
Baseline groundwater data will be collected prior to commencing works.
Daily field monitoring of the groundwater monitoring bores for pH, electrical conductivity, redox potential, dissolved oxygen, total titratable acidity and total alkalinity throughout the period of dewatering and will continue until demonstrated that normal groundwater elevations have been returned.
Submission of groundwater samples from the groundwater monitoring bores weekly (during dewatering operation) for analysis of total titratable
Works Approval: W6115/2018/1
IR-T04 Decision Report Template v2.0 (July 2017)
25 Control Description
acidity/alkalinity, total dissolved solids, sulfate, chloride, dissolved (filtered) metals (aluminum, arsenic, cadmium, chromium, iron, manganese, nickel, selenium and zinc), ammoniacal nitrogen, total nitrogen, total phosphorus and filterable reactive phosphorus.
After completion of the works, groundwater field monitoring and laboratory analysis will continue quarterly for six months.
Sediment
Sediment that forms in the sedimentation or infiltration basins will be sampled and analysed prior to decommissioning of the basin. If suitable, it will be reused onsite. If it is unsuitable, it will be removed offsite to an appropriate disposal facility.
Management Actions In the event that groundwater parameters differ from the baseline groundwater data by ±10%, a re-test will be undertaken within 24 hours. If the re-sampled data is still ±10% from baseline data, ground disturbing works will be stopped until an appropriate management or corrective action can be discussed with the Department.
Action criteria adopted by the Applicant is shown below in Figure 8.
Works Approval: W6115/2018/1
IR-T04 Decision Report Template v2.0 (July 2017)
26 Figure 8: Action criteria (taken from DER ASS Guideline)
Works Approval: W6115/2018/1
IR-T04 Decision Report Template v2.0 (July 2017)
27 Key findings
The Delegated Officer has reviewed the information regarding dewatering impacts from the Premises and has found:
1. Dewatering at the Premises has the potential to expose ASS/PASS to oxygen.
2. The preferred construction method (installation of secant or sheet piling or equivalent water-tight methods) is considered to minimise the exposure of ASS/PASS to oxygen by decreasing the size of the cone of depression.
3. Controls for dewatering proposed in the Applicant’s Management Plan are predominantly aligned with the DER ASS Guideline.
4. Design requirements for the sedimentation basin and infiltration basin are not provided with the Application, it is understood that these will be completed as part of the final construction plans.
5. The proposed monitoring program outlined in the Applicant’s Management Plan are also predominantly aligned with the DER ASS Guideline.
6. It is noted that groundwater well design and construction methods are not provided in the Management Plan.
Consequence
Based upon the investigation conducted and the current quality of groundwater in the vicinity of the Premises, the Delegated Officer has determined that the impact of dewatering may have mid-level on-site impacts. Therefore, the Delegated Officer considers the consequence to be moderate.
Likelihood of consequence
Based upon the dewatering controls proposed by the Applicant and considering the close proximity of ground and surface water receptors, the Delegated Officer has determined that the likelihood of impacts to waters and land from dewatering and treatment of dewatering effluent could occur at some time. Therefore, the Delegated Officer considers the
consequence to be possible.
Overall rating
The Delegated Officer has compared the consequence and likelihood ratings described above for the Risk Criteria and determined that the overall rating for the risk of dewatering impacts on environmental receptors during construction is medium.