Decision Report
8. Risk assessment
8.8 Risk Assessment – Wastewater (applicable to operations only)
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.
Works Approval: W6115/2018/1
IR-T04 Decision Report Template v2.0 (July 2017)
28 Groundwater feeds Broz Lake and there is an assumed hydrological connectivity between groundwater and the Helena River located approximately 300m downgradient from the proposed dewatering activities. This overflow has potential to infiltrate groundwater and subsequently into surrounding surface water bodies including the Helena River which discharges into the Swan River.
Overflow wastewater emissions may result in reduced water quality, impacts on ecosystems including potential impacts to flora and fauna, as well as potential public health and amenity impacts.
Criteria for assessment
Although the Premises is not located within or near a Public Drinking Water Source Area, groundwater below and within the vicinity of the site is considered to be fresh which has a beneficial use for drinking water and/or domestic use. The Department of Health’s Non- Potable Groundwater Use (NPUG) guidelines (2014) may be considered as an assessment criteria for this Premises.
The ANZECC & ARMCANZ (2000) Guidelines for Fresh Waters (ANZECC) may also be considered as an assessment criterion to assess the potential impact on groundwater and surface water.
Applicant controls
The Applicant has proposed the following controls for potential wastewater overflow discharges.
Table 11: Applicant controls for wastewater discharges
Site Infrastructure Description Operation details
WWPS Emergency overflow storage
(existing storage tanks and proposed underground emergency storage tanks) Concrete well liners, concrete base, with a concrete cover slab and will be completely underground.
270 kL capacity (based on 25 L/s flow for a minimum 3 hours)
Existing emergency overflow basin
After the emergency storage has reached maximum capacity, the wastewater will overflow into the existing overflow discharge basin.
Overflow from this basin will then enter the existing stormwater drainage system on Lakeside Drive which eventually discharges to the surface water body within Broz Park.
Monitored alarm system
The pumping station will be fitted with a 24 hours day, 7 days a week monitored alarm system to alert the Applicant of any malfunctions or abnormalities or fault
The system has the following different alarms which assist in alerting key personnel through a hierarchy of incident warnings:
Control cabinet open without
Works Approval: W6115/2018/1
IR-T04 Decision Report Template v2.0 (July 2017)
29 Site Infrastructure Description Operation details
response. authorisation alarm.
Power failure alarm.
Pump run with no flow alarm.
Pump failure alarm.
High level alarm (sewage reaches invert level of lowest incoming gravity sewer).
Extra high level Alarm (sewage reaches 150mm above invert of lowest incoming gravity sewer).
Flow to underground storage alarm.
Underground storage half full alarm.
Overflow to environment imminent Alarm
Maintenance staff are on standby at all times and can attend incidents as required.
Consequence Residences
Based upon the sensitivity of the surrounding residential community, residences may be impacted by mid-level impacts to amenity. Therefore, the Delegated Officer considers the consequence to be moderate.
Environmental receptors
Based upon the sensitivity of the surface water receptors including the Helena and Swan Rivers, the Delegated Officer has determined that the impact of wastewater overflows may cause high level onsite impacts with mid to low level offsite impacts. Broz Lake is groundwater fed which may result in discharges to groundwater. The consequence criteria may be
exceeded in an overflow for both surface water and groundwater. Therefore, the Delegated Officer considers the consequence to be major.
Likelihood of consequence Residences
Based upon the Applicant’s proposed controls and considering proximity to receptor, the discharge of wastewater overflow is only expected to occur in exceptional circumstances.
Therefore, the delegated officer considers the likelihood to be rare.
Environmental receptors
Based upon the Applicant’s proposed controls and considering proximity to receptor, the discharge of wastewater overflow is only expected to occur in exceptional circumstances.
Therefore, the delegated officer considers the likelihood to be rare.
Overall rating Residences
The Delegated Officer has compared the consequence and likelihood ratings described above
Works Approval: W6115/2018/1
IR-T04 Decision Report Template v2.0 (July 2017)
30 for the Risk Criteria and determined that the overall rating for the risk of wastewater impacts on human receptors during operation is medium.
Environmental receptors
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 wastewater impacts on sensitive receptors during operation is medium.