Decision Report

Definitions of terms and acronyms

Minister The Minister responsible for the EP Act and related regulations Noise Regulations The Protection of the Environment (Noise) Regulations 1997 (WA). Premises refers to the premises to which this Decision Report applies, as specified on the front of this Decision.

Purpose and scope of assessment

Application details

Phase B: Phase B works include revetment of levees on the Secondary Dam (WWP2) and Maturation Dam (WWP3), upgrades to weirs, and installation of a final rock filter at WWP3 outlet. The works also include the relocation of the chlorination unit to the WWP3 outlet (refer to figure 2 below). Phase A and Phase B renovation works and the associated activities when the plant is in operation.

Figure 2: Secondary and Tertiary pond upgrade

Background

Overview of Premises

Operational aspects

The IDEA plant was built in 2001 by Water Corporation and Simon Engineering under a Design, Build & Operate (DBO) contract. Cross-linked sewage received on site during a power outage or exceeding the hydraulic capacity of the IDEA plant (i.e. peak flows during storm events) will also be routed to Wastewater Pond Lagoon 1. handled by the IDEA factory according to current operations.

Cat 61 tanker waste streams of approximately 5 m3/day will be diverted to the IDEA plant for the duration of Phase A works. Prior to commencement of Phase "A" works, the license holder will be required to submit an application for license amendment to DWER requiring disposal of Category 61 waste directly at the IDEA plant. During the renovation phase of lagoons 2 and 3, 80% of the wastewater inflow will be directed to the IDEA plant and 20% (including trade waste) to the renovated pond 1 lagoon, under normal operation.

A temporary pumping station will be installed to transfer partially treated pond effluent to the IDEA plant for dilution with IDEA effluent. A Compliance Report for Phase "A" Works and a Permit Amendment are required to prescribe the Phase A infrastructure as per the application before the partially treated effluent from Pond Lagoon 1 is routed to the IDEA facility.

Figure 3: Historic WWTP inflows, 2011 - 2019 Flows during planned works

Infrastructure

Legislative context

Part V of the EP Act

The property has a history of shallow groundwater detected within the existing groundwater monitoring drilling network. Permit conditions 4.1.1 (IR1) and 4.1.1 (IR2) were added to the permit in 2016, requiring the permit holder to carry out an assessment of the seepage rate of the main wastewater containment infrastructure on site. The results showed that approximately 16.8% of the total sewage flows leaked from the IDEA installation and approximately 15% from the ponds.

Consultation

Location and siting

Siting context
Residential and sensitive receptors
Specified ecosystems
Groundwater and water sources
Soil type
Meteorology

It is possible that the containment/treatment infrastructure leaks and affects the local groundwater regime. Specified ecosystems are areas of high conservation value and special significance that may be affected as a result of activities on or emissions and discharges from the site. Hannan's Lake The nearest major natural body of water is Hannan's Lake, a normally dry ephemeral lake immediately east of the site.

Area of public drinking water supply The facility is not located in the area of public drinking water supply (OBJVV). Based on the Kalgoorlie 1:100,000 map series, the geology beneath the PPCN is mapped as predominantly colluvium (map unit: Czc) with Quaternary alluvium (map unit: Qa) occurring in the southern third and northeastern corner of the PPCN. The colluvium present consists of extensive sheets of gravel, sand, silt, and clay formed by weathering, erosion, and transport of many rock types.

The nearest available wind data for the area can be obtained from the Kalgoorlie Boulder Airport weather station (number 012038) which is located 8.4 km northwest of the site. The Bureau of Meteorology (BoM) provides the 09:00 and 15:00 wind speed and direction for Kalgoorlie Boulder airport weather station. Prevailing winds are to the east, north and south-east in the mornings, and to the west, south-east and north-west in the afternoons.

The nearest weather station for rainfall and temperature data is Kalgoorlie Boulder Airport 012038, located 8.4 km north-west of the premises.

Risk assessment

Determination of emission, pathway and receptor
Consequence and likelihood of risk events
Acceptability and treatment of Risk Event
Risk Assessment – Seepage/overflow of untreated sewage and treated
Risk Assessment – Nitrogen and phosphorous release
Summary of acceptability and treatment of Risk Events

The renovation of the existing dams is considered to have a low level of risk of spills outside of containment systems. The Delegated Officer is of the opinion that this phased approach to construction will reduce any risk of spills. In order to ensure that construction takes place through the proposed phased approach, thereby ensuring that the risk is minimized, the Delegated Officer will apply the Applicant's construction commitments as conditions for the work approval.

The delegated officer is of the opinion that the concrete floor of the paved surface will contain sufficient potential fuel leaks to minimize the chance of direct discharge to land and surface water. The Delegated Officer has reviewed the Department of Health's approval of the Reuse Scheme and considers it appropriate for the regulation of public health impacts. For risk events, a risk classification will be determined in accordance with the risk classification matrix in Table 12 below.

DWER will undertake an assessment of the consequence and likelihood of the Risk Event in accordance with Table 13 below. The following criteria were used to determine the probability of the risk event. Seepage of waste water from the storage ponds and crossing of ponds will release a range of contaminants into the environment leading to contamination of the surrounding soils and groundwater.

Therefore, the Delegated Officer considers the consequence of the overflow of dams with untreated and treated wastewater to be Moderate. Based on the proposed infrastructure and management controls, the Delegated Officer has determined that the likelihood of wastewater releases is unlikely to occur in most circumstances. The Delegated Officer compared the consequence and probability ratings described above with the risk rating matrix (Table 12) and determined that the overall rating for the risk of seepage/overflow of untreated sewage and treated wastewater from ponds is Medium.

Description of the risk event for the release of nitrogen and phosphorus During irrigation, the release of nitrogen (TN) and phosphorus (TP) from overland flow and subsurface seepage (trail) can cause vegetation adjacent to the site and groundwater sources (receptors) to suffer from a change in soil chemistry, inundation of the root zone and accumulation of groundwater in the surrounding area. Should a nutrient overload occur, the Chief Executive Officer has determined that the impact of a change in soil chemistry, root zone flooding and groundwater accumulation in the local area will cause low impacts locally and minimal impacts elsewhere. The delegated officer therefore considers the consequences of the release of nitrogen and phosphorus to be minor.

The delegated official has determined that impacts from nutrient loading are likely to occur at one time. Therefore, the delegated official considers the possibility of release of high concentration of nitrogen and phosphorus through irrigation possible.