Soil and Groundwater Contamination Report

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GRWP Works Approval Application

CH2M Beca // 25 March 2021 6583830 // page 120

Appendix G

Soil and Groundwater Contamination Report

Report

Soil and Groundwater Contamination Assessment Prepared for Western Water

Prepared by CH2M Beca Ltd

18 December 2020

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Soil and Groundwater Contamination Assessment

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Revision History Revision Nº Prepared By Description Date

1 Monique Farmer, Anastasia Rastorgueva

Draft soil and groundwater contamination assessment report

23/09/2020

2 Jo Cuttler Updated with new information (reports and photographs) provided by Western Water

16/11/2020

3

4

5

Document Acceptance Action Name Signed Date

Prepared by Monique Farmer Anastasia Rastorgueva

15/09/2020 21/09/2020

Reviewed by Kate Dowsley Jo Cuttler

22/09/2020 15/09/2020 16/11/2020

Approved by

on behalf of CH2M Beca

© CH2M Beca 2020 (unless CH2M Beca has expressly agreed otherwise with the Client in writing).

This report has been prepared by CH2M Beca on the specific instructions of our Client. It is solely for our Client’s use for the purpose for which it is intended in accordance with the agreed scope of work. Any use or reliance by any person contrary to the above, to which CH2M Beca has not given its prior written consent, is at that person's own risk.

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CH2M Beca // 18 December 2020 6583830 // page ii

Executive Summary

Soil assessment Based on the desktop review of existing conditions at Gisborne RWP (the Site), the potential for existing land contamination is possible. The presence and extent are unknown due to limited records and absence of soil sampling and analysis for contaminants of potential concern. Historic and current land uses indicate the area may contain shallow soil contamination associated with the potential use of lead-based paints and asbestos containing material during the construction of the plant in the 1980s, contamination associated with historic uncontrolled filling across the Site, and historic spills / leaks / releases of chemicals stored and handled at the Site, and historic biosolid storage.

In general, construction and operation of new infrastructure will have minimal impact on soils at Gisborne RWP. It should be noted that any potential soil impacts are related to storage and handling of hazardous chemicals on-site, earthwork and their associated machinery / equipment, and the creation of dust pollution. Management measures should be employed during the construction and operation of Gisborne RWP to reduce potential environmental impacts and protect beneficial uses.

Intrusive soil sampling investigations should be undertaken prior to construction to identify the potential presence and extent of contamination (if any) at the Site. This will further assist with estimating soil quantities that can be re-used on Site, and those that should be disposed of.

Groundwater assessment The groundwater conceptual model highlights the presence of shallow groundwater, and areas of nutrient contamination in groundwater. Groundwater discharges to the environment (Jacksons Creek) at the northern boundary of the RWP, and this is the receptor for impacts caused by groundwater disturbance. Stage 1 Upgrade works are expected to include 13 excavations, which have the potential to intersect groundwater.

The following risks were assessed:

◼ Potential for excavations to intersect groundwater ◼ Volume of groundwater inflows to excavations ◼ Extent of groundwater drawdown caused by dewatering of excavations and impact on groundwater

discharge to Jacksons Creek ◼ Ability to dispose of groundwater to the existing licensed discharge point to Jacksons Creek.

The assessment quantified these risks using existing groundwater information and found that during construction:

◼ The only structure expected to intersect groundwater was the new secondary effluent pipeline, which is anticipated to be installed 2 m below the watertable

◼ Groundwater inflows to the pipe trench were predicted to be 194 m3/day in the first five days of excavation, and 24 m3/day in the following 10 days when the pipe is under construction. Over the 15 day construction period, a total of 1,208 m3 is expected to flow into the excavation and require dewatering and disposal

◼ Dewatering the excavation results in drawdown of groundwater levels in the surrounding aquifer. Analysis showed that drawdown extended from the pipeline trench to Jacksons Creek, which could result in a 6.3 m3/day reduction in groundwater discharge to the creek. This represents <1% of total creek flow at the RWP (or up to 2% in the worst case analysis). The impact will be temporary and occur over the short-term, and in the context of annual streamflow considered to be negligible

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◼ Negligible impacts on Jacksons Creek mean that no mitigation measures are necessary to reduce dewatering or groundwater drawdown associated with construction of the pipeline

◼ Groundwater extracted via dewatering of the excavation was compared to EPA license discharge conditions for the Gisborne RWP and found to be suitable quality in terms of nutrient concentrations for disposal to Jacksons Creek via the licensed discharge point. Metals concentrations (cobalt, iron and zinc) exceed guideline levels for aquatic ecosystems and pose a risk to Jacksons Creek. Alternative measures are required, such as additional treatment to remove metals, or off-site disposal.

Once constructed, structures will be impermeable and therefore impacts to/from groundwater are unlikely. Since groundwater levels are below most of the structures, there is no risk of groundwater pressure impacts destabilising structures. While groundwater levels are above the new secondary effluent pipeline, the pipeline is expected to be sufficiently small that groundwater flow would not be restricted, and no stability impacts on the pipeline would be expected.

These results are based on the reference design drawings and advice specified in this report. If excavation or construction timeframes increase, for example a delay between excavating the trench and completing the pipeline construction, volumes of groundwater inflow will also increase. Likewise, if the trench is deeper than assumed in this analysis, groundwater inflow rates will be higher. The assessment would need to be repeated to determine impacts for any such changes in design and/or construction.

The risk of recycled water in lagoons 2, 3 and 4 causing contamination to groundwater was assessed as part of a previous study (Jacobs 2019). While some leakage was likely, evidence of groundwater contamination was low. As the Stage 1 upgrade works would improve water quality in these lagoons, there would be no change to the risk profile, such that groundwater contamination risk would remain low, and beneficial uses of groundwater would not be impacted.

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Contents

1 Introduction 1 1.1 Project Description 1 1.2 Objective 3 1.3 Soil Regulations 3 1.4 Groundwater Regulations 5

2 Description of Stage 1 upgrade works 8

3 Existing conditions 10 3.1 Soil 10 3.2 Groundwater 17

4 Potential soil impacts 22 4.1 Construction and Operation Impacts on Soil 22 4.2 Mitigation and Management 25

5 Potential groundwater impacts 29 5.1 Risks 29 5.2 Approach 37 5.3 Results 39

6 Summary and conclusions 47 6.1 Soil assessment 47 6.2 Groundwater assessment 47

7 References 49

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CH2M Beca // 18 December 2020 6583830 // page 1

1 Introduction

1.1 Project Description Gisborne Recycled Water Plant (RWP) is owned and operated by Western Water (WW) and is located on Haywood Drive, Gisborne approximately 15 km north-west of Sunbury and 40km north-west of Melbourne. The site is bordered by rural and agricultural land to the north, low density rural living to the east and south and the Calder Freeway to the west.

The Gisborne RWP treats wastewater from Gisborne, New Gisborne and Macedon townships and treated effluent produced by the plant is either supplied to customers through the Gisborne recycled water network or discharged under an EPA licence to Jacksons Creek. The plant is currently reaching the limit of its capacity. Growth in flows and loads to the plant are projected to increase in line with population from approximately 14,000 people to approximately 21,000 in 2035. To avoid negative impacts on Jacksons Creek, it is proposed to undertake the following upgrades to the plant:

◼ New common inlet screening and grit removal facility ◼ New odour control facility for inlet works consisting of a biotrickling filter with activated carbon polishing ◼ New membrane bioreactor (MBR) activated sludge process with provisions for future capacity

augmentations ◼ Ferric sulphate dosing facility (or other suitable chemical phosphorus removal chemical) ◼ New effluent discharge pipework to Jacksons Creek ◼ Sludge dewatering, including polymer dosing facilities ◼ Electrical and ancillary services ◼ Site power supply upgrade ◼ Site road works The concept design layout of the Stage 1 upgrade works is shown in Figure 1-1. This layout may be modified during detailed design, so this assessment has considered potential impacts within a broader study area to allow for future flexibility in plant design and layout. The study area is shown in Figure 1-1.

This assessment has been prepared to support a works approval application which is required from the Victorian Environment Protection Authority for any plant modifications that are likely to change the emissions from the plant to the environment.

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Figure 1-1 Concept design layout and study area

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1.2 Objective To support the Works Approval (exemption) application, an assessment of the soil and groundwater risks that may arise from the Stage 1 upgrade is required. The scope of the upgrade includes multiple excavations that will have implications for soil and groundwater management during construction, as well as new infrastructure that may cause impacts to soil and groundwater during future operation.

This report characterises the existing conditions at the Gisborne RWP and summarises the likely extent of soil and groundwater disturbance during construction along with any necessary management requirements. Potential impacts to beneficial uses and appropriate mitigation measures are assessed in accordance with the relevant legislation.

The objectives of this assessment are to:

◼ During construction: – Determine the volume and quality of soil to be excavated during site works, and recommend

appropriate re-use or disposal criteria – Review groundwater levels and excavation dimensions to determine where groundwater will be

encountered during construction – Assess likely groundwater inflow volumes and quality to excavations and recommend possible

disposal options – Determine groundwater drawdown around excavations and assess potential impacts on Jacksons

Creek ◼ During operation:

– Determine potential for additional soil and groundwater impacts as a result of new infrastructure operation

It is understood that all new infrastructure will be sealed to prevent groundwater ingress. However, design considerations for high groundwater levels must also be determined.

This report is structured as follows:

◼ Section 1: introduction and objectives ◼ Section 2: summary of the Stage 1 upgrade works that have the potential to affect soil and groundwater ◼ Section 3: existing soil and groundwater conditions on-site ◼ Section 4: potential soil impacts associated with the upgrade works ◼ Section 5: potential groundwater impacts associated with the upgrade works ◼ Section 6: summary and conclusions

1.3 Soil Regulations The following legislation, policies and guidelines were used in this soils assessment:

◼ State Environment Protection Policy (SEPP) (Prevention and Management of Contamination of Land) No. S95 (2002)

◼ National Environment Protection (Assessment of Site Contamination) Measure (2013 amendment) ◼ EPA Publication 859, Prevention and Management of Contamination of Land (2002) ◼ EPA Publication IWRG701, Sampling and Analysis of Waters, Wastewaters, Soils and Wastes, (2009)

A range of general uses of land in Victoria is provided by the SEPP (Prevention and Management of Contamination of Land). Land use categories listed in the SEPP include:

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◼ Parks and Reserves – encompasses parks and forested areas as defined by the relevant Commonwealth or State regulatory authority

◼ Agricultural - includes land utilised for agriculture of horticultural practices ◼ Sensitive use – consists of land used for residential purposes, a childcare centre, pre-school, or primary

school ◼ Recreation/Open space – includes open space and recreational areas used by the public ◼ Commercial – consists of land used for commercial and business activities ◼ Industrial - which covers land used for utilities and a range of industrial activities

Beneficial uses are to be protected for each of the identified land use categories listed above. Table 1 of the SEPP (Prevention and Management of Contamination of Land) outlines these beneficial land uses (see Table 1-1) which include:

◼ Maintenance of natural ecosystems, modified ecosystems and highly modified ecosystems ◼ Human health ◼ Buildings and structures ◼ Aesthetics ◼ Production of food, flora and fibre

Gisborne RWP land use is Public Use Zone 1 - Service and Utility (PUZ1). As the construction and operation of Gisborne RWP will be occurring on the same parcel, the beneficial land use most suited to the site is Industrial. Therefore, the beneficial uses would include:

◼ Maintenance of Ecosystems (highly modified) ◼ Human Health ◼ Buildings and Structures

Table 1-1: Indicators for beneficial land use

Beneficial Use Indicators Objectives

Maintenance of ecosystems

Chemical substances or waste identified through the application of the National Environment Protection (Assessment of Site Contamination) Measure (Schedule B(2), Appendix 1) or any other chemical substance or waste

Contamination must not adversely affect the maintenance of relevant ecosystems and the level of any indicator must not be greater than (a) any regional Ecological Investigation Level developed in accordance with the National Environment Protection (Assessment of Site Contamination) Measure and published by the Authority for a region in which the site is located. Until such time that regional Ecological Investigation Levels applicable to the site are published, the Interim Urban Ecological Investigation Levels nominated in the National Environment Protection (Assessment of Site Contamination) Measure shall be used in place of any regional Ecological Investigation Level, or (b) levels derived using the risk assessment methodology described in the National Environment Protection (Assessment of Site Contamination) Measure, or (c) levels approved by the Authority.

Human health

Chemical substances or wastes identified through the application of the National Environment

Protection (Assessment

of Site Contamination)

Measure (Schedule B(2),

Contamination must not cause an adverse effect on human health and the level of any indicator must not be greater than – (a) the investigation level specified for human health in the National Environment Protection (Assessment of

Site Contamination) Measure, or (b) levels derived using a risk assessment

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Beneficial Use Indicators Objectives

Appendix 1) or any other chemical substance or waste.

methodology described in the National Environment

Protection (Assessment of Site Contamination)

Measure, or (c) levels approved by the Authority

Buildings and structures

pH, sulfate, redox potential, salinity or any chemical substance or waste that may have a detrimental impact on the structural integrity of buildings or other structures.

Contamination must not cause the land to be corrosive to or adversely affect the integrity of structures or building materials.

Aesthetics¹ Any chemical substance or waste that may be offensive to the senses

Contamination must not cause the land to be offensive to the senses of human beings.

Notes: 1 Aesthetics is not considered a beneficial use for Industrial land use, however at the Gisborne RWP,

the use, handling and storage of chemicals occurs during operations, therefore it has been included as a beneficial use.

1.3.1 Acid Sulfate Soils

The following legislation, policies and guidelines are relevant to this policy area:

◼ Industrial Waste Management Policy (Waste Acid Sulfate Soils) (1999) (Vic) (IWMP (WASS))– framework to guide the management of waste acid sulfate soils (ASS) in Victoria

◼ EPA Publication 655, Acid Sulfate Soil and Rock (2009) (Vic) – gives guidance to landowner, developers and consultants regarding disturbance of soil, sediment, rock and/or groundwater where ASS may be disturbed

1.3.2 Other Guidelines

Additional EPA guidelines utilised during the assessment included:

◼ EPA Publication 1698, Liquid Storage and Handling Guidelines (2018) (Vic) – information pertaining to proper storage and handling methods for liquids, such as hazardous substances, in in order to reduce and control risks

◼ EPA Publication IWRG621, Soil Hazard Categorisation and Management (2009) (Vic) – outlines soil waste characterisation including the importance of assessing the site usage history, and how to select laboratory analytical

◼ EPA Publication 480, Environmental Guidelines for Major Construction Sites (1996) (Vic) – best practice and guidelines on implementing practices to minimise environmental impacts from construction activities

◼ EPA Publication 275, Construction Techniques for Sediment Pollution Control (1991) (Vic) – outlines recommendations on strategies to reduce sediment pollution from construction sites

1.4 Groundwater Regulations Groundwater quality in Victoria is protected under the 2018 SEPP Waters, issued under the Environment

Protection Act 1970 and administered by EPA Victoria. The SEPP Waters defines a range of protected beneficial uses for defined segments of the groundwater environment, which are based on groundwater

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salinity (Table 2 1). Beneficial uses of groundwater are considered to be precluded when relevant groundwater quality thresholds set out in the Groundwater SEPP for those beneficial uses have been exceeded.

Clause 15 of the SEPP Waters sets out the beneficial uses of groundwater and the circumstances in which the EPA may determine that beneficial uses do not apply. These include:

◼ If there is insufficient aquifer yield to sustain the beneficial use ◼ If the application of groundwater, such as for irrigation, may be a risk to beneficial uses of land or the

broader environment due to the soil properties ◼ If the beneficial use specified in the definition of water dependent ecosystems and species relates to

stygofauna and troglofauna ◼ The background level of an environmental quality indicator would not provide for the protection of the

beneficial use

A review of groundwater salinity at the Gisborne RWP indicates that groundwater is relatively fresh near Jacksons Creek (500 – 1500 mg/L Total Dissolved Solids (TDS)) and more saline towards the south of the site. Low salinity means that groundwater falls in SEPP Waters Segment A2 (601 to 1,200 mg/L TDS), which requires protection for all beneficial uses, which includes:

◼ Water dependent ecosystems and species ◼ Agriculture and irrigation (irrigation) ◼ Agriculture and irrigation (stock) ◼ Industrial and commercial ◼ Water-based recreation (primary contact recreation) ◼ Traditional owner cultural values ◼ Cultural and spiritual values ◼ Buildings and structures

Beneficial uses that may be excluded based on application of Clause 15 of the SEPP Waters are the use of groundwater for potential water supply and mineral water supply, and geothermal properties of groundwater. This is because aquifer properties (yields and temperature) are not sufficient for these uses.

Water resources in Victoria are protected under the Water Act 1989 (Water Act), which is administered by the Department of Environment, Land, Water and Planning (DELWP) and their regional delegates, including Southern Rural Water (SRW) who has jurisdiction in the area of the Gisborne RWP. The Water Act sets out water allocation and licensing requirements, providing for groundwater access, take, trade and disposal. Multiple policy and guidance documents are available to specify rules for various parts of the Water Act.

Given that groundwater at the RWP flows into Jacksons Creek, the key policy document relevant to this assessment is the ‘Guidelines for groundwater licensing and the protection of high value groundwater dependent ecosystems’ (DELWP 2015). While Jacksons Creek would not be considered a high value ecosystem due to the modified catchment, exotic species, extensive grazing and recycled water discharge, the guideline still provides a framework for assessing impacts on streamflow. Impacts categories specified in the guidelines are:

◼ Minor: Q90 (10th percentile) flows are reduced by less than 1% ◼ Moderate: Q90 (10th percentile) flows are reduced by between 1 and 10% ◼ Major: Q90 (10th percentile) flows are reduced by more than 10%

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2 Description of Stage 1 upgrade works

Construction and operation of any below ground infrastructure involves disturbance of soil and groundwater, and therefore presents the potential for impacts on the beneficial uses of soil and groundwater. A number of below ground structures are planned as part of the Stage 1 upgrade works, as summarised in Table 2-1 and shown in Figure 2-1. The location and dimensions of these structures were obtained from drawings 6583830-GAX-G-201 and 6583830-GAX-M-001 dated 13 August 2020 and 25 September 2020 respectively, and from emails from CH2M Beca design engineers dated 8 September 2020.

Construction of this infrastructure will involve the initial excavation, removal and stockpiling of soil, and construction of the structure in the excavation. If the excavation is below the watertable, then the construction phase will also involve management of any groundwater inflows, including dewatering and disposal of pumped groundwater. The volume of groundwater inflows will depend on the dimensions of the excavation, how long it is open for, and aquifer properties. The possible disposal options will depend on the quality and volume of groundwater pumped from the excavation.

Operation of the infrastructure is expected to have minimal potential for impacts, as the structures will be designed to be impermeable such that no seepage into the surrounding soil or groundwater should occur. The impermeability of the finished structures also means that no groundwater will flow into the structures during operation. Depending on the weight of the structure and the groundwater head, there is potential for uplift of the structure. Groundwater drainage around the structure may be required to mitigate this risk.

The infrastructure shown in Figure 2-1 and described in Table 2-1 is assessed in this report to determine the potential impacts to/from soil and groundwater on construction, operation and the environment.

Figure 2-1 Gisborne RWP with below ground structures in Stage 1 upgrades shown in red

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Table 2-1: Preliminary footprint and depth of below ground structures

# Structure Excavation footprint

Proposed invert depth (meters below ground level (mBGL)) and elevation (meters Australian height datum (mAHD))

1 New DN710 HDPE pipeline, connecting between the existing Header Pit to the new inlet works

1m x 80m ◼ Pipe to be installed with 1.0 m of cover and cross the drainage trench above ground (pipe bridge)

◼ 2.0 mBGL in underground sections 2 New Inlet works Structure (including grit

chamber) 13m x 23m ◼ Foundation will be ~1.0 mBGL

◼ -Base of main structure at 413.5 mAHD / 411.5 mAHD (grit chamber)

3 New Pipeline connecting between the new Inlet works and new Bioreactors

- ◼ Above ground

4 New Bioreactors 1 & 2 (plus reserved area for future bioreactor)

50m x 26m ◼ Base of structure 3.0 mBGL (406.5 mAHD) ◼ Potentially up to 5.0 mBGL

5 New Membrane Maintenance and Blower Building (incl. Control Room)

~30m x 15m ◼ Foundation about 2.0 mBGL (411.1 mAHD)1

6 New Membrane Tanks 16m x 12m ◼ Foundation ~ 1.0 mBGL (411.1 mAHD)1

7 New MBR Effluent Transfer Pipeline (OD426 mm), connected to the existing Clarifier Effluent Pipe (near the bioreactors)

200m x 1m ◼ ~2 mBGL installed with 1.0 m cover ◼ Lowest invert elevation ~ 402.0 mAHD

8 New access road 190 m long ◼ Excavation up to 3.4 mBGL ◼ Road excavation elevation ~ 405.0 – 420.0

mAHD 9 New DN375 (OD426 mm) pipeline to

discharge secondary effluent directly to creek (east)

150m x 1m ◼ ~2.0 mBGL installed with 1.0 m cover ◼ Lowest invert elevation ~397.5 mAHD

10 Chemical area 23m x 7m ◼ Base of building footings will be ~ 1.0 mBGL (406.6 mAHD)1

11 Pump room 28m x 6m ◼ Base of building footings will be ~ 1.0 mBGL (407.1 mAHD)1

12 Dewatering area 20m x 10m ◼ Building footings will be ~ 1.0 mBGL (404.6 mAHD)1

13 Odour control 8.6m x 19.2m ◼ Foundation will be 0.5 mBGL (409.0 mAHD)

Notes: 1 The likely maximum invert elevation was calculated based on top of concrete design elevation minus

the expected concrete thickness which is 0.4 m. 2 BGL – below ground level

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3 Existing conditions

3.1 Soil

3.1.1 Potential for contamination from historical land uses

A review of historical reports and aerial photographs are discussed below. No analytical results were available for chemical evaluation of soils at the Site at the time of reporting.

3.1.1.1 Aerial Photography Review

A Lotsearch report (2020), including historical aerial photography from 1946 – 2010, was obtained for this assessment. A review of aerial photography is outlined in Table 3-1.

Table 3-1: Aerial photography review

Year Site Surrounds

1946 The Site appears to be grassed with trees scattered across the northern half. A creek is visible, cutting across the north-eastern boundary.

Immediately north of the Site is a creek. The embankments appear to be quite steep. To the east, south and west the land is predominately vacant, grassed. To the south of the Site, a straight line of trees is visible.

1950 The Site does not appear to have changed significantly from the 1946 photograph.

The surrounding area does not appear to have changed significantly from the 1946 photograph with the exception of a small rectangular structure present to the west of the Site.


The surrounding area does not appear to have changed significantly from the 1950 photograph with the exception of development of land to the west of the Site. The land surrounding the rectangular structure appears to have undergone landscaping works.


The surrounding area does not appear to have changed significantly from the 1969 photograph.


The surrounding area does not appear to have changed significantly from the 1974 photograph with the exception of shaded areas visible to the south-east of the Site - potentially earthworks.

1984 The Site has undergone extensive changes. Six square lagoons located at the east of the site, and two long rectangular objects are present at the centre of the site. Two small rectangular structures are located south of the rectangular structures. A road has been constructed leading from the southern boundary to the centre of the Site and extends east along the lagoons. A cluster of small figures are visible on the southern boundary – possibly livestock.

The surrounding area has undergone significant changes. Various rectangular structures (likely residential) with landscaped surrounds are location to the east and south of the Site. Driveways appear to have been constructed, connecting the houses to a road. Earthworks appear to be occurring to the west of the Site. A shaded stretch of land extending north to south is apparent. Small automobile-like structures are visible on this

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Year Site Surrounds stretch of land - likely tractors and excavators.

1997 The Site has undergone some structural and landscape changes. Multiple rectangular structures are present on the eastern portion of the site. One large structure, possible residential, is located on the southern boundary of the site. Additional vegetation is present across the site.

The surrounding land has undergone some changes. Land to the east and south has undergone some residential and landscaping development. Land to the west of the Site consists of a highway running north-south.

2010 The Site has undergone some changes. An additional oval-shaped excavation has been established in the centre of the site. A formalised access road has been built, providing site access from the south. Additional rectangular structures are present on a hard-stand area on the north-west of the Site.

The surrounding land has undergone significant changes. Land to the north appears to have undergone residential and landscape development, with clear property lines observed. Further residential and landscaping works have been undertaken to land to the east and south. A large, oval-shaped track and field is present to the south of the Site.


The surrounding land does not appear to have changed significantly from the 2010 photograph.


The surrounding land does not appear to have changed significantly from the 2014 photograph.

The aerial photography review indicates that the land associated with Gisborne RWP was rural, grassed land. It is possible that it was historically used for temporary farming / grazing. The land did not undergo significant changes until 1984 where the construction of multiple lagoons and rectangular structures is apparent. This corresponds with the construction on Gisborne RWP in the early 1980s. Further developments to the site appear to have occurred up until 2014 where minimal changes to the land is visible.

Surrounding land use appears to consist of rural, grassed land from 1946 until 1984 where major residential and landscaping works are apparent. Further residential works appear to have occurred up until 2014 where minimal changes to the land is visible.

3.1.1.2 EPA Records and Licenced Activities

A search of the EPA Priority Sites and Pollution Notices Register indicated that there were no listed sites within the Gisborne RWP boundary or within a 1 km radius of the site. The site holds an EPA licence for Sewage Treatment (A03) activities.

3.1.1.3 PFAS Investigation and Management Programs

There were no registered PFAS Investigations and Management Programs, Waste Management Facilities and Landfills, or Former Gasworks and Liquid Fuel Facilities on-site or within a 1 km radius of the site (Lotsearch 2020).

3.1.1.4 Historical Business Directories

Business directory records listed a variety of business activities located within a 1 km radius of Gisborne RWP. Businesses included hospitality venues, accommodation, and motor garage and service station and

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were operating between 1950 and 1970. The motor garage and service station were located along the Calder Freeway, approximately 130 m west of Gisborne RWP (Lotsearch 2020).

3.1.1.5 Acid Sulfate Soils

There is an Extremely Low (1 – 5 %) probability of the occurrence of Acid Sulfate Soils beneath Gisborne RWP and surrounding land (Lotsearch 2020).

3.1.1.6 Recorded incidents (chemical spills)

On 17 November 2015, a float valve failed in the header tank of the ferric tank and the entire tank (6,000 L) was pumped into the first two lagoons. As part of the incident response, within an hour of the incident, Lagoon 1 was pumped into one of the Storm Overflow Lagoons. The tank release was fully contained within the lagoons and no ferric sulphate overflowed to surrounding land, this is evidenced by photographs taken after the incident on 17 November 2015 and provided by Western Water for this assessment. Western Water said the majority of the spilled material was recycled back through the plant. In 2017, material from the base of Lagoon 1 was sampled and analysed (see Section 3.1.1.7). It was classified as “Fill Material” per

IWRG621 Industrial Waste Resource Guidelines - Soil Hazard Categorisation and Management (2009). Sediment sampling from other lagoons is unknown.

3.1.1.7 Previous Investigations

CH2M Beca had access to reports outlining previous environmental investigations undertaken at the Site. A summary of the information is provided below:

Out-Task Environmental Pty Ltd (Out-Task), Recycled Water Plants Groundwater and Soil Monitoring Programs and Risk Assessment Report, August 2016

Out-Task was engaged to undertake an environmental groundwater and soil risk assessment for six RWP operations; one being Gisborne RWP. The assessment considered including from wastewater treatment tanks/lagoons, recycled water storage and irrigation area, sludge/biosolids processing, and ancillary facilities such as chemical storages, maintenance depots as well as legacy grits and screenings and other potential waste burial on site.

The report notes that in relation to soil impacts at Gisborne RWP, potential leaks / spills / releases at Gisborne RWP, anecdotal evidence of a recent release of ferric sulphate from the on-site storage container indicates that existing infrastructure could cause potential contamination to soils surrounding the infrastructure. The impact on soil from this release is unknown. In general, potential historic leaks / spills from inlet works, treatment tanks, clarifier and associated tanks in the past was likely but has not been documented well, and therefore the impact on surrounding soil is unknown.

Out-Task concluded that Gisborne RWP is a potential source of soil impacts due to storage and handling of chemicals on-Site, as well as accidental spills / leaks / releases from existing infrastructure on-site.

Senversa Pty Ltd (Senversa), Waste Soil Categorisation, Gisborne Recycled Water Treatment Plant, Letter Report, August 2017

Senversa was engaged to assess soil excavated (est. 1,500 m3) from the base of “a treatment lagoon” for the purposes of waste categorisation. A photograph of the lagoon and the stockpile inserted in Senversa’s report (2017), although not identified or labelled, looks to be a photograph of Lagoon 1. The landscape features in the photo align with other photographs of Lagoon 1, which was one of the receiving lagoons of the ferric sulphate release in November 2015.

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Ten samples were collected from the stockpile (identified as SP01) and two quality control samples were also submitted for analysis. The stockpile consisted of a mixture of inferred reworked alluvial swamp deposits, natural alluvial silty sand deposits and basaltic clay from the lagoon floor and batters. The samples were collected from a representative range of soil compositions.

Analysis indicated that the stockpiled material from the base of a treatment lagoon met the criteria for “Fill Material” as per IWRG621 Industrial Waste Resource Guidelines - Soil Hazard Categorisation and

Management (2009), given that all concentrations were below the adopted thresholds.

Construction and Remediation Advisory Services Pty Ltd (CARAS), Western Water Gisborne RWP Project Risk Register Report, September 2019

CARAS undertook a high-level Project Risk Register (PRR) to support proposed Gisborne RWP upgrades. The PRR considered work health and safety, environment, constructability, sustainability and existing operations.

In relation to soil impacts from the proposed upgrades, CARAS found that historical earthworks, cut and filling activities has been undertaken across the site with imported materials from unknown sources. Further, there were no records of engineering supervision for excavating and backfilling areas across the site and therefore it is likely that the site contains ‘uncontrolled fill’ materials.

EP Risk, Soil Investigation Report Gisborne Recycled Water Plant, September 2019

EP Risk were engaged by CARAS to undertake test pitting works at Gisborne RWP, with the aim being to determine the extent of uncontrolled fill and impacted soils at the Site.

Fieldwork involved test pitting across Gisborne RWP at six locations. Samples were collected and screened for volatile organic compounds (VOCs) with a photoionisation detector (PID). PID detects were low with a maximum recording of 0.3 parts per million (ppm). Test pitting works undertaken at the site did not identify any visual or olfactory evidence of contamination, however no samples were analysed by a laboratory.

Fill in the form of subangular gravels with clayey sands was observed at all test pit locations. Test pit locations TP04, TP05 and TP07 were established on the western portion of Gisborne RWP where the proposed Stage 1 construction will be undertaken. Fill was observed in this area from the surface to 0.3 – 0.7 meters below ground level (mBGL) until refusal on Basalt rock.

Hazard Alert, Asbestos Pre-Demolition (Division 6) and Hazardous Materials Survey Report for Western Water, Gisborne Depot – Haywood Dr, Gisborne (Nominated Buildings), April 2020

Hazard Alert undertook an asbestos survey and risk assessment at Gisborne RWP in April 2020. Three structures were reviewed at the site including the Main Building, Open Shed and Storage Shed. Additional hazardous materials including lead, silica and polychlorinated biphenyls (PCBs) were assessed also.

There was no asbestos containing material identified within the three structures assessed. Silica was considered to be present in flooring material, ceiling insulation and splashback within the structures. External areas where concrete slab and formwork exist were presumed to contain asbestos containing material and silica, however these areas were not sampled.

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Out-Task, Draft 2020 Annual Monitoring Program draft report, August 2020 Out-Task reports that Western Water has advised that a lagoon previously located immediately to the south of the current drying bed once received drilling muds, clarifier construction spoil and possibly other fill. In 2011, this lagoon was filled in and covered with topsoil. It is not known if the contents of the lagoon were removed. The footprint of this old lagoon was located approximately where the historic Biosolids Storage area is indicated in Image 1. Out-task recommended that the former sludge lagoon and any other legacy waste burial onsite should be clearly mapped and considered for future soil and groundwater contamination investigations and risk-based monitoring and/or mitigation where required. Anecdotal evidence, Gisborne RWP Potential Contaminated areas discussion, September 2020

Image 1: Areas of potential contamination

A discussion between Western Water and CH2M Beca on 10 September 2020 resulted in the identification of additional areas of potential contamination, as shown in Image 1. Western Water contributors were Luke Hateley (Project Manager), Rod Curtis (Manager, Water Systems and Solutions) and William Rajendram (Senior Environmental Engineer). Four areas were identified associated with historic usage of the Site. The areas are as follows:

◼ Concrete, rubble and asbestos concrete (AC) pipe fragments have been identified at the north-western portion of Gisborne RWP. Material of this kind is likely to be associated with uncontrolled backfilling and dumping of hard rubbish in the area (see Image 2), from an unknown source. It is possible that this material contains contaminants such as heavy metals and asbestos. It was recommended by Western Water that this area is not used as a laydown area, given the area has been used as a waste stockpiling at various times and soils are potentially contaminated.

◼ Concrete, rubble and pipe fragments have been identified at the south-western portion of Gisborne RWP, south of the current access road. Material of this kind is likely to be associated with uncontrolled backfilling in the area, from an unknown source. It is possible that this material contains contaminants such as heavy metals and asbestos.

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◼ Possible diesel spill area has been identified adjacent to the Gisborne RWP Depot. Diesel spills / leaks / releases in this area may have resulted in surface soil contamination.

◼ Historic biosolids storage area located south of the Sludge Drying Bed and west of the Stormwater Overflow lagoon. Biosolids stored in this area may have leached into the soils, resulting in surface soil contamination.

Image 2 Dumped hard rubbish on north western portion of Gisborne RWP

Historic activities at the site have not been well documented. No information was provided regarding intrusive soil investigations in these four areas, and it is unclear the potential presence and extent of soil contamination as a result of these historic activities.

The Site was used as a council depot (pre-1990’s) and the history of the Site during this time is unknown. There has been no vehicle servicing carried out by Western Water at the Site.

Summary

Based on the historical reports provided to CH2M Beca the following information has been deduced:

◼ Presence of uncontrolled fill is present at varying depths across the site, the extent of which is unknown ◼ Storage and handling of chemicals at the Site have led to spills / leaks / releases in the past, the extent of

impact is unknown, with the exception of the 6,000L release of ferric sulphate in 2015. This tank release was fully contained within the lagoons and recycled back through the plant.

◼ Lithology at the Site comprised gravelly sandy clay fill material underlain by clay. Fill was observed in the western portion of Gisborne RWP up to 0.7 mBGL until refusal on Basalt rock

◼ Previous sampling indicated no visual or olfactory signs of contamination.

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◼ It is possible that asbestos containing material is present across the site in uncontrolled fill, asbestos concrete and fragments of asbestos concrete pipelines.

◼ Waste materials (drilling muds, clarifier construction spoil and possibly other fill) was buried in the former sludge lagoon. This area was also historically used for Biosolid Storage.

3.1.2 Potential for contamination from historical land uses

From the review of historical reports, aerial photographs and research undertaken by CH2M Beca, a summary of potential for contamination from historical land uses is provided in Table 3-2.

Table 3-2: Potential for contamination from historical land use

Activity Potential Issues / Source Contaminants of potential concern

Gisborne RWP and associated infrastructure construction

Construction of the Gisborne RWP was undertaken in the early 1980s. Review of aerial photographs and anecdotal evidence indicated that the sites’ construction potentially involved volumes of uncontrolled fill or anthropogenic materials were imported across the Site from unknown sources.

Asbestos Heavy metals Petroleum hydrocarbons (TPH) Benzene, toluene, ethylbenzene and xylenes (BTEX) Polycyclic aromatic hydrocarbons (PAH) Volatile chlorinated hydrocarbons (VCHs) PCBs OCPs/OPPs

Lead-based paints As the site was constructed in the 1980s it is possible that lead-based paints, other paint types containing metals (e.g. zinc chromate) and lead jointing were present on infrastructure at the site including former buildings and pipelines.

Heavy metals

Operation of Gisborne RWP (including storage of chemicals) (Out-Task, 2016)

Gisborne RWP was opened in the 1980s. It is assumed chemicals were / are currently utilised and stored on-site during the operation of Gisborne RWP which have the potential to spill / leak and impact the site.

Heavy metals TPH Ammonia Herbicides Ferric sulphate Chloride Hydrogen peroxide Hypochlorite

Depot at Gisborne RWP (Out-Task, 2016)

Minor quantities of fuels and chemicals currently stored on-site during the operation of Gisborne RWP which have the potential to spill / leak and impact the site.

Lubricants Solvents Herbicides Cleaners

Historic Biosolid Storage

Anecdotal evidence reported historic storage of biosolids on an area located south of the Sludge Drying Bed and west of the Stormwater Overflow lagoon. Biosolids stored in this area may have leached into the soils, resulting in surface soil contamination.

Organics (nitrogen, phosphorus, potassium and sulphur) Heavy metals

Waste management Waste materials have been buried in various locations on site (e.g. in the former sludge lagoon).

Asbestos Heavy metals TPH, BTEX, PAH, VCHs, PCBs, OCPs/OPPs

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3.2 Groundwater A comprehensive data review was completed to develop a conceptual hydrogeological model for the Gisborne RWP (Jacobs, 2019), which is presented in Table 3-3. The key purpose of the conceptual model is to understand groundwater elevations, flow directions and receptors at the site to inform risks associated with proposed infrastructure upgrades at the RWP.

Groundwater flow at the Gisborne RWP is influenced by the thin sequence of alluvium between outcropping basalts in the south and Jacksons Creek in the north (Figure 3-2). Shallow groundwater levels in the alluvium indicate that on site excavations are likely to intersect groundwater, resulting in pit inflows and potentially the need for dewatering. The cross-section in Figure 3-1 provides an overview of the stratigraphy of the site and demonstrates the hydraulic connection between the alluvial aquifer and Jacksons Creek. Therefore, if dewatering occurs impacts on the creek will need to be investigated.

The risk of recycled water in the lagoons causing contamination to groundwater was assessed as part of a previous study (Jacobs 2019). While some leakage from the lagoons was likely, evidence of groundwater contamination was low. As the Stage 1 upgrade works would improve water quality in these lagoons, there would be no change to the risk profile, such that groundwater contamination risk would remain low, and beneficial uses of groundwater would not be impacted. Table 3-3: Summary of conceptual model for Gisborne RWP

Aspect Description

Topography The topography at the site is steeply sloping towards Jacksons Creek which forms the northern site boundary, with ground elevations ranging from 433 m at the south-eastern border to 398 m at the Creek. The RWP lagoons and infrastructure is located at the base of the hill on the alluvial flats.

Surrounding Land Use

Land use surrounding the site is primarily low-general density rural residential living. A major freeway (Calder Freeway) bounds the property to the west.

Geology The surface geology at the site is Quaternary Newer Volcanics basalt and Quaternary alluvial sediments (Figure 3-2). These formations overlie Ordovician-aged basement rock consisting of sandstone and siltstone. The basalt outcrops on the inclining topography on the southern part of the site. They consist of a relatively thick layer (up to 13.8m) of basaltic clay at the top of the unit which overlies harder rock with variable degrees of weathering and fracturing. Basalt thickness ranges from around 20 – 45 m in the elevated areas to the south of the RWP and 6 – 8m in lower-lying areas around the Creek and lagoons. The alluvial sediments comprise silt, clay and sand and cobbles and are present adjacent to Jacksons Creek as well as across the majority of the RWP including beneath lagoons and other RWP infrastructure (DELWP, 2014). The sediments near Jacksons Creek are sandier and around 4m thick, whereas further south on the site clay is more prevalent. The boundary between alluvium and basaltic clays is unclear as alluvium can also be clayey, but the sandier sediments in the north provide higher potential for interaction with the creek.

Relevant aquifers

The primary aquifers at the site are the alluvial sediments and the basalt. The watertable is present in the alluvial sediments that underlie the RWP and in the basalts where they outcrop in the southern (elevated) portion of the site. Alluvium is largely absent at Jacksons Creek, where basalt is visible in the creek bed (Jacobs site inspection, May 2018). On the terrace to the immediate south, alluvium is up to 4m thick in bores MW1AS, MW1AD and MW2A, and becomes thinner to the south until it pinches out against the basalt on the southern edge of the lagoons. Basalt at Jacksons Creek is approximately 8m thick and becomes thicker towards the south as surface elevation increases. Siltstone forms the basement formation beneath the basalt, and acts as a barrier to groundwater flow.

Aquitards Basaltic clays, which are generally present at the top of the basalt formation as part of a weathered zone, are likely to impede lateral and vertical groundwater flow to some extent. This layer is likely to limit the exchange of groundwater between the alluvium and basalt aquifers. Clays were less prevalent in bores near the creek where the alluvium is more sandy. Further south, clay is more common, with thicknesses of 2.5 to 13.8m overlying the basalt. This is interpreted to mean that:

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Aspect Description ◼ Infiltration from lagoons to groundwater is likely, particularly closer to the northern boundary ◼ Connection between the alluvium and basalt is likely to be strong in the northern part of the site

nearer the creek, and weaker in the elevated southern areas where clay is more prevalent. Observation bores

Seven on-site observation bores are distributed around the site and are identified as MW1AS, MW1AD MW2A, MW3 – MW7. Their locations are displayed in Figure 3-2. The up-gradient bores monitor the basalt (MW4 and MW6), with MW3 and MW7 monitoring the weathered clay at the top of the basalt. Closer to the creek, MW1AS, MW2A and MW3 monitor the alluvium. MW1AD monitors the basalt underlying the alluvium at the edge of the creek, which is conceptualised as being connected to the alluvium in this location (and therefore the creek). Bores MW4 and MW6 are considered to be background bores as they lie up-gradient on the southern property boundary. However, they monitor the basalt in the southern part of the site, which is conceptualised as limited interaction with overlying alluvium due to an intervening clay layer. Therefore, MW4 and MW6 may not reliably represent groundwater quality in the alluvium closer to the creek.

Watertable Groundwater levels at the site range from <1 mBGL1 in low-lying areas (MW2A) to 10.6 mBGL in elevated areas (MW6). Shallow depth to water is observed in the alluvial aquifer near Jacksons Creek, and also in the basalt aquifer near Stephenson’s Creek. Deeper groundwater levels were observed in the basalt aquifer in areas of higher topography south of the lagoons. Water levels have varied by approximately 1 m over the 10 years of monitoring, as seen in the graph below. There is not enough data to determine seasonal trends as the groundwater sampling has occurred mostly on an annual basis. Since 2018, the groundwater monitoring occurred in May, prior to then the wells were mostly monitored in December. Though no monitoring has occurred during the wet season, approximately 0.5 m difference in water level is observed between May and December.

Groundwater flow

Regionally, the inferred groundwater flow direction is generally north to north-easterly towards Jacksons Creek following topographical elevation changes. Watertable contours were compiled and are displayed in Figure 3-2 using regional watertable geometry from the DELWP report “Secure Allocations, Future Entitlements” (2012), modified and enhanced using available bore data and the treatment pond elevations. Based on the top water level elevation in maturation lagoon 1 (402.6 mAHD) there is a potential local groundwater mound at the lagoon, which results in a slight hydraulic gradient south towards MW03. Otherwise, flow is typically towards the north.

Groundwater quality (salinity) & segment

Regional salinity mapping suggests a groundwater TDS of 500-3,500 mg/L across the site (DELWP, 2018), which falls into SEPP Segments A2 and B, meaning that all beneficial uses apply. TDS concentrations in site monitoring bores range from 1,100 – 4,500 mg/L which includes background (i.e. up-gradient) monitoring bores. It should be noted, MW4 obtained an unusually high TDS result (8,500 mg/L TDS) in the 2020 monitoring round.

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Aspect Description

Groundwater management

The site is not within a groundwater protection zone or management area, however, active management of groundwater quality is occurring on the site. The EPA issued a Pollution Abatement Notice (PAN 7802) for the RWP in 2017 in response to reports of nutrient contamination in groundwater. A subsequent risk assessment found that the risk to beneficial uses was low (in particular, to ecosystems in Jacksons Creek), and as a result the PAN was withdrawn in 2019.

Existing groundwater contamination

Potential contaminant sources present on site include the maturation lagoons, sludge lagoon/drying pan, overflow lagoon, holding tanks and associated infrastructure, inlet works and a licensed discharge point (to Jacksons Creek). The treatment and sludge lagoons are considered to be the primary potential contaminant sources via leakage into groundwater. The base of the lagoons is in connection with the watertable. A detailed review of groundwater contamination, flow, and existing infrastructure suggested that RWP operations may have impacted groundwater south of the maturation lagoon 1 in MW3, evident as elevated nitrate and watertable mounding; and north of the lagoons/sludge pan area evident as elevated total phosphorous and nitrate in bores MW1, MW1AS and MW1AD. Bores MW5 and MW6 (background) also have elevated nitrate concentrations.

Receptors Jacksons Creek is the key receptor given its close proximity to the lagoons (60 m) and its location down-hydraulic gradient of the RWP. Downstream non-consumptive users (i.e. aquatic ecosystems) are at risk. No downstream surface water users or nearby consumptive groundwater users have been identified (Western Water 2017). The groundwater dependence of Jacksons Creek was assessed by Jacobs (2019), with groundwater discharge estimates of 0.7 ML/year, or approximately 6% of total flow (at times of very low flows).

Pathways The unconsolidated sediments of the alluvial aquifer comprise the likely key pathway for contaminant migration to Jacksons Creek (the key receptor); the mechanism being seepage/infiltration from sources to groundwater, and then subsequent transport down the hydraulic gradient towards discharge points at Jacksons Creek. The other pathway for impacts to Jacksons Creek is via the licensed discharge point in the north of the site. No irrigation occurs at this RWP due to space constraints.

Information gaps/ uncertainties

There is uncertainty on vertical groundwater flux at the site given that there is some remaining uncertainty on survey levels at the nested site. Bore MW1AD displays a groundwater elevation higher than the shallow bore MW1AS in the nested pair (and anomalously higher than ground level) and so the survey heights at this nested site require confirmation. Background wells MW4, MW6 and MW7 are not installed into the alluvium, which is at greatest risk of contamination compared to the basalt aquifer. Further monitoring events at these bores will assist in clearing data uncertainties and help to confirm ambient groundwater quality.

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Figure 3-1 Cross-section facing west showing geology, watertable and RWP

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Figure 3-2 Gisborne RWP geology, groundwater levels and existing RWP infrastructure

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4 Potential soil impacts

4.1 Construction and Operation Impacts on Soil The following sections summarise the assessment, listing potential soil and land impacts associated with construction and operation activities. The assessment relates to the Stage 1 construction and operation of Gisborne RWP and was based on standard construction and operational practices; however, it did not take into account the mitigation measures. If mitigation measures (Section 4.2) were employed, the likelihood and consequence of the impacts would be lower.

4.1.1 Construction

Five construction activities likely to be employed during development of Gisborne RWP are listed in Table 4-1, along with their associate impact pathway.

Table 4-1: Potential impacts during construction of Gisborne RWP

Activity Impact Pathway

Transport and use of chemicals at the Site

Accidental spill / leak / release of chlorine impacting on human health and safety or the environment

Construction equipment and vehicles Accidental spills and leaks from construction equipment and vehicle impacting on human health or the environment

Earthworks Production of dust during earthworks impacting human health or the environment

Excavating, trenching, tunnelling and decommissioning

Encountering contaminated soils impacting on human health or the environment. Creation of dust during earthworks could cause potential airborne contaminants which could impact on human health or the environment.

Stockpiled material (Site-generated)

Stockpiled material (i.e. excavated soils) intended for re-use on-site may mix with excavated material planned for off-site disposal Stockpiled soil may become eroded by weather, and runoff into Jacksons Creek and impacting human health or the environment Open excavations providing a fall-risk (safety) to on-site users

Stockpiled material (imported) Imported material (i.e. clean Fill, construction material, sand, aggregate) may mix with site soils, reducing the quality of the material for use on-site Stockpiled material may become eroded by weather, and runoff into Jacksons Creek and impacting human health or the environment

4.1.2 Operation and Plant Processes

Positive effects from the construction of Gisborne RWP would be reduced likelihood of spills / leakages / release of liquids from upgraded infrastructure with new fittings, and excavation and removal of potentially contaminated soils and uncontrolled fill at the Site.

Potential impacts of the new infrastructure functions on soil at Gisborne RWP are discussed in Table 4-2.

Table 4-2: Potential Impacts of Stage 1 Infrastructure on soil

New Infrastructure Element

Function Soil Impact

New DN710 HDPE pipeline, connecting between the existing

The new pipeline will provide a connection between the existing Header Pit to the new Inletworks

Nil. Provision of new infrastructure with new fittings will limit the likelihood of accidental leaks / spills / releases and

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Header Pit to the new Inletworks

impacts to soil or land surrounding the pipeline.

New Inlet works (incl. two grit chambers)

Inlet works undertakes screening and grit removal from flow from the Gisborne catchment. Flow is filtered through a 6 mm and 2 mm fine screen before flowing into the new Bioreactors. Two grit chambers will store the inorganic and organic material filtered from the flow. Screen washing compactors and grit cleaning systems and bins for disposal of grit.

Collection, storage and disposal of inorganic and organic material may lead to accidental releases of this material resulting in impacts to surface soil. Potential releases of the inorganic and organic material could result during cleaning processes resulting in the creation of surface soil and dust pollution.

New pipeline connecting between the new Inlet Works and new Bioreactors

The new pipeline will provide a connection between the Inlet works and the new Bioreactors

Nil. Provision of new infrastructure with new fittings will limit the likelihood of accidental leaks / spills / releases and impacts to soil or land surrounding the pipeline.

Bioreactors 1 & 2

Two single-pass bioreactors are provided in Stage 1. Fine-screened influent and membrane return activated sludge (RAS) from the membrane tanks is divided into each bioreactor by the splitting chamber. The bioreactors are configured for biological phosphorus removal and chemical phosphorus removal with ferric sulphate.

Storage and handling of ferric sulphate on-site may lead to accidental leaks / spills / releases resulting in impacts to surface soil.

Membrane Maintenance and Blower Building (incl. Control Room)

Provides a facility to house bioreactor aeration blowers, membrane air scour blowers, membrane clean-in-place facilities, and new electrical switchroom.

Storage and handling of equipment and cleaning products on-site may lead to accidental leaks / spills / releases resulting in impacts to surface soils

Membrane tanks

Mixed liquor from the bioreactors enters the membrane tanks via the membrane feed pump. RAS flows back to the bioreactors by gravity. Sodium hypochlorite and citric acid will be utilised as a clean-in-place system to remove organic and inorganic accumulations in the membrane tanks.

Storage and handling of sodium hypochlorite and citric acid on-site may lead to accidental leaks / spills / releases resulting in impacts to surface soil. Note: current storage of sodium hypochlorite and citric acid is within bunded tanks.

New Effluent Transfer Pipeline

The new pipeline will provide a connection between the Membrane Bioreactor and the existing Clarifier Effluent Pipe


Access road Provide access to demolition and construction teams via a bitumen road.

Nil. Confining access to the site for demolition and construction plant and machinery will minimise impact to soil at the site.

New pipeline to connect existing discharge, duplicating a similar pipeline nearby

Provides a connection between Membrane Tanks and existing Clarifier Effluent Pipe


General processes

Recycled water disinfection and pump station. hypochlorite dosing system Dewatering of waste activated sludge (WAS) and disposal of WAS.

Storage and handling of hypochlorite on-site may lead to accidental leaks / spills / releases resulting in impacts to surface soil.

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Biosolids Stabilisation and Dewatering Unit. Sludge will be stored in a sludge storage bin for 2-3 days before it is transferred to Melton for further processing

Storage and handling of liquid polymer on-site may lead to accidental leaks / spills / releases resulting in impacts to surface soil. Storage and handling of dewatered sludge on-site may lead to accidental leaks / spills / releases resulting in impacts to surface soil. Storage of undigested dewatered sludge for 2-3 days will generate odour.

4.1.3 Potential for Soil Impacts from Project activities

During the construction and operation of Gisborne RWP, there is the potential for stored chemicals, fuels and machinery to result in accidental spillage. Table 4-3 lists the types and approximately storage quantities of materials that are likely to be present at the Site.

Management and mitigation measures, which intend to prevent loss chemicals used and/or stored at the Site should be developed in order to minimise the likelihood of this event occurring, and the potential impact on human health and the environment.

Table 4-3: Chemicals stored on-site

Chemical Volume Location

Ferric sulphate 10,000 L Dosing bund/next to clarifier

Magnesium hydroxide liquid 3,000 L Dosing trailer next to control building

Sodium hypochlorite 5,000 L UV Building chemical dosing

500 L Dosing trailer

4.1.4 Storage and Usage of Excavated Soils

Soils will be excavated during the construction of the new infrastructure at Gisborne RWP. Approximate soil volumes created from these excavations are outlined in Table 4-4.

Table 4-4: Proposed infrastructure and quantity of soil removed


Approx. Area Size Approx. Depth Approx. Quantity Soil Removed

Bioreactors 1 & 2 50 x 26 m 3 m 3,000 – 5,000 m³

Membrane tanks ~30 x 15m 2 m 800 m³


200 m (length) x 1 m (width) 2 m 400 m³

Access road 190 m NA. Varying cut / fill depths throughout the length of the road

3,640 m³

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Approx. Area Size Approx. Depth Approx. Quantity Soil Removed

New pipeline to connect to existing discharge, duplicating a similar pipeline nearby

150 m (length) 3 m 480 m³

Soil analytical data was not provided at the time of reporting. It is advised that in-situ soil sampling is undertaken prior to intrusive works at the site. This will provide a preliminary understanding of soil waste classification to advise if soil can be disposed or reused on site. Following excavation of soils, the material should be sampled and analysed in accordance with EPA IWRG621 legislation. Stockpiled soil should be managed in accordance with EPA Publication 275. This will be documented in the Construction Environmental Management Plan.

4.2 Mitigation and Management

4.2.1 Contaminated Land Risks and Mitigation

Risks identified in the Stage 1 construction phase include encountering unidentified contaminated soils during excavation, which may result in an impact upon human health and the environment. Limited information exists regarding the potential for contaminated soils at the Site. In-situ sampling and laboratory analysis of soils across the site is recommended prior to earthworks to assess the presence and extent of existing contamination at the Site. Further, it will allow for a better estimation of what soils can be re-used on-Site and what will require disposal. The assessment should be undertaken in accordance with the SEPP (Prevention and Management of Contamination of Land) and the National Environment Protection (Assessment of Site Contamination) Measure.

During construction and operation, management measures should be employed to reduce likelihood of potential leaks / spills / releases of stored chemicals on-Site associated with their storage, handling and usage.

Where soils are to be imported to the Site (i.e. during the construction of new access road or temporary construction requirements), all soils should be documented and shall comply with the requirements of SEPP (Prevention and Management of Contamination of Land) and EPA Publication 627.

The Site Environmental Management Plan (EMP) should be updated to incorporate and manage changes to the Site in relation to new infrastructure and processes that have the potential to contaminate the Site during construction work and in future operations. A standalone Construction Environmental Management Plan (CEMP) will be implemented for the construction phase and it will document the management and mitigation measures recommended in this report. Construction and operation activities accompanied by the impact pathway and management measures are shown in Table 4-5.

Excavated material from the construction will be visually inspected, and where necessary, the soil will be sampled and analysed for potential contaminants for the purpose of categorisation. The CEMP will specify the requirements for appropriate stockpiling of any waste materials generated, or encountered during the works, and any classified waste will be disposed off-site to a licenced waste facility. No waste material will be stored on site permanently unless it is categorised as “Fill Material” per IWRG621 Industrial Waste Resource

Guidelines - Soil Hazard Categorisation and Management (2009). Soils with all contaminant levels below the IWRG621 Table 2 Total Concentration (TC0) threshold are categorised as clean fill. Clean fill material will be reused or repurposed on site.

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Table 4-5: Activity, impact pathway and management measures

Activity Impact Pathway Management Measures

Construction Works

Transport and use of chemicals Construction at the Site

Accidental spill / leak / release of chlorine impacting on human health and safety or the environment

Store hazardous chemicals in primary (storage container) and secondary containment (bunding). Store and handle hazardous chemicals on sealed surfaces.

Construction equipment and vehicles

Accidental spills and leaks from construction equipment and vehicle impacting on human health or the environment

Ensure machinery and equipment is properly serviced prior to driving onto the Site. Operate and store machinery and equipment on sealed surfaces where possible.

Earthworks Production of dust during earthworks impacting human health or the environment

Apply water to soil during earthworks in order to supress dust generation. Avoid earthworks on windy days.

Excavating, trenching, tunnelling and decommissioning

Encountering contaminated soils impacting on human health or the environment. Creation of dust during earthworks could cause potential airborne contaminants which could impact on human health or the environment.

Undertake soil investigation prior to intrusive works. Establish a plan to manage unexpected finds if uncovered. Apply water to soil during earthworks in order to supress dust generation. Avoid earthworks on windy days.

Stockpiled material (Site-generated)

Stockpiled material (i.e. excavated soils) intended for re-use on-site may mix with excavated material planned for off-site disposal Stockpiled soil may become eroded by weather, and runoff into Jacksons Creek and impacting human health or the environment Open excavations providing a fall-risk (safety) to on-site users

Ensure material bought on site is tracked and stored correctly. Store stockpiled material on a sealed surface (hardstand or black plastic). Cover stockpiled material if storing on-site for prolonged periods of time, or if windy weather exists. Fence-off open excavations.

Stockpiled material (imported) Imported material (i.e. clean Fill, construction material, sand, aggregate) may mix with site soils, reducing the quality of the material for use on-site Stockpiled material may become eroded by weather, and runoff into Jacksons Creek and impacting human health or the environment

Operation

New DN710 HDPE pipeline, connecting between the existing Header Pit to the new Inletworks

Nil. Provision of new infrastructure with new fittings will limit the likelihood of accidental leaks / spills /

-

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Activity Impact Pathway Management Measures releases and impacts to soil or land surrounding the pipeline.

New Inlet works (incl. two grit chambers)

Collection, storage and disposal of inorganic and organic material may lead to accidental releases of this material resulting in impacts to surface soil. Potential releases of the inorganic and organic material could result during cleaning processes resulting in the creation of surface soil and dust pollution.

Store and handle inorganic / organic material on sealed surfaces. Undertake cleaning in a contained environment. Avoid cleaning on windy days.

New pipeline connecting between new Inlet works and new Bioreactors


-

Bioreactors 1 & 2

Storage and handling of ferric sulphate on-site may lead to accidental leaks / spills / releases resulting in impacts to surface soil.


Membrane Maintenance and Blower Building (incl. Control Room)

Storage and handling of equipment and cleaning products on-site may lead to accidental leaks / spills / releases resulting in impacts to surface soils.


Membrane tanks

Storage and handling of sodium hypochlorite and citric acid on-site may lead to accidental leaks / spills / releases resulting in impacts to surface soil.




-

Access road

Nil. Confining access to the site for demolition and construction plant and machinery will minimise impact to soil at the site.

-

New pipeline to connect existing discharge, duplicating a similar pipeline nearby


-

General processes

Storage and handling of hypochlorite on-site may lead to accidental leaks / spills / releases resulting in impacts to surface soil.


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Activity Impact Pathway Management Measures

Storage and handling of liquid polymer on-site may lead to accidental leaks / spills / releases resulting in impacts to surface soil. Storage and handling of dewatered sludge on-site may lead to accidental leaks / spills / releases resulting in impacts to surface soil. Storage of undigested dewatered sludge for 2-3 days will generate odour.

Ensure odour-control procedures are in place.

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5 Potential groundwater impacts

5.1 Risks The upgrade works at Gisborne RWP include a number of structures that will require excavation below ground level, as described in Section 2. Depending on the location and depth of excavation, groundwater may be intersected. If this is the case, dewatering may need to occur during the excavation and construction period to keep the pit dry until an impermeable lining is put into place. The volume of groundwater that may enter an excavation depends on the:

◼ Dimensions of the excavation ◼ Length of time the excavation is open to groundwater flow ◼ Groundwater head above invert depth ◼ Hydraulic properties of the geological formation intersected.

These details are outlined in Table 5-1 for each of the proposed excavations, along with the expected groundwater head above or below the structure. Groundwater levels were based on groundwater contours in Figure 3-2, and nearest bores where available.

Comparison with groundwater levels shows that six of the planned excavation will intersect groundwater and may need to be dewatered to allow for construction. Depending on the volume and duration of dewatering, groundwater receptors on site may be impacted by reduced groundwater levels. Risks of dewatering will therefore be assessed for these excavations.

Table 5-1: Summary of excavations and potential for intersecting groundwater

# Structure Area Proposed excavation depth (mAHD)1

GW elevation (mAHD)2

Head above invert (m)3

Aquifer material

1

New DN710 HDPE pipeline, connecting between the existing Header Pit to the new inlet works

DN710 80 m length

◼ Pipe to be installed with 1.0 m of cover and cross the drainage trench above ground (pipe bridge)

◼ ~ 2.0 mBGL in underground sections

402.5 – 405.0 mAHD

- (watertable at least 2 m below pipeline excavation – discussion in Section 5.1.1)

Weathered basalt

2

New Inlet works Structure (including grit chamber)

13m x 23m

◼ Foundation at least about 1.0 mBGL

◼ 413.0 mAHD main structure / 411.0 mAHD (grit chamber)

404.0 mAHD

- (watertable at least 5 m below base of structure)

Alluvium/ weathered basalts

3

New Pipeline connecting between the new Inletworks and new Bioreactors

- Above ground - -

4

New Bioreactors 1 & 2 (plus reserved area for future bioreactor)

50m x 26m

◼ 3.0 mBGL (potentially up to 5.0 mBGL)

◼ Base of excavation 405.6mAHD

404.0 mAHD

For excavation 3.0 mBGL: watertable at least 0.6 m below


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Aquifer material

excavation – discussion in Section 5.1.2) 1.4 m for excavation 5.0 mBGL

5

New Membrane Maintenance and Blower Building (incl. Control Room)

~30m x 15m

◼ Foundation ~ 2.0 mBGL ◼ Base of excavation

410.6 mAHD 405.0 mAHD

- (watertable at least 4 m below excavation)


6 New Membrane Tanks

16m x 12m

◼ Foundation ~ 1.0 mBGL ◼ Base of excavation

406.6 mAHD 404.0 mAHD

- (watertable at least 1.6 m below excavation – discussion in Section 5.1.3)

Alluvium

7

New MBR Effluent Transfer Pipeline, connected to the existing Clarifier Effluent Pipe (near the bioreactors)

DN375 (OD426 mm) 200 m length x 1 m width

◼ ~2.0 mBGL installed with 1.0 m cover

◼ Base of excavation ~ 402.0 mAHD

400.0 – 405.0 mAHD

- (watertable at base of excavation near clarifier – discussion in Section 5.1.4)

Alluvium

8 New access road 190 m long

◼ Excavation up to 3.4 mBGL Base of excavation ~ 405.0 – 420.0 mAHD

402.5 – 412.5 mAHD

- (watertable at least 1.5 m below base of road excavation – discussion in Section 5.1.5)

Weathered basalts

9

New DN375 (OD426 mm) pipeline to discharge secondary effluent directly to creek (east)

DN375 (OD426 mm) 150 m length x 1 m width

◼ ~2.0 mBGL installed with 1.0 m cover

◼ Base of excavation ~397.5 mAHD

397.5 – 400.0 mAHD

2.0 m (discussion in Section 5.1.6)

Alluvium

10 Chemical area 23m x 7 m

◼ Building footings ~ 1.0 mBGL

◼ Base of excavation 406.1 mAHD

404.0 mAHD

- (watertable ~1.1 m below base)

Alluvium

11 Pump room 28 x 6



404.0 mAHD - (watertable at least 1.6

Alluvium

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Aquifer material

m below excavation)

12 Dewatering area 20m x 10 m



402.0 mAHD

- (watertable 1.1m below excavation)

Alluvium

13 Odour control 8.6 x 19.2

◼ Foundation 0.5 mBGL ◼ Base of excavation

408.1 mAHD 404.0 mAHD

- (watertable at least 3.1 m below base)

Alluvium

Notes: 1 calculated as base of structure plus 0.5m 2 groundwater elevation interpreted from watertable contours (Figure 3-2), and closest bore 3 calculated as base of excavation compare to adopted groundwater level, plus 1m to account for

seasonal variation The structures identified in Table 5-1 requiring excavations deeper than 1 m and where the depth to watertable below the base of excavation is less than 2 m (i.e. #1, #4, #6, #7, #8, #9) are discussed further below. The relationship between invert depth and depth to groundwater at each structure is discussed in more detail to determine whether the watertable will be intersected. Structures where the building footings are proposed at 1 mBGL (i.e. #10, #11, #12, #13) were not further analysed as they are considered as shallow excavations and pose a low risk of intersecting groundwater.

5.1.1 #1 – New HDPE pipeline

The excavated area for the new HDPE effluent transfer pipeline will be 80.0 m long, 1.0 m wide and no more than 2.0 m deep (based on a pipe diameter of 710 mm, with 1m of cover). The design of the underground section of the invert is assumed based on comments from the design team. Surface elevation ranges between 407.4 to 416.3 mAHD, with the highest elevation at the header pit at the western end of the pipeline.

Groundwater elevation is expected to be between 402.5 and 405.0 mAHD along the length of the pipeline. Shallowest groundwater levels occur at the drainage channel, however it is expected that the pipeline will be above ground at this location. Groundwater levels may vary up to 1.0 m across the site during wet season, resulting in groundwater elevation of 403.5 to 406.0 mAHD along the pipeline. Figure 5-1 illustrates

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groundwater levels with pipe trench depths, showing that groundwater will be at least 2 m below the excavation. Therefore, no further consideration of groundwater impacts is necessary for this pipeline.

Figure 5-1 Cross-section of the new HDPE pipeline and comparison with groundwater levels

5.1.2 #4 – New Bioreactors 1 & 2

The new bioreactors area is west of the sludge drying bed and aerobic/anoxic tanks. The excavation for the bioreactors is anticipated to be 50 m length, 20 m width and 3 m depth. The depth of base of the structure is 406.1 mAHD, with the base of excavation assumed to be 405.6 mAHD. The depth and dimensions were determined based on design drawing 6583830-GAX-G-003 (15/11/2019). Surface elevation at the bioreactors is approximately 409.5 mAHD, which was determined based on depth below ground.

Groundwater contours in the area suggest groundwater elevation at the location of the proposed bioreactors is between 402.5 to 405 mAHD. The bioreactors are located between the two contours and a groundwater elevation of 404.0 mAHD was adopted. Seasonal variations are expected, and groundwater elevation can be up to 405.0 mAHD. Based on these predictions, the bioreactor excavation is unlikely to intersect groundwater. The watertable is expected to be between 0.5 - 1.5m below the base of the excavation for the bioreactors. This suggests dewatering will not occur as part of construction and groundwater levels will remain unchanged and there is no risk to groundwater receptors such as Jacksons Creek. The relationship between the excavation and groundwater levels can be seen in Figure 5-2.

It should be noted, the design for the new bioreactors mentions a possible invert depth of 5.0 mBGL for some parts of the structure. Based on observed groundwater elevation at MW07, the unsaturated thickness

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of the aquifer beneath the surface is estimated to be up to 5.0 m at the proposed location for the bioreactors. Therefore, for construction purposes, it should be assumed that groundwater will be intersected if the depth of the invert is 5.0 mBGL.

Figure 5-2 Bioreactor and excavation depth compared to groundwater levels

5.1.3 #6 – New Membrane Tanks

The location of the new membrane tanks is west of the sludge drying bed and aerobic/anoxic tanks, south of the proposed new bioreactors. The proposed size of the tanks is 16 x 12 m and up to 1 mBGL (~407.1 mAHD). The excavation depth is 406.6 mAHD. The depth and dimensions of the tanks were determined based on design drawing 6583830-GAX-G-003 (15/11/2019). The surface elevation was determined based on base of tank elevation of 407.5 mAHD. The ground surface is approximately 408.5 mAHD, based on invert level and proposed excavation depth below ground.

Groundwater contour mapping suggests groundwater elevation beneath the excavation is between 402.5 – 405.0 mAHD (see Figure 3-2). Since the membrane tanks are between the contours, a groundwater elevation of 404.0 mAHD was adopted, which equates to a depth of approximately 4.5 mBGL. Seasonal groundwater fluctuations of up 1.0 m are possible, which could result in groundwater levels of 3.5mBGL. The relationship between the excavation and groundwater levels can be seen in Figure 5-3.

Given the shallow depth of the membrane tanks and interpolated groundwater levels, it is unlikely that groundwater will be intersected at this location. The watertable is expected to be between 1.5 – 2.5 m below the base of the excavation for the structure. Therefore, no further analysis will be undertaken for the membrane tanks.

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Figure 5-3 Depth of new membrane tanks and comparison with groundwater levels

5.1.4 #7 – New MBR Effluent Transfer Pipeline

The new MBR effluent transfer pipeline trench will be 200 m long, approximately 1 m wide and up to 2 m deep (based on a pipe outer diameter of 426 mm, with 1m of cover). The lowest elevation part of the trench was estimated by the design team at ~402.0 mAHD. Surface elevation along the length of the pipeline ranges between 403.1 to 408.5 mAHD, with the higher elevation observed near the North Region Depot. Based on the surface elevation contours, the lowest excavation depth is likely at 401.1 mAHD.

Groundwater contours suggest groundwater elevation will be between 400.0 and 405.0 mAHD along the alignment. Depth to groundwater is expected to be between 1.0 – 3.5 m BGL. Shallow groundwater is likely to occur at the section of the pipeline near the clarifier. Groundwater levels may be up to 1 m higher during the wet season, reducing the depth to groundwater to 0.1 to 2.5 mBGL. Figure 5-4 shows the relationship between surface, trench and groundwater.

Based on the provided invert depth, groundwater is not expected to be intersected in this excavation. If groundwater is intersected it will likely occur over a small section of the alignment near the existing clarifier tank, as shown in Figure 5-4. The rest of the alignment is located on an elevated surface at the existing Northern Region Depot and is unlikely to intersect groundwater as a) the invert depth is assumed to be 2 m BGL therefore elevation of the invert will be approximately 406.5 mAHD and b) groundwater levels at that location are expected to be 404.0 mBGL. Hence there is expected to be 3.0 m of unsaturated sediments below the base of the trench and the watertable. Therefore, groundwater impacts will not occur and there is no need for further analysis.

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Figure 5-4 Depth of new MBR pipeline and comparison with groundwater levels

5.1.5 #8 – New access road

The proposed new access road is 190 m long with a design surface which ranges between 405.0 – 420.0 mAHD. The proposed design requires up to 3.4 m of excavation below the existing surface. Groundwater levels beneath the proposed location for the new road range between 402.5 and 412.5 mAHD as shown in Figure 3-2.

To best understand the relationship between the existing surface, design surface and groundwater elevation, a simple cross-section was created. Groundwater levels at each section of the design were interpreted using the groundwater contours (Figure 3-2). As seen in Figure 5-5, groundwater is between 3.0 m and 8.0 m below the design surface elevation of the road and is therefore unlikely to be intersected during the excavation for the new access road. No change to groundwater levels is expected to occur as a result of the construction works, and therefore no further analysis is required for this structure.

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Figure 5-5 Access road excavation and comparison to groundwater level

5.1.6 #9 – New Pipeline to discharge effluent

The excavation for the new MBR effluent transfer pipeline will be approximately 150 m long, 1 m wide and up to 2m deep (based on an outer diameter of 426 mm, with an assumed 1m of cover). The lowest part of the trench is at approximately 397.5.0 mAHD (~2.5 - 3.5 mBGL). Surface elevation along the length of the pipeline is between 400.0 – 401.0 mAHD.

Groundwater contours suggests groundwater elevation will be between 397.5 and 400.0 mAHD. Groundwater elevation of 399.6 mAHD was adopted as most likely, based on the water level observed in MW02A. The relationship between groundwater levels and trench depth is shown in Figure 5-6. Shallow groundwater levels are expected at this location, and up to 2.0 m of groundwater head above the invert depth is possible. It is expected that dewatering will be necessary to keep the pit dry for construction of the pipeline. As such, estimated inflow volumes have been calculated using the Thiess equation (see Section 5.2).

This location is in close proximity to the creek and groundwater drawdown associated with dewatering of the excavation may impact groundwater discharge to Jacksons Creek. Analysis of impacts to Jacobs Creek using the Darcy equation is presented in Section 5.3.

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Figure 5-6 New effluent discharge pipeline and comparison with groundwater levels

5.2 Approach Dewatering of excavations is likely to be required during construction where excavations are below the watertable. Dewatering will lower groundwater levels around the excavation, which may cause impacts to other groundwater users, such as connected surface water systems, vegetation, and existing bores. These risks need to be assessed. The objectives of this project with regard to groundwater during construction can therefore be described as follows:

◼ 1. Identify the potential volume of dewatering associated with construction works ◼ 2. Identify the potential impacts of lower groundwater levels on groundwater receptors

The groundwater head likely to be encountered at each excavation is stated inTable 5-1, indicating that only the new pipeline to discharge secondary effluent (#9) will intersect groundwater. Further analysis of groundwater inflows and impacts is required.

5.2.1 Groundwater inflows

Analytical modelling was used to estimate the potential volumes of groundwater inflows and associated impacts. The Thies (1935) equation is an analytical solution to transient groundwater flow that relates groundwater inflow to aquifer properties and groundwater drawdown. In this analysis, the solution was used to determine the pumping rate (or the rate of inflow to excavations) that would be required to keep the groundwater level at or below the base of the excavation.

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A Thies analysis has been undertaken for each of the structures which were identified as likely to intersect the water table (i.e. structure #9 in Table 5-1). The excavation dimensions and depth, as well as timing for excavation and construction were based on drawings and advice from the CH2M Beca design team as of 09/09/2020.

The intersected aquifer is alluvium and/or weathered basalt, as stated in Table 5-1. The alluvial sediments consist of silt, clay, sand and cobbles of fine to medium density. The alluvial sediments are about 4 m thick and sandier in proximity to Jacksons Creek, however clay is more prevalent south of the lagoons. Since the upper parts of the basalt are weathered to clay, the alluvium and the weathered basalt were considered as a single aquifer unit for the purposes of the analysis.

There has been no on-site testing of aquifer properties, so typical values for fine to medium grained alluvium were adopted in the analysis. Based on hydraulic conductivity data compiled from multiple projects around Victoria, the most likely hydraulic conductivity for alluvium was estimated as 0.44 m/day. Transmissivity was calculated as hydraulic conductivity by saturated thickness. A typical storativity for unconfined aquifers was selected (0.1).

To characterise uncertainty in aquifer properties, in particular hydraulic conductivity, groundwater inflows were quantified for a ‘most likely’ scenario, as well as a ‘worst case’ scenario. The hydraulic conductivity for these scenarios was 0.44 m/day as the most likely, and 1m/day as the worst case. The worst case scenario is not expected to occur, but rather is intended to represent the highest likely groundwater inflows and drawdown possible at the site.

A number of uncertainties are associated with this method of estimating inflows. Assumptions for the method include the homogenous, isotropic, inform thickness and infinite nature of the aquifer. The aquifer properties such as hydraulic conductivity, transmissivity and storativity have been estimated based on previous studies, however no investigation was conducted on the hydraulic properties of the aquifer at the site and therefore real-life conditions may differ. Additionally, depth to groundwater has been measured at the RWP mostly in May or December and maximum groundwater levels post wet season may be higher.

Despite the uncertainties, the Thies equation is an appropriate analytical method for estimating inflows and drawdown in an aquifer. Outcomes from the Thies analysis will be estimated inflow volumes for the duration of construction period, as well as drawdown of groundwater around the structures. As the infrastructure will be functionally impermeable, inflows during operation are not expected. However, comment will be made of the potential impact of groundwater pressure on below-ground structures, and the need for groundwater drainage.

5.2.2 Drawdown impacts

An assessment to determine the dewatering impacts on Jacksons Creek was carried out where groundwater levels are likely to be lowered due to dewatering of excavations. Darcy’s Law estimates flow in a saturated medium (i.e. an aquifer) along a hydraulic gradient, based on the hydraulic conductivity of the medium. The equation is:

𝑄 = −𝐾𝐴𝑑ℎ

𝑑𝑙

Where: K = hydraulic conductivity 𝑑ℎ

𝑑𝑙 = hydraulic gradient

A = cross sectional area of aquifer Q = discharge

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Darcy’s Law was used to calculate groundwater discharge to the creek under two conditions: unimpacted and with drawdown from excavation dewatering during construction. The difference between the two conditions indicates the volumetric impact on Jacksons Creek.

5.3 Results The comparison of groundwater levels to likely excavation depths shows that only one structure is expected to intersect groundwater: the new pipeline to discharge secondary effluent to the creek (#9 in Table 5-1). The analysis methods described above have been applied to this structure in the following sections, to determine:

◼ Estimated groundwater inflows during excavation and construction of the trench ◼ Groundwater drawdown around the trench during construction ◼ Potential reduction in flow in Jacksons Creek as a result of groundwater drawdown

5.3.1 Groundwater inflows

The trench for the new pipeline to discharge secondary effluent to the creek will be excavated to 397.5 mAHD along the entire length of the pipeline. With groundwater levels of 399.6 mAHD, approximately 2.1 m of drawdown needs to be achieved uniformly along the trench to maintain dry conditions for construction. The aquifer at the pipeline location consists of 4 m of alluvium, underlain by basalt to an estimated depth of 10m, based on borelogs and aquifer mapping (vvg.org.au). Inflows have been considered for two time periods using different methods:

◼ During excavation of the trench, which is assumed to take 5 days. This analysis uses the Thiess solution, which enables an estimate of inflow rates and groundwater drawdown around the excavation.

◼ Construction of the pipe inside the trench, taking an additional 10 days. This analysis uses the Darcy solution, which recognises that inflow rates will decline over the construction period, as the excavation is dewatered, lowering groundwater levels and the hydraulic gradient, and ultimately resulting in declining inflow rates.

The inflow volume for the new pipeline excavation was initially estimated using the Thies equation to simulate drawdown of groundwater levels to the base of the trench after 5 days of excavation. Input values and inflow results are shown in Table 5-2. The analysis estimates that approximately 194 m3/day will need to be pumped from the trench to keep it dry for construction under the most likely scenario (and 304 m3/day under the worst case scenario). It should be noted that some of this groundwater will be removed together with the excavated soil and so total actual inflow volumes will be slightly lower.

Table 5-2: Analytical modelling inputs and results for Thies analysis of groundwater inflows during excavation

Structure Excavation size (L x W m)

Excavation depth (mAHD)

Construction period (days)

Groundwater level (mAHD)

Drawdown required (m)

Aquifer parameters

Average daily inflow rates (m3/day)

Total inflows (m3)

#9 - New Pipeline to discharge effluent

150 x 1 397.5 5 days to excavate

399.6 2.1 Likely case: K = 0.44 m/day T = 3.5 m2/day S = 0.1 Worst case: K = 1 m/day T = 8 m2/day S = 0.1

Likely: 194 m3/day Worst case: 304 m3/day

More likely: 968 m3 Worst case: 1,520 m3

Transmissivity (T) = hydraulic conductivity (K) * saturated thickness (b)

The rates of dewatering shown in Table 5-2 lowers groundwater levels around the trench. The extent of drawdown (taken as the 0.1 m drawdown location) occurs at a distance of 35m from the trench for the likely scenario, and 50 m for the worst case scenario. The impacts of this groundwater drawdown are discussed further in Section 5.3.2.

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To maintain groundwater level at the base of the trench for the following 10 days while construction is occurring, dewatering will need to continue. However, dewatering during excavation will have already lowered groundwater levels, resulting in a lower hydraulic gradient towards the trench, and therefore, lower inflow rates.

To account for the lower hydraulic gradients as a result of earlier dewatering, the Darcy solution was used to calculate inflows during construction of the pipe inside the trench (Thies solution does not allow for variable flow rates). The construction period was estimated to take 10 days. Inputs and results of the analysis are shown in Table 5-3.

As for the Thies analysis, two scenarios were analysed for a likely hydraulic conductivity (0.44 m/day) and a higher hydraulic conductivity (1 m/day). The hydraulic gradient was calculated as the distance from the trench to edge of the drawdown cone after the 5 day excavation period (30 m) and the total groundwater drawdown at the trench (2 m). Area is calculated as the length of the trench by the aquifer thickness beneath the trench (8 m – 2 m drawdown from previous 5 day period). Total inflows expected over the 10 day construction period are 240 m3/day (likely case) or 390 m3/day (worst case).

Table 5-3 Analytical modelling inputs and results for Darcy flow to estimate

Structure Excavation size (L x W)

Excavation depth (mAHD)

Construction period (days)

Hydraulic conductivity (K)

Hydraulic gradient (dh/dl)

Area (m2) Average daily inflow rates (m3/day)

Total inflows (m3)


150m x 1m

397.5 (2m) 10 days to construct impermeable lining

K = 0.44 m/day (likely case) K = 1 m/day (worst case)

2.1 m / 35 m = 0.07

150 x (8-2) = 900 m2

Likely: 24 m3/day Worst case: 39 m3/day

Likely: 240 m3 Worst case: 390 m3

Expected groundwater inflows (and dewatering volumes) are summarised in Table 5-4. Over the 15 day period of trench excavation and pipe installation, the predicted groundwater inflow is approximately 1,208 m3 (likely case) or up to 1,910 m3 (worst case).

Table 5-4 Estimated inflow rates and volumes

Structure Time period Average daily inflow rates (m3/day) Total inflows (m3)


5 day excavation period Likely: 194 m3/day Worst case: 304 m3/day

Likely: 968 m3 Worst case: 1,520 m3

10 day construction period Likely: 24 m3/day

Worst case: 39 m3/day Likely: 240 m3 Worst case: 390 m3

Total Likely: 1,208 m3 Worst case: 1,910 m3

The above volumes can be used a starting point for assessing the potential inflow volumes expected to achieve the required drawdown of 2.1 m. If more certainty is required, additional hydraulic testing may be necessary. The ‘more likely’ scenario is recommended to be used for planning of dewatering and disposal options. The ‘worst case’ estimate should be considered as a contingency. If excavation or construction timeframes increase, for example a delay between excavating the trench and completing the pipeline construction, volumes of groundwater inflow will also increase. Likewise, if the trench is deeper than assumed in this analysis, groundwater inflow rates will be higher.

5.3.2 Potential impacts

Given the likely need for dewatering of the pipe trench, the potential impacts of groundwater drawdown on groundwater receptors must be assessed. Jacksons Creek is a groundwater dependent ecosystem, which relies on groundwater inflows for baseflow, and to maintain permanent pools over summer. Given the

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proximity of Jacksons Creek to the site and its hydraulic connection with the alluvial aquifer and consequently groundwater, dewatering of excavations may have an impact on the creek.

To construct the new pipeline for effluent discharge (#9) up to 2 m of drawdown is required to keep the excavation dry during digging and construction which is estimated to take 15 days in total. The Thies (1935) method was used to estimate the expected drawdown at the creek under the most likely (1208 m3) and worst case (1920 m3) dewatering conditions at the site. Drawdown was estimated using the Thies equation, and assuming that the total calculated groundwater inflow volumes occurred at a consistent rate over the 15 day period (Thies only allows a constant rate). The results of the modelling are detailed in Table 5-5.

Under both the most likely and worst case scenarios, groundwater drawdown intersects Jacksons Creek, indicating that groundwater discharge to the creek may be reduced as a result of dewatering. The likely case predicts that an average drawdown of 0.2 m will affect a 180 m stretch of the creek. For the worst case, an average drawdown of 0.4 m affects 230 m of the creek. Table 5-5 summarises these drawdown results.

Table 5-5 Estimated drawdown at Jacksons Creek

Structure Construction period Drawdown at Jacksons Creek (m) Length of creek affected by >0.1m drawdown


After 5 day of excavating

Most likely: 0.2 m Worst case: 0.4 m

180 m 230 m

Darcy’s Law was used to estimate the change in groundwater inflows to the creek caused by the drawdown estimates in Table 5-5, for likely and worst case scenarios. The hydraulic gradient was derived by assuming that groundwater levels at the creek are 3.6 m lower (~396.0 mAHD) than groundwater levels at the location of the pipeline, as seen in Figure 5-7. This level is representative of the lowest point of the creek in the east, however the hydraulic gradient is lower in the west. Current groundwater levels at the pipeline were determined using observed levels at MW01As, MW02A and MW05 which range between 399.46 – 399.65 mAHD (399.6 mAHD adopted). The nearest distance from the creek to the pipeline alignment is 30 m, which has been adopted to determine the hydraulic gradient. The saturated thickness of the aquifer is 8.0 m based on bore logs from MW01AD. The saturated thickness of the aquifer beneath the creek is assumed to be 7.0 m, which accounts for the creek invert depth.

The results in Table 5-6 show that a reduction in groundwater discharge of 6.3 m3/day is expected, or 16.1 m3/day worst case. These estimates represent the maximum daily impact, which would occur at the end of construction (i.e. at day 15). After construction is complete and dewatering ceases, groundwater levels will recover and the daily impact on streamflow will gradually reduce to zero.

Table 5-6 Analytical modelling inputs and results for Darcy flow analysis of groundwater discharge to Jacksons Creek

Scenario Hydraulic conductivity (K m/day)

Hydraulic gradient (dh/dl)

Area (m2) Average groundwater discharge volume (m3/day)

Reduction in groundwater discharge (m3/day)

Current groundwater discharge

0.44 399.6-396 / 30 = 0.12 180 10.3 m3/day -

Groundwater discharge under likely scenario

0.44 397.5-396 / 30 = 0.05 180 4 m3/day 6.3

Current groundwater discharge

1 399.6-396 / 30 = 0.12 230 27.6 m3/day -

Groundwater discharge under worst case scenario

1 397.5-396 / 30 = 0.05 230 11.5 m3/day 16.1

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Figure 5-7 Hydraulic gradient between pipeline location and Jacksons Creek

The predicted reduction in groundwater discharge to Jacksons Creek as a result of dewatering for the pipeline construction has been compared to total streamflow to assess whether the reduction is significant. Streamflow in Jacksons Creek is monitored at the site via gauge 230233 located at the downstream end of the RWP and discharge volumes from the RWP are monitored at the licensed discharge point (Figure 5-8). Subtracting these data sources gives the upstream flow in Jacksons Creek, which has a median value of 4.1 ML/day and a low flow (10th percentile) value of 0.8 ML/day.

Dewatering of the pipeline excavation is estimated to reduce flow upstream of the gauge by 6.3 m3/day and 16.1 m3/day (worst case). The impact of this reduction on streamflow is summarised in Table 5-7, showing that the maximum impact on streamflow is a 2% reduction in flow. It is more likely that the impact will be 1% lower streamflow, or less. This level of impact is less than the level of measurement accuracy and is considered to be minor in terms of impact to streamflow (according to the ecosystems GDE guidelines (DELWP 2015). Added to this, the maximum impact will occur at the end of construction (after 15 days) and groundwater levels will then recover. As such, the maximum impact would occur for only a short time period, and would be negligible in terms of annual flow and ecosystem impacts.

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Figure 5-8 Jacksons Creek streamflow upstream and downstream of the RWP

Table 5-7 Impact of reduced groundwater discharge on flow in Jacksons Creek

Streamflow Streamflow (ML/day)

Scenario Reduction in streamflow (ML/day)

Remaining streamflow (ML/day)

Reduction in streamflow (%)

Median flow (50th percentile)

4.1 Likely case 0.0063 4.0937 0.2%

Median flow (50th percentile)

4.1 Worst case 0.016 4.084 0.4%

Low flow (10th percentile)

0.8 Likely case 0.0063 0.7937 0.8%

Low flow (10th percentile)

0.8 Worst case 0.016 0.784 2.0%

5.3.3 Groundwater disposal

The analytical modelling determined that up to 1,208 m3 (likely case) and 1,910 m3 (worst likely) of groundwater inflows will need to be disposed of as a result of dewatering the excavation for the new pipeline.

Groundwater quality at the site has been assessed against the current disposal licence conditions at Gisborne RWP which allows the RWP to discharge into Jackson Creek. The discharge of groundwater inflows to Jackson Creek, is most preferred as it is the easiest out of all potential options. Groundwater quality and volume of inflows needs to adhere to the conditions of the licence, the comparison for which is presented in Table 5-8.

Groundwater wells MW01AS, MW01AD and MW02A were used to establish the average groundwater quality likely to be encountered at the pipeline alignment. These wells were tested in 2017 (MW02A only), 2018, 2019 and 2020. The groundwater quality has been determined as the 90th percentile concentrations, which is a conservative estimate. MW05 was not considered since the well is installed in basalt and analyte concentrations are likely to be lower than those in the alluvium aquifer.

0.001

0.01

0.1

1

10

100

1000

Jack

son

s C

reek

flo

w (

ML/

d)

Streamflow d/s discharge point (230233) Streamflow u/s discharge point (230233 - RWP discharge)

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The comparison in Table 5-8 shows that the volumes of groundwater to be disposed of are sufficiently minor that they would not cause the daily discharge flow rate to be exceeded. Groundwater concentrations of ammonia, E.coli and maximum pH are within both maximum and annual median licensed discharge limits. The total phosphorous concentration in groundwater exceeds the licensed annual median limit, however, this will be diluted by the significantly larger volumes of effluent being discharged, which are below the licensed limit. A mass balance calculation shows that 1.9ML of effluent at 0.2 mg/L TP with 0.3ML of groundwater at 1.8 mg/L TP results in a total concentration of 0.42 mg/L, which is within the median limit. The impact on annual concentration will be negligible given that groundwater will only be added over the 15 day construction period. Similarly, the addition of total nitrogen concentrations in groundwater will be within licensed limits, as concentrations in groundwater and effluent are below the maximum limit, and will also be below the annual median, given the very small volume of groundwater compared to effluent being discharged on an annual basis (mass balance shows concentration of 6.75 mg/L total nitrogen over the construction period). Low groundwater pH will also be mitigated by the higher effluent pH so that the total discharge is within licence limits.

Groundwater on site is not analysed for BOD or suspended solids, therefore a comparison of groundwater quality to those guidelines has not been established. However, addition of the predicted small volumes of groundwater to effluent for a 15 day period is not likely to exceed limits that are calculated over an annual timeframe.

Table 5-8: Comparison of groundwater quality to effluent discharge and licence limits.

Indicator Limit type Discharge Limit Effluent quality discharge Groundwater quality

Flow rate Max Daily flow (ML/D) 2.4 1.9 ML/day (average)

2.1 ML/day (90th percentile)

More likely: 0.2 ML/day

Worst case: 0.3 ML/day

Ammonia Annual Median (mg/L) 2 0.6 0.3

Ammonia Maximum (mg/L) 3

Biochemical oxygen demand (BOD) (5 day)

Annual Median (mg/L) 5 4.7 Not analysed

E.Coli Annual Median (org/ml) 200 0 1

Suspended solids

Annual Median (mg/L) 10 2.0 Not analysed

Total nitrogen Annual Median (mg/L) 10 7.9 4.0

Total nitrogen Maximum (mg/L) 15

Total phosphorus

Annual Median (mg/L) 0.5 0.2 1.8

pH Maximum 9 7.2 (90th percentile) 7.4 (laboratory pH) (90th percentile)

pH Minimum 6 7.0 (10th percentile) 5.8 (laboratory pH) (10th percentile)

Given the long history of use at the RWP, groundwater quality for other parameters (i.e. not just those listed in the licence) should also be assessed to understand risk of groundwater discharge on ecology in Jacksons Creek. Metal concentrations in the groundwater wells nearest the proposed pipeline (MW01AS, MW01AD, MW02A) have been compared with default guideline values for protection of freshwater ecology (95 and 90%

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protection values, ANZECC 2018) in Table 5-9. A mass balance has been calculated assuming 0.2 ML/d groundwater is added into Maturation Lagoon 4 (3.38ML capacity) for 15 days (estimated pipeline construction period) in Table 5-10. Concentrations that exceed the guideline level for protection of 90% of aquatic species are highlighted pink.

The disposal of groundwater to maturation lagoon 4 would cause guideline levels for metals to be exceeded in maturation lagoon 4. This creates a risk to aquatic ecosystems in Jacksons Creek from the licensed discharge point. Further dilution could be achieved by distributing extracted groundwater between lagoons. If the groundwater discharge of 0.2 ML/d is spread between all four maturation lagoons during construction, dilution is sufficient to reduce most metal concentrations to below the guideline values for aquatic ecosystems. However, concentrations of cobalt, iron and zinc in the lagoons would still exceed aquatic protection guideline levels. Disposal of groundwater pumped from the pipeline excavation is therefore not suitable for addition to lagoons. It is recommended that alternative options are examined, such as treatment to remove metals, or off-site disposal.

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Table 5-9: Effect of groundwater discharge on water quality in maturation lagoon 4 after 15 days of dewatering

Table 5-10: Effect of groundwater discharge on water quality when added to all four lagoons

Alu

min

ium

(fi

lte

red

)

An

tim

on

y (f

ilte

red

)

Ars

en

ic (

filt

ere

d)

Bar

ium

(fi

lte

red

)

Be

ryll

ium

(fi

lte

red

)

Bo

ron

(fi

lte

red

)

Cad

miu

m (

filt

ere

d)

Ch

rom

ium

(II

I+V

I)

(fil

tere

d)

Co

bal

t (f

ilte

red

)

Co

pp

er

(fil

tere

d)

Iro

n (

filt

ere

d)

Man

gan

ese

(fi

lte

red

)

Me

rcu

ry (

filt

ere

d)

Mo

lyb

de

nu

m (

filt

ere

d)

Nic

kel (

filt

ere

d)

Sele

niu

m (

filt

ere

d)

Silv

er

(fil

tere

d)

Stro

nti

um

(fi

lte

red

)

Thal

liu

m (

filt

ere

d)

Tin

(fi

lte

red

)

Tita

niu

m (

filt

ere

d)

Van

adiu

m (

filt

ere

d)

Zin

c (f

ilte

red

)

mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

EQL 0.01 0.001 0.001 0.001 0.001 0.02 0.0002 0.001 0.001 0.001 0.01 0.001 0.0001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

ANZECC 2018 FW 95% 0.055 0.009 0.024 0.37 2E-04 0.003 0.001 0.001 0.3 1.9 6E-04 0.034 0.011 0.011 5E-05 3E-05 0.003 0.006 0.008

ANZECC 2018 FW 90% 0.08 0.094 0.68 4E-04 0.002 2.5 0.002 0.013 0.018 1E-04 0.015

Volume (ML)

Groundwater (90%ile) 3 0.2 0.008 0.003 0.097 0.008 1.165 ND ND 0.025 0.002 6.201 1.35 ND 0.005 0.025 0.002 ND 0.31 ND 0.015 ND 0.007 0.029

Maturation lagoon 4 3.38 0.01 ND ND 0.01 ND 0.07 ND ND ND ND 0.216 0.121 ND 0.003 0.007 ND ND 0.087 ND ND ND 0.001 0.013

Total concentration (weighted) 0.099 0.004 0.002 0.051 0.004 0.585 - - 0.012 0.001 3.030 0.699 - 0.004 0.015 0.001 - 0.192 - 0.008 - 0.004 0.021

Alu

min

ium

(fi

lte

red

)

An

tim

on

y (f

ilte

red

)

Ars

en

ic (

filt

ere

d)

Bar

ium

(fi

lte

red

)

Be

ryll

ium

(fi

lte

red

)

Bo

ron

(fi

lte

red

)

Cad

miu

m (

filt

ere

d)

Ch

rom

ium

(II

I+V

I)

(fil

tere

d)

Co

bal

t (f

ilte

red

)

Co

pp

er

(fil

tere

d)

Iro

n (

filt

ere

d)

Man

gan

ese

(fi

lte

red

)

Me

rcu

ry (

filt

ere

d)

Mo

lyb

de

nu

m (

filt

ere

d)

Nic

kel (

filt

ere

d)

Sele

niu

m (

filt

ere

d)

Silv

er

(fil

tere

d)

Stro

nti

um

(fi

lte

red

)

Thal

liu

m (

filt

ere

d)

Tin

(fi

lte

red

)

Tita

niu

m (

filt

ere

d)

Van

adiu

m (

filt

ere

d)

Zin

c (f

ilte

red

)

Lead

(fi

lte

red

)

mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

EQL 0.01 0.001 0.001 0.001 0.001 0.02 0.0002 0.001 0.001 0.001 0.01 0.001 0.0001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001

ANZECC 2018 FW 95% 0.055 0.009 0.024 0.37 2E-04 0.003 0.001 0.001 0.3 1.9 6E-04 0.034 0.011 0.011 5E-05 3E-05 0.003 0.006 0.008 0.003

ANZECC 2018 FW 90% 0.08 0.094 0.68 4E-04 0.002 2.5 0.002 0.013 0.018 1E-04 0.015 0.006

Volume (ML)

Groundwater (90%ile) 3 0.2 0.008 0.003 0.097 0.008 1.165 ND ND 0.025 0.002 6.201 1.35 ND 0.005 0.025 0.002 ND 0.31 ND 0.015 ND 0.007 0.029 ND

All lagoons 26.88 0.01 ND ND 0.01 ND 0.07 ND ND ND ND 0.216 0.121 ND 0.003 0.007 ND ND 0.087 ND ND ND 0.001 0.013 ND

Total concentration (weighted) 0.029 0.002 0.001 0.018 0.002 0.180 - - 0.003 0.001 0.817 0.244 - 0.003 0.008 0.001 - 0.109 - 0.002 - 0.002 0.015 -

Metals

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6 Summary and conclusions

6.1 Soil assessment Based on the desktop-based review of existing conditions at Gisborne RWP, the potential for existing land contamination is possible. The presence and extent are unknown due to limited records and absence of soil sampling and analysis for contaminants of potential concern. Historic and current land uses indicate the area may contain shallow soil contamination associated with the potential use of lead-based paints and asbestos containing material during the construction of the plant in the 1980s, contamination associated with historic uncontrolled filling across the Site, and historic spills / leaks / releases of chemicals stored and handled at the Site, and historic biosolid storage.

In general, construction and operation of new infrastructure will have minimal impact on soils at Gisborne RWP. It should be noted that any potential soil impacts are related to storage and handling of hazardous chemicals on-site, earthwork and their associated machinery / equipment, and the creation of dust pollution. Management measures should be employed during the construction and operation of Gisborne RWP to reduce potential environmental impacts and protect beneficial uses.

Intrusive soil sampling investigations should be undertaken prior to construction to identify the potential presence and extent of contamination (if any) at the Site. This will further assist with estimating soil quantities that can be re-used on Site, and those that should be disposed of.

6.2 Groundwater assessment The groundwater conceptual model highlights the presence of shallow groundwater, and areas of nutrient contamination in groundwater. Groundwater discharges to the environment (Jacksons Creek) at the northern boundary of the RWP, and this is the receptor for impacts caused by groundwater disturbance.

The potential for the following impacts was assessed:

◼ Potential for excavations to intersect groundwater ◼ Volume of groundwater inflows to excavations ◼ Extent of groundwater drawdown caused by dewatering of excavations ◼ Impact of groundwater drawdown on groundwater discharge to Jacksons Creek ◼ Ability to dispose of groundwater to the existing licensed discharge point to Jacksons Creek

The only structure expected to intersect groundwater was the new secondary effluent pipeline, which is anticipated to be installed 2 m below the watertable. Groundwater inflows were predicted to be 194 m3/day in the first five days of excavation, and 24 m3/day in the following 10 days when the pipe is under construction. Over the 15 day construction period, a total of 1,208 m3 is expected to flow into the excavation and require dewatering and disposal.

To account for uncertainty in aquifer parameters, a ‘worst case’ scenario was also analysed, which indicated inflows of 304 m3/day in the first five days, 39 m3/day in the following 10 days, and a total of 1,910 m3 inflow over 15 days. These results represent an upper limit on inflow estimates, and while they are not expected to occur, they should be considered in contingency planning.

Dewatering the excavation results in drawdown of groundwater levels in the surrounding aquifer. Analysis showed that drawdown extended from the pipeline trench to Jacksons Creek, indicating that impacts on groundwater discharge to the creek could occur. These impacts were calculated as a 6.3 m3/day reduction in

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groundwater discharge to the creek, which represented <1% of total creek flow at the RWP (or up to 2% in the worst case analysis). The impact on Jacksons Creek is therefore considered to be minor.

The simplest disposal option for groundwater extracted from the trench is to direct it into one of the maturation lagoon 4 where it will be disposed of to Jacksons Creek at the licensed discharge point. Comparison of expected groundwater quality with EPA licence limits shows that most groundwater nutrient concentrations are within licensed limits. Where groundwater concentrations are higher than licence concentration limits (total phosphorous and total nitrogen), the small volume of groundwater is diluted by the much larger volume of effluent, resulting in the total disposal meeting annual concentration limits. Disposal to maturation lagoon 4 is therefore acceptable under EPA license conditions.

However, metals concentrations in groundwater are higher than guideline values for the protection of aquatic ecosystems. In particular, concentrations of cobalt, iron and zinc in groundwater would cause water quality in maturation lagoons (and at the licensed discharge point) to exceed aquatic protection guideline values, and discharge of water to the creek therefore presents a risk to creek ecosystems. Disposal of groundwater pumped from the pipeline excavation is therefore not suitable for addition to lagoons. It is recommended that alternative options are examined, such as treatment to remove dissolved metals, or off-site disposal.

In summary:

◼ Apart from the new secondary effluent pipeline, none of the Stage 1 upgrade structures present a risk to groundwater.

◼ The new secondary effluent pipeline is expected to have negligible impacts on groundwater because: – The construction period is short (15 days), resulting in relatively minor groundwater inflow volumes – Although groundwater drawdown reaches Jacksons Creek, the reduction in groundwater discharge to

the creek is negligible relative to total creek flow. – No mitigation measures are necessary to reduce dewatering or groundwater drawdown associated

with construction of the pipeline. ◼ Groundwater extracted via dewatering of the excavation is not suitable quality for disposal to Jacksons

Creek via the licensed discharge point.

Once constructed, structures will be impermeable and therefore impacts to/from groundwater are unlikely. Since groundwater levels are below most of the structures, there is no risk of groundwater pressure impacts destabilising structures. While groundwater levels are above the new secondary effluent pipeline, the pipeline is expected to be sufficiently small that groundwater flow would not be restricted, and no stability impacts on the pipeline would be expected.

These results are based on current design drawings and advice specified in this report. If excavation or construction timeframes increase, for example a delay between excavating the trench and completing the pipeline construction, volumes of groundwater inflow will also increase. Likewise, if the trench is deeper than assumed in this analysis, groundwater inflow rates will be higher. The assessment would need to be repeated to determine impacts for any such changes in design and/or construction.

The risk of recycled water in lagoons 2, 3 and 4 causing contamination to groundwater was assessed as part of a previous study (Jacobs 2019). While some leakage from the lagoons was likely, evidence of groundwater contamination was low. As the Stage 1 upgrade works would improve water quality in these lagoons, there would be no change to the risk profile, such that groundwater contamination risk would remain low, and beneficial uses of groundwater would not be impacted.

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7 References

Construction and Remediation Advisory Services (CARAS), 2019, Western Water Gisborne RWP Project Risk Register Report

DELWP, 2014, Victoria - Seamless Geology 2014

DELWP, 2018, Victorian Aquifer Framework (VAF) 3D Surfaces (Groundwater Salinity).

EP Risk, 2019, Soil Investigation Report Gisborne Recycled Water Plant

Hazard Alert, 2020, Asbestos Pre-Demolition (Division 6) and Hazardous Materials Survey Report for Western Water, Gisborne Depot – Haywood Dr, Gisborne (Nominated Buildings)

Jacobs, 2019, RWP Groundwater Compliance Assessment: analysis of potential impacts and risks at RWPs, report for Western Water, May 2019

Out-Task Environmental, 2016, Recycled Water Plants Groundwater and Soil Monitoring Programs and Risk Assessment Report

Out-Task, 2020, Draft Annual Monitoring Program draft report

Senversa, 2017, Waste Soil Categorisation, Gisborne Recycled Water Treatment Plant, Letter Report

Western Water, 2020, Gisborne RWP existing equipment, chemical register

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