APPENDIX P: TECHNICAL REPORT FOR POLLUTANT UTFALL … · 2019. 11. 28. · TNG Darwin Processing Facility Outfall Pollutant Outfall Modelling Report 26 November 2019 ... has been

APPENDIX P: TECHNICAL REPORT FOR POLLUTANT OUTFALL

MODELLING

TNG Darwin Processing Facility Outfall Pollutant Outfall Modelling Report

26 November 2019 | 13135.101.R2.Rev0


© 2019 Baird Australia Pty Ltd as Trustee for the Baird Australia Unit Trust (Baird) All Rights Reserved. Copyright in the whole and every part of this document, including any data sets or outputs that accompany this report, belongs to Baird and may not be used, sold, transferred, copied or reproduced in whole or in part in any manner or form or in or on any media to any person without the prior written consent of Baird.

This document was prepared by Baird Australia Pty Ltd as Trustee for the Baird Australia Unit Trust for Animal Plant Mineral Pty Ltd. The outputs from this document are designated only for application to the intended purpose, as specified in the document, and should not be used for any other site or project. The material in it reflects the judgment of Baird in light of the information available to them at the time of preparation. Any use that a Third Party makes of this document, or any reliance on decisions to be made based on it, are the responsibility of such Third Parties. Baird accepts no responsibility for damages, if any, suffered by any Third Party as a result of decisions made or actions based on this document.

Commercial in Confidence

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Prepared for: Prepared by:

Animal Plant Mineral Pty Ltd 3/25 The Broadway Ellenbrook WA 6069 Contact: Mitch Ladyman Phone: +61 8 6296 5155

Baird Australia Pty Ltd as Trustee for the Baird Australia Unit Trust ACN 161 683 889 | ABN 92 798 128 010 For further information, please contact Jim Churchill at +61 8 6255 5080 [email protected] www.baird.com

13135.101.R2.Rev0 Z:\Shared With Me\QMS\2019\Reports_2019\13135.101.R2.Rev0_DarwinOutfall_PollutantModellingReport.docx

Rev. Date Status Comments Prepared Reviewed Approved

A 16/10/2019 Draft Issued For Client Review JC / RW DT JC

0 26/11/2019 Final Issued with Updated Figure Resolution JC RW JC



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Executive Summary The TNG Darwin Processing Facility Outfall Project proposes to establish a processing facility on land adjacent the Elizabeth River Bridge near Darwin in the Northern Territory. The operation of the processing facility will produce a waste stream that is proposed to be directed through a pipeline to an outfall location in Darwin Harbour which will discharge wastewater at a rate of 34.6 ML a day. Baird Australia Pty Limited (Baird) has been engaged by Animal Plant Mineral Pty Ltd to develop a hydrodynamic modelling program to support the environmental approvals process to assess: • Dilution contours around the outfall in typical wet and dry season conditions; • Predicted mixing zones required to meet the level of ecological protection of the waters surrounding

the mixing zone (salinity and temperature); and. • Provide outputs and reporting in a format suitable to support the development of a Marine

Environmental Quality Monitoring and Management Plan to support the environmental approvals for the Project (to be completed by O2 Marine).

To achieve the above requirements, a high-resolution three-dimensional numerical model system has been developed to assess the mixing and dispersion of the wastewater from two proposed outfall locations in Elizabeth River and Darwin Harbour. To fully account for plume dispersion, the modelling has been completed using a nearfield (in direct proximity to the outfall) and farfield modelling approach (all of Darwin Harbour). The modelling has been undertaken for two representative seasons, wet season and dry season, to ensure inter-seasonal effects are taken into account, covering one month within each season. Cumulative effects of the wastewater have also been investigated by carrying out 6 month long runs for each season.

The proposed outfall location in Elizabeth River was considered not to be suitable, as the plume recirculation effects were considerable in the model, and model outcomes indicate that an outfall with the wastewater discharging at the location would lead to accumulation of the chemicals carried in the waste stream in the river system over time. Further investigation was made into the Darwin Harbour outfall location, with the 3D model outcomes demonstrating that there is good mixing of the plume through East Arm. Based on the model outcomes, it is concluded that the East Arm outfall would result in an increase in the background concentration in the Harbour of the respective chemicals in the waste stream, with dilution reaching an equilibrium within approximately 6-months.



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Table of Contents

1. Introduction ............................................................................................................................. 1

1.1 Background 1

1.2 Project Overview 2

1.3 Outfall Modelling Scope 2

2. Background Information ........................................................................................................ 3

2.1 Measured Data Sources 3

3. Wastewater Characteristics .................................................................................................. 6

3.1 Description of the Wastewater 6

4. Model Setup ............................................................................................................................. 7

4.1 Model System and Approach 7

4.1.1 Nearfield Model 7

4.1.2 Farfield Model 7

4.2 Determination of the Discharge Requirements in the Model 9

4.2.1 Scenario Modelling 9

4.2.2 Wastewater Outfall Location 10

4.2.3 Discharge Calculations 10

4.3 Model Settings for Nearfield Modelling – CORMIX 12

4.3.1 Hydrodynamic Cases 12

4.3.2 Outfall Design 13

4.4 Model Settings for Farfield Modelling in Delft3D 16

5. Modelling Outcomes ........................................................................................................... 17

5.1 Case 1 - Elizabeth River Outfall Location 17

5.1.1 Model Results 17

5.2 Case 2 - Darwin Harbour Outfall Location 20

5.2.1 Nearfield Model Results 20

5.2.2 Integration of Nearfield and Farfield Model Results 22

5.2.3 Farfield Model - Plume Spatial Extents 23



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5.2.4 Plume Dispersal 26

5.2.5 Modelled Tracer Concentration at locations in Darwin Harbour and Elizabeth River for a 6-month simulation period 27

5.2.6 Wastewater Plume – Analysis of 3D Depth Concentration 32

5.3 Model Outputs 34

5.3.1 Mixing Zones 34

5.3.2 Dilution Contours 35

5.3.3 Salinity Plume Extent 38

5.3.4 Temperature Plume Extent 41

6. Conclusions ......................................................................................................................... 45

7. References ............................................................................................................................ 47

Summary of Final Dilution Mapping Datasets

Tables Table 2.1: Measured Data Summary .................................................................................................................3

Table 3.1: Wastewater Properties ......................................................................................................................6

Table 4.1: Delft Model setup summary (adopted from Baird, 2019) .................................................................9

Table 4.2: Wastewater outfall required flow rate calculations ........................................................................ 12

Table 4.3: Percentile current speeds at the proposed Darwin Harbour outfall location (Depth 8.1m) ......... 12

Table 4.4: Hydrodynamic scenarios determined for assessment in CORMIX .............................................. 13

Table 4.5: CORMIX model setup summary – East Arm Location (Case 2) .................................................. 14

Table 4.6: Model Settings – Farfield Model .................................................................................................... 16

Table 5.1: Nearfield Modelled Dilution Rate at edge of Nearfield (CORMIX) for cases based on a the Discharge Current speeds at East Arm (Outfall Case 2). .............................................................................. 21

Table 5.2: Nearfield and Farfield (depth averaged) Models Comparison of Modelled Dilution Rate at edge of Nearfield ....................................................................................................................................................... 23

Table 5.3: Salinity Range in Darwin Inner Harbour ........................................................................................ 38



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Table 5.4: Temperature Range in Darwin Inner Harbour .............................................................................. 41

Figures Figure 1.1: Project Location in Darwin Harbour ................................................................................................1

Figure 2.1: Measured Data Locations ................................................................................................................5

Figure 4.1: Local Hydrodynamic Model - Domain Decomposition Grid setup .................................................8

Figure 4.2: Wastewater outfall locations assessed in the study (Google Earth) ........................................... 10

Figure 4.3: Time series demonstrating when the hydrodynamic conditions at the Darwin Harbour outfall location are suitable for wastewater discharge across a 14-day period in the dry season (December 2018). .......................................................................................................................................................................... 11

Figure 4.4: Diffuser geometry as modelled in CORMIX ................................................................................. 15

Figure 4.5: CORMIX 3D Plume result for the adopted single port diffuser for design current case ............. 15

Figure 5.1: Elizabeth River Outfall location (case 1). Upstream measurement location in Elizabeth River shown in relation to the proposed outfall locations (Google Earth). .............................................................. 17

Figure 5.2: Plots of Darwin Harbour showing the oscillation of the tracer plume around the outfall location and the Elizabeth River Bridge. The plume travel towards the harbour when wastewater is being released (ebb tides) and the plume is redirected back up Elizabeth River (with some being directed into the creeks along Elizabeth River) on flood tides .............................................................................................................. 18

Figure 5.3: Tracer Concentration (depth-averaged) at the Elizabeth River outfall location .......................... 19

Figure 5.4: Tracer Concentration (depth-averaged) Elizabeth River Upstream Location (3.5km upstream of the outfall location) ........................................................................................................................................... 19

Figure 5.5: Plot of CORMIX modelled dilutions at the end of the nearfield vs current velocity .................... 21

Figure 5.6: CORMIX 2D Plume result for Outfall Design ............................................................................... 22

Figure 5.7: Modelled depth averaged dilutions from conservative tracer for a current velocity at 0.08ms-1 (approximate 5th percentile). East Arm outfall location. .................................................................................. 24

Figure 5.8: Modelled depth averaged dilutions from conservative tracer for a current velocity at 0.2ms-1 (approximate 50th percentile). East Arm outfall location. ................................................................................ 24

Figure 5.9: Modelled depth averaged dilutions from conservative tracer for a current velocity at 0.4ms-1 (approximate 80th percentile). East Arm outfall location. ................................................................................ 25

Figure 5.10: Modelled depth averaged dilutions for percentile cases (P5, P50 and P80) showing extent and shape of 300 dilution contours. ....................................................................................................................... 25



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Figure 5.11: Time series of outfall release through the ebb tide cycle, 11 Aug 2019 08:00 to 14:00. Depth averaged currents are shown steering the outfall effluent plume in the farfield model. ................................ 26

Figure 5.12: Model Reporting Locations for Darwin East Arm outfall (Case 2). Tracer concentration assessed over the six-month model simulation period. ................................................................................. 27

Figure 5.13: Tracer Level (following on an input value of 100 kg/m3) within East Arm Darwin Harbour, following wastewater discharge from Darwin Harbour ................................................................................... 29

Figure 5.14: Tracer Level (following on an input value of 100 kg/m3) 2km upstream of the Elizabeth River Bridge at Eliz River Upstream, following wastewater discharge from Darwin Harbour ................................ 29

Figure 5.15: Tracer Level (following on an input value of 100 kg/m3) at the Darwin Harbour Tide Gauge, following wastewater discharge from Darwin Harbour ................................................................................... 30

Figure 5.16: Tracer Level (following on an input value of 100 kg/m3) within the Inner Harbour area, following wastewater discharge from Darwin Harbour ................................................................................... 30

Figure 5.17: Tracer Level (following on an input value of 100 kg/m3) within the Outer Harbour area, following wastewater discharge from Darwin Harbour ................................................................................... 31

Figure 5.18: Tracer Level (following on an input value of 100 kg/m3) upstream within Middle Arm (Middle Arm 1), following wastewater discharge from Darwin Harbour ...................................................................... 31

Figure 5.19: Tracer Level (following on an input value of 100 kg/m3) within Middle Arm close to the inner harbour (Middle Arm 2), following wastewater discharge from Darwin Harbour ........................................... 32

Figure 5.20: Cross section taken through the 33m grid and the 6m grid to produce the depth dependent plots taken from the 3D models ...................................................................................................................... 33

Figure 5.21: Salinity levels taken from the model cross section shown in Figure 5.20 across a discharge cycle at 19:30 on the 12th of July (top) and 00:00 on the 13th of July (bottom) .............................................. 33

Figure 5.22: Temperatures taken from the model cross section shown in Figure 5.20 across a discharge cycle at 19:30 on the 12th of July (top), 00:00 on the 13th of July (middle) and 01:00 on the 13th of July (bottom) ............................................................................................................................................................ 34

Figure 5.23: 60m mixing zone centred on the proposed outfall location in the Darwin Inner Harbour ........ 35

Figure 5.24: Dilution Contours for the dry season scenario based on analysis of the 5th percentile dilution rate over the 4-week simulation. Upper Plot: Seabed layer, Lower Plot Surface Layer. .............................. 36

Figure 5.25: Dilution Contours for the wet season scenario based on analysis of the 5th percentile dilution rate over the 4-week simulation. Upper Plot: Seabed layer, Lower Plot Surface Layer. .............................. 37

Figure 5.26: East Arm Dry Season Salinity Contours, showing where the plume reaches back to the model background salinity (defined as the median salinity in the data measured in the Inner Harbour during the Dry Season) in the depth averaged model ..................................................................................................... 39



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Figure 5.27: East Arm Dry Season Salinity Contours, showing where the plume reaches back to the model background salinity (defined as the median salinity in the data measured in the Inner Harbour during the Dry Season) in the 3D model, at the surface layer......................................................................................... 39

Figure 5.28: East Arm Wet Season Salinity Contours, showing where the plume reaches back to the model background salinity (defined as the median salinity in the data measured in the Inner Harbour during the Wet Season) in the depth averaged model .................................................................................................... 40

Figure 5.29: East Arm Wet Season Salinity Contours, showing where the plume reaches back to the model background salinity (defined as the median salinity in the data measured in the Inner Harbour during the Wet Season) in the 3D model, at the surface layer ........................................................................................ 40

Figure 5.30: East Arm Dry Season Temperature Contours, showing where the plume reaches back to the model background temperature (defined as the median temperature in the data measured in the Inner Harbour during the Dry Season) in the depth averaged model ..................................................................... 42

Figure 5.31: East Arm Dry Season Temperature Contours, showing where the plume reaches back to the model background temperature (defined as the median temperature in the data measured in the Inner Harbour during the Dry Season) in the 3D model, at the surface layer ......................................................... 42

Figure 5.32: East Arm Wet Season Temperature Contours, showing where the plume reaches back to the model background temperature (defined as the median temperature in the data measured in the Inner Harbour during the Wet Season) in the depth averaged model .................................................................... 43

Figure 5.33: East Arm Wet Season Temperature Contours, showing where the plume reaches back to the model background temperature (defined as the median temperature in the data measured in the Inner Harbour during the Wet Season) in the 3D model, at the surface layer ........................................................ 43



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1. Introduction

1.1 Background

TNG Limited (TNG) have submitted a Notice of Intent (NOI) to the Northern Territory Environment Protection Authority (NT EPA) and Department of Lands Planning and Environment (DLPE) to construct and operate the Darwin Processing Facility (processing facility).

TNG proposes to establish the Processing Facility on land adjacent the Elizabeth River Bridge. Land to the north, south and west contains intertidal areas and mangrove forests that connect the site to the Elizabeth River and Darwin Harbour, and have been zoned for Conservation.

The Processing Facility will process magnetite concentrate to produce: • vanadium pentoxide - for use in steel, non-ferrous alloys, chemicals, catalysts and energy storage

(vanadium redox batteries); • titanium dioxide pigment - for use in paint, plastics, paper and inks; and • pig iron - for use in steel making.

The three products will be exported through the Port of Darwin’s East Arm Wharf to international customers.

Figure 1.1: Project Location in Darwin Harbour (TLNG)



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1.2 Project Overview

Baird Australia (Baird) were appointed by TNG to deliver a numerical modelling assessment of the wastewater outfall from the processing facility. The objective of the outfall modelling is to support the environmental assessment for the site. The processing facility outfall will discharge wastewater into the Darwin Harbour system at a higher temperature than the ambient receiving waters in the Darwin Harbour system and have a lower salinity than marine water and will be close to that of fresh water.

Baird have developed a hydrodynamic model of the Darwin Harbour which has been validated against measured data available from around the project location as reported in the TNG Darwin Processing Facility Outfall – Hydrodynamic Modelling Report (Baird 2019). The validated hydrodynamic model has been developed to characterise the local hydrodynamic conditions of Darwin Harbour around the project site and determine an outfall design and wastewater release strategy. This report presents numerical modelling outcomes of dilution and mixing of the wastewater discharge into Darwin Harbour to assist with the assessment of water quality impacts around the outfall location (nearfield) and considers the farfield effects of the wastewater plume through Darwin Harbour. For this study, Baird has utilised a specialised nearfield dilution model (CORMIX) as well as the calibrated Delft3D model to examine dilution and mixing in the farfield.

Modelling of the wastewater from the outfall is presented in this report to provide: • Dilution contours around the outfall in typical wet and dry season conditions; • Predicted mixing zones required to meet the level of ecological protection of the waters surrounding

the mixing zone (salinity and temperature); and • Analysis of the influence on the receiving waters of Darwin Harbour from the planned outfall release

over seasonal periods of one month duration and 6-months duration. The outputs and reporting are provided to support the development of a Marine Environmental Quality Monitoring and Management Plan (MEQMMP) to support the environmental approvals for the Project (to be completed by O2 Marine).

1.3 Outfall Modelling Scope

Baird was engaged to assess the outfall strategy and to examine dispersion of the wastewater plume based on the requirements of the Processing Facility.

Baird have established a farfield hydrodynamic model of Darwin Harbour as outlined in the Hydrodynamic Modelling Report (Baird 2019). To analyse the wastewater plume from the outfall, the farfield model has been applied in conjunction with a ‘nearfield’ model to fully account for the mixing zone around the wastewater outfall and through the wider Darwin Harbour extent.

The tasks undertaken for the wastewater outfall modelling were as follows: 1. Establishment and validation of the Hydrodynamic model, as outlined in Baird’s 2019 Hydrodynamic

Modelling Report – see Baird (2019); 2. Outfall Options Assessment to determine an optimal wastewater release strategy; 3. Plume stability assessment in the ‘nearfield’. CORMIX nearfield modelling undertaken of the outfall

release to investigate the stability of the plume in the immediate area around the outfall location (0m to approximately 70m); and

4. Modelling of the plume in the ‘farfield’ area. Farfield modelling using a Delft3D model (integrating the nearfield model outcomes) to examine the outfall release through the wider Darwin Harbour region analysing dispersion of conservative tracer, salinity and temperature.

The modelling program was completed for representative seasonal environmental conditions (wet season / dry season) with output designed to support the environmental approvals process (MEQMMP).



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2. Background Information

2.1 Measured Data Sources

Measured datasets in Darwin Harbour applied in this study were sourced from the Department of Environment and Natural Resources (DENR), the Bureau of Meteorology (BoM), the National Tidal Centre (NTC) and O2Marine (O2M) as summarised in Table 2.1.

Table 2.1: Measured Data Summary

Dataset

Baseline Metocean Data

Elizabeth River Aquadopp (O2M) • Approximately 1.2km downstream of the Elizabeth River Bridge in

Palmerston, in 5.7m depth (MSL, MGA50 713734E, 8613513S) • Deployment covering the period 27 March 2019 – 22 May 2019 • Current speed and direction through the water column, Water Level,

Temperature Darwin Harbour Aquadopp (O2M) • Approximately 1.5km offshore of the East Arm Boat Ramp, in 8.2m

depth (MSL, MGA50 707986E, 8617540S) • Deployments covering period 22 May 2019 – 24 July 2019 • Current speed and direction through the water column, Water Level,

Temperature Elizabeth River ADCP (DENR) • Approximately 1.2km downstream of the Elizabeth River Bridge in

Palmerston, in 5.7m depth (MSL, MGA50 713754 8613566) • Deployments covering November 2015 to September 2017 (some

gaps exist) • Current speed and direction through the water column, Water Level,

Temperature

Baseline Water Quality

Elizabeth River Seabird CTD Instrument (DENR) • Approximately 1.2km downstream of the Elizabeth River Bridge in

Palmerston, in 5.7m depth (MSL, MGA50 713754 8613566) • Deployments covering April 2015 to April 2016 (some gaps existing) • Temperature, Salinity, pH, DO, PAR, Turbidity, Fluorescence Darwin Harbour (Darwin Harbour Pt2) WQ logger (DENR) • Approximately 1.5km offshore of the East Arm Boat Ramp, in 8.2m

depth (MSL, MGA50 707429E, 8617859S) • Deployments covering December 1986 to October 2018 (intermittent -

many large gaps in the data) • Salinity, NTU, Temperature, pH, TDS etc.



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Dataset

Measured Wind Data from Darwin Airport

Bureau of Meteorology Data (BoM) Data for the period 16 August 2012 to 20 May 2019 from the Darwin Station location at the Darwin Airport (station 14015), northwest of the project site (MGA 705453E 8626273S) • Wind Speed / Direction data Half hourly data converted, measured 10-minute average at 10m height

Measured Tide, Water Quality and Wind Data on Fort Hill Wharf

National Tidal Centre Data (NTC) Data for the period 01 Jan 1991 to 31 July 2019 from the Darwin NTC AWS (including tide gauge) on Fort Hill Wharf (NTC AWS (IDO71014) MGA 700647E 8620547S) • Sea Level (mAHD) • Water Temperature (ºC) • Pressure (hPa) • Wind Speed (m/s) • Wind Gust (m/s) • Wind Direction (ºN)

The locations of the measured metocean data is shown in Figure 2.1.



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Figure 2.1: Measured Data Locations

Darwin Airport (BOM 014015)

DENR Darwin Harbour Pt2

O2 Darwin Harbour Adopp

DENR ElizRiv01

O2 Middle Arm Adopp

Darwin NTC AWS (BOM IDO71014)

O2 Elizabeth River Adopp



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3. Wastewater Characteristics

3.1 Description of the Wastewater

The operation of the processing facility will produce a waste stream that will be directed through a pipeline to an outfall location in Darwin Harbour. The waste stream properties are summarised in Table 3.1.

The total amount of wastewater produced is 34.6 ML a day. The waste stream will contain a range of chemicals associated with the treatment process, with a salinity of 12 ppt (part per thousand) at a temperature up to 50˚C. The wastewater salinity is marginally above fresh water and when released into the receiving waters of Darwin Harbour. Due to the lower relative salinity and density of the wastewater compared to the Darwin Harbour receiving waters the wastewater plume will rise rapidly to the upper water column.

The waste stream volumes in Table 3.1 were provided by the project (SMS Group 2019) and are based on the operating assumptions for the refinery (including a 20% margin for abnormal operation e.g. due to cleaning) and the pigment plant.

Table 3.1: Wastewater Properties

Wastewater Property Waste Stream Projection

Flow Rate 34.6 ML / day

Salinity 12 ppt (1.2% Salinity)

Temperature 50˚ C maximum



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4. Model Setup

4.1 Model System and Approach

For this study, a high-resolution three-dimensional numerical model system has been developed to assess the mixing and dispersion of the wastewater from the outfall and determine the extent of the mixing zone around the outfall location. Due to the complex nature of the plume dispersion, the wastewater outfall modelling has been completed using a nearfield (in direct proximity to the outfall) and farfield modelling approach (all of Darwin Harbour). The nearfield and farfield models are integrated to provide a model system that simulates the plume behaviour around the outfall during discharge cycles and which accounts for the interaction of processes including density differences between the wastewater and receiving waters, the effect of local currents and water levels on plume dispersion as well as recirculation effects of the remnant plume back to the outfall location under certain conditions.

The model system was developed to assess wet and dry season scenarios across the full spring and neap tide cycles. The model simulations apply conservative tracer in the model system to examine the temporal and spatial varying concentrations of selected constituents of the wastewater discharge around the immediate outfall location and through the wider extent of Darwin Harbour. The tracer modelling is used to predict the dilution rate around the outfall and can be applied as a mass conservative proxy for determining the dilution of toxicants released in the waste stream (metals and metalloids). The model system also examines the plume dispersion in terms of temperature and salinity around the outfall to define the extent of the mixing zones for water quality in the environmental assessment (MEQMMP).

4.1.1 Nearfield Model

To model the wastewater plume dispersion in the immediate vicinity of the outfall location, the CORMIX model has been applied. CORMIX is a rule based expert system used to provide mixing zone analysis for a range of discharges into bodies of water, with particular emphasis on the geometry and dilution characteristics of plumes defining the nearfield mixing zone (Doneker et al 2007). In this case, CORMIX has been used solely to define the characteristics of the nearfield mixing zone dominated by the buoyancy and mass flux of the jet discharging from the wastewater outfall. CORMIX has the ability to describe the smaller scale fluid motions in this zone where strong initial mixing occurs as a result of the buoyancy difference between the wastewater jet and the receiving environment, as well as mixing created by the momentum of the outfall jet.

4.1.2 Farfield Model

As the distance from the outfall increases, the properties determining the behaviour of the discharge close to the outfall (i.e. the physical properties of the wastewater effluent, the characteristic design of the outfall) become less important than the ambient conditions within Darwin Harbour in determining the behaviour of the wastewater plume. Description of the wider hydrodynamics influencing the advection and dispersion of the wastewater within the harbour requires a model capable of incorporating metocean parameters including tides and winds.

The hydrodynamic model developed for the Darwin Outfall Project (Baird 2019) has been established with the Delft3D modelling system (Deltares 2018). Delft3D is an integrated modelling suite, which simulates two-dimensional (in either the horizontal or a vertical plane) and three-dimensional flow, sediment transport and morphology, waves, water quality, and ecology and can handle the interactions between these processes.



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Complete detail of the hydrodynamic model setup can be found in Baird (2019) but in general the overall model system consists of:- • A regional scale model extending across the entire northern coast of the Northern Territory, through

Darwin Harbour into the Elizabeth River in the vicinity of the project site (Delft Flexible Mesh, DFM); and

• A local scale hydrodynamic model established over the Darwin Harbour area in a Delft3D domain decomposition grid arrangement with increasing grid resolution from offshore to the project site (100m grid, 33m Grid, 6m Grid);

The regional model is used to define the boundary conditions in the local scale model and wind forcing is applied to the model based on measured data from East Arm. Wave forcing has not been included in any of the models and this is generally a conservative approach as wave energy would enhance farfield mixing. Due to the sheltered nature of the discharge location, waves will have little impact on the nearfield mixing processes.

The local scale model setup for the wastewater dispersion modelling is shown in Figure 4.1.

Figure 4.1: Local Hydrodynamic Model - Domain Decomposition Grid setup

An overview of the key parameters in the Delft3D model setup is shown in Figure 4.1. The local model is run in 2D (depth-averaged) and 3D (sigma-layer) mode in the modelling analysis. The specifications for the 2D and 3D models is presented in Table 4.1.

The model shows very close agreement to measured data from the wet season and dry season, with high validation statistics against measured water level and current data from Elizabeth River and East Arm locations. A complete description of the farfield model setup and validation against measured data in Darwin Harbour is presented in the Hydrodynamic Modelling Report (Baird 2019).

100m Grid 33m Grid

6m Grid



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Table 4.1: Delft Model setup summary (adopted from Baird, 2019)

Feature Description

Grid size / type Domain Decomposition (DD) - Regular Grids at 100m, 33m and 6.6m

Grid Extent Outer Grid: 52km x 33km

3D sigma layer model 5-vertical sigma layers with each layer thicknesses 20% of the water column.

Vertical Datum Mean Sea Level (m MSL) which is approximately Australian Height Datum (AHD)

Horizontal eddy diffusivity coefficient 100m Grid = 10 m2/s, 33m Grid = 3 m2/s, 6.6m Grid = 0.5 m2/s

Horizontal eddy viscosity coefficient 100m Grid = 10 m2/s, 33m Grid = 5 m2/s, 6.6m Grid = 1 m2/s

Vertical eddy viscosity / diffusivity k-ε turbulence closure model

Time step (2D model) 0.25 mins (15 secs)

Time step (3D z-layer) 0.2 mins (12 secs)

Bed friction Chezy 65m1/2/s

4.2 Determination of the Discharge Requirements in the Model

4.2.1 Scenario Modelling

The modelling program for the wastewater outfall assessment is based on scenario modelling. The scenario modelling approach has been adopted to optimise the model run times for the farfield model, as continuous modelling of environmental conditions over the full year would be impractical due to the long run times of the model system (Baird 2019).

In developing the scenario approach, modelling cases of four weeks duration have been selected that are representative of the dry season and wet season in Darwin to examine the influence of the varying metocean conditions (winds and currents) on the wastewater plume characteristics. The two scenario cases established in Baird 2019 were as follows: • Wet Season: The period selected as representative of the wet season period was 01 January 2016 –

31 January 2016. The month of January 2016 was selected based on the following: • The winds in January from the measured data represent generally the key wind directions over the

wet season months November, December, January and February. Analysis of the measured data



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indicates the wind speed characteristics in the January period selected is generally descriptive of the average of the wet season months.

• Dry Season: The period selected as representative of the dry season period was 20 June – 20 July 2019. The period was selected based on the following: • The winds from the measured data represent generally the key wind directions over the Dry

season months May, June July and August. Analysis of the measured data indicates the wind speed characteristics are generally descriptive of the average of the dry season months.

It is noted that the wet season scenario case does not include catchment-based flows (i.e. runoff through creeks and rivers). This approach has been adopted as a conservative basis for modelling flow velocities as wet season catchment-based flows through the Elizabeth River and East Arm regions of interest would result in increased flow velocity and enhance the mixing potential around the outfall.

4.2.2 Wastewater Outfall Location

Two locations for the wastewater outfall in Darwin Harbour were examined: • Outfall Case 1: Darwin Harbour in the branch of the Elizabeth River near the process plant site • Outfall Case 2: Darwin Harbour in East Arm.

The wastewater is directed through a seabed pipeline to an outfall at the seabed in both cases. The examination of the plume dispersion characteristics of each respective location is outlined in the sections to follow. The outfall locations are shown in Figure 4.2.

Figure 4.2: Wastewater outfall locations assessed in the study (Google Earth)

4.2.3 Discharge Calculations

The wastewater discharge specifications are summarised in Table 3.1, indicating the processing facility releases 34.6 ML/day. To calculate the discharge requirements for each of the respective outfall locations,



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the modelled water level and velocity at the respective sites was analysed based on a reporting location in the validated hydrodynamic model (Baird 2019).

The discharge regime was calculated at the Elizabeth River outfall location (Case 1) and Darwin Harbour outfall location (Case 2) over the two seasonal cases (wet / dry season). Based on the analysis of the modelled environmental conditions at the intended discharge sites it was determined that discharge at the outfall would not be continuous. The discharge regime has been developed based on release of the wastewater only on the ebb tide (outgoing tide) to ensure the wastewater plume is directed away from the outfall in the direction of the Harbour entrance under the general tidal currents when discharging.

The modelled seasonal scenarios were used to calculate when the ebb tide conditions were met, and the wastewater outfall could be discharging. Figure 4.3 shows an example of a 14-day period in the dry season scenario model, with grey vertical lines demonstrating periods of time when ebb tide conditions permit discharge from the wastewater outfall.

Figure 4.3: Time series demonstrating when the hydrodynamic conditions at the Darwin Harbour outfall location are suitable for wastewater discharge across a 14-day period in the dry season (December 2018).

The daily discharge objective of the processing facility is 34.6ML. This requirement was used to determine how much flow was required through the outfall, in accordance with the environmental conditions (ebb tide discharge only) across the full month of wet and dry season. For periods where the outfall was not discharging, the wastewater is required to be stored on site by TNG. The discharge requirements are summarised in Table 4.2.



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Table 4.2: Wastewater outfall required flow rate calculations

Process Facility Defined Flow Rate = 34.6ML/day

Discharge Regime Required Flow Rate

Dry Season Required Flow Rate

Wet Season

Elizabeth River – Ebb tide discharge 1.02 m3/sec 1.16 m3/sec

Darwin Harbour – Ebb tide discharge 0.89 m3/sec 1.01 m3/sec

4.3 Model Settings for Nearfield Modelling – CORMIX

4.3.1 Hydrodynamic Cases

CORMIX requires input of ambient environmental data to assess the interaction of an effluent plume with the receiving environment. The CORMIX conservative pollutant module was applied in this study, to assess the behaviour of the nearfield mixing zone from a set of hydrodynamic cases based on the potential range of modelled water level and ambient velocity at the outfall site.

Assessment of a range of hydrodynamic scenarios, defined by the range of conditions experienced at the outfall location during the model simulations was established. Table 4.3 outlines the modelled depth averaged current speeds experienced at the Case 2 outfall site in East Arm during ebb tides; these current speeds were then used to determine upper and lower bounds for current speeds to be assessed in CORMIX. Some higher speeds were also examined to ensure that the dilution rates experienced in the farfield model (D3D) could be compared against the CORMIX outcomes (Section 5.2.2).

The depth averaged velocity in Table 4.3 shows average current velocity is approximately 0.21 ms-1 with an upper range (95th percentile) value of approximately 0.78ms-1 and a lower bound velocity (5th percentile) of approximately 0.08ms-1.

Table 4.3: Percentile current speeds at the proposed Darwin Harbour outfall location (Depth 8.1m)

Percentile Current Velocity

5th 0.08 ms-1

20th 0.12 ms-1

50th 0.21 ms-1

80th 0.38 ms-1

95th 0.57 ms-1

99th 0.78 ms-1

The farfield model was used to examine current speeds in Table 4.3 and determine the potential range of associated water levels at the outfall location, to establish the CORMIX scenarios assessed as listed in Table 4.4. These cases represent the general hydrodynamic conditions based on current velocity influencing the plume dispersion: • Current Velocity 0.08ms-1 (5th Percentile). Very low current velocity, very limited mixing potential in

water column, largest plume generated. Can occur over a range of water levels; • Current Velocity 0.5ms-1 (50th Percentile). Median current velocity, reasonably good mixing potential;



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• Current Velocity 0.8ms-1 (95th Percentile). Highest current speeds, strong mixing potential and smallest plume. Occurs over a very limited water level range

During the ebb tide, the current velocity is directed from east to west at the East arm location (Outfall Case 2).

Table 4.4: Hydrodynamic scenarios determined for assessment in CORMIX

Current Velocity Water Levels (m MSL)

0.08 ms-1 -3.7 -2.5 -1.5 0 1.5 2.5 3.4

0.1 ms-1 -3.6 -1.5 0 1.5 3.3

0.2 ms-1 -3.2 -1.5 0 1.5 3.25

0.3 ms-1 -1 0 1 3.2

0.4 ms-1 -2.4 -1.5 0 1.6 3.2

0.5 ms-1 -1.3 0 1.2

0.6 ms-1 -0.75 0.5 1.25

0.7 ms-1 0.75 1.5

0.8 ms-1 0.5

The set of cases defined in Table 4.4 was assessed in CORMIX to examine the plume characteristics as summarised in Table 5.2. The CORMIX cases allowed comparison between the CORMIX dilution and mixing of conservative tracer in the nearfield region to be compared with corresponding length scales in the farfield model cases (Delft3D). This process was used to calibrate the horizontal diffusivity terms in the farfield model to ensure agreement in dilution rates at the edge of the nearfield boundary (CORMIX) and the interface with the farfield (D3D). This process is outlined in Section 5.2.2.

4.3.2 Outfall Design

A number of sensitivity cases were run to determine the diffuser design best suited to producing the maximum amount of mixing within the nearfield mixing zone for the outfall regime.

A range of diffuser types were examined within CORMIX including single port diffusers, multi-port diffusers and surface diffusers. A thorough modelling assessment was completed to test the range of diffuser types across a range of design hydrodynamic cases to examine which performed best in terms of delivering the highest rate of dilution around the outfall. This determined that a single-port diffuser produced the optimal dilution rate for the site under the environmental conditions and discharge regime.

Due to the lower relative density of the wastewater plume compared to the receiving environment, as well as the sensitivity cases being run based on a ‘worst-case scenario’ of the lowest expected combination of water level and current velocity, the greatest plume stability was achieved with a single port diffuser, located at the bottom of the water column (ie seabed) discharging at 90º to the prevailing current direction and angled to discharge level with the seabed.

The parameters used to define model cases in CORMIX include diffuser geometry, the discharge and salinity characteristics of the effluent, and the characteristics of the ambient receiving environment. A summary of the key inputs and assumptions is outlined in Table 4.5.



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Table 4.5: CORMIX model setup summary – East Arm Location (Case 2)

Feature Description

Effluent Parameters

Effluent Type Conservative Pollutant

Flow rate 0.89 m3/s (as determined in Table 4.2)

Discharge concentration (excess) Difference between outfall pollutant concentration and background pollutant concentration.

Effluent Salinity 12 ppt

Effluent Temperature 50ºC

Ambient Parameters

Discharge depth 8.2m MSL

Ambient velocity Assessed for the range of modelled current speeds that the diffuser will be in operation (see Table 4.3 and Table 4.4)

Darcy-Weisbach friction factor 0.0228

Indicative Discharge Parameters

Geometry Single Port Diffuser

Diffuser distance from shore 1750 m

Diffuser port height 0 m above seabed

Diffuser port diameter 0.25 m

Discharge velocity ≈ 8 m/s

Diffuser port orientation

Single port per riser

Port angle in horizontal plane: 90º to dominant current direction

Port angle in vertical plane: 0º

This diffuser geometry, as well as the geometry in relation to the dominant flow direction, is shown in Figure 4.4. The intersection of the x, y and z planes represents the water level. A schematic representation of the flow field from the design case for the single-port diffuser is shown in Figure 4.5 showing the full 3D plume, demonstrating the turbulent mixing from the initially weakly deflected jet created by the single-port diffuser, followed by a change in the mixing regime once the jet is strongly deflected by the ambient current, and finally the integrated mixing once the jet is entrained into the predominant current flow at the end of the nearfield region (NFR).



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Figure 4.4: Diffuser geometry as modelled in CORMIX

Figure 4.5: CORMIX 3D Plume result for the adopted single port diffuser for design current case



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4.4 Model Settings for Farfield Modelling in Delft3D

The wastewater outfall in the farfield Delft3D model has been schematised to be input at the seabed in the 3D model scenarios. Velocity is not specified as a model input for Delft3D, however tidal velocity in the hydrodynamic model immediately directs the discharge in the ebb tide current direction at time of release. Over the length scale of the nearfield mixing region (discussed in the next section) the input discharge immediately proceeds to dilute in the receiving waters and rise to the surface levels as a light plume before being mixed through the water column.

A summary of the governing model settings for the wastewater outfall modelling, additional to the settings described in Baird’s 2019 Hydrodynamic Modelling Report (Baird 2019), is provided in Table 4.6.

Table 4.6: Model Settings – Farfield Model

Feature Description

Boundary Conditions and background receiving waters in the model

Salinity and temperature for each of the seasonal cases has been assigned in the model based on analysis of the available measured data at the site, as outlined in Baird 2019. Model background Salinity and temperature values are calculated 50th percentile statistics from DENR datasets between 2015 and 2016 (Elizabeth River) and a temporally sparse dataset captured sporadically between 1986 and 2018 (Darwin Harbour).

Wet Season Salinity: 32.1 ppt

Temperature: 31.1 oC

Dry Season Salinity: 35 ppt

Temperature: 27 oC

Discharge Conditions (Wastewater Characteristics)

Discharge flow rate (m3/s), salinity (ppt), temperature (º Celsius) and tracer concentration (kg/ m3) defined at 10-minute timestep from discharge points in the model representing the discharge from the diffuser (refer Figure 4.3).

Salinity: 12 ppt

Temperature: 50 ºC

Discharge Locations

Case 1

Elizabeth River

Depth 5.7 m MSL

Case 2

East Arm

Depth 8.2 m MSL

Co-ordinates MGA52

713,746

8,613,518

Co-ordinates MGA52

707,990

8,617,566



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5. Modelling Outcomes

5.1 Case 1 - Elizabeth River Outfall Location

The Elizabeth River location is in close proximity to the processing facility location and downstream of the Elizabeth River bridge (Figure 5.1). Assessment of the suitability of this location for the wastewater outfall was initially undertaken in the farfield model.

Figure 5.1: Elizabeth River Outfall location (case 1). Upstream measurement location in Elizabeth River shown in relation to the proposed outfall locations (Google Earth).

5.1.1 Model Results

The farfield simulation of the Elizabeth River outfall case was completed for the dry season scenario. The wastewater plume was analysed in the model across the four-week period, and it was noted the modelled tracer plume tended to oscillate around the Elizabeth River outfall location, without the plume significantly dispersing and being carried out of the Elizabeth River through to Darwin Harbour. During release periods (ebb tides) the tracer plume is directed downstream from the outfall in Elizabeth River, but does not have sufficient time to exit the branch of the river system before flood tides redirect the remnant plume back upstream. When the redirected plume moves back upstream on the flood tides, it concentrates upstream of the Elizabeth River Bridge as well as being directed into the creek systems attached to the Elizabeth River around the TNG site. The modelled plume concentration in the vicinity of the Elizabeth River Bridge is observed in Figure 5.2 as it moves through successive flood tide and ebb tide sequences in the model over a 15 hour period.

The modelled level of tracer concentration was assessed over the month at the main outfall location and at the Elizabeth River upstream location (refer Figure 5.1). The level of tracer at the two locations was analysed over the 1-month dry season simulation which showed the tracer level continued to increase



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through the 4-week period with recirculation of the plume contributing to the tracer concentration accumulating at the outfall location and in the upper reaches of the Elizabeth River.

Figure 5.2: Plots of Darwin Harbour showing the oscillation of the tracer plume around the outfall location and the Elizabeth River Bridge. The plume travel towards the harbour when wastewater is being released (ebb tides) and the plume is redirected back up Elizabeth River (with some being directed into the creeks along Elizabeth River) on flood tides

FLOOD TIDE

FLOOD TIDE

EBB TIDE

EBB TIDE



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To further assess the accumulation of tracer upstream in the model simulations, the model time was extended to run over six months continuously (extending the Dry Season model scenario with the wastewater released on the ebb tide). For the extended model, the time series plot of the tracer concentration at the Elizabeth River outfall location is shown in Figure 5.3 . The tracer concentration for the upstream Elizabeth River location over the six-month simulation period is shown in Figure 5.4.

Figure 5.3: Tracer Concentration (depth-averaged) at the Elizabeth River outfall location

Figure 5.4: Tracer Concentration (depth-averaged) Elizabeth River Upstream Location (3.5km upstream of the outfall location)



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The time series plot of tracer concentration for the six-month simulations shown in Figure 5.3 and Figure 5.4 confirmed that there is an accumulation of tracer observed at both locations over time. The 10-day and 30-day moving averages were calculated for each location to more clearly show the ongoing accumulation trend at each location, indicating that an average steady state concentration has still not been reached at these locations after six months. At the end of the six-month simulation, the minimum dilution rate at the outfall and upstream locations equates to approximately 40 dilutions (seen in the final days of September, at the end of the run) with an average dilution rate at this time of 50.

In summary, the recirculation effects were considerable in the model, with the remnant plume redirected upstream on the flood tide back to the outfall location, resulting in a concentration of tracer at the outfall location and further upstream past the Elizabeth River Bridge. The model outcomes indicate that an outfall with the wastewater discharging at this location would not be flushed out of the Elizabeth River, instead the wastewater plume would lead to accumulation of the chemicals carried in the waste stream in the river system over time.

5.2 Case 2 - Darwin Harbour Outfall Location

The alternative location examined for the outfall is approximately 8km from the process facility, immediately west of the Elizabeth River entrance in the East Arm section of the harbour (refer Figure 5.1). The location has been selected to be in the primary flow path away from the mouth of the Elizabeth River and outside of the navigation channel, with the outfall located on the seabed at 8.1m MSL depth. Initial modelling of the wastewater release from the location using the farfield model was used to determine the optimal position for the outfall location which was then examined with both the nearfield and farfield models.

5.2.1 Nearfield Model Results

The setup and establishment of the CORMIX model to apply to the analysis of the Darwin Harbour East Arm location is specified in Section 4.3. The modelled velocity and associated water level at the location over a full month of modelled seasonal conditions was used as a basis for establishing the hydrodynamic conditions at the discharge location. The outfall discharge on the ebb tide (outgoing tide) was used in determining the hydrodynamic cases for CORMIX.

The representative hydrodynamic cases assessed in the model for the Darwin Harbour East Arm site in the CORMIX model are summarised in Table 5.1. The model dilutions at the edge of the nearfield region (NFR) are shown for each respective case representing the approximate point at which the turbulent mixing around the outfall reaches stability and horizontal turbulence and mixing in the farfield dominates.

The results in Table 5.1 indicate the nearfield region is consistently in the range of 25m – 70m. Dilutions range from 31 dilutions to 128 dilutions in the nearfield region between the range of typical hydrodynamic scenarios (5th and 95th percentile). For the 50th percentile current velocity of 0.2ms-1 the nearfield region extends 25m from the outfall and approximately 50 dilutions are achieved.

A plot of the CORMIX modelled dilutions at the end of the nearfield vs the current velocity in Figure 5.5 shows a strong linear relationship, with the number of dilutions increasing at the edge of the nearfield as velocity increases. A similar relationship is generally noted for the size of the nearfield extent vs velocity.



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Table 5.1: Nearfield Modelled Dilution Rate at edge of Nearfield (CORMIX) for cases based on a the Discharge Current speeds at East Arm (Outfall Case 2).

Representative Current Velocity Water Level and Current Velocity

CORMIX Nearfield Distance

Dilutions at

Nearfield CORMIX

5th Percentile -0.5m MSL, Current Velocity = 0.08ms-1 56m 30

20th Percentile 0m MSL, Current Velocity = 0.1ms-1 56m 34



95th Percentile 0.5m MSL, Current Velocity = 0.6ms-1 60m 128

99th Percentile 0.5m MSL, Current Velocity = 0.8ms-1 71m 146

Figure 5.5: Plot of CORMIX modelled dilutions at the end of the nearfield vs current velocity

In Figure 5.6 the plume footprint and modelled dilution along the plume centreline is shown from CORMIX, based on the 80th percentile current of 0.4 m/s, and water level of 0m MSL examined in the outfall design process (Section 4.3.1). The contours in Figure 5.6 show a 2D representation of the dilution isolines following full development of the nearfield mixing, as determined by the rules-based system in CORMIX. The dilution isoline reaches the edge of the nearfield region (NFR) at a distance of 45m from the outfall.



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Figure 5.6: CORMIX 2D Plume result for Outfall Design

5.2.2 Integration of Nearfield and Farfield Model Results

To integrate the results from the nearfield CORMIX model with the farfield results from Delft3D, the plume footprint and dilution rate along the centreline of the plume were compared with the dilution rates at the through the plume footprint in the farfield model.

To integrate the modelled rates of dilution from the farfield model (Delft3D) with the nearfield model (CORMIX), representative test cases were examined across the range of typical current velocities at the outfall. The test cases are summarised in Table 5.2. A direct comparison of the dilution rates in the NFR from the nearfield and farfield model cases in equivalent hydrodynamic conditions were examined at the defined edge of the nearfield. Calibration of the Delft3D model through adjustments to the horizontal diffusivity terms was undertaken to closely align the nearfield dilutions rates in the test cases (refer Table 4.1).

Overall a close agreement in dilution rates at the edge of the nearfield boundary between CORMIX and the farfield D3D was achieved as highlighted in Table 5.2. The results in Table 5.2 show the Delft3D model dilution rates in the nearfield scale to nearfield results from CORMIX closely for the mid-range to high current velocity cases (P50 to P95) with general agreement shown for the lowest current cases (P5 and P20) noting the dilution rates in the low velocity conditions are critical for the plume dispersal (ie represent ‘worst case’ dispersion conditions).

The overall agreement between models for the test cases in Table 5.2 indicates that the horizontal and vertical turbulence models and parameter coefficients adopted in the Delft3D model are well prescribed for the range of energy conditions critical for plume impacts.

Outfall



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Table 5.2: Nearfield and Farfield (depth averaged) Models Comparison of Modelled Dilution Rate at edge of Nearfield

Representative Current Velocity Water Level and Current Velocity

CORMIX Nearfield Distance

Dilutions CORMIX

Dilutions Delft3D

5th Percentile -0.5m MSL, Current Velocity = 0.08ms-1 56m 34m 12m

30 24 19

51 39 24

20th Percentile 0m MSL, Current Velocity = 0.1ms-1 56m 34m 22m

34 30 25

54 43 37

50th Percentile 0m MSL, Current Velocity = 0.2ms-1 25.1m 50 54

80th Percentile 0m MSL, Current Velocity = 0.4ms-1 45.8m 97 102

95th Percentile 0.5m MSL, Current Velocity = 0.6ms-1 59.9m 128 136

5.2.3 Farfield Model - Plume Spatial Extents

Examples of the modelled plume extents from the farfield model are shown in Figure 5.7 for the current speed of 0.08ms-1 (5th percentile case), Figure 5.8 for the current speed of 0.2ms-1 (50th percentile case) and Figure 5.9 for the current speed of 0.4ms-1 (80th percentile case). The plume extents have been determined based on analysis of the modelled depth averaged tracer dilutions and correspond with cases presented in Table 5.2.

In all modelled plume extents shown, it can be clearly seen that the plume is directed to the west under the hydrodynamic forcing, with the plume shape and extent strongly influenced by the current velocity. The lowest velocity case shows a wide plume extent that spreads from the outfall, whilst the higher velocity cases result in an elongated narrow plume shape aligned with the currents.

In Figure 5.10 a combined figure showing the shape and extent of the modelled 300 dilution contour is presented for the 3 cases for direct comparison of the shape and extent.



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Figure 5.7: Modelled depth averaged dilutions from conservative tracer for a current velocity at 0.08ms-1 (approximate 5th percentile). East Arm outfall location.




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Figure 5.10: Modelled depth averaged dilutions for percentile cases (P5, P50 and P80) showing extent and shape of 300 dilution contours.



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5.2.4 Plume Dispersal

The timed release of the outfall allows for the plume to be directed with the ebb tide current out of the harbour. In Figure 5.11 the ebb tide sequence on 10 August 2019 is shown illustrating the plume being directed by the ebb tide currents towards the west flowing out into Darwin Harbour over the 6 hours of timed release. This is the general pattern of release for the wastewater plume though the scenario models in spring and neap tides. When the outfall has ceased and the flood tide currents redirect flows into East Arm, there is a remnant plume that is directed back through East Arm over the outfall location. Over the time of the seasonal model simulation (one month) the recirculation of the plume through East Arm leads to a gradual increase in the background level of tracer. This has been examined in greater detail for a longer model period of 6 months to understand the longer-term effects.

Figure 5.11: Time series of outfall release through the ebb tide cycle, 11 Aug 2019 08:00 to 14:00. Depth averaged currents are shown steering the outfall effluent plume in the farfield model.

Bladin Pt Bladin Pt

Bladin Pt Bladin Pt

x Outfall x Outfall

x Outfall x Outfall



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5.2.5 Modelled Tracer Concentration at locations in Darwin Harbour and Elizabeth River for a 6-month simulation period

The tracer concentration was analysed for locations in Darwin Harbour and Elizabeth River from an extended model simulation period of 6-months (Dry Season model scenario with the wastewater released on the ebb tide). The extended model was run to analyse recirculation effects of the plume and accumulation of the tracer in the model in the farfield model. Locations around the outfall and through Darwin Harbour as shown in Figure 5.14 were analysed for modelled tracer concentration over the extended simulation period.

Figure 5.12: Model Reporting Locations for Darwin East Arm outfall (Case 2). Tracer concentration assessed over the six-month model simulation period.

For the extended model simulation of 6 months, the time series plot of the tracer concentration at locations through the model are shown in Figure 5.13 to Figure 5.19. The plots show the modelled tracer



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concentration oscillates around a general mean level. The peak and trough of the concentration are associated with the flood and ebb tide, the discharge regime and the dispersal and recirculation of the plume over time. The oscillation effects are most strongly observed at the outfall location (Figure 5.13). The highest concentration in the modelled tracer is associated with the ebb tide when the discharge of wastewater is occurring. The plume is dispersed around the outfall location under the hydrodynamic forcing in the model. During flood tides, the discharge from the outfall has stopped and the concentration level in the model is based on the remnant plume that is redirected back across the outfall.

The model outcomes show the following: • For the East Arm location, the modelled dilution rate appears to achieve an equilibrium level within 6

months at a dilution rate of approximately 255 dilutions; • For the Elizabeth River upstream location, the modelled dilution rate achieves equilibrium level within 5

months at a dilution rate of approximately 295 dilutions; • For the Darwin tide gauge location, the modelled dilution rate achieves equilibrium level within 4.5

months at a dilution rate of approximately 1050 dilutions; • For the Inner Harbour location, the modelled dilution rate achieves equilibrium level within 5 months at

a dilution rate of approximately 2700 dilutions; • For the Outer Harbour location, the modelled dilution rate achieves equilibrium level within 5 months at

a dilution rate of approximately 12500 dilutions; • For the Middle Arm 1 location, the modelled dilution rate achieves equilibrium level within 6 months at

a dilution rate of approximately 1430 dilutions; and • For the Middle Arm 2 location, the modelled dilution rate achieves equilibrium level within 6 months at

a dilution rate of approximately 1430 dilutions.

Releasing the wastewater from the outfall at the East Arm location allows for a higher rate of initial dilution at the outfall, seen in the lower levels of tracer concentration in the model following release in comparison to release at the Elizabeth River site, as well as levelling off of accumulation in relatively less time than the Elizabeth River location. This can be seen in Figure 5.13 where dilution is seen to be increased approximately tenfold at the point of release, as well as in Figure 5.14 where the accumulation of tracer in the upstream reaches of the Elizabeth River (2km upstream of the Elizabeth River Bridge, as also shown for the Elizabeth River outfall location in Figure 5.4) is seen to level off within approximately five and a half months following the initiation of discharge from the outfall. The results confirm that whilst the background level of tracer has increased over the one-month seasonal scenarios, for the longer-term model cases (6-months) there is an equilibrium reached where the background level reaches stability for the locations examined in East Arm, upstream Elizabeth River and sites in Darwin Harbour.

In conclusion, the discharge of wastewater from the East Arm outfall location will raise the background concentration in Darwin Harbour of wastewater elements (toxicants). The background level will increase and then stabilise within approximately six months. Based on the tracer modelling a new equilibrium background concentration is projected based on the wastewater discharge with dilution rates of approximately 255 dilutions in East Arm, 300 dilutions in upper reaches of Elizabeth River and 2700 dilutions in Darwin Harbour.



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Figure 5.13: Tracer Level (following on an input value of 100 kg/m3) within East Arm Darwin Harbour, following wastewater discharge from Darwin Harbour

Figure 5.14: Tracer Level (following on an input value of 100 kg/m3) 2km upstream of the Elizabeth River Bridge at Eliz River Upstream, following wastewater discharge from Darwin Harbour



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Figure 5.15: Tracer Level (following on an input value of 100 kg/m3) at the Darwin Harbour Tide Gauge, following wastewater discharge from Darwin Harbour

Figure 5.16: Tracer Level (following on an input value of 100 kg/m3) within the Inner Harbour area, following wastewater discharge from Darwin Harbour



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Figure 5.17: Tracer Level (following on an input value of 100 kg/m3) within the Outer Harbour area, following wastewater discharge from Darwin Harbour

Figure 5.18: Tracer Level (following on an input value of 100 kg/m3) upstream within Middle Arm (Middle Arm 1), following wastewater discharge from Darwin Harbour



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Figure 5.19: Tracer Level (following on an input value of 100 kg/m3) within Middle Arm close to the inner harbour (Middle Arm 2), following wastewater discharge from Darwin Harbour

5.2.6 Wastewater Plume – Analysis of 3D Depth Concentration

A series of cross-sectional plots from the 3D model to show the mixing in East Arm around the outfall are presented along the line shown in Figure 5.20. The cross section plots shown in Figure 5.21 to Figure 5.22 show temperature and salinity during a four hour period where the outfall is discharging on the ebb tide. The plots demonstrate that stratification effects around the outfall are very localised to the immediate area around the outfall after which the temperature and salinity become quite well mixed through the water column. The high level of mixing experienced across the harbour over time is particularly evident in Figure 5.22 where the middle and bottom plots show the temperature across the cross section just before and just after the wastewater discharge ceases for that cycle; almost immediately following wastewater discharge ceasing, the temperature plume drops almost back to the background temperature.

Note that the white sections in Figure 5.21 and Figure 5.22 show the regions where the 6m and 33m grid enclosures interface across the cross section. The results indicate that due to the strong prevailing currents at the site, in the farfield the plume is expected to be well mixed in the water column.



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Figure 5.20: Cross section taken through the 33m grid and the 6m grid to produce the depth dependent plots taken from the 3D models

Figure 5.21: Salinity levels taken from the model cross section shown in Figure 5.20 across a discharge cycle at 19:30 on the 12th of July (top) and 00:00 on the 13th of July (bottom)

33m Grid

6m Grid

Cross Section



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Figure 5.22: Temperatures taken from the model cross section shown in Figure 5.20 across a discharge cycle at 19:30 on the 12th of July (top), 00:00 on the 13th of July (middle) and 01:00 on the 13th of July (bottom)

5.3 Model Outputs

5.3.1 Mixing Zones

The MEQMMP requires examination of the mixing zone around the wastewater outfall to define the dilutions achieved from the model for the designated level of ecological protection (LEP). For the East Arm outfall location, the recommendation for the region closest to the outfall location is based on the nearfield mixing results from CORMIX (Table 5.1). The recommended number of dilutions achieved within a 60m radius from the outfall consistent with the approximate 5th percentile value is 25 dilutions, based on the hydrodynamic cases reported in Table 5.1. For reference the dilutions at the edge of the 60m mixing zone are predicted for the 50th percentile case to be approximately double (59 dilutions) and for the 95th percentile case approximately five times as high (128 dilutions).



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Figure 5.23: 60m mixing zone centred on the proposed outfall location in the Darwin Inner Harbour

5.3.2 Dilution Contours

The calculation of the dilution contours around the outfall was completed for the wet season and dry season cases respectively in the 3D farfield model in a format to inform the MEQMMP. The dilution contours represent the calculated 5th percentile dilution level (i.e. 95th percent concentration) across the model grid over the entire month in each respective simulation. The dilution is calculated from the 3D model results in the upper water column (surface layer) and seabed (lowest layer).

The final contours do not represent a moment in time, rather they are an analysis of each point on the model grid to assess the dilution result across the entire simulation period, calculating the 5th Percentile value at each respective point location. The final contours are shown in Figure 5.24 for the dry season and Figure 5.25 for the wet season.

The comparison of the shape and extent of the dry season and wet season dilution contours confirms: • the wet season cases have a larger extent compared to the dry season. This is a result of the winds

that steer the surface layer further to the east; and • the surface layer dilution contours extend across a larger area than those at the seabed. This outcome

is due to the density difference that causes the plume to rise and concentrate more in the surface layers in the immediate vicinity of the outfall.



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Figure 5.24: Dilution Contours for the dry season scenario based on analysis of the 5th percentile dilution rate over the 4-week simulation. Upper Plot: Seabed layer, Lower Plot Surface Layer.



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Figure 5.25: Dilution Contours for the wet season scenario based on analysis of the 5th percentile dilution rate over the 4-week simulation. Upper Plot: Seabed layer, Lower Plot Surface Layer.



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5.3.3 Salinity Plume Extent

Investigation into the zone of influence over which the salinity levels reduce back to background levels was undertaken for the MEQMMP alongside the tracer modelling. As noted in Section 5.2.6 the stratification is very localised in the model indicating within 500m of the outfall there is good mixing through the water column noted in the 3D model scenarios.

As defined in Baird 2019, the range of salinities experienced within the Darwin Inner Harbour, measured at a location very close to the proposed East Arm outfall location, can be characterised by the statistical measures presented in Table 5.3. This statistical data shows the relatively greater range in salinity levels experienced in Darwin Harbour during the wet season, as well as the generally lower salinity levels experienced during the wet season.

Table 5.3: Salinity Range in Darwin Inner Harbour

Wet Season Salinity

ppt Dry Season Salinity

Ppt

10th Percentile 29.1 34.0





The 50th percentile (median) salinity in Table 5.3 was used to define the background salinity for the wet season and dry season models respectively. The model scenarios for each of the respective seasonal model cases were analysed to determine the spatial extent describing where the modelled salinity level reverts to the background level across the full month-long run.

The final contours do not represent a moment in time, rather they are an analysis of each point on the model grid to assess the minimum salinity level reached in that grid point across the entire simulation period (i.e. the salinity that is furthest from the model background salinity, as a result of the modelled plume effects from the outfall) : • Figure 5.26 demonstrates the modelled spatial extent surrounding the wastewater outfall to the

modelled background level for the depth averaged model. Contours of increasing difference from the background salinity are shown moving toward the outfall location based on the model outcomes.

• Figure 5.26 shows the same contour levels at the same successive intervals, except it is depicting the results of the 3D modelling, showing the plume extent in the surface layer of the water column.

• Figure 5.28 shows similar salinity contour levels as Figure 5.26, as experienced in the water surrounding the outfall in the wet season.

• Figure 5.29 shows these wet season contour levels again, except it is depicting the results of the 3D modelling, showing the plume extent in the surface layer of the water column.

Stratification of the water column around the outfall can be seen in comparing the extents of the contours in the 2D depth averaged and 3D surface layer outcomes. This confirms the conclusions in Section 5.2.6 demonstrating localised effects around the outfall after which mixing occurs as the wastewater rises from the outfall at the seabed to the top of the water column, resulting in generally well mixed conditions within a short distance of the outfall.



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Figure 5.26: East Arm Dry Season Salinity Contours, showing where the plume reaches back to the model background salinity (defined as the median salinity in the data measured in the Inner Harbour during the Dry Season) in the depth averaged model

Figure 5.27: East Arm Dry Season Salinity Contours, showing where the plume reaches back to the model background salinity (defined as the median salinity in the data measured in the Inner Harbour during the Dry Season) in the 3D model, at the surface layer



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Figure 5.28: East Arm Wet Season Salinity Contours, showing where the plume reaches back to the model background salinity (defined as the median salinity in the data measured in the Inner Harbour during the Wet Season) in the depth averaged model

Figure 5.29: East Arm Wet Season Salinity Contours, showing where the plume reaches back to the model background salinity (defined as the median salinity in the data measured in the Inner Harbour during the Wet Season) in the 3D model, at the surface layer



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Figure 5.26, Figure 5.27, Figure 5.28 and Figure 5.29 show the relatively small difference in salinity in East Arm caused by the outfall wastewater, as the contours shown demonstrate that salinity levels increase back to well within the 20th percentile salinity close to the outfall (the 20th percentile salinity level is so close to the outfall it cannot be shown in the contours), and to within the 50th percentile by the contour shown in red. This shows that the modelled changes to salinity caused by the wastewater are within the natural salinity variance experienced within the harbour.

The comparison of the shape and extent of the dry season and wet season dilution contours confirms: • the wet season cases have a larger extent compared to the dry season. This is a result of the winds;

and • the surface layer salinity contours extend across a larger area than those at the seabed. This outcome

is due to the density difference that causes the plume to rise and concentrate more in the surface layers.

• Once the plume has reached the background salinity in both the 2D and 3D model, the extent of which is quite similar, the 3D model demonstrates it is well mixed through the water column, and therefore that the 3D and 2D models are comparable from this point.

5.3.4 Temperature Plume Extent

Investigation into the zone of influence over which the temperature reduces back to background levels was also undertaken alongside the tracer modelling. The temperature range experienced within Darwin Harbour have also been defined according to percentiles, as detailed in Table 5.4 from the available measured data (Baird 2019). This statistical data shows the relatively greater range in temperatures experienced in Darwin Harbour during the dry season, as well as the generally lower temperatures experienced during the dry season.

Table 5.4: Temperature Range in Darwin Inner Harbour

Wet Season Temperature

oC Dry Season Temperature

oC






The 50th percentile (median) temperature was used to define a background temperature according to the wet season and dry season models. The runs for both seasons were analysed to determine the area over which temperature decreases back to the background level across the full month-long run, incorporating the mixing following each release during the intermittent discharge at the wastewater outfall. As with the salinity contours, the final temperature contours do not represent a moment in time, rather they are an analysis of each point on the model grid to assess the maximum temperature reached in that grid point across the entire simulation period (i.e. the temperature that is furthest from the model background temperature, as induced by the wastewater outfall discharging at 50˚C). • Figure 5.30 demonstrates the 2D depth averaged modelled contours where waters surrounding the

wastewater outfall are above the background temperature, with successively increasing difference from the background temperature shown as the contours become closer to the outfall location. Figure 5.31 shows the wet season contour levels of the 3D modelling, showing the plume extent in the surface layer of the water column.



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• Figure 5.32 shows similar temperature contours, as experienced in the water surrounding the outfall in the wet season for 2D depth averaged model. Figure 5.33 shows the wet season contour levels of the 3D modelling, showing the plume extent in the surface layer of the water column.

Figure 5.30: East Arm Dry Season Temperature Contours, showing where the plume reaches back to the model background temperature (defined as the median temperature in the data measured in the Inner Harbour during the Dry Season) in the depth averaged model

Figure 5.31: East Arm Dry Season Temperature Contours, showing where the plume reaches back to the model background temperature (defined as the median temperature in the data measured in the Inner Harbour during the Dry Season) in the 3D model, at the surface layer



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Figure 5.32: East Arm Wet Season Temperature Contours, showing where the plume reaches back to the model background temperature (defined as the median temperature in the data measured in the Inner Harbour during the Wet Season) in the depth averaged model

Figure 5.33: East Arm Wet Season Temperature Contours, showing where the plume reaches back to the model background temperature (defined as the median temperature in the data measured in the Inner Harbour during the Wet Season) in the 3D model, at the surface layer



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As in Section 5.3.3, a small level of stratification is evident in the 3D plume extents, but the stratification is minimal, as demonstrated by the similar and overlapping areas of the contours within the outer median contour (red).

Both Figure 5.31 and Figure 5.33 show the relatively small difference in temperature caused by the outfall wastewater, as the contours shown demonstrate that temperature increase back to well within the 20th percentile temperature close to the outfall, and to within the 50th percentile by the contour shown in red. This shows that the changes to temperature caused by the wastewater are within the natural temperature variance experienced within the harbour.

The comparison of the shape and extent of the dry season and wet season dilution contours confirms: • The wet season cases have a larger extent compared to the dry season. This is a result of the winds;

and • The surface layer temperature contours extend across a larger area than those at the seabed. This

outcome is due to the density difference that causes the plume to rise and concentrate more in the surface layers.

• Once the plume has reached the background temperature in both the 2D and 3D model, the extent of which is quite similar, the 3D model demonstrates it is well mixed through the water column, and therefore that the 3D and 2D models are comparable from this point.



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6. Conclusions TNG Limited (TNG) have submitted a Notice of Intent (NOI) to the Northern Territory Environment Protection Authority (NT EPA) and Department of Lands Planning and Environment (DLPE) to construct and operate the Darwin Processing Facility (processing facility). The operation of the processing facility will produce a waste stream that is proposed to be directed through a pipeline to an outfall location in Darwin Harbour which will discharge wastewater at a rate of 34.6 ML a day. The waste stream will contain a range of chemicals associated with the treatment process, and discharge with a salinity of 12 ppt (parts per thousand) at a temperature up to 50˚C.

Baird Australia (Baird) has undertaken a numerical modelling assessment of the outfall as part of the environmental assessment process. Baird have developed a hydrodynamic model of the Darwin Harbour which has been validated against measured data available from around the project location as reported in the TNG Darwin Processing Facility Outfall – Hydrodynamic Modelling Report (Baird 2019). The validated hydrodynamic model was used to develop an outfall design and wastewater release strategy to assess two potential locations in the harbour for the outfall at Elizabeth River and East Arm.

Based on the analysis of the modelled environmental conditions at the intended discharge sites it was determined that discharge at the outfall would not be continuous. The discharge regime has been developed based on release of the wastewater only on the ebb tide (outgoing tide) to ensure the wastewater plume is directed away from the outfall in the direction of the Darwin Harbour entrance under the general tidal currents when the wastewater is being released.

For this study a high-resolution three-dimensional numerical model system has been developed to assess the mixing and dispersion of the wastewater from the outfall location and into Darwin Harbour. To fully account for plume dispersion, the modelling has been completed using a nearfield (in direct proximity to the outfall) and farfield modelling approach (all of Darwin Harbour). The nearfield and farfield models are integrated to provide a model system that simulates the plume behaviour around the outfall during discharge cycles and which accounts for the interaction of processes including density differences between the wastewater and receiving waters, the effect of local currents and water levels on plume dispersion, as well as recirculation effects of the remnant plume back to the outfall location under flood tide conditions.

The model assessment examines four-week simulation periods representative of the wet and dry season incorporating two full spring and neap tide cycles. Conservative tracer is applied in the model to represent the wastewater volume, discharged with temperature and salinity defined by the production process. The temporal and spatial varying concentrations of tracer through the model scenarios is used to predict the dilution rate around the outfall for analysis of toxicants planned for release in the waste stream (metals and metalloids). The model analysis examines the plume dispersion in terms of temperature and salinity around the outfall to define the extent of the mixing zones for water quality.

The initial outfall location modelled in the Elizabeth River showed the modelled tracer plume tended to oscillate around the Elizabeth River outfall location, without the plume significantly dispersing and being carried out of the Elizabeth River through to Darwin Harbour. During release periods (ebb tides) the tracer plume is directed downstream from the outfall in Elizabeth River, but does not have sufficient time to exit the branch of the river system before flood tides redirect the remnant plume back upstream. When the redirected plume moves back upstream on the flood tides, it concentrates upstream of the Elizabeth River Bridge as well as being directed into the creek systems attached to the Elizabeth River around the TNG site. The plume recirculation effects were considerable in the model, and model outcomes indicate that an outfall with the wastewater discharging at the location would lead to accumulation of the chemicals carried in the waste stream in the river system over time.

The second location examined for the outfall is approximately 8km from the process facility, immediately west of the Elizabeth River entrance in the East Arm section of the harbour (refer Figure 5.1). Releasing the wastewater from the outfall at the East Arm location resulted in a higher rate of modelled dilution



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around the outfall in comparison to release at the Elizabeth River site due to the increased flows through East Arm and exchange with Darwin Harbour. The 3D model outcomes demonstrate that there is good mixing of the plume through East Arm and at a short distance from the discharge location there is good vertical mixing of the tracer plume through the water column. The outfall location in East Arm was analysed in detail to determine the dilution contours around the site with the nearfield model (CORMIX) used to provide a recommendation on the number of dilutions predicted within a 60m radius of the outfall.

Analysis of the modelled tracer concentration in the farfield model scenarios was completed to determine the shape and extent of the dilution contours around the outfall across the two seasonal scenarios, with the wet season showing marginally larger extents compared with the dry season. Dilution at the surface from the 3D model shows larger spatial extents compared with the bed level due to the salinity difference of the wastewater compared to the receiving waters.

The farfield model results were analysed for the two seasonal scenarios to determine the shape and extent of the temperature and salinity plumes. The 3D model outcomes demonstrate that there is good mixing of the plume through East Arm - at the discharge location there is stratification around the immediate vicinity of the outfall, but the water column is vertically well mixed a short distance away. For both temperature and salinity, the modelled changes caused by the wastewater are within the natural temperature variance experienced within the harbour over the wet and dry season periods.

A 6-month simulation case was analysed to investigate the tracer concentration for locations upstream and downstream of the outfall in East Arm, upstream Elizabeth River and sites through Darwin Harbour. The tracer modelling showed that at locations upstream and downstream of the outfall the tracer concentration increased over time and then reached equilibrium within approximately six months. Based on the model outcomes, it is concluded that the East Arm outfall would result in an increase in the background concentration in the Harbour of the respective chemicals in the waste stream, with dilution reaching an equilibrium within approximately 6-months.



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7. References ANZECC & ARMCANZ (2000). Australian and New Zealand Guidelines for Fresh and Marine Water Quality. Volume 1: The Guidelines. (Australian and New Zealand Environment and Conservation Council (ANZECC) & Agriculture and Resource Management Council of Australia and New Zealand (ARMCANZ).

ANZECC & ARMCANZ (2000) Australian and New Zealand Guidelines for Fresh and Marine Water Quality. Volume 2: Aquatic Ecosystems – Rationale and Background Information. (Australian and New Zealand Environment and Conservation Council (ANZECC) & Agriculture and Resource Management Council of Australia and New Zealand (ARMCANZ).

Baird (2019). TNG Darwin Processing Facility Outfall – Hydrodynamic Modelling Report. Prepared for TNG. Ref 13135.101.R1.Rev0.

Deltares (2014). Delft3D-FLOW: Simulation of multi-dimensional hydrodynamic flows and transport phenomena, including sediments. User Manual. Hydro-Morphodynamics. Version: 3.15.32923

Doneker, R.L. and G.H. Jirka (2007). CORMIX User Manual: A Hydrodynamic Mixing Zone Model and Decision Support System for Pollutant Discharges into Surface Waters", EPA-823-K-07-001.

NTEPA (2013). Guidelines on Mixing Zones.

SMS Group 2019, TNG-FEED-SDE SAT-TNG-00008 Waste water, TNG Front-End Engineering and Design for Beneficiation and Processing Plant for Mt.Peak Ti/V/Fe Project, Email sent Friday 13 September 2019, B.Nowak-P.Kreppold.



13135.101.R2.Rev0 Appendix A

Summary of Final Dilution Mapping Datasets

Supplied Datasets File Filename Date

3D Dilution Contours – Surface Layer 16/10/2019

3D Salinity Contours – Surface Layer 16/10/2019

3D Temperature Contours – Surface Layer 16/10/2019

Documents

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