Volume 3, Chapter 21: Water environment Appendices€¦ · 2.5 Lakes and reservoirs 8 3. Water quality objectives and Regulatory guidance 9 3.1 Introduction 9 3.2 Objectives of the

Heathrow Expansion PRELIMINARY ENVIRONMENTAL INFORMATION REPORT

Volume 3, Chapter 21: Water environment

Appendices

Appendix 21.1 Surface water quality assessment

Appendix 21.2 Groundwater impact assessment

Appendix 21.3 Preliminary WFD assessment

Appendix 21.4 Flood risk assessment

Appendix 21.5 Drainage impact assessment

Appendix 21.6 Screening of abstractions and discharges

Appendix 21.7 Lower Colne modelling report

Appendix 21.8 Lower crane Modelling report

1.1 © Heathrow Airport Limited 2019

Heathrow Expansion PRELIMINARY ENVIRONMENTAL INFORMATION REPORT Appendix 21.1: Surface water quality assessment

Appendix 21.1 © Heathrow Airport Limited 2019

Document Control

APPENDIX 21.1 SURFACE WATER QUALITY ASSESSMENT



CONTENTS

1. Introduction 1

1.1. Context 1

1.2 Purpose of the Surface Water Quality Assessment 1

1.3 Purpose of the Interim SWQA 1

1.4 Report structure 2

2. Project setting 4

2.1 Project area 4

2.2 Waterbodies in the vicinity of the Airport 4

2.3 Surface water: rivers and streams 4

2.4 Discharge of airfield runoff 5 Southern airfield drainage catchment 6 Western airfield drainage catchment 6 Clockhouse Lane Pit 6 Mayfields Farm treatment facility 7 Eastern airfield drainage catchment 7

2.5 Lakes and reservoirs 8

3. Water quality objectives and Regulatory guidance 9

3.1 Introduction 9

3.2 Objectives of the SWQA 9

3.3 Regulatory guidance 10

3.4 Existing Discharge permits 11 Southern and Western Catchment permits 11 Eastern catchment permit 12 North Western Reservoir 12 South East Catchment, and North East Catchment 13

3.5 Environmental Quality Standards 13

3.6 WFD classification and water quality parameters 13

4. Conceptualisation of potential surface water quality sources, pathways and receptors 16

4.2 Additional aspects of the conceptual model 19 New catchments and airfield discharges 19 Mixing of river waters 20 Changes to baseflow quality into the river 20 Operational site - Land-take for increased Airport footprint 21

4.3 Parameters of interest 22



5. Assessment of the Construction Phase 24

5.1 Introduction 24 Impact assessment and environmental measures 24

6. Assessment of permanent changes in road drainage 25

6.1 Introduction 25

6.2 Assessment of road drainage 25

6.3 Assessment at ES stage 25

7. Assessment of BOD in operational airfield discharges 27

7.1 Introduction 27

7.2 Assessment of potential BOD concentrations 27

7.3 Summary of PEIR numerical assessment of BOD 29 Effect of diverting the rivers and lining the channels 29 New discharges for the DCO Project to the Colne and Colne Brook catchments 29 Eastern Catchment discharge to the River Crane 29 Clockhouse Lane Pit discharge to Felthamhill Brook 29 Potential environmental measures within the CLP 30

7.4 Next steps 31

8. Assessment of Orthophosphate in operational airfield discharges 32

8.1 Introduction 32

8.2 Orthophosphate water quality standards 32

8.3 Orthophosphate concentrations in Clockhouse Lane Pit 33

8.4 Clockhouse Lane Pit: Orthophosphate for the with development scenario 35 Potential risks 35 Risk assessment and potential environmental measures 35

8.5 Discharges to the west of the Airport: Orthophosphate for the ‘with development’ scenario 36 Potential risks 36 Risk assessment and potential environmental measures 37

8.6 Discharges from the Eastern Catchment 37

8.7 Next steps 38

9. Assessment of PFOS in operational airfield discharges 39

9.1 PFOS background information and WFD assessment 39

9.2 PFOS concentrations: rivers around Heathrow 40 Environment Agency monitoring data 40 DCO Project baseline monitoring data 40

9.3 PFOS concentrations: airfield waters 42 Heathrow monitoring data 42 DCO Project baseline monitoring data 42



9.4 Discussion of PFOS data 43

9.5 Potential sources of PFOS 44 Non-Heathrow sources of PFOS 46

9.6 Assessment of potential PFOS effects 46 Historic source only 46 On-site continuing sources 47 Up-gradient sources of PFOS 48 Combination of sources 48

9.7 Management of PFOS in surface water discharges 48

9.8 Next steps: PFOS assessment at ES 48 Water quality monitoring 48 Detailed on-site inventory and sampling of PFOS 49 Engagement with the Environment Agency 49

10. Assessment of PAHs in operational airfield discharges 50

10.1 Introduction 50

10.2 WFD limits for PAH species 50

10.3 Baseline PAH concentrations in rivers: Environment Agency data 51

10.4 Baseline PAH concentrations in airfield discharge waters 52

10.5 Comparison of Environment Agency and DCO baseline data 53

10.6 Potential PAH concentrations 54

11. Assessment of atmospheric deposition on water quality 55


11.2 Baseline nitrate concentrations 55 Lakes and reservoirs 55

11.3 Numerical assessment of NOx deposition 57 Discussion 59

12. Wastewater treatment and discharge 61


12.2 WwTP discharge 61

12.3 Next steps 62

13. Conclusions 63

13.2 Construction activities, dewatering and runoff discharge 63

13.3 Roads 63

13.4 River diversions and lined channels 63

13.5 Airfield discharges 64 BOD 64 Orthophosphate 65 PFOS 66



PAHs 67 13.6 Lakes, reservoirs and atmospheric deposition 67

13.7 Wastewater treatment plant 68

14. Glossary 69

Model scenarios 5 Assessment criteria 5

14.1 River network – PEIR Models 8 Defining initial flows 12 Flow gauging network 12

TABLE OF TABLES

Table 3.1 Summary of Heathrow surface water discharge permits 11 Table 4.1 Source-Pathway- Receptors summary 17 Table 8.1 Orthophosphate concentration ranges for WFD Classes 32 Table 10.1 WFD EQS values for Specific Pollutant and Priority Substances 50 Table 11.2 Historic (2000-2016) water quality sampling data gathered by the Environment Agency56

ANNEXES Figures Annex A SIMCAT Modelling Annex B Cumulative percentage plots of BOD concentration At Discharge locations Annex C Baseline SIMCAT Model v01, Graphs of FLow results Annex D Baseline SIMCAT M0del v01, Graphs of BOD results Annex E Baseline SIMCAT Model v03, Graphs of BOD results for Best Fit Model Annex F Baseline SIMCAT Model v07, Graphs of BOD results for Felthamhill Brook/Portlane Brook at reduced CLP discharge Annex G SIMCAT Models, Graphs of Flow Output for ‘With-Development’ Scenario, Preferred Approach v01 Annex H SIMCAT Model, Graphs of BOD concentration Output for ‘With-Development’ Scenario, Preferred Approach v01 Annex I SIMCAT Model, Graphs of BOD concentration Output for ‘With-Development’ Scenario, Alternative Approach v01 Annex J Preferred Approach SIMCAT Models, Graphs of BOD Output for ‘With-Development’ Scenarios for Clockhouse Lane Pit Annex K Graphs of orthophosphate concentrations in Clockhouse Lane Pit and Portlane Brook Annex L Environment Agency monitoring data: PAH results for rivers in the vicinity of HEathrow Annex M DCO Baseline monitoring: PAH results for site waters


Appendix 21.1-1 © Heathrow Airport Limited 2019

1. INTRODUCTION

1.1. Context 1.1.1 Heathrow Airport Limited proposes to remodel and expand the current two runway,

four terminal, Heathrow Airport (‘the Airport’) by adding a third runway and associated development (‘the DCO Project’).

1.1.2 This interim Surface Water Quality Assessment (SWQA) report accompanies the Preliminary Environmental Information Report (PEIR), forming one of the appendices to Chapter 21: Water Environment. The SWQA also informs the assessment of surface water quality risk in Appendix 21.3: Water Framework Directive Assessment, Volume 3 and Appendix 21.5: Drainage Impact Assessment, Volume 3 (DIA).

1.2 Purpose of the Surface Water Quality Assessment

1.2.1 The overall aim of the DCO Project with respect to surface water quality is for there to be no significant deterioration in surface water quality as a result of the DCO Project, and for there to be betterment where possible.

1.2.2 The DCO Project is a large engineering project, and whilst much of the proposed changes are associated with the third runway and the expansion of the airfield, the DCO Project also includes a number of components outside of the expanded airfield footprint.

1.2.3 The SWQA presents an assessment of potential risks of changes to surface water quality that may arise as a result of the DCO Project, either during the development period or the long-term operational period. This includes potential risks to rivers, lakes, streams and reservoirs in the area around Heathrow and downstream locations that could also be affected.

1.3 Purpose of the Interim SWQA

1.3.1 This report in its current version is an ‘Interim SWQA’ report that has been developed to inform the PEIR assessment. This Interim SWQA has been completed based on consideration of the DCO Project components and activities with regards to potential contaminant sources, pathways and receptors during development and operation. A review of water quality monitoring data for the airfield and wider area has been undertaken and identified several water quality parameters that could change as a result of the DCO Project. The range of potential effects are set out in Section 4.



1.3.2 This is an interim document based on the available data with respect to the DCO Project design and the available monitoring data at the time of the study. Therefore, any results and conclusions will be preliminary. The SWQA will be updated for the ES as the DCO Project plans are advanced, as additional survey data become available, and as other DCO Project studies become available with pertinent information.

1.4 Report structure

1.4.1 The report structure and description of the sections is as follows:

Section 2 Project setting: this section describes the surface water bodies around Heathrow in their current configuration. This includes descriptions of the rivers, lakes and reservoirs around Heathrow, and also the on-site water reservoirs and surface water treatment facilities at Heathrow.

Section 3 Water Quality Objectives and Regulatory Guidance: this section summarises the objectives of the scheme with respect to water quality and the regulatory drivers for water quality.

Section 4 Conceptualisation of potential surface water quality sources, pathways and receptors: this section describes the conceptualisation of potential surface water quality effects following the Source-Pathway-Receptor (SPR) approach.

Section 5 Assessment of the construction phase: this section summarises the contaminant sources and risks to surface water quality during the construction phase, and presents a high-level overview of the measures to be put in place to mitigate against those effects.

Section 6 Assessment of permanent changes in road drainage: this section discusses the potential changes to road runoff quality as a result of changes to the road network and increased traffic.

Section 7 Assessment of BOD in operational airfield discharges: This section presents an assessment of potential water quality changes with respect to Biochemical Oxygen Demand (BOD) that may occur as a result of the development, looking at the operational period, post-development.

Section 8 Assessment of orthophosphate in airfield discharges: this section presents baseline orthophosphate concentrations in the local rivers and site discharges, and discusses potential changes to water quality as a result of the DCO Project.

Section 9 Assessment of Perfluorooctane sulfonic acid (PFOS) in operational airfield discharges: this section presents baseline PFOS concentrations in the local rivers and site discharges, and discusses potential changes to water quality as a result of the DCO Project.

Section 10 Assessment of Polycyclic Aromatic Hydrocarbons (PAHs) in operational airfield



discharges: this section presents baseline PAH concentrations in the local rivers and site discharges, and discusses potential changes to water quality as a result of the DCO Project.

Section 11 Assessment of atmospheric deposition on water quality: this section presents a numerical assessment of the potential effects to the lakes and reservoirs from increased atmospheric nitrous oxide (NOx) deposition.

Section 12 Waste water treatment and discharge: this section describes relevant water quality considerations associated with the potential inclusion of a waste water treatment plant in the DCO Project design.

Section 13 Conclusions

Section 14 Glossary



2. PROJECT SETTING

2.1 Project area

2.1.1 An overview of the proposed area of development is shown in Figure 6.1, Volume 2.

2.2 Waterbodies in the vicinity of the Airport

2.2.1 With respect to WFD waterbodies, rivers will be subject to the largest degree of modifications from diversions and discharges, and are therefore the primary waterbodies that are assessed in this SWQA. However, several lakes and reservoirs have also been considered with respect to potential water quality changes. Detailed descriptions of the water environment are presented in the PEIR report as follows:

1. The existing river network is described in detail in Chapter 21: Water environment. The planned changes to the river networks as a result of the DCO Project are described in Chapter 6: DCO Project description and Chapter 21: Water environment

2. The existing airfield drainage network and runoff management is described in PEIR report Appendix 21.5, including descriptions of Clockhouse Lane Pit, Mayfield Farm and the Eastern Balancing Reservoir. Changes to the airfield network as a result of the DCO Project are described in Appendix 21.5.

3. The existing lakes and reservoirs around Heathrow are described in Chapter 21: Water environment. Changes to lakes as a result of the DCO Project are described in Chapter 6: DCO Project description and Chapter 21: Water environment.

2.2.2 These chapters and appendices should be referred to for the detailed description of these features. However, for the SWQA, certain key features are pertinent to this assessment and therefore have been described in greater detail within the report, particularly for specific characteristics of those features as they interact with different waterbodies, receive different discharge flows and in relation to water quality.

2.3 Surface water: rivers and streams

2.3.1 The surface water network of rivers that lie upstream and downstream of the site, is summarised as follows:



1. To the west of the airport is the River Colne and the associated catchment. The River Colne is an often braided system of river channels that split and re-merge several times. In the vicinity of the airport the River Colne bifurcates in turn to the Wraysbury River, the Upper Duke of Northumberland’s River, the Longford River and the Surrey Ash. The Wraysbury River re-converges with the River Colne upstream of the confluence with the River Thames

2. The Colne Brook is another bi-furcation of the Colne and flows approximately north to south to the west of the River Colne and Wraysbury River, in the vicinity of the site. A portion of the flow from the Wraysbury River enters the Colne Brook via the Poyle Channel

3. The River Crane flows north to south past the eastern perimeter of the Airport, and to the south of the Airport there is a confluence of the River Crane with the Upper Duke of Northumberland River

4. Portlane Brook and Felthamhill Brook are located to the south of the Airport

5. All of the rivers ultimately flow to the River Thames.

2.3.2 The DCO Project requires that rivers to the northwest and west of the current Airport be diverted, namely the River Colne, Wraysbury River, Bigley Ditch, Colne Brook, Duke of Northumberland’s River and Longford River. This will include sending all rivers excepting the Colne Brook into a Covered River Corridor (CRC) beneath the third runway, described in Chapter 6: DCO Project. The CRC will contain two channels – the combined Colne / Wraysbury flow on the west and the combined Duke of Northumberland and Longford flow to the east. These new combined channels continue east past the expanded western perimeter of the new airport, until eventually re-entering their baseline channels. Further detail on the river diversions can be found in Chapter 6: DCO Project and Chapter 21: Water environment.

2.4 Discharge of airfield runoff

2.4.1 Surface water runoff from the existing Airport is managed by the on-site drainage network that captures and routes runoff flows. The current Airport operational area is sub-divided into three main catchment areas that determine where airfield runoff water drains prior to discharge. A schematic representation of the Airport catchments is shown in Figure 21.23, Volume 2.

2.4.2 There are also three smaller catchments around the periphery of the airfield. The North Western Catchment (Figure 21.23, Volume 2) comprises landside drainage around Terminal 5, and therefore does not drain runoff from the runways. Water is collected in the North Western Reservoir and is discharged 500 m west of the Airport via a culvert, into the Wraysbury River. There are also two smaller



catchments, the South East Catchment and the North East Catchment, on the eastern perimeter of the Airport. These small catchments are around the perimeter of the Airport and drain by gravity to the River Crane.

Southern airfield drainage catchment 2.4.3 The airfield’s Southern Catchment drains to the Southern Balancing Reservoir also

known as Clockhouse Lane Pit (hereafter referred to as the CLP). This catchment area will remain largely unchanged as a result of the DCO Project and will continue to discharge to the CLP.

2.4.4 De-icer chemicals in water give rise to elevated BOD concentrations, and discharge of high BOD concentrations in discharge flows can lead to oxygen depletion in the receiving waterbodies. Existing runoff with elevated BOD concentrations (from de-icing fluids) is currently diverted to Mayfield Farm for treatment prior to discharge to CLP. This will continue under the DCO Project.

Western airfield drainage catchment 2.4.5 Surface runoff from this catchment currently enters the Storm Water Outfall Tunnel

(SWOT), which sends flows to CLP. Currently, there is no on-site treatment option for Western Catchment runoff. Automated monitoring measures organic carbon in the runoff as a proxy for BOD. If the flows have high organic carbon levels they are diverted to Spout Lane Lagoon where they can then be discharged to Mogden Sewage Treatment Works via Bath Road Sewer. The Spout Lane Lagoon is a raised holding reservoir southwest of the Western Catchment.

2.4.6 The DCO Project may increase the area of this catchment, with the inclusion of a small portion of the third runway catchment. Also, Spout Lane Lagoon will be removed, so there will be no discharge of airfield water to Mogden STW via Bath Road Sewer. Instead, all flow from the Western Catchment will be sent via the SWOT towards the CLP. The Mayfields Farm treatment system will be expanded to accommodate the additional flows anticipated from the Western Catchment.

Clockhouse Lane Pit 2.4.7 Clockhouse Lane Pit is a former gravel pit to the south of the Airport. Currently

runoff flows from the Southern and Western catchments are discharged to CLP, with the exception of high BOD flows from the Western Catchment, which are discharged to sewer.

2.4.8 When water levels in the CLP are elevated the CLP lake system can outfall (surface water discharge) to the Feltham Relief Sewer (via both gravity and pumped connections, depending upon water level of the lake). The Feltham Relief



Sewer is understood to discharge into Felthamhill Brook, which shortly thereafter flows into Portlane Brook.

2.4.9 As a former gravel pit, CLP is likely to be in hydraulic continuity with the surrounding groundwater. The CLP can therefore act as a zone of groundwater recharge, such that when airfield discharge flows increase the lake water level a portion of the CLP water can recharge into the surrounding gravel aquifer (depending upon the relative difference in water levels between the CLP and groundwater). Recharge to groundwater could account for a substantial proportion of the flows discharged to the CLP, although the proportion of water recharging into the gravels as opposed to leaving the CLP via the CLP Outfall is not currently known; this will be considered subsequent to PEIR.

Mayfields Farm treatment facility 2.4.10 The Heathrow Constructed Wetlands Facility at Mayfield Farm is an actively

managed facility which utilises sustainable reed bed treatment processes to treat contaminated flows arising from the airfield, primarily surface water contaminated with de-icing fluid with high BOD concentrations. The facility comprises a number of elements, including balancing storage, high intensity aeration, and reed beds, and is currently effective at reducing contamination to levels suitable for discharge to the CLP.

Eastern airfield drainage catchment 2.4.11 The airfield’s Eastern Catchment drains by gravity directly into the Eastern

Balancing Reservoir (EBR), which is a former gravel pit. The EBR is split into several sections; an upper, middle and lower pond.

2.4.12 Water in the EBR discharges by gravity into the River Crane via a surface water outfall. However, as the EBR is an old gravel pit it is likely to be in hydraulic continuity with the surrounding groundwater. When water is flowing into the EBR it may act as a zone of groundwater recharge, leading to a portion of the flow from the Eastern Catchment infiltrating into groundwater.

2.4.13 The sequence of EBR ponds provides attenuation of flows and provides some treatment in the form of oil separation, aeration and settlement. Water with high BOD is diverted to the middle pond to increase its residence time.

2.4.14 The DCO Project is not expected to substantially change the drainage area or drainage quality of the Eastern Catchment.

2.4.15 Heathrow are in the process of developing a treatment solution for BOD removal for the EBR as part of a scope of improvement works for the current Airport, which is being undertaken separately to the DCO Project. It is understood that the



treatment will comprise a Moving Bed Biofilm Reactor (MBBR) treatment plant to remove BOD. The new treatment plant is in place, but has not yet been used..

2.5 Lakes and reservoirs

2.5.1 There are currently a number of lakes and reservoirs in the vicinity, principally to the northwest, southwest and south of the current Airport. The majority of the lakes are former gravel pits that have been allowed to fill with water and are assumed to be in hydraulic continuity with groundwater. Of these lakes, only Wraysbury Lake and Wraysbury No. 2, to the southwest of Heathrow, are classified by the Environment Agency as Water Framework Directive (WFD) waterbodies. Their water quality is a function of groundwater quality in the Lower Thames Gravels aquifer, but there may also be some influence from Horton Brook and Colne Brook, rivers that may contribute water to these lakes.

2.5.2 There are also five man-made reservoirs operated and maintained by Thames Water. The Queen Mother and Wraysbury reservoirs are fed by abstraction from the Thames at Sunnymeads. King George VI and Staines reservoirs (North and South) are fed by abstraction from the Thames near Egham, further downstream. The reservoirs are surrounded by raised embankments and are assumed not to be connected to groundwater. Since they are embanked, they also do not have significant surface water catchments. Their water quality will primarily be influenced by the quality of the source water (the River Thames) with some changes potentially occurring due to mixing or biological activity.



3. WATER QUALITY OBJECTIVES AND REGULATORY GUIDANCE

3.1 Introduction

3.1.1 This section summaries the regulatory regime, Heathrow’s existing discharge permits, and sets out the objectives for the SWQA.

3.2 Objectives of the SWQA

3.2.1 With respect to potential changes to water quality aspects, the DCO Project will involve:

1. New discharges of airfield waters to surface water to the west of the site

2. Potential changes to the discharges to Felthamhill Brook via CLP

3. Substantial physical changes to several rivers to the west of Heathrow as a result of diversions, construction, the CRC etc

4. Other developments and land uses under the DCO Project.

3.2.2 Potential risks of deterioration to water quality during the construction phase also need to be considered.

3.2.3 The details of new and revised discharges will be subject to the permitting process.

3.2.4 Therefore, the purpose of the SWQA is as follows:

1. To identify and describe those aspects of the DCO Project that could result in changes to surface water quality, with respect to sources / causes and also specific parameters of concern where possible

2. To determine whether those potential changes could be considered significant with regard to the relevant Environmental Quality Standard (EQS) values

3. For parameters whose potential changes may be considered significant, where possible, identify potential environmental measures, such as design changes or treatment, that could remove or reduce the magnitude of potential changes to water quality

4. Where successful environmental measures cannot be identified at PEIR, to determine the further evaluation that needs to be undertaken for ES in order to identify viable solutions.



3.2.5 For PEIR, assessment of the potential for significant changes to water quality are based upon the requirements of WFD, and the assessment has primarily been undertaken against WFD EQS values, as set out in the WFD Directions 20151. This is also supported by Environment Agency guidance documents on river basin management plans (Position 1340 16) and on no deterioration (Position 200 13).

3.2.6 In addition, the water quality assessment will consider other EQS standards as appropriate based on protected status or other use of other relevant receptors or waterbodies. This applies to the surface water reservoirs to the southwest of Heathrow, which are not WFD waterbodies but are used for drinking water supply; potential changes to the reservoirs have been assessed against Drinking Water Standards.

3.2.7 If, as part of the PEIR assessment, the SWQA identifies parameters that may not meet the objectives above, then further assessment and consideration of effects and environmental measures will be undertaken when preparing the ES. There will also be engagement with the Environment Agency to assess potential risks and agree a way forward with respect to permitting the site discharges, and any specific parameters that may be assessed as having residual risk of deterioration.

3.3 Regulatory guidance

3.3.1 This Interim SWQA forms part of the PEIR submission, and the final version will form part of the ES for the DCO submission. There are two main regulatory consultees with respect to potential water quality effects associated with the Project:

1. The Environment Agency

2. Natural England.

3.3.2 The Environment Agency have the lead responsibility on considering the development and potential effects on surface water quality. The Environment Agency will consider the compliance of the DCO Project with respect to WFD and whether the effects of the development do not conflict with the River Basin Management Plan (RBMP) for the Thames basin. The WFD limits with respect to water quality are set out in the WFD Directions 2015.2

3.3.3 Natural England are the government’s adviser for the natural environment in England. They are a “competent authority” under the Habitats Regulations, and must undertake a formal assessment of any plans or projects that may be capable of affecting the designated interest features of European sites (for example,

1 http://www.legislation.gov.uk/uksi/2015/1623/pdfs/uksiod_20151623_en_auto.pdf 2 http://www.legislation.gov.uk/uksi/2015/1623/pdfs/uksiod_20151623_en_auto.pdf



Special Protection Area (SPA), Special Areas of Conservation (SAC) or Ramsar Sites). Since there are designated sites downstream of the DCO Project, Natural England will need to determine whether any changes in water quality associated with the DCO Project could affect achievement of conservation objectives at those designated sites.

3.4 Existing Discharge permits

3.4.1 In accordance with statutory requirements, Heathrow has permits to discharge water from the existing airport. The permit numbers and the locations of the discharge point and monitoring points are shown in Table 3.1 and are described in the following sub-sections.

Table 3.1 Summary of Heathrow surface water discharge permits

Permit & Variation No.

Discharge location Discharge Point / Monitoring Point NGR

WR1243/V001 Southern Catchment to Clockhouse Lane Pit

TQ 0769 7297*

N/WR0614/V001 Western Catchment to Clockhouse Lane Pit TQ 0770 7295*

CP3033/V001 Eastern Catchment to River Crane TQ 10863 75227*

CP3034 North Western Reservoir to Wraysbury River

TQ 0404 7630

CP3031 South East Catchment to River Crane TQ 1036 7583

CP3032 North East Catchment to River Crane TQ 1012 7619

*Permit locations that require monitoring

Southern and Western Catchment permits 3.4.2 There are two permits for discharging water from the airfield into CLP – one from

the Southern Catchment (WR1243/V001) and one from the Western Catchment (N/WR0614/V001). The discharge points and monitoring locations are adjacent to each other.

3.4.3 The permits apply the limits of discharge quality to both discharge flows to CLP. The permitted limits are shown in Table 3.2.

3.4.4 The footnotes to the permits for the discharges to CLP include allowances for extreme weather conditions that adversely affect the operation of the pollution control system. The footnotes state that so long as the operator has taken all appropriate measures to minimise the emission and to mitigate any adverse effect,



the results of discharge samples collected during that time shall not be used in determining compliance with the emission limits.

Eastern catchment permit The Eastern Catchment of Heathrow drains to a series of ponds known as the Eastern Balancing Reservoir (EBR), that then discharges by gravity to the River Crane. The limits applied to the permit for the EBR are shown in Table 3.2.

Table 3.2 Summary of Heathrow surface water discharge permits

Permit & Variation No.

Discharge location Parameter Discharge limit

WR1243/V001 Southern Catchment runoff discharging to Clockhouse Lane Pit

BOD 40 mg/l

Suspended solids 100 mg/l

pH Minimum = pH 6 Maximum = pH 9

Visible oil or grease No significant trace

N/WR0614/V001 Western Catchment runoff discharging to Clockhouse Lane Pit

BOD 40 mg/l

Suspended solids 100 mg/l

pH Minimum = pH 6 Maximum = pH 9


CP3033/V001 Eastern Catchment discharge from the Eastern Balancing Reservoir to the River Crane


CP3034 North Western Reservoir to Wraysbury River Oil or grease 20 mg/l

CP3031 South East Catchment to River Crane Oil or grease 20 mg/l

CP3032 North East Catchment to River Crane Oil or grease 20 mg/l

North Western Reservoir 3.4.5 Heathrow hold a consent to discharge from the North Western Reservoir via a

culvert into the Wraysbury River. The water in the North Western Reservoir is from landside drainage around Terminal 5, and therefore is understood to not contain glycol / de-icing chemicals from the airfield runoff.



3.4.6 The consent for the North Western Reservoir stipulates a limit of no more than 20mg/l of oil or grease.

South East Catchment, and North East Catchment 3.4.7 Heathrow holds two consents to discharge to the River Crane at locations

upstream of the discharge from the Eastern Balancing Reservoir. Surface water runoff flows from these small catchments receive runoff from roads and verges and do not include runoff from the runways. The flows pass through oil interceptors prior to discharge. The consents stipulate a limit of no more than 20mg/l of oil or grease.

3.5 Environmental Quality Standards

3.5.1 The WFD Directions (2015) set out environmental quality standard (EQS) values for water quality parameters for assessing the ecological and chemical status of WFD water bodies. The classification for the water quality elements contributes to the overall assessment of the WFD status for the WFD waterbodies, as described by UKTAG, 20073. Environment Agency guidance also sets out other EQS limits for screening water quality for potential pollutants4.

3.5.2 The airport discharges and associated surface water features are not classed as WFD waterbodies (as discussed in the PEIR Appendix 21.3). However, comparison of their water quality with the WFD Direction EQS values and other EQS values provides a useful benchmark for assessing whether those discharges could give rise to potential environmental effects. Where values greater than the EQS limits occur, this indicates that further assessment should be undertaken to determine whether there is a potential risk of an environmental effect. This process of screening water quality data has been undertaken and identified several parameters for further assessment, as described in Section 4.

3.6 WFD classification and water quality parameters

3.6.1 WFD water quality parameters and determinands are grouped into three broad categories that contribute to different elements of the overall WFD classification for each waterbody:

1. Physico-chemical water quality parameters, such as pH, BOD, dissolved oxygen, temperature, and reactive phosphate, are specified in WFD Directions

3 https://www.wfduk.org/sites/default/files/Media/Characterisation%20of%20the%20water%20environment/Recommendations%20on%20surface%20water%20status%20classification_Final_010609.pdf 4 https://www.gov.uk/guidance/surface-water-pollution-risk-assessment-for-your-environmental-permit

https://www.wfduk.org/sites/default/files/Media/Characterisation%20of%20the%20water%20environment/Recommendations%20on%20surface%20water%20status%20classification_Final_010609.pdf

https://www.wfduk.org/sites/default/files/Media/Characterisation%20of%20the%20water%20environment/Recommendations%20on%20surface%20water%20status%20classification_Final_010609.pdf

https://www.gov.uk/guidance/surface-water-pollution-risk-assessment-for-your-environmental-permit



Schedule 3 Part 1. These parameters contribute to the assessment of the Ecological status. Each of the physico-chemical determinands have different criteria for each of High, Good, Moderate, Poor or Bad status

2. Specific pollutants as set out in WFD Directions (2015) Schedule 3 Part 2 comprise a list of chemical determinands that are assessed based on long-term average concentrations and a short term (95th percentile) limit. Specific pollutants also contribute to determining the Ecological status. If all values are below the limits the waterbody is classed as being at High status with respect to Specific Pollutants, whereas parameters that exceed the limits would be classed as Moderate status for the Specific Pollutant element

3. Good Chemical Status is assessed on the basis of Priority substances and other pollutants as set out in WFD Directions (2015) Schedule 3 Part 4. These are assessed based on Annual Average EQS (AA-EQS values) and Maximum Allowable Concentration EQS (MAC-EQS) values. Passing or failing the AA-EQS criteria determines whether the waterbody achieves Good Chemical Status or not. For assessing the overall status of the waterbody, being at Good Chemical Status is classed as being at High Status for that element. Not achieving Good Chemical Status is classed as being at Moderate status for that element. Waterbodies not achieving Good Chemical Status can only achieve an overall classification of Moderate or worse.

3.6.2 The Environment Agency’s 2016 WFD classifications5 for the waterbodies in the vicinity of the Airport have been reviewed to inform this assessment.

3.6.3 The river surface water bodies were each classified as Moderate, with the exception of the River Crane and River Thames (Egham to Teddington) that were classed as Poor. The less than Good status for these locations included Moderate or Poor status for one or more Ecological elements.

3.6.4 For the physico-chemical assessment parameters, each of the river waterbodies of the study area around Heathrow were classed as at either Moderate or Poor status for reactive phosphate (orthophosphate). The River Crane and Yeading Brook were classed as at Poor and Bad status respectively for dissolved oxygen, and Yeading Brook was also at Poor for ammonia. All of the rivers were classed as being at Good Chemical Status based on the occurrence of priority substances and other pollutants.

3.6.5 There are seven waterbodies classed as lakes in the vicinity of Heathrow, comprising five water supply reservoirs, and two flooded former gravel pits. The lakes were classified as being at Moderate status with the exception of Wraysbury

5 https://environment.data.gov.uk/catchment-planning/

https://environment.data.gov.uk/catchment-planning/



Lake, which was classed as Poor. The lakes were typically at Moderate or Poor status with regard to one or more Ecological classification elements. The reservoirs were classed as Poor or Bad for total phosphorous. Wraysbury Lake was Good with respect to total phosphorous, for the physicochemical elements and for Chemical status; the Poor classification was due to Biological Quality elements.

3.6.6 The Lower Thames Gravels groundwater aquifer was classed as being at overall Good status, including Good status with respect to the various chemical assessment criteria.



4. CONCEPTUALISATION OF POTENTIAL SURFACE WATER QUALITY SOURCES, PATHWAYS AND RECEPTORS

4.1.1 The DCO Project design available at PEIR has been reviewed and the potential effects on surface water quality have been assessed based on the standard ‘Source-Pathway-Receptor’ (SPR) approach for assessing potential risks to surface water quality:

1. Sources: the potential sources and activities that could release contaminants / solutes that could alter surface water quality.

2. Pathways: these are routes by which the migration of contaminants and/or particulate matter can arrive at the receptor(s). The pathway may also include the receiving river itself where there are sensitive receptors downstream of the discharges.

3. Receptors: with respect to surface water quality these are the surface water bodies (streams, rivers, lakes, reservoirs etc) that may be affected by the source activity. These are the receiving water course for either runoff or discharge of airfield waters. Receptors may also be based upon water users and ecosystems.

4.1.2 Due to the complex nature of the DCO Project there are multiple sources, pathways and receptors that could cause changes to surface water quality.

4.1.3 In some instances, the sources are point sources, such as discharge points of airfield runoff into receiving waters, such as the River Crane. This will also include the discharge of dewatering flows and runoff from areas of ground disturbance during construction; there may be multiple points of discharge for different work areas/zones through the construction phase, however, each discharge will be a distinct point source. For other aspects, the sources are classed as ‘diffuse’, as they are spread over an area and do not originate from a single location. Diffuse sources include atmospheric deposition of NOx species and agricultural contaminants that occur across large areas. The various SPR linkages identified with respect to surface water quality are presented in Table 4.1.



Table 4.1 Source-Pathway- Receptors summary

Source Pathway Receptors Description of potential sources and contaminant issues

Construction activities: soil materials and stockpiles

Mobilisation in overland runoff

River Colne, Colne Brook, Wraysbury River, Duke of Northumberland, Longford River, River Crane

Potential water quality issues associated with areas of disturbed ground include: Surface water runoff from areas of disturbed ground or from in-river or near river activities that mobilises contaminants and/or particulate matter. Accidental release of fuel and oils. Uncontrolled release of chemicals or water that has contacted chemicals or reactive materials.

Groundwater and landfill leachate

Dewatering abstractions to discharge point


Discharge of dewatering flows from excavations, where the groundwater could be contaminated or contain particulate matter from historic landfills or former gravel pits.

Roads (contaminants, dust etc released from vehicles)

Mobilisation in runoff then draining to road drainage network


Runoff from roads can contain traces of fuels, oils and other contaminants. Drainage of those flows to surface waters can have an effect on water quality

New airfield discharges

Point discharges from new airfield drainage network via one or more airfield drainage and pollution control areas


Airfield runoff can contain various contaminants, including de-icer chemicals (glycol, acetate, formate) that increases BOD of the water. Other solute species in the site discharges can include orthophosphate, PFOS and PAHs. Changes to the discharge volumes will occur due to the increased Airport footprint. This will increase the potential solute loading and treatment requirements. In addition, the DCO Project will include new discharges to rivers to the west of the Airport.

Discharges to Clockhouse Lane Pit

Point discharges from Southern and Western airfield catchments into CLP

Initial receptor is CLP, however, CLP discharges to Felthamhill Brook, Portlane Brook

Vehicle and aircraft Atmospheric deposition All rivers, lakes and Increased air traffic and road traffic associated with the Project will generate



Source Pathway Receptors Description of potential sources and contaminant issues

exhausts (dry and wet) reservoirs in the vicinity of Heathrow

additional air emissions, including Nitrogen Oxide (NOx) compounds. Atmospheric deposition of NOx emissions could increase nutrient content in rivers and lakes (when the NOx converts to nitrate).

Wastewater Treatment Plant

Point discharges from WwTP


Treated effluent may still contain elevated nutrients and BOD.

Discharges from other off-airport development (excl. roads)

Surface runoff discharging via point discharges to surface water network


Ancillary facilities associated with the DCO Project, such as car parks, buildings, fuel storage depots will all have runoff drainage components that could contain traces of fuels, oils and other contaminants. These flows may discharge to surface water bodies.



4.1.4 There are other facilities associated with the DCO Project that are not within the Airport footprint and that will have drainage of surface water runoff and could include activities that may discharge water as trade effluent or similar. Such facilities will include fuel depots, and other off-airfield developments. It is anticipated that these discharges will be managed by conventional approaches such as oil interceptors and SuDS methods. Details of discharges from those facilities will be evaluated for ES.

4.2 Additional aspects of the conceptual model

4.2.1 Table 4.1 sets out the conventional SPR aspects that may alter surface water quality. The physical alteration to the site and waterbodies also gives rise to other potential mechanisms for changes to surface water quality, described as follows:

New catchments and airfield discharges 4.2.2 The development will expand the footprint of the airport to the west and north of

the existing airport. This additional area will increase the runoff volume to be discharged and that may require treatment.

4.2.3 For PEIR, Heathrow have set out two approaches for the management of runoff flows and the potential treatment requirements. These approaches are described in greater detail in PEIR Appendix 21.5 and are summarised below.

1. Preferred Approach. Under this approach the new airfield area will divided into two distinct catchments: the smaller Northwest catchment will encompass the airfield area to the west of the M25, and the remainder of the new airfield area. This design allows runoff to drain primarily via gravity with only limited requirements for pumping. Runoff will be managed in shallow sub-surface drainage channels (an additional SWOT tunnel is not planned for this approach). Each of the two catchments will have dedicated surface water attenuation and treatment areas: the Northwest catchment will drain to the Northwest attenuation and treatment area, and this will discharge to the Colne Brook, whilst the larger catchment will drain southwest, to the Southwest attenuation and treatment area. The Southwest attenuation and treatment area would primarily discharge either to the River Colne or to the Wraysbury, or a combination of both, and with an option of pumped discharge to the Twin Rivers to balance flows as required. The precise locations of the discharges have not been identified at PEIR

2. Alternative Approach. Under this approach a single facility to the north of the third runway would be constructed for drainage and pollution control. The new airfield area will effectively be treated as a single new catchment. Airfield runoff for the entirety of the new catchment would be collected and transferred north



via a second SWOT tunnel and then would be pumped to a surface retention pond. Water complying with discharge limits would be discharged directly, whilst non-compliant water would be sent for treatment prior to discharge. The water would likely be discharged to the CRC channels, either to the Colne-Wraysbury channel or split between the Colne-Wraysbury and the Longford-Duke of Northumberland channels. The precise locations of the discharges have not been identified at PEIR.

4.2.4 Each attenuation and treatment area has been designed with aerated gravel reed beds for treatment and reduction of BOD concentrations for treating runoff flows when necessary.

Mixing of river waters 4.2.5 There will be two channels constructed for the DCO Project to pass through the

CRC: one for the River Colne and Wraysbury Rivers, and another for the Upper Duke of Northumberland’s River and Longford River. Details of the CRC are described in Chapter 6 and Chapter 21.

4.2.6 Combining rivers can potentially lead to changes to water quality, for instance where one river is of poor quality and the other of good quality, there is potential for the combined flows to be of reduced quality relative to the good quality river. However, this will not be the case for the river diversions and the CRC as the rivers are derived from the same source; formed by the bifurcation of the River Colne, and therefore will effectively be the same composition at the point at which the flows diverge. There may be subsequent changes in water quality arising from subsequent mixing with other rivers, or from different discharges into the separate rivers.

Changes to baseflow quality into the river 4.2.7 The diverted river channels will pass through areas used as landfill. Baseflow of

groundwater from those areas into the river could therefore pose a risk of groundwater contaminated by leachate migrating to the rivers. Alternately, if river water recharged into groundwater in those areas of historic landfill, the recharge could increase the mobilisation of landfill leachate and could also alter the groundwater flow directions, causing groundwater contamination to migrate.

4.2.8 The historic landfills in the area were often unlicensed. Many were not constructed according to modern environmental standards and records of the materials disposed of into the landfills are often partial at best. Ground investigations and groundwater quality monitoring as part of the baseline study (Chapter 14: Land Quality) have been undertaken to identify areas where contaminants are present in groundwater.



4.2.9 To prevent the potential hydraulic continuity between river and groundwater in the areas of the landfill, these new river channels will be lined, thereby disconnecting the river from the aquifer. This will prevent groundwater baseflow from those areas entering the river, and prevent the rivers from recharging into groundwater (depending upon the relative water levels in the river and gravel aquifer).

Operational site - Land-take for increased Airport footprint 4.2.10 The DCO Project will increase the Airport footprint and therefore the area of land

where runoff drainage is directly managed by Heathrow. The land-take will include areas that are currently agricultural land as well as commercial, industrial and residential land. Those land uses have an associated drainage quality, although it will mainly be runoff from the agricultural land that drains directly to the river catchments, whilst runoff from the commercial, industrial and residential land being captured in managed drainage and sewage systems.

4.2.11 Thus the DCO Project will result in changes to the drainage quality, and the site will be subject to drainage and pollution management as appropriate: runoff will be captured and monitored, and if required will be treated prior to discharge. Drainage will be managed within individual land parcels. The rates of flow discharge will also be managed to reduce the risk of flooding, as per the findings of the DIA (PEIR Appendix 21.5) and FRA (PEIR Appendix 21.4: Flood Risk Assessment, Volume 3).

4.2.12 Runoff from agricultural land can lead to release of fertilisers (nitrate and phosphate), pesticides, suspended solids / particulate matter, and release of organic carbon. Agricultural runoff typically drains to rivers and can contribute to reductions in surface water quality.

4.2.13 Reducing the agricultural land will reduce the loading of nutrients, pesticides, organic carbon and suspended solids. Therefore, the potential effect of the DCO Project with regard to changes in land-use could be to reduce loading of those parameters associated with agricultural land. This reduction in loading from agricultural land would primarily occur in the catchment of the River Colne.

4.2.14 The Colne catchment upstream is considerably larger than the land-take area associated with the DCO Project (see Figure 21.5, Volume 2). Therefore, a reduction in agricultural land could give rise to some degree of betterment, but relative to the overall catchment area and river flow in the Colne, any improvement would be negligible or at least would be unlikely to be discernible relative to the background conditions and other sources within the catchment.



4.3 Parameters of interest

4.3.1 The SWQA has set out to assess potential risks to water quality as a result of the DCO Project. The design of the DCO Project and the supporting ground investigations are ongoing, and not all of the baseline studies are complete at the time of the PEIR submission.

4.3.2 Based on a review of the available baseline water quality data, particularly airfield discharge water quality data, and on the understanding of the processes that may affect water quality data, a sub-set of key parameters of interest were identified for evaluation and discussion at PEIR, as follows:

1. BOD: concentrations can be elevated in the airfield’s surface water runoff from the use of de-icing chemicals, including glycols, formate and acetate compounds. The use of de-icer is extensive in winter in response to low temperatures (below 4ºC). De-icer is used by some airlines year-round but in much smaller quantities. The potential sources and BOD concentrations in discharges waters are discussed further in Section 7

2. Orthophosphate: concentrations of orthophosphate in the EBR and CLP discharge may be classed in the Moderate range under the WFD classification. The potential sources of orthophosphate in discharge water and associated concentrations is discussed further in Section 8

3. PFOS: this substance has been measured in the water samples from the wider catchment and current airfield discharges at concentrations greater than the WFD AA-EQS limit. The WFD AA-EQS limits for PFOS are low, at 0.00065µg/l. The potential occurrence of PFOS in discharges waters and in local surface waters are discussed further in Section 9

4. PAHs: these substances are the by-products of combustion of hydrocarbons, and therefore are ubiquitous in typical urban environments. Several of these substances have low WFD EQS limits. Monitoring of the airfield discharge waters and of river waters around Heathrow have identified that several PAH substances are present at concentrations that exceed the WFD limits. The potential PAH concentrations in discharge waters and local surface waters are discussed further in Section 10

5. Nitrate: nitrate is a nutrient important for plant growth. Sources include fertilisers, sewage effluent and atmospheric deposition. With respect to the DCO Project, release of nitrogen oxides from increased vehicle emissions could increase nitrate concentrations in waterbodies. Atmospheric deposition of nitrate to water bodies is discussed in Section 11.



4.3.3 Baseline monitoring for the DCO Project commenced in late 2017 and will continue beyond the PEIR submission. Data collection and review is ongoing and will be reported at ES.



5. ASSESSMENT OF THE CONSTRUCTION PHASE

5.1 Introduction

5.1.1 The groundworks and construction activities for the DCO Project will be extensive and complex, as described in Chapter 6: DCO Project description. Activities / sources that could potentially have an effect on surface water quality include:

1. Mobilisation of soils and particulate matter from disturbed ground or stockpiled material, such as during excavations and land-stripping

2. Mobilisation of soils, sediment and other particulate matter from near-river, riverbank or in-river activities. This includes the river diversions, construction near rivers, and CRC and road construction activities where in-river supports will be necessary.

3. Increased leaching and mobilisation of solutes from disturbed ground, stripped land or stockpiled materials, such as the release/mobilisation of nutrients (e.g. nitrate, phosphate, ammonia), dissolved organic carbon, and pesticides/ herbicides from agricultural land

4. Release/mobilisation of solutes from areas of ground contamination, including landfill areas, into surface water runoff

5. Discharge of contaminated groundwater by dewatering abstractions. This includes for dewatering abstractions from historic landfill areas, former gravel pits (Old Slade Lake, Orlitts Lake), and for dewatering for constructing foundations and infrastructure

6. Spillages of fuel, oils and chemicals (including cement and concrete) associated with the development activities that could then be mobilised in runoff and discharged to receiving waters.

Impact assessment and environmental measures 5.1.2 A draft Code of Construction Practice (CoCP) has been developed to include the

requirements for protection of the environment, including water quality effects. This includes measures for managing surface runoff and dewatering flows to remove suspended matter, and for water treatment if and where necessary. The draft CoCP outlines the approach to good practice guidance in the construction phase. Further work prior to the ES in relation to construction and water is outlined in Chapter 21: Water environment.



6. ASSESSMENT OF PERMANENT CHANGES IN ROAD DRAINAGE

6.1 Introduction

6.1.1 The M25 and several other major roads around the Airport will be moved and upgraded as part of the DCO Project. Alterations to the road networks are discussed in PEIR Chapter 6: DCO Project description.

6.1.2 Road runoff can mobilise trace quantities of contaminants, such as fuel and oil, as well as metals (particulate and soluble forms) from brake materials or other sources. Road runoff will be diverted to drains, which will flow to appropriate interceptors as recommended under the Design Manual for Roads and Bridges (DMRB) guidance6.

6.2 Assessment of road drainage

6.2.1 Assessment of road runoff is typically undertaken following the Highways Agency Water Risks Assessment Tool (HAWRAT). This is a tiered approach that initially looks at the potential concentrations of contaminant species in routine runoff prior to any dilution in a waterbody. The second tier considers the effects of any in-river dilution. The first two steps assume that no environmental measures are applied. The third tier then considers the effects of environmental measures to reduce the risks.

6.2.2 Information on the road layout and design was not available at sufficient detail to conduct a numerical risk assessment of potential water quality changes at PEIR, or to describe any environmental measures that could be necessary. However, it can be reasonably assumed for the purposes of the PEIR assessment that road drainage water quality will be managed in accordance with appropriate guidance.

6.3 Assessment at ES stage

6.3.1 The potential effects of the planned changes to the road network will need to be assessed as part of the ES and is expected to be undertaken using the HAWRAT methodology.

6.3.2 The HAWRAT assessment would be used to assess the potential risks to water quality from road runoff, and then depending upon the outcome of the assessment, appropriate environmental measures would be designed for any

6 http://www.standardsforhighways.co.uk/ha/standards/dmrb/

http://www.standardsforhighways.co.uk/ha/standards/dmrb/



potential effects, in the form of SuDS measures such as wetlands, swales and detention basins.



7. ASSESSMENT OF BOD IN OPERATIONAL AIRFIELD DISCHARGES

7.1 Introduction

7.1.1 The natural presence of organic matter in water has an associated BOD concentration, which is related to the oxygen demand corresponding to the breakdown of that organic matter. Increases to the BOD, for instance from anthropogenic sources, can lead to depleted oxygen levels in rivers or streams.

7.1.2 The use of glycol and other de-icers can elevate BOD concentrations which can subsequently impair oxygen levels and the ecological function of a waterbody. Heathrow monitor and manage the runoff from the airfield to achieve BOD concentrations within their permitted discharge levels, and therefore limit potential effects to receiving waters. Changes to the airfield and rivers from the DCO process may alter those flows and could increase BOD loading if not appropriately managed. Potential BOD concentrations in the discharge flows and in the receiving water have therefore been assessed at PEIR to provide a provisional assessment of changes to BOD loading and whether these may pose a risk to water quality.

7.2 Assessment of potential BOD concentrations

7.2.1 The WFD Directions set out the limits for BOD concentrations for different WFD classes as shown in Table 7.1 for evaluation against the 90th percentile BOD concentration. These concentrations are for Type 3, 5 and 7 rivers, with the rivers around Heathrow typically being Type 5 or 7 based upon the altitude of the rivers and typical alkalinity concentrations. The limits are applicable to WFD waterbodies to determine the baseline WFD class with respect to BOD, and to assess whether the class may change as a result to changes arising from the DCO Project. The limits in Table 7.1 have also been used for screening the BOD concentrations in the EBR, CLP and site discharges to assess whether they could present a risk of environmental effects.

Table 7.1 BOD concentration ranges for WFD classes

Parameter High Good Moderate Poor Bad

BOD (90th percentile, mg/l) <4 4-5 5-6.5 6.5-9 >9



7.2.2 The WFD Directions note that BOD concentrations should not be used by the regulators in classifying the status of water bodies. This is because dissolved oxygen is the more relevant parameter with respect to compliance and ecosystem health, and that the relationship between BOD and dissolved oxygen is complex. The UKTAG guidance7 indicates that assessment of BOD should be used, when necessary, for deciding on actions to improve dissolved oxygen compliance. The numerical assessment at PEIR has evaluated BOD to assess potential changes to surface water quality associated with de-icer use; assessing dissolved oxygen without assessing BOD would not be practical. Therefore, at PEIR, numerical models have assessed BOD concentrations and how these may change as a result of airfield discharges and compared this against the WFD class values in Table 7.1, providing an indication of where there could be changes in BOD levels. The numerical modelling also represents changes to the river connectivity, and changes to river baseflow in areas where sections of the river channel may be lined.

7.2.3 Surface water quality modelling of the river network has been completed using the Environment Agency’s SIMCAT (SIMulated CATchment) modelling software. SIMCAT is a modelling tool routinely deployed by the Environment Agency for planning, permitting and regulatory guidance for evaluating changes to existing discharges or new discharges into rivers. The SIMCAT models for the SWQA were developed from SIMCAT models of the Thames River Basin (TRB) developed by the Environment Agency and provided to Heathrow.

7.2.4 BOD concentrations in airfield runoff are highly variable due to the seasonal nature of the source: cold winters with several periods of low temperatures will require higher de-icer use and more water will be sent for treatment than for milder winters. It is not possible to predict de-icer use or BOD concentrations of runoff and so instead the models used historic BOD data to represent potential future concentrations. This assumes that de-icer use and BOD concentrations are similar to previous years. The new airfield may include improved measures to capture de-icer fluids, such as dedicated de-icer stands. Those measures have not been included in the numerical assessment.

7.2.5 A detailed description of the SIMCAT modelling process and output is presented in Annex A, with graphical output in Annexes B to J.

7 https://www.wfduk.org/sites/default/files/Media/Environmental%20standards/Environmental%20standards%20phase%201_Finalv2_010408.pdf

https://www.wfduk.org/sites/default/files/Media/Environmental%20standards/Environmental%20standards%20phase%201_Finalv2_010408.pdf




7.3 Summary of PEIR numerical assessment of BOD

7.3.1 The output and interpretation of the SIMCAT modelling of potential changes to BOD are summarised in the following sub-sections.

Effect of diverting the rivers and lining the channels 7.3.2 The models indicated that the overall effect of lining the channels and diverting the

rivers would give rise to limited changes in BOD concentration relative to the baseline conditions. Those changes are also small in comparison to potential changes that may arise from changes to the site discharges.

New discharges for the DCO Project to the Colne and Colne Brook catchments 7.3.3 The SIMCAT modelling indicates that the proposed new discharges from the new

airfield catchments into the Colne and Colne Brook catchments to the west of Heathrow are not expected to give rise to significant changes to BOD concentrations. This is because the discharge flows are low in comparison to the flows in the receiving waters (5% or less).

7.3.4 However, this conclusion should be confirmed by updating the models with additional data as it becomes available.

Eastern Catchment discharge to the River Crane 7.3.5 The Eastern Catchment is not anticipated to change as part of the development,

and therefore the models do not indicate a significant change to BOD concentrations in the River Crane. The models have not assessed the influence of the recently installed BOD treatment at the EBR and so provide an estimate based on the current situation, however, BOD treatment at the EBR would be expected to reduce BOD concentrations relative to the current conditions.

7.3.6 Overall, the DCO Project development is not anticipated to significantly increase BOD concentrations in the River Crane.

Clockhouse Lane Pit discharge to Felthamhill Brook 7.3.7 The SIMCAT modelling has shown that the DCO Project may lead to an increase

in BOD concentrations in Felthamhill Brook and Portlane Brook.

7.3.8 The potential for BOD concentrations are dependent upon:

1. The degree of attenuation of BOD that occurs within the CLP, where an increase in flow to the CLP could reduce residence time and BOD attenuation

2. The proportion of airfield discharge to the CLP that discharges from the CLP via the CLP Outlet to Felthamhill Brook.



7.3.9 If it is assumed that all of the airfield discharge into the CLP is discharged via the CLP Outlet (as has been applied in the models to be conservative), then the BOD concentration in the CLP water influences BOD concentrations in Portlane Brook; this is because the discharge from the CLP could contribute a large proportion of the flow to Portlane Brook.

7.3.10 A model has been run that assumes that the BOD treatment removal and attenuation within the CLP occurs to the same degree as present, and applies the monitored range of BOD concentrations to the CLP Outlet. When this BOD concentration profile is discharged to the Felthamhill Brook at 100% of the discharge rate from the Southern and Western catchments to the CLP, the resulting concentration in Felthamhill Brook predicts an initial increase in BOD (with a corresponding decrease in WFD class with respect to BOD from High to Good). This change occurs for a short distance until Felthamhill Brook meets Portlane Brook, and the resulting mixture reduces the BOD concentration and the WFD status for BOD returns to within the ‘High’ range. This is in line with the Baseline model output, and does not indicate a change from existing conditions.

7.3.11 To evaluate the sensitivity of the models, alternative scenarios have been run that assume BOD attenuation has been substantially reduced: BOD concentrations at the CLP Outlet were set to be 50% higher than the monitoring data across the entire range of BOD concentrations (as a conservative approach to represent a reduction in BOD attenuation). The results from this scenario predicts that the 90th percentile BOD concentrations increase to the ‘Poor’ WFD status range for BOD until the confluence with Portlane Brook, at which point the mixing reduces BOD concentrations to the Good range – this is an overall prediction of a decrease of WFD status with respect to BOD relative to the Baseline model.

7.3.12 However, this scenario assumes all the flow entering the CLP is discharged to Felthamhill Brook. A further scenario where the BOD concentration is increased by 50% but the discharge flow to Felthamhill Brook is only 50% of the discharge to the CLP does not indicate an overall WFD Class change with respect to BOD.

7.3.13 In short, the predictions of potential effects are dependent upon the behaviour of flow in the CLP and BOD attenuation. The model will be refined for ES with further work proposed subsequent to PEIR to evaluate flows into and out of the CLP.

Potential environmental measures within the CLP 7.3.14 The model scenarios have shown that an increase of BOD concentrations in

Portlane Brook is possible under a combination of unfavourable conditions: high flows containing high BOD leading to reduced attenuation in the CLP and all flows exiting the CLP via the CLP surface water outfall.



7.3.15 However, the models have not taken into account further measures that could be implemented to reduce BOD concentrations, if required. These could include:

1. A first line of environmental measures could be to achieve a higher level of BOD removal at the treatment plant at Mayfield Farm, thereby reducing the BOD load in the CLP. The additional treatment capacity at Mayfield Farm for the Western Catchment flow has a design target of 30mg/l BOD, which is lower than the current permitted discharge limit of 40mg/l for the Southern Catchment

2. In addition, within the CLP there is infrastructure in place that can allow increased BOD attenuation if necessary; there are aerators installed in the CLP that can increase oxygen levels to facilitate BOD degradation. It is also possible to re-circulate water to the ponds nearer to the discharges from the airfield, increasing the residence time to increase BOD removal. Further aerators and a pumping system could therefore be installed to increase BOD attenuation when this is necessary.

7.4 Next steps

7.4.1 For ES the SIMCAT BOD model will be finalised with respect to the design discharge locations, and any further estimates of potential discharge flow rates. The location of the discharge point from Feltham Relief Sewer to Felthamhill Brook will be confirmed and finalised in the SIMCAT model. Further assessment of the CLP will also provide a greater understanding of flow behaviour and BOD attenuation in CLP, which will be used to refine the predictions of BOD in Portlane Brook. At ES the SIMCAT model will also be used to evaluate dissolved oxygen concentrations in the rivers around Heathrow. This will further inform the WFD assessment and will also be used for the ecological assessments undertaken at PEIR.



8. ASSESSMENT OF ORTHOPHOSPHATE IN OPERATIONAL AIRFIELD DISCHARGES

8.1 Introduction

8.1.1 A review of available water quality data has identified the potential for elevated orthophosphate concentrations in CLP and in the discharges from the airfield into CLP.

8.1.2 Comparison of data from the Western and Southern catchments suggest the main source of orthophosphate to CLP may not be the airfield itself, but could arise from the treatment for BOD at Mayfield Farm as the treatment system requires dosing with orthophosphate to facilitate BOD removal.

8.1.3 This section presents a summary review of the ortho-phosphate data and a preliminary assessment of the potential changes to orthophosphate concentrations that may occur post-development due to the DCO Project. This includes at CLP and downstream receptors, and to the rivers that may receive discharge flows from the new airfield catchments.

8.2 Orthophosphate water quality standards

8.2.1 The WFD Directions specify EQS limits for different WFD classes for orthophosphate based upon the annual mean concentration. However, the various class limits are calculated based upon the alkalinity concentration in the river and on the altitude of the monitoring locations, and therefore vary slightly for each of the locations (typically by less than 0.01 mg/l as P). The calculated range of orthophosphate concentrations for the rivers around Heathrow are summarised in Table 8.1 (based on the monitoring locations in Table 8.2 and https://originalshrewsbury.co.uk/visit/dana-prison).

Table 8.1 Orthophosphate concentration ranges for WFD Classes


Mean orthophosphate (mg/l

as P) <0.05 0.05-0.09 0.09-0.21 0.21-1.09 >1.09



8.3 Orthophosphate concentrations in Clockhouse Lane Pit

8.3.1 The monitoring data for orthophosphate at CLP available at PEIR has been limited. Heathrow have analysed surface water samples from CLP for orthophosphate, typically on a quarterly basis, but this analysis commenced relatively recently: most of the CLP locations monitored by Heathrow have orthophosphate data only from 2016 onwards. The CLP Outlet has data for 2014-2015, and 2017 onwards. The Environment Agency monitoring of the discharges into the CLP does not include orthophosphate.

8.3.2 Heathrow’s monitoring data for orthophosphate is summarised in Table 8.2 and shown in Graphic 21.1K.1 in Annex K. The data shows that the Southern Catchment discharge has reported substantially higher orthophosphate concentrations (up to around 6mg/l) than the Western Catchment or the other CLP locations.

8.3.3 It should be noted the data does not indicate that the discharge of Southern Catchment water with higher orthophosphate into CLP has led to increases in orthophosphate in the discharge from CLP or within Portlane Brook: the sequence of monitoring locations from the discharges towards the CLP Outlet show reducing orthophosphate concentrations; the attenuation is likely due to dilution within the CLP along with nutrient uptake by plants and settlement of particulate bound orthophosphate. The CLP Outlet concentrations are lower than at the airfield discharge inlets, reporting an average concentration of 0.10 mg/l, lower than the average concentration for Portlane Brook (0.15 mg/l as P, see Table 8.3).

8.3.4 Orthophosphate concentrations in Portlane Brook are summarised in Table 8.3 and shown in Graphic 21.1K.2 in Annex K. The average concentration is 0.15 mg/l for 2010 to 2018, similar average concentrations occur in the years 2016 and 2017. This would classify Portlane Brook as being of Moderate WFD status with respect to orthophosphate.

Table 8.2 Summary of orthophosphate concentrations for the CLP

CLP monitoring location Count First date Last date

Orthophosphate concentration (mg/l as P)

Minimum Average Max

CLP Southern Catchment Inlet 10 25/02/2016 31/08/2018 0.04 1.43 5.80

CLP Western Catchment Inlet 10 25/02/2016 17/05/2018 0.05 0.16 0.49

CLP Inside Weir Notch 10 25/02/2016 17/05/2018 0.033 0.39 2.2

CLP Northern 10 25/02/2016 17/05/2018 0.02 0.18 0.61



CLP monitoring location Count First date Last date


Minimum Average Max

Peninsular

CLP Mid Point 4 27/11/2017 17/05/2018 0.10 0.29 0.58

CLP Outlet 11 10/01/2014 21/08/2018 0.01 0.10 0.25

Table 8.3 Summary of orthophosphate concentrations for Portlane Brook

Environment Agency monitoring location Count First date Last date


Minimum Average Max

PTHR0054 Portlane Brook 25 22/01/2010 28/06/2018 0.036 0.15 0.37

8.3.5 The difference between the high orthophosphate concentrations in the Southern Catchment discharge and the lower concentrations in the Western Catchment discharge are considered to be due to the Mayfield Farm treatment system. The treatment system includes addition of nutrients (phosphate and ammonia) to facilitate BOD removal. Excess nutrients not consumed during the treatment could give rise to elevated orthophosphate in the discharge to the CLP. Orthophosphate may be attenuated in the CLP, but this may include by sorption and settlement of particulate matter, which could then be re-suspended in the CLP water column. It has not been confirmed that Mayfield Farm is the source of the additional orthophosphate. However, given the absence of orthophosphate from the Western Catchment, the timings of the peaks and the absence of other potential sources, Mayfield Farm is considered to be a potential source.

8.3.6 The Mayfield Farm facility remains ready for use from November to April, when there may be a requirement to treat for BOD. Requirements for treatment and discharge from Mayfield Farm are sporadic, in response to cold periods and high BOD concentrations from de-icer use. During the operational period, the process is maintained in readiness for use; for the majority of the time the water in the treatment process is recirculated within the system at optimum conditions to maintain the biological conditions. During this period there is limited discharge of water from the plant. It is only during cold periods when de-icers are in use that runoff flows pass through the system and will be dosed with nutrients to facilitate BOD removal. The process aims to balance the nutrient requirements to the



system, however, the data indicates that there can still remain an excess of orthophosphate in the discharged flows.

8.4 Clockhouse Lane Pit: Orthophosphate for the with development scenario

Potential risks 8.4.1 The baseline orthophosphate concentrations described in Section 8.3 indicate

that the Southern Catchment discharge to the CLP may contain elevated orthophosphate concentrations, and that this could be associated with nutrient dosing at the Mayfield Farm treatment system.

8.4.2 Under the DCO Project the Western Catchment discharge will be treated at the expanded Mayfield Farm treatment system (previously Western Catchment runoff with high BOD would have been discharged to the Thames Waters sewers). This means that the quantity of treated water being discharged during the winter months to the CLP could approximately double (based upon relative sizes of the catchment areas).

8.4.3 The CLP could therefore potentially receive increased flows to the CLP (which would reduce the residence time within the CLP) and increases to orthophosphate loading to the CLP from the treated Western Catchment flows.

Risk assessment and potential environmental measures 8.4.4 The data with regards to orthophosphate concentrations in the discharge is limited,

and the factors that control those concentrations and the orthophosphate attenuation is not well constrained. A qualitative assessment has been undertaken for PEIR, with numerical assessment to be undertaken for ES.

8.4.5 It is assumed at PEIR that the expanded Mayfield Farm treatment system will include measures for good management of nutrient dosing, monitoring of orthophosphate in the discharge and could include measures to reduce orthophosphate levels in the discharge to CLP. This would result in lower peak concentrations than have been reported. However, concentrations of orthophosphate in the discharged flows from the Western Catchment may still be higher than at present as a result of potential treatment.

8.4.6 Further assessment will be undertaken at ES to evaluate whether the additional flows to the CLP or orthophosphate loading from the Western Catchment could increase orthophosphate concentrations in the discharge from the CLP Outfall.

8.4.7 If it is determined that there could be an increase in orthophosphate at the CLP Outfall then that could increase concentrations in Portlane Brook, then there are



environmental measures that can reduce orthophosphate levels in CLP and at the CLP Outfall:

1. Recirculation of effluent within Mayfield Farm treatment circuit to reduce orthophosphate concentrations in the effluent

2. Increase aeration and biological activity within the CLP to increase uptake of orthophosphate. This could include planting of reeds or other plant species that would increase uptake of orthophosphate

3. Recirculation of water within CLP, by pumping water back to the discharge points, in order to increase residence time and to increase attenuation within the CLP

4. Additional active treatment stage added to the process circuit specifically to remove orthophosphate from the effluent.

8.5 Discharges to the west of the Airport: Orthophosphate for the ‘with development’ scenario

Potential risks 8.5.1 Under the DCO Project there will be one or more discharge points for airfield

runoff to be discharged to rivers to the west of the Airport. The locations for these discharge points have not yet been determined, however, could be to one or more of the following rivers; the River Colne and Wraysbury River, the Colne Brook, the diverted Colne-Wraysbury channel, or the Duke of Northumberland’s River.

8.5.2 Mean orthophosphate concentrations in these rivers are summarised for the Environment Agency’s monitoring locations in Table 8.4. The mean concentrations are consistently in the Poor WFD classification (where Poor status typically ranges from approximately 0.21 to 1.1 mg/l as P for these rivers).

Table 8.4 Summary of mean orthophosphate concentrations (mg/l) in the rivers to the west of Heathrow

Monitoring period

Environment Agency monitoring location 2012 to 2018 2015 2016 2017

PCNR0025 Colne above Thames 0.29 0.35 0.22 0.27

PCNR0100 Wraysbury River above Colne 0.31 0.31 0.24 0.26

PCNR0421 Colne Brook above Thames 0.33 0.34 0.26 0.35



8.5.3 The new airfield surface water drainage catchment(s) will have dedicated treatment for the removal of BOD. A similar approach as to that undertaken at the existing treatment facility at Mayfield Farm is proposed, involving aerated gravel reed beds, and therefore will also likely involve addition of nutrients to facilitate BOD removal. Nutrient dosing of the new discharges could give rise to elevated orthophosphate concentrations in the treated effluent.

8.5.4 Good management of nutrient dosing could reduce the potential orthophosphate concentrations to levels lower in new discharges than have been reported for the existing Mayfield Farm discharge, however, residual orthophosphate concentrations could nevertheless occur in the new discharges. These flows will discharge from the new treatment facility direct to the receiving river water where it will be mixed and diluted.

Risk assessment and potential environmental measures 8.5.5 There is no data on the potential orthophosphate concentrations that may occur in

the discharge from the new airfield catchments, and therefore undertaking detailed numerical assessment of the discharges and mixing in the receiving waters has not been undertaken for PEIR.

8.5.6 Potential concentrations of orthophosphate in the new airfield discharges and effects upon the receiving water will be assessed at ES. Should this indicate potentially elevated orthophosphate concentrations, these could be reduced through one or more of the following environmental measures:

1. Improved monitoring and management of nutrient dosing levels for the treatment to reduce excess orthophosphate concentrations

2. Recirculation of effluent within the treatment circuit to reduce orthophosphate concentrations in the effluent


8.6 Discharges from the Eastern Catchment

8.6.1 The DCO Project will not give rise to substantial changes to either the Eastern Catchment or the discharges from the Eastern Balancing Reservoir to the River Crane. Therefore, the DCO Project will not alter the orthophosphate concentrations in the runoff water at the EBR.



8.7 Next steps

8.7.1 For ES, additional monitoring data will be reviewed to assess the feasible range of orthophosphate concentrations in the discharge flows for CLP and this will be used to infer potential orthophosphate concentrations for the new airfield discharges. Where possible and appropriate, numerical modelling of potential orthophosphate concentrations in the receiving waters will be undertaken and assessed against WFD limits. Appropriate environmental measures will then be determined if required.



9. ASSESSMENT OF PFOS IN OPERATIONAL AIRFIELD DISCHARGES

9.1 PFOS background information and WFD assessment

9.1.1 PFOS is a polyfluorinated surfactant compound that is part of the Per- and Polyfluoroalkyl substances (PFAS) group of compounds.

9.1.2 PFAS compounds have had many industrial uses: common uses of PFAS includes in Aqueous Fire Fighting Foams (AFFF), cleaning products and stain prevention materials, metal plating, and in aviation fluids. Several PFAS compounds, including PFOS, are designated as Persistent Organic Pollutants (POP). The use of PFAS is now restricted, although remains permissible for certain products, including in aircraft hydraulic fluids.

9.1.3 Under the WFD Directions (2015), from 22 December 2018 the list of Priority Hazardous Substances includes ‘Perfluorooctane sulfonic acid and its derivatives (PFOS)’. The discussion in this report focuses on Perfluorooctane sulfonic acid (PFOS), CAS No. 1763-23-1, although also includes discussion of other PFAS species and sources.

9.1.4 The WFD Directions sets the AA-EQS value for inland waters for PFOS at a concentration of 0.00065µg/L. The MAC-EQS is 36µg/L. The AA-EQS concentration is very low, and analysis to such low detection limits is technically challenging; currently there are no commercial or Environment Agency laboratories that are accredited to conduct the analysis to the AA-EQS limit.

9.1.5 Monitoring data indicates that the presence of PFOS is widespread in the rivers around Heathrow at concentrations greater than the AA-EQS, including the rivers upstream of Heathrow. Environment Agency monitoring also shows that PFOS is ubiquitous in rivers in the London area and beyond and, therefore, that there are or have been widespread sources of PFOS release into the wider environment.

9.1.6 PFOS was only introduced as a Priority Hazardous Substance in late 2018, in the middle of a River Basin Management Plan (RBMP) cycle (the current RBMP was published in 2015, and the next RBMP update is due in 2021). This means that the baseline for PFOS has yet to be defined by the Environment Agency, and the Chemical Status with respect to PFOS has not been formally assessed.



9.1.7 PFOS is an ‘emerging contaminant’8 and the assessment of PFOS risks in the environment and for the DCO Project are still at an early stage. Heathrow are engaging with the Environment Agency to understand the presence of PFOS in the regional context, and to determine a practical means to assess potential water quality changes associated with current and future airfield discharges. The outcome of these discussions will inform the assessment of potential risks and changes to water quality from the airfield discharges. This in turn will inform whether environmental measures are needed to manage PFOS concentrations at Application.

9.2 PFOS concentrations: rivers around Heathrow

Environment Agency monitoring data 9.2.1 The Environment Agency have conducted PFOS analysis on river waters across

the Southeast Region, and the nearest of those monitored to Heathrow are:

1. On the River Colne, PFOS has been analysed at Colne at Hampermill, Oxhey (PCNR0028), approximately 20 km upstream of the airport

2. On the River Thames, at the Thames above Molesey Weir (PTHR0070), which is downstream from where the Longford River enters the Thames.

9.2.2 The PFOS concentrations for these locations are summarised in Table 9.1. Both locations report average PFOS concentrations higher than the AA-EQS. The average PFOS concentrations are higher for the upstream location on the River Colne relative to the Thames. This indicates there are ongoing and/or historic sources of PFOS in these catchments.

Table 9.1 Summary of Environment Agency PFOS data

Location ID

Location name/description First date Last date No. Minimum

(µg/l) Average

(µg/l) Maximum

(µg/l)

PCNR0028 Colne at Hampermill, Oxhey 10/05/2018 02/10/2018 7 0.011 0.014 0.018

PTHR0070 Thames above Molesey Weir 16/05/2016 16/11/2016 7 0.0065 0.0084 0.011

DCO Project baseline monitoring data 9.2.3 PFOS concentrations from the DCO Project baseline monitoring are presented in

Table 9.2 for a subset of data for selected locations on the River Colne, River 8 ‘Emerging contaminants’ are substances that may pose a risk to human or environmental health that may pose risks that are not fully understood. Risks from these substances typically have not been historically monitored or assessed.



Crane and the River Thames. This data provides an insight into PFOS concentrations in the rivers around Heathrow. These locations are shown in Figure 21.1.1.

9.2.4 The PFOS concentrations in Table 9.2 are all greater than the AA-EQS but well below the MAC-EQS. As with the Environment Agency data, the values in Table 9.2 indicate there are sources of PFOS in the wider catchment and upstream of Heathrow.

Table 9.2 PFOS concentrations (µg/l) for selected river waters from the DCO Project baseline water quality monitoring

River Location ID Location name/description

Round 2 Jan/Feb 2018

Round 5 April 2018

Round 8 July 2018

Round 11

Oct 2018

River Colne

HEP-SW-2 River Colne, upstream of Heathrow 0.0073 0.0035 0.0037 0.0036

HEP-SW-84 River Colne to west of Heathrow 0.0036 0.004 0.0047 0.0048

HEP-SW-16 River Colne, downstream of Heathrow

0.0056 0.0051 0.0047 0.0066

River Crane

HEP-SW-17 River Crane, below Grand Union Canal 0.010 0.0099 0.021 0.012

HEP-SW-18 River Crane, upstream of Heathrow and EBR discharge

0.020 0.0094 0.013 0.010

HEP-SW-20 River Crane, downstream of EBR discharge

0.013 0.014 0.058 0.063

HEP-SW-21 River Crane, downstream of confluence with Duke of Northumberland

0.0051 0.011 0.0079 0.012

HEP-SW-24 River Crane upstream of confluence with the Thames (at PCRN0006)

0.0092 0.018 0.012 0.019

Duke of Northumberland

HEP-SW-48 Upper Duke of Northumberland 0.0038 0.0052 0.0043 0.0052

HEP-SW-22 Upper Duke of Northumberland above confluence with River Crane

0.0034 0.0030 0.0046 0.0045

River Thames HEP-SW-25 Thames above Egham 0.004 0.005 0.005 0.005

HEP-SW-26

Thames downstream of River Colne 0.0038 0.005 0.0042 0.006



9.3 PFOS concentrations: airfield waters

Heathrow monitoring data 9.3.1 Heathrow have conducted sampling and analysis of PFOS and a wider range of

related PFAS compounds. Summary data for PFOS concentrations from 2011 to early 2018 (i.e. before its inclusion within the list of Priority Hazardous Substances) are presented in Error! Reference source not found.Table 9.3.

Table 9.3 Summary of Heathrow’s routine PFOS monitoring

Location ID No. of results

No. below detection limit

Minimum (µg/l)

Average (µg/l)

Maximum (µg/l)

EBR Diversion Chamber 74 3 0.005** 0.29 0.81

EBR Outlet to River Crane* 80 1 0.050** 0.24 0.71

CLP Southern Catchment Inlet 72 13 0.005** 0.090 0.74

CLP Western Catchment Inlet 72 3 0.005** 0.16 0.55

CLP Outlet 65 0 0.043 0.096 0.27 *Note: the EBR Outlet to River Crane location changed name, but the results were grouped together as they represent the same flow of discharged water. **Minimum values at the analytical detection limits.

9.3.2 The PFOS analyses conducted for Heathrow’s routine monitoring have not been undertaken to the low AA-EQS concentration limit. For calculating the arithmetic mean concentrations in Table 9.3Error! Reference source not found., values reported as below the analytical detection limit were set as being equal to the analytical detection limit (this simplification does not substantially influence the results or the interpretation).

DCO Project baseline monitoring data 9.3.3 Baseline monitoring of surface water bodies has been undertaken in support of the

DCO Project, with sampling commencing monthly from December 2017. All of the DCO baseline monitoring locations are shown in Figure 21.4. Data collection is ongoing and will be reported at ES.

9.3.4 Due to the relevance of PFOS, and importance of understanding its distribution in the surface water environment, preliminary PFOS results from the baseline monitoring have been reviewed and are presented in Table 9.4Error! Reference source not found. for the DCO baseline monitoring data of the CLP and EBR.



The locations listed in Table 9.4Error! Reference source not found. are shown in Figure 21.1.1. Sampling is undertaken monthly but PFOS analysis is undertaken on samples once every three months, and hence data are presented for Rounds 2, 5, 8 and 11.

Table 9.4 PFOS concentrations (µg/l) of from baseline water quality monitoring undertaken for the DCO Project

Location ID Location name/description Round 2 Jan/Feb 2018

Round 5 April 2018

Round 8 July 2018

Round 11 Oct 2018

HEP-SW-99 Southern Catchment Diversion Chamber 0.0045 0.049 0.027 0.015

HEP-SW-94 Adjacent to SWOT discharge to CLP 0.028 0.045 0.24 0.058

HEP-SW-98 EBR Upper Pond 0.22 0.19 0.18 0.14

HEP-SW-97 EBR Middle Pond 0.082 0.093 0.12 0.039

HEP-SW-96 EBR Lower Pond 0.11 0.14 0.14 0.095

9.4 Discussion of PFOS data

9.4.1 Based on the PFOS monitoring data in Sections Error! Reference source not found. and 9.2 the following key observations can be made:

1. PFOS is present in the river waters at concentrations greater than the WFD AA-EQS but below the MAC-EQS:

a. PFOS concentrations in the River Colne were around 0.004 to 0.007µg/l

b. PFOS concentrations in the Duke of Northumberland’s River were around 0.003 to 0.005µg/l

c. PFOS concentrations in the River Crane were around 0.01µg/l, although higher concentrations (up to around 0.06µg/l) were reported at the location downstream of the EBR discharge

d. PFOS concentrations in the River Thames were around 0.004 to 0.006µg/l.

2. PFOS is present in the on-site waters at concentrations greater than the WFD AA-EQS but below the MAC-EQS. Based on the Heathrow data:

a. The highest average concentrations were reported for the EBR Diversion Chamber and the EBR Outlet to the River Crane, with average concentrations of 0.29 and 0.24µg/l respectively

b. The lowest average PFOS concentration for the airport drainage was for the Southern Catchment (0.091µg/l)



c. The average PFOS concentration at the CLP Outlet (0.099µg/l) was lower than from the Western Catchment and slightly higher than the Southern Catchment

d. PFOS concentrations in the airfield discharges were higher under the Heathrow monitoring data than the DCO Project baseline monitoring, particularly for the discharges to the CLP. The reason for the differences are not clear, although the samples were taken for different periods

e. A review of time-series trends of the Heathrow data back to 2011 does not indicate that PFOS concentrations in the discharge flows are declining over time.

9.4.2 Overall, this data indicates that there is a widespread presence of PFOS in the wider catchments around Heathrow at levels above the AA-EQS. The existing airfield discharges contain PFOS, but it is clear the discharges from the Airport are not the only source of PFOS in these catchments (because PFOS is present in the rivers upstream of the Airport at concentrations greater than the WFD AA-EQS).

9.4.3 As described earlier, PFOS is an emerging contaminant and the baseline levels of PFOS have not yet been defined and reconciled against the WFD limits for this substance. Therefore, it is too early to determine whether PFOS in the airfield discharges may be an issue for the current or future airfield discharges.

9.5 Potential sources of PFOS

9.5.1 Potential sources of PFOS in the airfield discharges can be divided into several categories based upon whether the source is on-site or off-site and whether the PFOS use is as a result of historic / legacy usage or is potentially ongoing.

9.5.2 The question of whether the sources are historic or ongoing is of relevance for assessing whether PFOS concentrations could decline (through natural attenuation) over time.

On-site sources of PFAS substances

9.5.3 With respect to the Airport there could be several potential on-site sources:

PFOS used in Aqueous Fire Fighting Foams 9.5.4 Fire-fighting foams had been used across the world for several decades prior to

restrictions being placed on PFOS use. At Heathrow, AFFFs have previously been commonly used for fire-fighting training exercises and also as required during emergency incidents.

9.5.5 The AFFF foams and their residue will have drained through the airport’s drainage network. It is understood from Heathrow that water containing AFFF was routed to



foul sewers during exercises. However, due to the retention of waters within the drainage network and the potential sorption behaviour of the PFAS chemicals, it is possible that PFAS substances could been retained in the drainage pipes and have entered the drainage ponds (the EBR and CLP). In addition, fire-fighting water and foam may have ponded on areas of soils or grass where it could infiltrate to the unsaturated sub-surface materials and into shallow groundwater.

9.5.6 Heathrow ceased use of AFFFs containing PFOS in 2013 and has since replaced its foam with non-PFAS alternatives. However, it cannot be currently ruled out that there are no third-party stocks or users of AFFFs containing PFOS within the airport.

Aviation hydraulic fluids

9.5.7 The use of PFAS chemicals is still currently permitted for aviation hydraulic fluids, particularly fluids related to the deployment of landing gear.

9.5.8 PFAS compounds are used in several hydraulic fluids, however it is not certain if this includes PFOS. The hydraulic fluid Skydrol LD-4, a commonly used hydraulic fluid, is reported to contain a ‘PFOS-related molecule’ (perfluoroalkyl sulphonate), but may not actually contain PFOS as mentioned in the EU regulation9. It is not clear if this ‘PFOS-related molecule’ could be considered to be a PFOS-derivative, or could degrade / convert to PFOS. There are a number of other brands / suppliers of hydraulic fluids that may contain PFAS substances including PFOS, but whose PFOS content is not clear. Therefore, it needs to be confirmed whether the aviation hydraulic fluids contain PFOS and other PFAS compounds, and then whether leakage of such fluids could present a potentially significant source of PFOS release into the Airport’s drainage network.

9.5.9 The hydraulic fluids used by airlines are not currently within Heathrow’s control and therefore it may not be viable to eliminate this potential source of PFOS.

9.5.10 Accidental spillage or leaks of hydraulic fluids upon landing or during maintenance could lead to the release of PFAS substances. Leaks upon landing could be mobilised into the surface water drainage network. It is not currently clear how frequently this could happen, how likely this source / pathway is as a potential cause of PFOS in discharge waters, and therefore whether increased air traffic could increase the rate of PFOS release from hydraulic fluid losses to the airfield.

9 https://circabc.europa.eu/webdav/CircaBC/env/pop/Library/01-meetings_authorities/16%20-%2016th%20Competent%20Authorities%20meeting%20on%20POPs%20-%2030%20May%202017/02%20-%20Documents/POP-CA_05-17_13-PFOS_EU_Annex-A.pdf

https://circabc.europa.eu/webdav/CircaBC/env/pop/Library/01-meetings_authorities/16%20-%2016th%20Competent%20Authorities%20meeting%20on%20POPs%20-%2030%20May%202017/02%20-%20Documents/POP-CA_05-17_13-PFOS_EU_Annex-A.pdf





Other chemicals

9.5.11 There may be other PFAS containing chemicals that are used at the Airport by third party tenants and contractors. Currently there is no information to suggest that this is the case, but the absence of such chemicals has not been confirmed, and so Heathrow have taken a precautionary approach and consider that these may be present on-site until demonstrated otherwise (see Paragraph 0).

Non-Heathrow sources of PFOS 9.5.12 As a Persistent Organic Pollutant, PFOS can be present in the environment from

historic usage and data indicates there are other sources upstream of Heathrow. PFOS can partition onto particulate solid matter, and therefore may be retained within soils (where it can then be leached to groundwater) or river sediment (where it can be remobilised to flowing water). The desorption of PFOS from those materials could give rise to an ongoing source of PFOS release over an extended period. Mobilisation of PFOS bound to particulate matter could also give rise to elevated concentrations in water samples.

9.6 Assessment of potential PFOS effects

9.6.1 The current discharge waters from the Airport contain PFOS at concentrations greater than the WFD AA-EQS. The potential for the DCO Project to increase PFOS in discharge flows is dependent on the source of PFOS.

Historic source only 9.6.2 The existing airfield drainage network is understood to include a proportion of

groundwater flow due to groundwater levels being relatively shallow (i.e. either at or above the level of some of the drainage pipes). This groundwater ingress into the drainage network could be contaminated from historic PFOS use for training and from emergency incidents. The drainage network may also retain residual PFOS, either bound to pipework or in entrained sediment, from historic fire-fighting activities.

9.6.3 If the primary source of PFOS is from historic legacy use of AFFFs and there is no new release of PFOS at surface (such as from hydraulic fluids), then the PFOS source is limited to the inventory of PFOS retained in the sub-surface drainage network, soil materials and in groundwater.

9.6.4 This legacy source of PFOS should decline over time, although as PFOS does not readily biodegrade, then any significant reduction in PFOS content could take a long time. This means that PFOS concentrations in the discharge from the EBR and the CLP could remain elevated.



9.6.5 With respect to the new airfield catchments, the new drainage network will not have been in contact with AFFFs, and so release of residual AFFF derived PFOS will not be a potential source of PFOS for the new catchment drainage. In addition, the ground investigations for the DCO Project indicate that PFOS concentrations in groundwater beneath the new catchments may be lower than in the existing airfield discharge flows. Therefore, any groundwater that could enter the new drainage network for the new airfield could contain lower PFOS concentrations than the existing discharges.

9.6.6 Overall, if the PFOS source is historic, then:

1. The DCO Project would not be expected to increase PFOS concentrations in the current discharges to the EBR or the CLP

2. The discharges for the new airfield catchments could be expected to contain lower PFOS concentrations than the EBR or CLP discharges: the only PFOS source should be historic groundwater contamination. This would need to be confirmed by further monitoring of the groundwater in the footprint of the new airfield. If high PFOS concentrations are identified in this area it may be possible to identify and remediate those sources to avoid such effects prior to development.

On-site continuing sources 9.6.7 If the main sources of PFOS are related to ongoing use of AFFFs, hydraulic fluids

or other chemicals, then the potential effects and potentially relevant environmental measures are different.

1. If the potential ongoing sources are due to continued use of AFFFs by Heathrow’s tenants, or from use of other chemicals that contain PFOS, then their use could be eliminated or restricted to areas that would not affect the drainage discharges

2. If the source is related to accidental release of hydraulic fluids, such as from minor leaks on aircraft then this problem would be more difficult manage, as Heathrow are not currently able to eliminate the use of such hydraulic fluids. Such losses could occur across the airfield, and so identifying and containing those losses would be hard to achieve. The addition of a third runway is to allow a greater number of flights and aircraft, which would increase the overall potential for fluid losses, such that, if this is a source of PFOS release, the DCO Project could increase concentrations in the airfield discharges. Enhanced monitoring and controls on maintenance of hydraulic fluids could be implemented to reduce the potential release of PFOS via this pathway. Further information is required on potential rates of losses of hydraulic fluids.



Up-gradient sources of PFOS 9.6.8 It could be that some or all of the PFOS in the airfield discharges are due to up-

gradient sources of PFOS that are not related to Heathrow. The airfield drainage network (drainage pipe system and the SWOT) is understood to capture groundwater that enters the drainage network. If this groundwater contains PFOS that is derived from contamination upgradient of Heathrow, then the main source may not be related to Heathrow activities. Surface water and groundwater monitoring data indicate there are PFOS sources up-gradient of the Airport, but the potential for up-gradient sources of PFOS to be the main source of PFOS in the airfield discharges would need to be demonstrated through further investigation.

Combination of sources 9.6.9 It could be that the PFOS concentrations reported in the airfield discharges arise

due to a combination of different sources: historic AFFF use on-site, minor losses of hydraulic fluids, and groundwater contaminated by PFOS up-gradient of the Airport.

9.6.10 Further investigation will be undertaken to characterise the different sources and understand the apportionment of the different contributions. Environmental measures, if required, would then need to take into account the contribution from different sources.

9.7 Management of PFOS in surface water discharges

9.7.1 As PFOS is an emerging contaminant and the studies into its presence and sources are at a preliminary stage, it is too early to determine whether there will need to be management of PFOS in the airfield discharges. As with the potential presence of PFOS, the potential need for and type of environmental measures that may be necessary to reduce PFOS concentrations are dependent upon the source of PFOS. Suitable mitigation measures will be considered as necessary as the investigation progresses.

9.8 Next steps: PFOS assessment at ES

9.8.1 Further assessment and characterisation of PFOS will be undertaken for ES, including the following steps:

Water quality monitoring 9.8.2 The DCO Project baseline monitoring and the routine Heathrow monitoring will

continue, collecting more samples and monitoring data with respect to PFOS in the airfield discharges, surface water and groundwater around Heathrow.



Detailed on-site inventory and sampling of PFOS 9.8.3 Heathrow have initiated further investigations into potential sources of PFAS on-

site, and into the distribution of PFAS in water across the site. The plan for the initial phase of investigation is as follows:

1. Product inventory. A survey of contractor’s / tenants will be undertaken to establish whether there is any on-site use or storage of chemicals that may contain PFOS

2. Monitoring. Water samples will be collected from the main drainage discharges, the main water storage locations, and a number of drains and manholes around the site. In addition, sediment samples will be collected from the EBR, CLP, River Crane and Portlane Brook. This survey will aim to identify any areas of elevated PFOS presence and identify dominant pathways, such as groundwater or surface water

3. Reporting. The outcome from the first two stages will be reported and this will feed into plans for further assessment of potential PFOS sources.

9.8.4 This work is planned to be undertaken in Q2/Q3 2019, and reported in Q3/Q4 2019. The outcome of the survey will be available for inclusion in the ES and will provide direction to subsequent investigations as necessary to characterise this potential risk.

Engagement with the Environment Agency 9.8.5 Heathrow have engaged with the Environment Agency with regards to the

widespread presence of PFOS identified in the baseline monitoring, which indicates that PFOS is ubiquitous at concentrations greater than the AA-EQS upstream, adjacent to and downstream of Heathrow.

9.8.6 Discussions will continue with the Environment Agency to determine a way forward for the assessment of PFOS with regards to baseline PFOS concentrations, and whether the airfield discharges for the DCO Project may require management or environmental measures to reduce PFOS concentrations at Application.



10. ASSESSMENT OF PAHS IN OPERATIONAL AIRFIELD DISCHARGES

10.1 Introduction

10.1.1 PAH species are by-products of combustion, including vehicle engines and also coal fires, and are therefore relatively common in the urban environment.

10.1.2 Baseline monitoring data for PAH species show that several of these species are present in river water and site waters at concentrations greater than the WFD limits.

10.1.3 This section presents a summary of the PAH monitoring data and discusses those PAH species that occur at concentrations greater than the WFD EQS limits, and whether airfield discharges could increase PAH concentrations in the discharges or in the receiving waters.

10.2 WFD limits for PAH species

10.2.1 The WFD Directions specify EQS values for several PAH species, as shown in Table 10.1, with EQS values for each species either as annual average (AA-EQS) values or as maximum allowable concentrations (MAC-EQS).

Table 10.1 WFD EQS values for Specific Pollutant and Priority Substances

Group Parameter Unit AA-EQS MAC-EQS

PAHs

Anthracene µg/l 0.1 0.7

Fluoranthene µg/l 0.0063 0.12

Naphthalene µg/l 2 130

Benzo(a)pyrene µg/l 0.00017 0.27

Benzo(b)fluoranthene µg/l * 0.017

Benzo(k)fluoranthene µg/l * 0.017

Benzo(g,h,i)perylene µg/l * 0.0082 *Benzo(a)pyrene is a marker for these parameters for the AA-EQS

10.2.2 It is important to note that the WFD EQS values for several of the PAH species are very low. Analytical improvements have been made in recent years in order to achieve the concentrations required by the WFD EQS limits, although these ultra-low detection limits are not always conducted as standard. Variations in laboratory detection limits through time and by different laboratories gives rise to



issues when comparing the analytical limits of detection against the WFD EQS values: a large portion of PAH analyses were conducted with detection limits of 0.01µg/l or greater (which is greater than the WFD EQS for the PAH species in Table 10.1, except for anthracene and naphthalene). This means that many results are reported as below detection limits but cannot be confirmed as being below respective EQS values.

10.3 Baseline PAH concentrations in rivers: Environment Agency data

10.3.1 The Environment Agency have analysed for PAH species infrequently and at a limited number of locations. Therefore, the number of analytical results for PAH species are limited compared to the number of samples collected or the number of results for other parameters.

10.3.2 The Environment Agency monitoring data for rivers around the site from 2010 onwards are summarised in tables in Annex L for monitoring locations on the River Colne, Colne Brook, River Frays, Horton Brook, River Crane and Duke of Northumberland’s River. The Environment Agency results are summarised as follows:

1. There are few results for anthracene and naphthalene although no values exceed the WFD limits

2. For fluoranthene, of the 114 results 55 were reported with detection limits greater than the AA-EQS and so cannot be assessed against the AA-EQS. There were 28 results that were above the AA-EQS and 31 that were below the AA-EQS. Horton Brook was predominantly below the AA-EQS (21 out of 22 results). Monitoring locations for the River Crane and Duke of Northumberland were typically greater than the AA-EQS

3. For benzo(a)pyrene there were 59 results with detection limits greater than the AA-EQS (and so cannot be assessed against the AA-EQS). Nine results reported values below the AA-EQS, all for the Horton Brook. There were 48 results greater than the AA-EQS, with concentrations exceeding the AA-EQS at the other locations

4. For Benzo(b)fluoranthene the detection limits were typically below the MAC-EQS. There were 4 out of 108 results that exceeded the MAC-EQS (one for the Colne Brook, one for Horton Brook and two at Pinn above Frays)

5. For Benzo(k)fluoranthene the detection limits were typically below the MAC-EQS. One out of 113 results exceeded the MAC-EQS (at Pinn above Frays)

6. For benzo(g,h,i)perylene there were 59 results with detection limits greater than the MAC-EQS. There were 48 results less than the MAC-EQS and 4



greater than the MAC-EQS (one on the Duke of Northumberland’s River, and three at Pinn above Frays).

10.3.3 Overall, the Environment Agency data shows that background concentrations of PAHs in rivers can exceed the WFD EQS values, with benzo(a)pyrene and fluoranthene frequently greater than their respective AA-EQS values. Only Horton Brook reported a substantial number of concentrations below the AA-EQS for fluoranthene and benzo(a)pyrene.

10.4 Baseline PAH concentrations in airfield discharge waters

10.4.1 The Environment Agency samples from Heathrow’s discharges have not been analysed for PAHs. The Heathrow routine monitoring has analysed for PAHs on a quarterly basis.

10.4.2 A review of the DCO Project baseline PAH data collected to the end of 2018 has been conducted at PEIR for the airfield discharge waters. The available results are presented in Annex M for the final EBR pond (SW-96), for the flow from the Southern Catchment to the CLP (SW-99) and from CLP adjacent to the discharge from the SWOT (SW-94).

10.4.3 The data presented in Annex M do not cover an entire year and so final conclusions in relation to AA-EQS values cannot be drawn, however, the available results are summarised as follows:

1. The results for anthracene and naphthalene do not exceed the WFD limits

2. For fluoranthene there are thirteen results reported as greater than the AA-EQS, and no results reported below the AA-EQS values, even where lower detection limits are achieved. There is one result for the Southern Catchment flow (SW-99) that exceeds the MAC-EQS

3. For benzo(a)pyrene there are no results reported as below the AA-EQS values, even where lower detection limits are achieved

4. For benzo(b)fluoranthene, benzo(k)fluoranthene and benzo(g,h,i)perylene there are two results that exceed the MAC-EQS, one for Southern Catchment flow (SW-99) and one for CLP (SW-94).

10.4.4 Overall, the data shows that the airfield waters can contain elevated PAH concentrations relative to the WFD EQS values, with benzo(a)pyrene and fluoranthene reporting concentrations consistently greater than the low values set for the AA-EQS.



10.5 Comparison of Environment Agency and DCO baseline data

10.5.1 The Environment Agency’s river monitoring and the baseline site monitoring data show that PAHs commonly occur at concentrations greater than the WFD AA-EQS limits, particularly for benzo(a)pyrene and fluoranthene, which have very low AA-EQS values (0.00017µg/l and 0.0063µg/l respectively).

10.5.2 Table 10.2 presents a comparison of average concentrations of benzo(a)pyrene and fluoranthene for the site discharges (DCO Project baseline monitoring undertaken for the DCO Project) and rivers (Environment Agency monitoring). Where there is more than one location on the River Crane and Duke of Northumberland’s River, these results have been combined to provide an average for the river. These averages have been calculated by excluding results that are below detection limits to prevent the calculated averages being skewed by detection limit values greater than the EQS limits. This has reduced the number of results that could be used in calculating the averages. The numbers in parentheses in Table 10.2 show the number of results used to calculate the average.

10.5.3 With the exception of fluoranthene for Horton Brook, the average PAH concentrations shown in Table 10.2 are greater than the AA-EQS values.

Table 10.2 Comparison of average PAH concentrations for site water and rivers

Monitoring location / river

Benzo(a)pyrene (µg/l) *

Fluoranthene (µg/l) *

Locations monitored for DCO Project.

HEP-SW-99, Southern Catchment Diversion

Chamber 0.050 (1) 0.060 (4)

HEP-SW-94,CLP near SWOT Outfall 0.013 (3) 0.035 (5)

SW-96, EBR final Pond 0.007 (1) 0.015 (4)

Environment Agency river monitoring

Colne - 0.014 (3)

Colne Brook - 0.054 (1)

Crane 0.0012 (4) 0.015 (5)

Duke of Northumberland 0.0039 (8) 0.011 (8)

Horton Brook 0.0003 (22) 0.0019 (20)

Pinn 0.0024 (20) 0.0067 (19) *The numbers in parentheses show the number of results used to calculate the average.



10.6 Potential PAH concentrations

10.6.1 Available monitoring data for PAHs is limited but indicates that the baseline PAH concentrations in the rivers and the airfield waters contain benzo(a)pyrene and fluoranthene at concentrations greater than the AA-EQS, and that concentrations of those species may be higher in the airfield waters than in the local rivers.

10.6.2 PAHs are typically released into the environment from exhausts as a result of incomplete combustion of fuels, but the specific sources and pathways of PAHs into the rivers and airfield runoff are not clear. PAH concentrations are also a function of the solubility of the species versus the capacity to bind/sorb onto solid surfaces; PAHs can also be mobilised on particulate matter. PAH concentrations in the new catchment runoff have not been estimated other than to assume they will be similar to those of the existing catchments.

10.6.3 Changes to the discharges from the airfield have been described in previous sections, and the relative proportions of expected discharge flows relative to the receiving waters have also been presented and discussed.

10.6.4 Based on the understanding of those flows, the potential effects of those changes on PAH concentrations are summarised as follows:

1. The new airfield catchments will discharge to rivers to the west of the airport. Calculations show that the discharge flows are small relative to the total flows in the River Colne, Wraysbury River or Duke of Northumberland. Therefore, the additional discharges may not substantially alter the PAH concentration in the receiving waters, although this will be assessed for ES

2. Southern and Western catchments. The Western Catchment total discharge rate may increase relative to pre-development levels. This could increase the discharge rates to Felthamhill Brook. There has not been PAH monitoring data for the CLP Outfall to assess the PAH concentrations leaving CLP. There is currently no basis to assume that PAH concentrations could substantially increase relative to current levels, but this will be assessed further at ES

3. Eastern Balancing Reservoir: the DCO Project is not expected to give rise to substantial changes to either the Eastern Catchment or the discharges from the Eastern Balancing Reservoir to the River Crane.

10.6.5 Further water quality data will become available for the ES. This initial assessment of potential risks associated with PAH will be updated to reflect additional data and any changes to the DCO Project design and/or conceptual understanding.



11. ASSESSMENT OF ATMOSPHERIC DEPOSITION ON WATER QUALITY

11.1 Introduction

11.1.1 Increases in aircraft and vehicle movements in and around the Airport once the third runway is complete will lead to an increase in the atmospheric release of oxides of nitrogen (NOx) from engines. An increase in the source of NOx would lead to an increase in atmospheric deposition of nitrogen compounds, primarily as nitrate, to the surfaces of rivers, lakes and reservoirs.

11.1.2 Lakes and reservoirs have a large surface area and volume, and also have a longer residence time (the average duration that a water molecule is retained within the waterbody) than rivers. By comparison, rivers are fast flowing and have less volume relative to their surface area. Lakes and reservoirs are therefore more likely than rivers to show a change in nitrate concentration as a result of atmospheric deposition of NOx compounds. The quantitative assessment of nitrate concentrations has therefore been completed initially for the lakes and reservoirs. If the assessment indicates that the lakes and reservoirs would not be substantially affected, then the rivers are not likely to be at risk from this process.

11.1.3 This section presents a quantitative assessment of the potential for increases to NOx concentrations to increase nitrate levels in local waterbodies.

11.2 Baseline nitrate concentrations

Lakes and reservoirs 11.2.1 The study area and the lakes and reservoirs that fall within the scope of this

investigation are described in Chapter 21: Water environment.

11.2.2 A summary of nitrate concentrations for the reservoirs is presented in Table 11.1, based upon Thames Water monitoring data.



Table 11.1 Historic (2015-2018) reservoir water quality sampling data gathered by Thames Water.

Reservoir Mean Nitrate-N (mg/l) 90th percentile Nitrate-N (mg/l)

King George VI 4.27 6.55

Queen Mother 6.42 7.7

Staines North 3.72 5.9

Staines South 1.67 5.3

Wraysbury 6.32 7.4

11.2.3 Table 11.2 presents summary data from Environment Agency monitoring of the River Thames upstream of the reservoir outtakes, the Horton Brook and Wraysbury II Gravel Pit.

Table 11.2 Historic (2000-2016) water quality sampling data gathered by the Environment Agency

Sample location Mean Nitrate-N (mg/l)

90th percentile Nitrate-N (mg/l)

Horton Brook above Colne Brook 0.83 1.73

Horton Brook above Tan House Stream 3.57 7.14

Horton Brook above Wraysbury 2 Gravel Pt 3.31 5.59

Thames above Water Intake, Egham 6.68 7.83

Thames at Sunnymeads Water Intake, Wraysbury 6.6 7.92

Wraysbury II Gravel Pit 0.62 1.47

11.2.4 The data in Table 11.1 and Table 11.2 cover different time periods but indicates that nitrate concentrations in the Thames at Sunnymeads and the Wraysbury and Queen Mother reservoirs can be very similar. This is to be expected, as these reservoirs receive water from the abstraction at Sunnymeads. However the data also indicates that nitrate concentrations in King George VI and Staines (North and South) reservoirs are lower than in the Thames at Egham. Since these reservoirs are fed by water from the abstraction at Egham, it might be expected that reservoir water quality would be very similar to that of the Thames at Egham.

11.2.5 Nitrate concentrations in the reservoirs and Wraysbury II gravel pit exhibit seasonal variability, with concentrations typically peaking in winter and at their lowest in the summer. The degree of variation differs between the waterbodies but



is typically 2 to 4 mg-N/l. It is also of note that the baseline nitrate concentration in Wraysbury No. 2 former gravel pit is very low in comparison to the reservoirs.

Reservoir and lake area and capacity 11.2.6 Thames Water have provided information on the areas and capacities of the

reservoirs, shown in Table 11.3.

Table 11.3 Thames Water reservoir area and capacity

Reservoir Surface area (ha) Max Dam Height (m) Volume at TWL (m3)

King George VI 138 16.4 18,732,000

Queen Mother 191 19.8 31,492,000

Staines North 70 12 6,678,200

Staines South 100 12 7,469,000

Wraysbury 183 16.2 26,910,000 Note: TWL = Top Water Level

11.3 Numerical assessment of NOx deposition

11.3.1 The potential change in nitrate concentration from atmospheric NOx deposition is a function of:

1. The concentration of NOx and its potential deposition rate

2. The surface area of the water body and

3. The residence time of water within the water body (which controls how much NOx could build up in the water body over time through atmospheric deposition).

11.3.2 The assessment has first considered lakes and reservoirs as these have larger surface areas and longer residence times than rivers. If the potential effect on the reservoirs and lakes is significant then the potential effect on the rivers will be investigated.

11.3.3 Two scenarios are considered for the assessment:

1. The additional nitrogen deposited on the surface of a lake or reservoir is completely mixed throughout the water column

2. The additional nitrogen deposited on the surface of a lake or reservoir is confined to a layer at the surface of the waterbody.



11.3.4 This latter scenario is intended to represent the situation that could occur if the water column stratifies, preventing complete mixing to depth. It is not known whether the lakes and reservoirs are subject to stratification in the spring and summer. In both cases, it has been assumed that all the nitrogen deposited on the waterbody is transformed to nitrate (NO3), and that water in the waterbodies has a residence time of 1 year (although the residence time is expected to be much shorter than this in reality due to the use of these reservoirs as a part of the public water supply network).

11.3.5 The concentration of nitrate in the waterbodies was then estimated as a function of baseline water quality and the predicted increase due to increased atmospheric deposition.

11.3.6 Nitrate retention is the removal of nitrate from the water column through denitrification: the microbial conversion of nitrate to nitrogen gas that acts to reduce nitrate concentrations in water. It can be difficult to determine when denitrification is occurring, and denitrification rates can be highly variable. For the purposes of this assessment it has been assumed that no denitrification will occur. This is a conservative assumption, which means that the increase in nitrate in the water columns of lakes and reservoirs may be less than is calculated.

11.3.7 Information from air quality assessment (PEIR Chapter 7: Air quality and odour) indicates that the maximum increase in atmospheric deposition of nitrogen at Wraysbury Reservoir that could occur due to development is 6.7kg-N/ha/yr. Note that this is a provisional figure used for the preliminary assessment at PEIR that may be revised at ES. Whilst there will be some spatial variation in deposition rates, this figure is considered to be representative of the other waterbodies.

11.3.8 The predicted increase in nitrate concentrations in the reservoirs post-development is as shown in Table 11.4. For the calculations assuming stratification has occurred in the water column, a mixing depth of 5 m has been assumed.

Table 11.4 Calculated nitrate concentrations from increased NOx emissions

Reservoir Change in nitrate

concentration (mg-N/l): Complete

mixing

Change in nitrate

concentration (mg-N/l): 5m mixing layer

Nitrate concentration (mg-N/l) post-development:

Complete mixing


5m mixing layer

King George VI 0.049 0.13 4.32 4.40

Queen Mother 0.041 0.13 6.46 6.55

Staines North 0.070 0.13 3.79 3.86



Reservoir Change in nitrate

concentration (mg-N/l): Complete

mixing

Change in nitrate

concentration (mg-N/l): 5m mixing layer


Complete mixing


5m mixing layer

Staines South 0.090 0.13 1.76 1.80

Wraysbury 0.046 0.13 6.37 6.45

11.3.9 Wraysbury No. 2 former gravel pit has a surface area of 53 ha (it is smaller than the smallest of the reservoirs; the Staines North reservoir at 70 ha). It is assumed to be in continuity with groundwater and may also be influenced by the water quality of Horton Brook. It is noted that nitrate concentrations in Horton Brook, although variable along its length, are lower than in the Thames and baseline nitrate concentrations in Wraysbury No. 2 are very low. Given the small surface area of the lake and the flushing effect of groundwater flow it is considered that the increase in nitrate concentration due to increased atmospheric deposition is likely to be smaller than that in the reservoirs.

Discussion 11.3.10 This section has considered the potential increase in nitrate concentrations in the

water columns of lakes and reservoirs as a result of increased NOx emissions.

11.3.11 The estimated increases in nitrate concentration are small, and assuming complete mixing throughout the water column, are uniformly less than 0.1mg-N/l. If it is assumed that the additional nitrate is confined to a mixing layer of depth 5m, the estimated increase in nitrate concentration is 0.13mg-N/l.

11.3.12 This latter figure equates to 2% to 4% of baseline nitrate concentrations in each of the reservoirs, except Staines South, which because baseline nitrate concentrations are very low, equates to an increase of 8%. These calculations have been conservative and do not include other processes such as denitrification. If denitrification was taken into account, the actual increase in nitrate concentration that is likely to result from the DCO Project would be less than has been calculated.

11.3.13 In each of the reservoirs, the predicted increase in nitrate concentration from potential increases to NOx emissions is less than the variability in nitrate concentrations observed in the historical record (such as comparing the difference between the mean and 90th percentile nitrate concentrations in Table 11.4).

11.3.14 The Drinking Water Standard for nitrate is 11.3mg-N/l. Even with the additional NOx emissions the predicted concentrations in the reservoirs do not exceed 60%



of the Drinking Water Standard for nitrate. Therefore, the calculations do not show a risk of significant increases in nitrate concentration in the reservoirs or lakes.

11.3.15 The rivers in the vicinity of the site have much smaller surface areas and much shorter residence times, and hence the potential effect of atmospheric NOx deposition on river water quality is expected to be negligible.



12. WASTEWATER TREATMENT AND DISCHARGE

12.1 Introduction

12.1.1 The majority of the existing airport foul wastewater is currently discharged via Bath Road Sewer to Mogden Sewage Treatment Works (STW). The DCO Project will significantly increase the number of airport personnel and passengers at the Airport, with a resulting increase in waste water production.

12.1.2 Two strategies are being considered for dealing with foul water from the expansion and possibly the existing airport facilities.

12.1.3 The preferred strategy is to continue the existing approach of discharging to the Thames Water Utilities Limited (TWUL) sewage system, with treatment being undertaken off-site at their sewage treatment works at Mogden. An ‘offset strategy’ is proposed whereby there would be no increase in flows to the TWUL system compared to the existing situation. This approach has been subject to preliminary consultation with TWUL and remains under review.

12.1.4 The alternative to the preferred strategy would be the creation of a new dedicated Waste Water Treatment Plant (WWTP) to serve the expanded airfield. This could take a number of forms, from treating airfield expansion foul water (only), to serving the entire airfield, and perhaps providing treatment for surface water as well. Tertiary treatment could be incorporated to produce treated sewage effluent for water re-use as grey water.

12.1.5 The currently preferred location of a new ‘Heathrow’ WWTP is to the north of the expanded airfield, adjacent to the extended Saxon Lake, as indicated in the parameter plans released as part of the AEC. A Discharge location from the potential WWTP are unconfirmed at this stage, and in any case could be flexible as outfall flows could be pumped. The point of discharge would be agreed with regulators.

12.2 WwTP discharge

12.2.1 The WwTP, if required, is to be located to the north of the third runway. Potential locations for discharge of treated waters have not been determined at PEIR: treated flows would likely be sent west to discharge into the diverted rivers, although there may be an option to pump flows to the River Crane to supplement base flow.

12.2.2 Provisional scenarios in the Foul Water Strategy indicate that the flows to be treated could vary from zero (all foul water to Bath Road Sewer) to around 9Ml/d mean flow, with peak flows of up to 31Ml/d. Within the treated water there could be



residual levels of BOD, orthophosphate and ammonia that could have an effect on river quality if the foul water was treated on-site and discharged to the local rivers.

12.3 Next steps

12.3.1 Preliminary calculations of potential effects from an on-site WwTP have not been undertaken at PEIR as the requirement for a WwTP and the potential range of discharge flows is not known. These flow and quality effects could be cumulative with the airfield discharges, and so would need to be assessed in combination with the effects of other discharges. If the WwTW is required under the DCO Project, potential changes to receiving water quality will be evaluated at ES. Should the evaluation identify the potential for substantial changes to the receiving water quality, then environmental measures will be included in the design to prevent those substantial changes occurring.



13. CONCLUSIONS

13.1.1 This SWQA report has presented a preliminary assessment of key potential risks to surface water quality for PEIR that could arise from the DCO Project development. A summary of potential sources and causes of changes to water quality are summarised below.

13.2 Construction activities, dewatering and runoff discharge

13.2.1 Groundwater dewatering flows and surface runoff captured within the groundworks could include a range of potential contaminants that could reduce water quality in the receiving waters. Potential contaminants could include increases to suspended particulate matter from ground disturbance, oils and fuel spillages from mobile plant, nutrient release from disturbed soils, and solutes mobilised from historic contamination, such as from historic landfills / waste disposal, historic industrial activities.

13.2.2 The management of surface runoff and dewatering flows during the construction will be managed following good practice measures outlined in the CoCP. Numerical modelling may be undertaken for ES if required and as plans are developed to sufficient detail.

13.3 Roads

13.3.1 The potential effects of the planned changes on the surface water environment from the road network were not assessed at PEIR due to lack of design detail. This will be assessed as part of the ES using the Highways Agency Water Risks Assessment Tool (HAWRAT).

13.4 River diversions and lined channels

13.4.1 Existing rivers will be diverted over areas of historic landfill. Potential contamination of the river channels by mobilising landfill leachate will be prevented by lining the new river channels to hydraulically disconnect them from the groundwater. This will also reduce groundwater baseflow into the lined zones. The preliminary assessment indicates the removal of baseflow from the lined zones does not alter river flow to a degree that there would be a substantial effect on water quality, as the flows in the rivers in these areas are much larger than the baseflow contributions.



13.5 Airfield discharges

13.5.1 New balancing lagoons and water treatment facilities will be constructed for managing surface runoff from the new airfield catchments, and these will discharge into the rivers to the west of the site; into rivers that have not previously received airfield runoff. Changes from the DCO Project may also increase the discharge flows to CLP from the Western Catchment. These new and altered runoff discharges may contain potential pollutants that could deteriorate water quality relative to the pre-development conditions.

13.5.2 Based on the understanding of the processes that may affect water quality, and from review of the available baseline water quality data, particularly airfield discharge water quality data, a sub-set of key parameters of interest were identified for evaluation and discussion at PEIR:

BOD 13.5.3 Increased use of de-icer chemicals during cold periods contributes to substantial

concentrations of BOD in airfield runoff. This is a known issue that Heathrow address through segregation and treatment of affected runoff flows to reduce BOD levels. Increasing the airport size and capacity will increase the volume of water that may be contaminated during cold periods.

13.5.4 A preliminary SIMCAT model has been developed to numerically assess the baseline conditions for the local rivers, and the influence of the airfield discharges on BOD concentrations in the rivers.

13.5.5 The numerical models indicate that discharges to the west of the site (River Colne, Wraysbury River or the CRC channels) are not likely to significantly increase BOD concentrations for either the Preferred Approach or the Alternative Approach. This is because, as long as the majority of the discharge is into the larger channels, the discharge flows are small in comparison to the flows in the receiving rivers.

13.5.6 The assessment is less certain with respect to discharges to CLP and Felthamhill Brook. The CLP is hydrologically complex, and further work is required to understand how much of the airfield water that is discharged to CLP ultimately drains via the CLP Outfall to Felthamhill Brook. Monitoring data for CLP indicates that BOD is attenuated within the CLP, further reducing the BOD concentrations from the discharges from the airfield into the CLP.

13.5.7 The potential for increases to airfield discharges to increase BOD concentrations in the Portlane Brook WFD waterbody is therefore dependent upon how much of the airfield runoff discharges via the CLP Outfall, and whether increases to discharge flows could substantially reduce the residence time and BOD attenuation within CLP. This will be assessed further at ES.



13.5.8 Should the ES assessment indicate potential for substantial increases to BOD concentrations at the CLP Outfall, there are environmental measures available that could increase the retention time and aeration of water in the CLP if required, and therefore can increase BOD attenuation in the CLP if necessary to reduce BOD concentrations at the CLP Outfall.

Orthophosphate 13.5.9 The airfield discharge flows from the Southern Catchment to the CLP occasionally

report elevated orthophosphate concentrations relative to WFD values and in comparison to the other airfield catchments. The source of the additional orthophosphate may be from nutrient dosing during cold periods to facilitate BOD removal at Mayfield Farm.

13.5.10 Monitoring locations around CLP indicate that the elevated orthophosphate concentrations are attenuated within the CLP and that concentrations at the CLP Outfall are generally lower than in Portlane Brook.

13.5.11 Following the airport expansion, high BOD Western Catchment flows will also be treated at Mayfield Farm, increasing the volume of water discharged to the CLP that may contain elevated orthophosphate concentrations. The increase in flows to the CLP could also decrease the residence time in the CLP, reducing the potential for attenuation of orthophosphate. This could present a risk of higher orthophosphate concentrations at the CLP Outfall and therefore higher orthophosphate concentrations in Portlane Brook.

13.5.12 Changes to orthophosphate concentrations in the CLP have not been numerically assessed at PEIR. Should the ES assessment indicate potential increases to orthophosphate concentrations in Portlane Brook, there are several environmental measures available that could reduce concentrations if required:

1. Improved monitoring and management of nutrient dosing levels at Mayfield Farm, to reduce and prevent high orthophosphate concentrations

2. Increase aeration and biological activity within CLP to increase uptake of orthophosphate. This could include planting of reeds or other plant

3. Recirculation of water within CLP, by pumping water back to the discharge points, in order to increase residence time and to increase attenuation within the CLP


13.5.13 BOD removal for the new airfield discharges to the west of Heathrow may use the same method as at Mayfield Farm. Therefore, the new discharges could also contain elevated concentrations of orthophosphate. Numerical assessment of the



orthophosphate concentrations has not been conducted for PEIR. This will be undertaken for the ES as additional data becomes available. Should the assessment indicate orthophosphate in the discharges could influence river quality, there are environmental measures that could reduce the discharge concentrations if required:

1. Improved monitoring and management of nutrient dosing levels for the treatment to reduce and prevent high excess orthophosphate concentrations

2. Recirculation of effluent within the treatment circuit to reduce orthophosphate concentrations in the effluent


13.5.14 Overall, the risks of increasing orthophosphate concentrations should be assessed further as part of the ES, and potential environmental measures developed accordingly.

PFOS 13.5.15 Monitoring data indicates that the presence of PFOS is widespread in the rivers

around Heathrow at concentrations greater than the AA-EQS (0.00065µg/l) but below the MAC-EQS (36µg/l), including the rivers upstream of Heathrow. Environment Agency monitoring also shows that PFOS is ubiquitous in rivers in the London area and beyond, and therefore indicates that there has been widespread historical release of PFOS into the wider environment.

13.5.16 Water quality monitoring also indicates that PFOS is present in the baseline airfield runoff discharge flows at concentrations greater than the WFD AA-EQS but also well below the MAC-EQS.

13.5.17 The potential sources of PFOS in the baseline airfield discharges have been considered (such as historic use of AFFF for fire-fighting or aircraft hydraulic fluids). Further investigation of those potential sources is to be undertaken subsequent to PEIR.

13.5.18 PFOS is an ‘emerging contaminant’ and the assessment of PFOS risks in the environment and for the DCO Project are still an early stage. The Environment Agency have yet to fully assess the baseline concentrations of PFOS in these catchments. Guidance on the implications of elevated PFOS concentrations with respect to Good Chemical Status in these rivers has, therefore, yet to be published. Consequently, it has not been possible to conduct a preliminary assessment as to whether PFOS in future airfield discharges may be an issue with respect to WFD compliance.



13.5.19 Heathrow are engaging with the Environment Agency to understand the presence of PFOS in the regional context, and to determine a practical means to assess potential water quality changes associated with current and future airfield discharges. The outcome of these discussions will inform the assessment of potential risks and changes to water quality from the airfield discharges. This in turn will inform whether environmental measures are needed to manage PFOS concentrations at Application.

PAHs 13.5.20 Monitoring data indicates that several PAHs, specifically benzo(a)pyrene and

fluoranthene are present in airfield runoff and in several rivers at concentrations greater than the WFD EQS limits. These compounds are products of combustion and are generally ubiquitous in the urban environment, and hence identifying specific sources is generally not possible. The controls on PAH concentrations are also influenced by the solubility of the species, and presence of PAHs bound to particulate matter.

13.5.21 PAH concentrations in the new airfield catchment runoff are assumed to be similar to that of the runoff from the existing airfield catchments. PAH concentrations from the Western, Southern or Eastern catchments are not anticipated to change substantially as a result of the DCO Project. The discharge flows from the new airfield catchments that will discharge to rivers to the west of the airport are small relative to the total flows in the River Colne or Wraysbury River. Therefore, the additional discharges may not substantially alter the PAH concentration in the receiving waters.

13.5.22 Further water quality data will become available for the ES. This initial assessment of potential risks associated with PAH will be updated to reflect additional data and any changes to the DCO Project design and/or conceptual understanding.

13.6 Lakes, reservoirs and atmospheric deposition

13.6.1 Increases in aircraft and vehicle movements near the site could lead to an increase in the release of oxides of nitrogen (NOx). An increase in the source of NOx would lead to an increase in atmospheric deposition of nitrogen compounds to the surfaces of rivers, lakes and reservoirs.

13.6.2 Rates of atmospheric deposition of NOx compounds to the local lakes and reservoirs were calculated and it was concluded that the DCO Project would not substantially increase nitrate concentrations in these waterbodies. This conclusion will be re-checked at ES if there is any revision to the atmospheric deposition number from the air quality assessment.



13.7 Wastewater treatment plant

13.7.1 An on-site dedicated wastewater treatment plant may be required to handle increases to foul water flows. Discharges from this facility would therefore need assessment with respect to potential water quality effects on receiving waters. The requirements and anticipated flows of the WwTP will be considered at ES stage.



14. GLOSSARY

Term Definition

Above Ordnance Datum (AOD)

Ordnance Datum is the vertical datum used by the Ordnance Survey as the basis for deriving the height of ground level on maps. Topography may be described using the level in comparison to ‘above’ ordnance datum.

Abstraction Removal of water from surface water or groundwater.

Aeration The introduction of air into a body of water

Aquifer A body of permeable rock that is capable of storing significant quantities of water; is underlain by impermeable material, and through which groundwater moves.

Atmospheric deposition Transference of substances in the air to land or water on the earth surface.

Attenuation Reduction in peak flow rate and increased duration of flow event.

Balancing pond A balancing Pond is a drainage system used to control flooding by temporarily storing flood waters.

Baseflow The sustained flow in a channel or drainage system.

Baseline conditions The conditions that would exist in the absence of the proposed project at the time that the project would be constructed/ operated. The definition of these baseline conditions should be informed by changes arising from other causes (e.g. other consented developments).

Bifurcation Division of a watercourse into two channels.

Biochemical oxygen demand (BOD)

The quantity of oxygen required to biodegrade the organic matter present in a volume of water. It is used as a measure of the pollution of a water sample and can be an indicator of a risk of oxygen depletion in a waterbody.

Biodegrade Substances that can be decomposed naturally by bacteria and other organisms

Braided system Watercourse channels that regularly divide and combine, resembling the form of a braid.

Capacity A system’s capability to accommodate a designated level of demand at a desirable level of service.

Channel morphology The shape and direction of the channel along its course.

Climate change A change in the state of the climate that can be identified (e.g. by using statistical tests) by changes in the mean and/or the variability of its properties, and that persists for an extended period, typically decades or longer. Climate change may be due to natural internal processes, to external forcing or to persistent anthropogenic changes in the composition of the atmosphere, ocean or in land use.

Conceptual Model Representation of a system used to simulate scenarios in a model.

Conveyance Movement of water from one location to another



Term Definition

Covered River Corridor (CRC)

The new structure that will convey the rerouted river under the North West runway.

Cumulative Effects The summation of effects that result from changes caused by a development in conjunction with other past, present, or reasonably foreseeable actions

Cumulative Effects Assessment

Assessment of impacts as a result of the incremental changes caused by other past, present and reasonably foreseeable human activities and natural processes together with the DCO Project.

Desk-based assessment A data collection exercise using existing sources of data. The purpose is to identify relevant known resources.

Development Consent Order (DCO)

This is the means of obtaining permission for developments categorised as Nationally Significant Infrastructure Projects, under the Planning Act 2008.

Discharge Release of effluent waste into a watercourse or waterbody.

Drainage catchment The area contributing surface water flow to a point on a drainage or river system. Can be divided into sub-catchments.

Drinking Water Standard

Drinking water standards describe the quality parameters set for ensuring safe and acceptable drinking water. The standards include specifications set in the EU drinking water directive and national specification which apply only in the UK.

Effluent water Waste or foul water

Environment Agency (EA)

EA are the statutory environmental regulator in England responsible for water quality and resources.

Environmental compensation

Whilst certain assets of the environment are damaged, the overall quality and condition of the environment remains the same.

Environmental measures

Indicators of the quality of the environment, particularly to monitor negative changes.

Environmental Quality Standards (EQS)

A value, generally defined by regulation, which specifies the maximum permissible concentration of a potentially hazardous chemical in an environmental sample, generally of air or water.

Environmental Statement (ES)

The ES sets out the developer’s assessment of the likely environment effects from the proposed development. An ES sets out the description of the project, and a description of the mitigation measures envisaged to avoid, or reduce significant adverse environmental effects.

Eutrophication The ageing of a lake or land-locked body of water that results in organic material being produced in abundance due to a ready supply of nutrients accumulated over the years.

Good Chemical Status A definition of water quality for a WFD waterbody based upon concentrations of priority substances and other pollutants.

Groundwater recharge The addition of water to the groundwater system by natural or artificial processes.

Hydraulic connectivity The degree of connection or interaction between two bodies of water, such



Term Definition

as between a river and the surrounding aquifer, or between different parts of the sub-surface.

Hydraulic continuity See hydraulic connectivity

Hydrogeology Hydrogeology is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust

Hydrology he study of the waters of the earth, their occurrence, circulation and distribution; their chemical and physical properties; their relation to the environment, including their relation to living things.

Infiltration The passage of water into the ground.

Leachate A liquid containing organic and inorganic chemicals, heavy metals and pathogens that can cause harm when they enter the environment.

Moving Bed Biofilm Reactor (MBBR)

A treatment technology used for the reduction of pollutants in wastewater by facilitating biological breakdown or removal of pollutants.

Organic carbon Carbon that occurs naturally in compounds and molecules.

Physico-chemical determinands (or water quality parameters)

Water properties such as pH, electrical conductivity, temperature, redox potential, and also concentrations of certain species such as dissolved oxygen, orthophosphate and ammonia.

Receiving waters Waterbodies that effluent water is discharged into.

Reservoir A lake, pond or impoundment used to store water.

Residence time The average length of time during which a substance is retained within a given waterbody

River Basin Management Plan (RBMP)

The Water Framework Directive requires European Union member states to put in Place RBMPs. Each RBMP applies a “river basin district” (an area of one or more neighbouring river basins (e.g. River Thames). The river basin process involves setting environmental objectives for groundwater and surface waters and devising programmes of measures to meet those objectives.

Runoff Water flow over the ground surface to the drainage system. This occurs if the ground is impermeable, is saturated, or if the rainfall is particularly intense.

Scoping Opinion The scoping opinion for the DCO Project adopted by the Secretary of State on 2 July 2018.

Scoping Report A report that presents the findings of an initial stage in the environmental impact assessment process. A Scoping Report was submitted by Heathrow to the Planning Inspectorate on the 21 May 2018. This report included the identification of the proposed methodologies for the further studies.

Sewer A pipe or channel taking domestic foul and/or surface water from buildings and associated paths and hardstanding from two or more curtilages and having a proper outfall.

SIMCAT Abbreviation for SIMulated CATchment. Environment Agency proprietary software for probabilistic assessment of river network flows and quality



Term Definition

Source Pathway Receptor (SPR)

An approach for evaluating pollution risks, based upon identifying a pollutant source, a receptor that may be affected by the pollutant, and a pathway by which the pollutant may move from the source to the receptor

Strata A layer of rock or soil

Surface water Water that appears on the land surface that has not seeped into the ground, i.e. lakes, rivers, streams, standing water, ponds, precipitation.

Storm Water Outfall tunnel (SWOT)

Tunnel that carries surface water from the development site through a tunnel to a discharge location.

Suspended solids General term describing suspended material, used as a water quality indicator.

Wastewater Treatment Plant (WwTP)

Facilities for the treatment of foul water.

Water framework Directive (WFD)

A substantial piece of EU water legislation that came into force in 2000, with the overarching objective to get all water bodies in Europe to attain Good or High Ecological Status. River Basin Management Plans have been created which set out measures and potential mitigation to ensure that water bodies in England and Wales achieve ‘Good Ecological Status’.

Watercourse A term including all rivers, streams, ditches, drains, cuts, culverts, dykes, sluices and passages through which water flows.



FIGURES

APPENDICES

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This drawing may contain Ordnance Survey Mastermap and Raster Data.OS Copyright Acknowledgement.Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown Copyright and database right. All rights reserved. Heathrow Airport Limited, O.S. Licence Number 100020071.

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Annex A-1 © Heathrow Airport Limited 2019

ANNEX A SIMCAT MODELLING



NUMERICAL MODELLING OF BOD WATER QUALITY

Introduction Numerical modelling of the surface water network has been undertaken to assess the concentrations of BOD in the river network as a result of the new discharges of airfield runoff and increases to the existing drainage discharges to CLP. The numerical modelling also represents changes to the river connectivity, and changes to river baseflow in areas where sections of the river channel may be lined.

Surface water quality modelling of the river network has been completed using the Environment Agency’s SIMCAT (SIMulated CATchment) modelling software. SIMCAT is a standard tool routinely deployed by the Environment Agency for planning, permitting and regulatory guidance for evaluating changes to existing discharges or new discharges into rivers. The SIMCAT models for the SWQA were developed from SIMCAT models of the Thames River Basin (TRB) provided by the Environment Agency.

SIMCAT models calculate the flow and quality of river water through a defined river network, and can include point discharges and diffuse sources of solute loading. Calculations/simulations are undertaken using a Monte-Carlo probabilistic approach that allows statistical assessment of potential risks.

Modelling objectives The aim of the numerical modelling was to assess the resulting river quality from the development and the risk of water quality exceedances in the rivers, where the rivers are the receptors and points of assessment. The model results present predictions of water quality along the river reaches. The results have been compared against EQS values, identifying where significant changes may occur, and indicating the approximate lengths of reaches that could be affected.

At PEIR, due to the complexity of the system and absence of detailed flow rates, the numerical models do not aim to provide a definitive assessment and prediction of surface water quality. Instead, at PEIR the aim of the modelling is to provide a screening assessment of where planned changes could give rise to potential exceedances of EQS values.

Where the models identify potential exceedances (or WFD class changes), then further assessment will be required for ES, and could drive additional environmental measures (e.g. further treatment or changes to water management plans) in order to maintain and/or improve WFD classification.



SIMCAT Monte Carlo Assessment

SIMCAT runs models using a Monte Carlo simulation/probabilistic approach to provide a statistical assessment of potential flows and water quality. This means that each attribute of river flow and quality is not defined by a single value but by a specified range of values, typically (but not exclusively) defined using mean values and a measure of variance from the mean, such as the standard deviation or the Q95. The variance is typically assumed to be log-normal, but other distributions can also be used. Where data is available and does not follow a parametric distribution, the data population can be entered as a non-parametric distribution file, thereby there is no assumption on the form of the distribution.

If the model were run a single time (a deterministic analysis), the model calculations would use a single value for each parameter and would generate a single set of results of flow and quality for each location. When set to Monte Carlo analysis the model is run a specified number of times (referred to as ‘shots’), and for each ‘shot’ the model extracts a value from the range of values from the statistical distributions for each input value (for flow and quality for every reach and discharge) and uses those values in the calculations. By running several hundred shots, each using different input values, SIMCAT can then provide a statistical output on the flow and concentrations that could be expected. The higher the number of shots, the more statistically representative the output values are for each given set of model inputs. The Environment Agency models were set to run 730 shots, and this number has not been altered for these models.

SIMCAT calibration and gap-filling process The developers of SIMCAT recognised that where models disagree with observed values that a likely cause of the discrepancies is ‘sampling error’, by which they mean a limited number of flow gauging locations and a limited number of samples is unlikely to capture all the variation that could occur in a natural system. To account for this, the SIMCAT developers included a ‘gap-filling’ process. The gap-filling process is a model calibration process where values are defined for some locations, and then the model is run to match those values by changing variables such as flows, discharge concentrations, baseflow concentrations or degradation rates.

There are many variables that could influence a given parameter concentration, and there may not be a defensible basis for altering parameters to achieve that fit beyond the improvement to the model calibration. Therefore, whilst this is an effective way of improving the ‘fit’ of the models to observed data, the gap-filling process has not been applied to these models.

Rather than allowing the models to automatically fit to the data, the models have been run as they were constructed. Where discrepancies from observed data are observed, then minor changes to certain input parameters have been assessed using different scenarios. The changes applied in the sensitivity runs have been undertaken to inform on potential



key controls on the models that could influence water quality and reconcile the differences between the observed and modelled data.

Model development and scenarios

SIMCAT models Three SIMCAT models have been constructed for the assessment of potential water quality effects from the proposed development scheme:

1. Baseline Model: The first model developed represents the pre-development river network and existing discharges. The Baseline Model has been developed by refinement and amendment of the larger TRB catchment model provided by the EA.

2. Preferred Approach: This model incorporates the key changes to the river network under the Preferred Approach. The main changes are with respect to the river network and the additional runoff discharges from the two new catchment areas and their associated attenuation and treatment areas, with discharges to the River Colne and Wraysbury River from the southwest attenuation and treatment area, and to the Colne Brook from the Northwest catchment.

3. Alternative Approach: This model incorporates the Alternative Approach design. It adopts the same river network as the Preferred Approach model, but has a single catchment and treatment facility associated with a Northern Treatment facility, with discharges to the CRC channel(s).

Model calibration and scenarios Assessing the accuracy of numerical models is necessary to ensure the models provide a reasonable representation of the flow and chemistry of the various interacting rivers and discharges.

The degree to which model calibration can be achieved is a function of the data available: defining the model based upon the number of calibration points/locations in the model network and the dataset associated with each location.

Baseline Model outputs have been compared against available flow and quality data. The flow data for assessing the model accuracy is discussed in Section 0, and the water quality data is discussed in Section 0. Calibration has been undertaken for the Baseline model in order to improve the fit of the data to the observed data where there is a valid basis for adjusting the changes to the input values and/or where the changes are based on conservative assumptions.



Model scenarios Several model scenarios have been run as part of the modelling process to ensure the model is working correctly, and for the Baseline model as part of the calibration process to achieve as reasonable a fit to observed data as possible. These include:

1. Applying different methods of defining model inputs, particularly for flow or quality data where SIMCAT inputs can be defined from different data sources or by different methods (e.g. as log-normal distributions or non-parametric distributions)

2. Evaluating different rates of discharge flows, particularly where there is uncertainty, such as the discharge rates from the CLP to Felthamhill Brook

3. Options for different discharge locations based on the Preferred Approach and Alternative Approach for the attenuation and treatment areas and their associated discharges, where the discharges could be sent to one or more rivers.

The different scenario runs have been applied to evaluate areas of uncertainty, and to establish if the models are sensitive to different processes.

Assessment criteria The WFD Directions set out the limits for BOD concentrations for different WFD classes as shown in Table 21.1A.1 for evaluation against the 90th percentile BOD concentration. These concentrations are for Type 3, 5 and 7 rivers, with the rivers around Heathrow typically being Type 5 or 7 based upon the altitude of the rivers and typical alkalinity concentrations.

The limits are applicable to WFD waterbodies to determine the baseline WFD class with respect to BOD, and to assess whether the class may change as a result to changes arising from the DCO Project. The limits Table 21.1A.1 have also been used for screening the BOD concentrations in the EBR, CLP and airfield discharges to assess whether they could present a risk of environmental effects.

Table 21.1A.1 BOD concentration ranges for WFD classes


BOD (90th percentile, mg/l) <4 4-5 5-6.5 6.5-9 >9

The WFD Directions note that BOD concentrations should not be used by the regulators in classifying the status of water bodies. This is because dissolved oxygen is the more



relevant parameter with respect to compliance and ecosystem health, and that the relationship between BOD and dissolved oxygen is complex. The UKTAG guidance10 indicates that assessment of BOD should be used, when necessary, for deciding on actions to improve dissolved oxygen compliance. The numerical assessment at PEIR has evaluated BOD to assess potential changes to surface water quality associated with de-icer use; assessing dissolved oxygen without assessing BOD would not be practical. Therefore, at PEIR, numerical models have assessed BOD concentrations and how these may change as a result of airfield discharges and compared this against the WFD class values in Table A.1, providing an indication of where there could be changes in BOD levels.

For WFD, the assessment of rivers as a receptor includes any portion of the river, although assessment points can often be in the lower portion of the river/waterbody. Where discharges cause deterioration in WFD class immediately downstream of the discharge but the quality improves further downstream, then the length of river reach that is degraded may become a factor in the assessment. Under the UKTAG recommendations on surface water classification schemes for WFD11, the spatial criteria for assessing the geographic extent of failures of one or more standards or conditions with respect to the ecological river status are summarised as follows:

1. Failure of a condition limit for ‘High’ in more than 0.5 km of contiguous length of river, or for more than 5% of river length. Where 5% of river length is less than 0.5 km, then the failure will apply if a total of more than 0.5 km or 100% of river length is affected, whichever is the smaller

2. Failure of a condition for Good, Moderate or Poor in more than 1.5 km of contiguous river length. Where 15% of the river length is less than 1.5 km, then the failure will apply if a total of more than 1.5 km or 100% of river length is affected, whichever is the smaller.

10 https://www.wfduk.org/sites/default/files/Media/Environmental%20standards/Environmental%20standards%20phase%201_Finalv2_010408.pdf 11https://www.wfduk.org/sites/default/files/Media/Characterisation%20of%20the%20water%20environment/Recommendations%20on%20surface%20water%20status%20classification_Final_010609.pdf





SIMCAT RIVER NETWORK AND FLOW MODELLING

River network – Baseline Model SIMCAT works by numerically defining the length and flow parameters for each reach of a river network, how the various reaches are connected and interact, along with representations of the various abstractions, discharges into the rivers, baseflow and runoff components etc. Each river is represented by one or more reaches depending upon the number of significant features, such as bifurcations and convergences of tributaries.

The Environment Agency provided SIMCAT models that represent the Thames River Basin (TRB), with the different models for assessing different solute parameters:

1. The first Environment Agency model (TH_Model2_Baseline.dat) is for evaluation of Biochemical Oxygen Demand (BOD), dissolved oxygen (DO) and ammonia (for simplicity, this model will subsequently be referred to as “the BOD Model”); and

2. The second model (TH_PostCal2_Mod1.dat) is for the evaluation of nitrate, phosphate and total phosphorous (“the Nutrient Model”).

The Environment Agency TRB models were extensive and complex, representing over 1300 river reaches in the Thames basin, and were designed to be a high-level tool to assess the entire river basin.

The Environment Agency’s BOD Model for the Thames River Basin has been used as the basis for the Baseline SIMCAT model. The original Environment Agency models have been heavily modified to achieve a smaller, flexible model that contains the necessary detail for the reaches of interest but allows more rapid evaluation: the model extent has been reduced from over 1300 reaches to around 30 reaches, focussing the model network around Heathrow and the catchments of interest. The reduced model includes the upstream reaches necessary to define the upstream boundaries of the model, and those downstream river reaches that may be influenced by the DCO Project.

The revised river network used in the Baseline SIMCAT model is shown in Figure 21.1.2 for the river network and discharge locations as they are currently configured (pre-development). Key features of the smaller river network are as follows:

1. To avoid the need to model and represent too much of the upstream reaches, the upper reaches of the River Colne and Colne Brook have been removed from the Environment Agency’s TRB model. In the models, the Colne flow starts approximately 1 km north of the proposed extent of the Airport, and the Colne Brook starts approximately 5 km north of the proposed extent. These locations have been chosen as they are upstream locations where the flows can be represented as a single channel. These locations are assigned as



‘headwaters’ in SIMCAT and flows are defined on the basis of the original Environment Agency TRB model (see Section 16.5)

2. The upstream portions of the Yeading Brook, which flows into the River Crane to the northeast of Heathrow, have been excluded from the model; with the start of the River Crane being the furthest upstream river to the east of Heathrow. As for the Colne and Colne Brook, the River Crane has been assigned as a ‘headwaters, and flows defined on the basis of the original Environment Agency TRB model

3. The Poyle Channel was not included in the original EA SIMCAT models. This channel has been included in these SIMCAT models, acting as a channel for flow from the Wraysbury to the Colne Brook

4. The lower reach of the River Crane has a bifurcation to the north, to the Lower Duke of Northumberland and Mogden STW – this bifurcation is not represented in the Environment Agency’s TRB model but has been included in these models.

Figure 21.1.3 shows the reach numbering as has been applied in the respective models. The reach numbers in the Baseline model have applied the same numbers as the Environment Agency’s TRB models where possible, but some reach numbers were changed in order to include the bifurcations at the Poyle Channel and Lower Duke of Northumberland.

The model river network extends to the rivers as far as the River Thames, but does not include the Thames itself. The purpose of the models is to identify risks to water quality and assess the potential changes to the receiving waters. Given the size of the Thames flow, for the contributing flows to result in degradation of the Thames relative to the baseline conditions the contributing flows would have to be considerably degraded in terms of quality. Such degradation of those rivers would not be acceptable within the DCO Project objectives, and therefore the Thames has not been assessed at this stage. The Thames may be included in the assessments at ES if there is a need to assess changes to water quality further downstream.

14.1 River network – PEIR Models

The river network within SIMCAT under the PEIR-scheme is shown in Figure 21.1.3, accounting for the re-alignment of the river reaches around the northwest of Heathrow.

For the Preferred and Alternative models, the reach numbering system in SIMCAT had to be adapted further from the original Environment Agency TRB numbering to account for the changes to the river network. Where practical the reach numbers were kept at their original values, but a number of changes had to be made in order to keep the numbering system sequential to allow the SIMCAT models to run.



The Preferred and Alternative models were also adjusted to include the new discharge locations under the respective configurations. The precise locations of the discharges are not known, and so the discharges were located at the downstream nearest points to the attenuation and treatment zones on the respective rivers. As will be discussed for the model outputs, the precise locations of the new drainage discharges does not cause a substantial problem for these models and will be evaluated further at ES once the discharge locations have been defined.

Bifurcations The river network includes a number of reaches that branch to form two separate rivers/reaches. These river bifurcations are defined in SIMCAT by treating each new branch as a new reach number and assigning a proportional split to each branch of the river.

The original Environment Agency TRB SIMCAT model included the majority of the bifurcations and these had proportions assigned to them. However, bifurcations for the Wraysbury to the Poyle Channel and for the River Crane to the Lower Duke of Northumberland were not represented in the original model.

Flow accretion surveys and hydrological modelling has been undertaken as part of the DCO Project studies (as summarised in Chapter 21: Water Environment), and has looked at the river network in detail, including defining the flow splits for river bifurcations. The SIMCAT model is using the same flow apportionment as the hydrological studies, although the SIMCAT model does not include the same resolution of channels.

Within SIMCAT, use of a single value to define the flow apportionment to each branch is an oversimplification, as it assumes that the proportional split remains constant for all flow rates, which is not the case (particularly at the lower reaches of the River Crane). However, there is typically insufficient data for the bifurcations to accurately define the flow split at different flow rates.

Discharges from the site

Baseline model Under the existing water management plans there are three airfield surface water catchments that discharge to the environment:

1. The Eastern Catchment which drains to the EBR which then discharges under gravity to the River Crane

2. The Southern Catchment, which drains to CLP, unless the water has high BOD when it is sent to Mayfield Farm for treatment prior to being discharged to CLP



3. The Western Catchment runoff drains southwards in the SWOT and is pumped out to the CLP, unless the water has high BOD, when it is sent to Spout Lane Lagoon for discharge to sewer.

Within SIMCAT, the discharge from the EBR to the River Crane is represented as a standard effluent discharge point. The representation of discharge flows from the airfield, into CLP and to Felthamhill Brook are not as straightforward:

1. The CLP is a lake/pond. SIMCAT is not intended to represent lakes / ponds

2. The CLP has some continuity with the surrounding gravels aquifer. This means that a portion of flow discharged into the CLP can recharge into the aquifer rather than be discharged via the CLP Outlet to the Feltham Relief Sewer. The rate of discharge from the CLP to the FRS is not known. Conversely, when groundwater levels are high, groundwater may discharge into CLP and then drain via the FRS

3. Water levels in the CLP are controlled by pump-assist gravity drainage to the Feltham Relief Sewer (FRS), which it is understood discharges to the Felthamhill Brook a short distance upstream of the confluence of Felthamhill Brook with Portlane Brook

4. The CLP is structured such that airfield runoff water entering the CLP is routed to have an extended residence time, having to migrate through several compartments of the CLP before it could arrive at the CLP pumphouse towards the southeast corner of the CLP. Dilution/mixing of the flows within the CLP, and the long flow path and extended residence time allows for attenuation of the flows to occur. This attenuation will smooth out short-lived peaks in flows and concentrations, and will allow the decay/degradation of certain solute species, such as degradation of BOD and oxidation of ammonia. This can be further enhanced as Heathrow operate several aerators within the CLP to increase dissolved oxygen levels. There is not sufficient data to characterise these various attenuation processes within the SIMCAT model

5. The CLP is not a WFD waterbody, and is not the appropriate receptor for assessment of water quality effects from the airfield discharge; Portlane Brook is the appropriate WFD water body for this assessment.

To represent the CLP within the SIMCAT the following approach has been taken and simplifying assumptions made:

1. The CLP has not been specified as a reach/waterbody represented within the SIMCAT models. The Feltham Relief Sewer is also not explicitly represented in the model



2. The CLP has been treated as the source of discharge flows, and assumes that the airfield discharges can be treated as a single discharge point to Felthamhill Brook

3. In the baseline models it will be assumed that all discharge flows from the airfield are discharged to Felthamhill Brook. This is a conservative assumption, as it ignores any losses of water from the CLP to groundwater, thereby overestimating the discharges to Felthamhill Brook

4. The water quality data do not allow attenuation rates between the discharge flows into the CLP and the discharge outlet from the CLP to be characterised. To represent the attenuation of solutes in the CLP, the baseline SIMCAT models will use the composition of the samples from the CLP Outlet

5. The influence of these assumptions on the discharge quality has been assessed using different scenarios to assess the sensitivity of the model outputs to those simplifying assumptions.

Groundwater ingress to surface discharges It is understood that the drainage network for each of the catchments can also capture and discharge groundwater. Anecdotal evidence describes that the surface drainage discharges to the EBR and the CLP continue to flow even during extended dry periods without rainfall, which has been assumed as being derived from groundwater entering the drainage network. Flow monitoring data for these discharges are not available, and so any groundwater contribution to the surface drainage discharges cannot currently be quantified within the models. Studies to assess the hydrology of CLP have been proposed subsequent to PEIR.

Monitoring of the site discharges would be valuable to assess groundwater contribution to surface water discharges.

SIMCAT flow modelling

Methodology River flows within the SIMCAT models are represented as follows:

1. Each Headwater is assigned an initial flow rate. River flow inputs are defined using Mean (arithmetic average) flows and the Q95 value (the 5th percentile low flow value which is exceeded in the river 95% of the time)

2. The model increases the river flow along the reaches by accreting ‘diffuse flows’. Diffuse flows represent a combination of catchment runoff and groundwater baseflow. SIMCAT does not differentiate between contributions from runoff as opposed to baseflow – they are effectively combined into a



single value. The diffuse flow inputs are defined as a rate per km of reach length, defined as a mean and Q95 value

3. Therefore, flow accumulates within the rivers, increasing downstream from diffuse flows, and from the joining of river reaches together

4. Discharges from point sources (e.g. sewage outfalls, water works, industrial effluent) are represented as point flows and defined either using values of mean flow and standard deviation. Alternately, discharges can be defined within ‘npd’ files (‘non-parametric distributions’). Discharges included within the baseline model include from the Iver and Mogden Sewage Treatment Works, and from the Kempton and Ashford Water Treatment Works.

Defining initial flows The modelled river network for these models has been reduced from the original Environment Agency TRB model, and this includes reducing the upstream extent of the River Colne and Colne Brook, and exclusion of the Yeading Brook upstream of the River Crane. Rather than include these additional reaches in the DCO Project models, the Environment Agency TRB model has been run and the flow results (mean and Q95 values) extracted at the new upstream headwater locations (for the River Colne, Colne Brook and River Crane). These extracted mean and Q95 values were then used to define the headwater flow values for those reaches.

The only other headwaters in the model are for Horton Brook (which has not been truncated), for which there is no other source of flow data or gauging data to calibrate to. Therefore, the input for SIMCAT for the Horton Brook reaches has been brought in as was, and not altered.

The accuracy of the flows in the respective rivers were then assessed against flow data from the Environment Agency’s flow gauging stations.

Flow gauging network In the vicinity of Heathrow there are several Environment Agency flow gauging locations, as shown in Figure 21.1.1. These same locations have been used for the evaluation of the flows within the baseline SIMCAT models.

Due to the complexity of the river network, with the number of confluences and bifurcations along its length, the flow gauging network is valuable for flows at the specific locations but does not allow for a detailed analysis of flows and river contributions. For example, the River Crane has two gauging locations, one upstream of Heathrow (FS Crane Cranford) another downstream (FS Crane Marsh Farm). Between these two gauging locations there are several key features: the EBR discharge; an inflow from the Upper Duke of Northumberland’s River; the bifurcation to the Lower Duke of Northumberland’s River. This configuration of one gauging station upstream and one downstream of multiple features



does not allow the contributions from different flows to be differentiated, such as the flow in the Upper Duke of Northumberland, the discharge flow from the EBR or the flow to the Lower Duke of Northumberland.

The situation to the west of Heathrow is similar, as the bifurcations along the Colne mean the flow gauges are not positioned to allow quantification of any of the individual reaches. Following the Colne upstream from Heathrow, the first flow gauge is encountered at Colne Denham and another at the Misbourne Denham location; both are over 8 km upstream from the Airport’s current northern perimeter, and there are no further downstream flow gauges on the Colne. Between those the gauges north of Heathrow and the Thames there are inflows from the River Pinn, bifurcations to the Colne Brook, Wraysbury River, Upper Duke of Northumberland, Longford River, River Ash, and also several confluences with the Wraysbury River.

There are several flow gauges that will allow comparison of the overall measured flows with the modelled flows at those specific points. But those flow stations cannot provide any further granularity on relative flow contributions to / from different branches, or for assessing potential causes of discrepancies between measured and modelled flow values. In addition, no flow gauging data has been available for Felthamhill Brook or Portlane Brook. The only flow data for these locations has been obtained from the Environment Agency’s SIMCAT models.

The Environment Agency’s SIMCAT model was developed/calibrated using flow data from the Environment Agency’s flow gauging network using data for the period 2010 to 2012. The flow records for the gauging locations around Heathrow are summarised in Table 21.1A.1 for gauging stations on the River Colne and Colne Brook, and in Table 21.1A.2 for the Yeading Brook and River Crane stations. Mean flow values are presented for several different periods: the entire flow record, 2010-2012, 2000-2016 and 2010-2016.

Mean flows in the tables show that 2010 to 2012 was a dry period relative to the other periods presented: the rivers to the west of Heathrow (Table 21.1.A.2) reported considerably lower mean flows, between 14 and 30% lower, for the 2010 to 2012 period in comparison to the entire flow record. In the rivers to the east of Heathrow), the smaller flows in the Yeading Brook show that the 2010 to 2012 period is comparable to the entire flow record. However, further downstream in the River Crane, the 2010 to 2012 period reports lower flows. The mean flows for the other periods presented (2000-2012, 2000-2016 and 2010 to 2016) show slightly higher flows when compared to the entire flow record.

For the purpose of the PEIR SIMCAT models, use of dry period flows is a conservative approach, as reduced flows allow for lower rates of dilution, and so should over-estimate potential effects from discharges.

Further data from fieldwork will become available as the Project proceeds to the ES and DCO studies, including additional flow gauges in the rivers to the west of Heathrow



(installed in 2018 as part of the DCO Project baseline investigations). If necessary for the ES, and can be supported by the additional field data, the flow data in the SIMCAT models can be adjusted and recalibrated to a different period.

Table 21.1A.2 Mean flows (Ml/d) in the Colne and Colne Brook catchment flow gauges

Period Colne at Denham

Misbourne Denham

Colne Brook Hythe End

Colne Staines Trading Estate

River Colne Misbourne Colne Brook Colne

First date 01/10/1952 01/07/1984 01/07/1991 01/04/1999

Last date 30/09/2016 30/09/2015 21/06/2017 21/06/2017

Entire record mean flow 357 23.2 178 152

2010-2012 mean flow 307 16.4 143 111

2000-2016 mean flow 396 26.7 192 153

2010-2016 mean flow 372 23.3 206 116

Proportion (%) of the 2010-2012 period relative to the entire flow record

86.0 70.6 80.4 73.3

Table 21.1A.3 Mean flows (Ml/d) in the Yeading Brook and River Crane catchment flow gauges

Period Yeading East Yeading West Crane at Cranford

Crane Marsh Farm

First date 01/10/1995 09/03/1994 03/04/1978 06/12/1977

Last date 30/09/2016 30/09/2016 30/09/2016 30/09/2015

Entire record mean flow 4.73 12.0 44.1 44.4

2010-2012 mean flow 4.81 11.6 36.7 31.0

2000-2016 mean flow 4.90 12.1 46.7 45.5

2010-2016 mean flow 4.67 13.7 47.2 37.3

Proportion (%) of the 2010-2012 period relative to the entire flow record

102 96.4 83.4 69.9



Airfield discharge flow data

Baseline model As described in Section 0, the Baseline SIMCAT models have two discharge points for airfield runoff into the environment:

1. The EBR discharge into the River Crane

2. The combined discharge of the Southern and Western Catchments to Felthamhill Brook (treating the CLP as a discharge point, rather than as a water body within the models).

Discharge flows from the Airport to the receiving waters are not metered, either from the EBR to the River Crane, or from the Western or Southern Catchments to the CLP. Nor is there flow monitoring of the discharge from the CLP to Felthamhill Brook (via Feltham Relief Sewer). Some flow monitoring has been conducted of high BOD flows around the Mayfield Farm treatment system during winter months, but not enough to characterise the discharge to CLP. Monitoring of the water levels in the CLP and of the inflows and discharges have been proposed subsequent to PEIR.

The Environment Agency’s TRB models do not include representative flow discharges from the Eastern Balancing Reservoir and the discharge from CLP is not represented.

For the purpose of the PEIR SIMCAT models, the airfield discharge flows have been generated using modelled data based on the available rainfall record. Runoff flows for the airfield catchments have been generated using the 4R model12. The 4R model is a tool for evaluating Rainfall, Runoff, Recharge and Routing, that was used for the groundwater modelling. The output from the 4R model was calculated as a runoff rate per unit area (m2) of the airfield, accounting for the proportion of hard-standing (low permeability, high runoff factor) and grass / vegetated areas (higher permeability and water retention, lower runoff factor).

The output from the 4R model calculates a daily rate of runoff based on the input rainfall time-series, which for the groundwater model ran from 1995 to 2016. Whilst 4R calculates a runoff rate, this output does not account for the lag/retention of runoff flows in the drainage system before being discharged, where the lag in to the drainage network would act to smooth peak flows by extending the duration of the ‘drain down’ from each rainfall event. To account for this lag, drainage retention calculations were conducted within Excel, and results exported for the required model period (2010-2012) to a .npd file for use by the SIMCAT model. The drainage lag calculations route runoff through a “store”, a fixed proportion of the water in which is released each day, thus generating a smoothed

12 Heathcote, J.A., Lewis, R.T. and Soley, R.W.N., 2004 Rainfall routing to runoff and recharge for regional groundwater resource models. QJEGH 37 113-130



discharge hydrograph relative to the runoff hydrograph. As there is no metered flow data for the discharge there is not a site-specific basis for using this relationship. The lag relationship used was based on professional judgement from similar projects / sites, but could be refined should there be monitoring of site discharge flows.

The areas of the drainage catchments used in the discharge calculations are shown in Table 21.1.A.4. Table 21.1A.4 also presents summary values for the discharge flows for each of the Baseline catchments output from the 4R model and drainage retention calculations.

Table 21.1A.4 Baseline airfield catchments, surface areas and summary discharge rates

Catchment Catchment area Discharges to

Summary of 4R simulated flows

Mean flow Standard deviation Q95

(ha) Ml/d Ml/d Ml/d

Eastern 427 EBR 6.7 6.3 0.5

Southern 314 Combined flow to CLP 9.5 8.9 0.7

Western 293

The discharge rates in Table 21.1A.4 are numerical predictions and cannot be verified as flow monitoring of the discharges is not undertaken. Flow monitoring of the discharges into and out of the CLP have been proposed subsequent to PEIR. The 4R model output presents the best available data, as discharge flow rates are not available from the site. Other Task Orders for the DCO Project, such as the Flood Risk Assessments, have also generated model predicted calculations of discharge. However, the output of flood risk assessments and drainage impact assessments is generally on high flow events, whereas SIMCAT models are concerned with the mean and low (Q95) flow conditions. Therefore, the drainage values used for the Flood risk assessments are not suitable for use in this study.

PEIR ‘with development’ models The expansion will increase the footprint of the airfield and will require the discharge of larger volumes of surface water. These discharge flows have been calculated following the same methodology as for the Baseline Model, with the flows re-calculated based on the anticipated catchment areas of the new catchments. This assumes that the proportion of hard-standing to vegetated/permeable areas remains the same. The footprints of the expanded airfield and the calculated discharge rates are presented in Table 21.1A.5 for the models for the Preferred Approach and Alternative Approach.



Table 21.1A5 PEIR model airfield catchments, surface areas and summary discharge rates

Approach Catchment Catchment area Discharges to

Summary of 4R simulated flows

Mean flow Standard deviation Q95

(ha) Ml/d Ml/d Ml/d

Preferred 3rd Runway 367 Colne and/or

Wraysbury 5.8 5.4 0.4

Northwest 63 Colne Brook 1.0 0.9 0.1

Alternative 3rd Runway 430 CRC Channels 6.7 6.3 0.5

Diffuse flows

Inflows to rivers from surface runoff and groundwater baseflow into the river channels is represented within SIMCAT by defining ‘diffuse flows’ for all reaches. The diffuse flows for the model reaches are assigned a per kilometre rate and water quality in the Environment Agency TRB model. The diffuse flow is then added to each reach as the model calculations progress down the reaches. The Baseline Model has used the rates as were set in the Environment Agency TRB models.

The CRC will pass through areas of landfill and so those reaches will be lined to hydraulically isolate them from the surrounding aquifer. Surface runoff to the CRC channels will also be very limited as they are covered and the runoff catchment will be small. Therefore, within the PEIR models, those reaches will be assigned a diffuse flow of zero to reflect that there will be no runoff and no baseflow entering the CRC channels.

In addition to the covered CRC channels, there will also be new channels that are not covered but that pass through areas of historic landfill. This includes sections of river upstream and downstream of the CRC and the diverted section of the Colne Brook. Those sections of new river channel will be lined to hydraulically isolate them from the surrounding aquifer. The proportion of contribution to diffuse flows from runoff relative to baseflow has not been characterised and is hard to evaluate in detail. In the absence of data, the diffuse flow rates for those lined reaches have been set at half of the diffuse flows from the Baseline Model. Because the lengths of the reaches that will be hydraulically isolated are short (<1 km), this will give rise to a limited reduction to the overall flows within these rivers.



RIVER WATER AND EFFLUENT DISCHARGE QUALITY DATA

Defining water quality inputs For the PEIR SIMCAT models, BOD data has been defined within SIMCAT in several ways:

1. For headwaters and diffuse flow, the concentrations are defined as mean values and standard deviations (typically assuming a log-normal distribution)

2. For effluent flows / discharges, the concentrations can also be defined as mean values and the standard deviation (typically assuming a normal or log-normal distribution)

3. Where data is available and does not follow a parametric normal or log-normal distribution, the BOD values can be entered as a ‘non-parametric distribution’ (npd) file. This involves placing all the results in a single file that is read by SIMCAT, with each Monte Carlo ‘shot’ using a separate value extracted from a point along the distribution.

Defining BOD discharge quality – Baseline Model BOD concentrations in the airfield runoff are strongly influenced by the use of de-icers during cold periods; BOD concentrations will be higher during cold winters, with the length of peak BOD concentrations determined by the length, frequency and intensity of cold periods during each winter. Dilution is also a factor in the concentration, where the volume of water that washes away the de-icer will determine the BOD concentration.

For the purpose of the numerical modelling, it has been assumed that the available monitoring data can be considered representative of typical conditions and the range of BOD concentrations that may occur in the discharge flows in future. There are several sources of data that were available for defining the BOD of the discharges from the site:

1. Heathrow routine monitoring data

2. Environment Agency monitoring data

3. DCO Project Baseline monitoring data

The numerical modelling for PEIR has mainly relied upon the Heathrow and Environment Agency monitoring datasets. The baseline monitoring data collected for the DCO Project includes monitoring locations at the EBR and at the CLP, however, less than a year’s worth of data were available at the time of preparation of the PEIR SIMCAT models / report, and so this data has not been used.




Heathrow’s routine monitoring of the Eastern Balancing Reservoir is summarised in Table 21.1A.6 for data from 2012 onwards. Locations include the inlet to the EBR (EBR Diversion Chamber) and discharge points from the EBR. Until 2017 the discharge was sampled at one outfall location (“EBR Outlet to River Crane”). From 2017/2018 adjustments were made to the outfalls, and the original location was replaced with several monitoring locations (EBR New Outlet, EBR Outlet North, EBR Outlet South). The Environment Agency’s monitoring data for the EBR (PCRR0039) is summarised in Table 21.1A.6. These locations are shown in Figure 21.1.1.

Comparison of the data for the EBR Diversion Chamber and the EBR Outlet to the River Crane indicates some attenuation occurs by reducing peak concentrations, however, the attenuation is limited, and concentrations are not significantly lower than at the inlet to the EBR.

A graph comparing the BOD sample population for the monitoring of the outfall from the EBR is presented in Annex B. The data compares Heathrow’s monitoring data (2012 to 2017) with the Environment Agency’s monitoring data (2001 to 2017). For each of the datasets the results have been ranked by concentration and presented as a cumulative population distribution. The background is colour coded in line with the WFD limits for BOD (for indicative purposes only, as the WFD limits do not apply to the CLP or site discharges). The graph shows that the population of the BOD results are broadly comparable, with a slight divergence at higher concentrations: the Environment Agency dataset covers a longer period and shows a larger proportion of higher concentrations. Around 70% of the samples would be classed as either being at High or Good WFD status, whilst around 10-20% of results show elevated concentrations that are within the Bad WFD status classification (note: the EBR is a discharge flow, not a WFD waterbody, and therefore the comparison with WFD class boundaries is for information only).

To be conservative, the EBR discharge is defined in the SIMCAT model using the Environment Agency’s monitoring data (PCRR0039 for 2001 to 2017).



Table 21.1A.6 Summary BOD values from Heathrow routine monitoring of the Eastern Balancing Reservoir

First date Last date No. of BOD results Minimum value (mg/l) Arithmetic mean (mg/l) Standard deviation 90th percentile Maximum

EBR Diversion Chamber

Jan 2012 Oct 2018 81 1.0 17.0 20 20 683

EBR Inlet to Lower pond

Jan 2012 Oct 2018 80 1.0 12.7 31 14 166

EBR Outlet to River Crane

Jan 2012 Dec 2017 68 1.0 13.0 40.3 9.39 260

EBR New Outlet Oct 2017 Dec 2017 3 1.6 2.10 0.70 2.68 2.90

EBR Outlet North

Jan 2018 Oct 2018 3 2.8 3.13 1.52 3.42 3.50

EBR Outlet South

Jan 2018 Oct 2018 10 1.0 11.5 29.7 15.7 96

Table 21.1A.7 Summary BOD values from Environment Agency monitoring locations around the site

EA location code

EA location name

First date

Last date

No. of BOD results

Minimum value (mg/l)

Arithmetic mean (mg/l)

Standard deviation

90th percentile

Maximum

PCRR0039 Heathrow Jan Dec 129 1 13.2 36.7 26.0 233



EA location code

EA location name

First date

Last date

No. of BOD results



Standard deviation

90th percentile

Maximum

Airport Eastern Lagoons at Outlet

2001 2017

PTHE0447

SWOT discharge to Clockhouse Lane Pit

Feb 2004 Sept

2018 88 1.2 10.2 31.5 11.8 235

PTHE0336

Heathrow Southern Balancing Pond, Middle Inlet: Bedfont

Nov 2001

Oct 2018 168 1 9.47 22.7 16.8 191

Table 21.1A.8 Summary BOD values from Heathrow monitoring of Clockhouse Lane Pit

First date Last date No. of BOD results



Standard deviation

90th percentile Maximum

CLP Southern Catchment Inlet

Jan 2012 Oct 2018 82 1 5.28 21.5 5.46 197

CLP Western Catchment Inlet

Jan 2012 Oct 2018 80 1 5.77 15.6 7.17 124



First date Last date No. of BOD results



Standard deviation

90th percentile Maximum

CLP Inside Weir Notch Jan 2012 Oct 2018 82 1 7.73 27.5 7.99 182

CLP Northern Peninsular

Jan 2012 Oct 2018 82 1 6.16 22.0 6.60 188

CLP Mid-point Oct 2017 Oct 2018 13 1 3.05 2.41 6.60 8.90

CLP Outlet Jan 2012 Oct 2018 77 1 2.72 1.87 5.00 9.80

Table 21.1A.9 Summary BOD values from Environment Agency monitoring locations around Heathrow

EA location code

EA location name First date Last

date

No. of BOD

results



Standard deviation

90th percentile

Maximum

PCNR0025 Colne Above Thames Jan 2000 Sept

2014 173 1 1.89 1.14 3.04 9.40

PCNR0100 Wraysbury River Above Colne Jan 2000 Dec

2006 78 1 1.66 0.77 2.66 4.40

PCNR0063 Horton Brook Jan 2000 Nov 2007 92 1 2.12 1.59 4.29 11.2

PCNR0039 Colne Brook Jan 2000 Nov 2007 93 1 1.69 0.73 2.70 4.10

PCRR0030 Duke of Northumberlands

Jan 2000 Jul 2004 55 1 2.12 1.13 3.00 8.40



EA location code

EA location name First date Last

date

No. of BOD

results



Standard deviation

90th percentile

Maximum

River at River G

PTHR0265 Longford River at High Street, Hampton

Jan 2000 Mar 2010 122 1 1.79 0.70 2.80 4.40

PTHR0004 Ash above Ashford Common Waterworks

Jan 2000 Jun 2007 91 1 1.65 0.75 2.80 4.20

PTHR0005 Ash Above Thames

Jan 2000 Jun 2007 91 1 1.60 0.72 2.54 4.30

PTHR0054 Portlane Brook Jan 2000 Jun 2007 68 1 1.60 0.72 2.41 4.10

PCRR0084 Yeading Brook at North Hydes Road

Jan 2000 Sept 2018 161 1 2.66 2.16 4.70 17.4

PCRR0111 Crane above Eastern Balancing Reservoir

Sept 2013 Sept 2018 56 1 2.45 2.77 3.97 17.7

PCRR0014 Crane Below Eastern Balancing Reservoir

Nov 2001 Sept 2018 110 1 4.20 9.15 7.12 95.6

PCRR0020 Crane above Duke of Northumberlands

Jan 2000 Dec 2017 144 1 3.33 2.68 6.35 14.7

PCRR0006 Crane at Northcotes Road

Jan 2000 Dec 2007 96 1 2.55 1.24 3.76 8.3



Clockhouse Lane Pit Heathrow’s routine monitoring of CLP is summarised Table 21.1A.8 for data from 2012 onwards, with several monitoring locations around the CLP. The Environment Agency’s monitoring of the CLP is summarised in Table 21.1A.6 for data from 2001 onwards. For the CLP the Environment Agency have monitored the outlet from the SWOT (PTHE0477, for the Western Catchment discharge), and the discharge from the Southern Catchment (PTHE0336).

Graphs comparing the BOD sample population for the monitoring of the site discharges to the CLP is presented in Annex B. The graph includes the Heathrow monitoring of discharges from the Southern and Western catchments, the CLP Outlet location, and the Environment Agency monitoring of the PTHE0336 and PTHE0477. The graphs present 2012 to 2018 data so that the datasets are comparable.

The graph comparing BOD concentrations for the Western Catchment discharge to the CLP shows that the Heathrow and Environment Agency data are broadly comparable; the Environment Agency data from the SWOT are higher than the Heathrow data from the Western Catchment. At lower concentrations this is because the Environment Agency data has a higher detection limit (around 3 mg/l) whereas the Heathrow monitoring had a lower detection limit (1 mg/l). At higher concentrations (approximately above 4 mg/l) the Environment Agency data reports higher concentrations than the Heathrow data.

The graph comparing BOD concentrations for the Southern Catchment discharge to the CLP shows larger differences between the Heathrow and Environment Agency data than for the Western Catchment discharge. The differences become more apparent at higher concentrations, with the Environment Agency monitoring reporting higher concentrations than the Heathrow monitoring data. The reason for this difference is not apparent from the available data.

In both sets of graphs for the CLP data, BOD concentrations at the CLP Outlet are lower than concentrations from the discharge points, indicating that BOD is attenuated within the CLP. This could happen through a combination of degradation and dilution processes.

Within the baseline SIMCAT model it would not be representative to assign the BOD concentration of the discharges into the CLP to the flow that exits the CLP to Felthamhill Brook. Instead, as the monitoring data indicates attenuation of BOD occurs in the CLP, the discharge BOD concentrations will be represented in SIMCAT by the CLP Outlet monitoring data. This data was set in the model based upon the mean and standard deviation, and then a separate model run with the data input as a non-parametric distribution (.npd file).



Defining BOD of discharges for the PEIR Models

A combination of on and off-stand de-icing operations will continue to be operated to achieve safe aircraft operations. The majority of aircraft de-icing is currently undertaken on stand, with four supplementary remote aircraft de-icing pads located on the airfield. These remote pads are generally used during the more extreme weather events and during weather events that require ‘second wave’ aircraft to be de-iced. (‘Second wave’ aircraft de-icing occurs when the temperature remains sufficiently low that aircraft, other than the first ones departing in the morning, require de-icing; it is these aircraft that have a more constrained time slot on stand, and hence remote pads are sometimes utilised to release required stand capacity).

Through the DCO Project, all new aircraft stands will be equipped to safely facilitate aircraft de-icing, with the necessary pollution control and drainage safeguards. Additional off-stand aircraft de-icing will be required, not only to accommodate ‘second wave’ aircraft de-icing, but also due to more modern composite construction aircraft typically having a shorter ‘hold-over’ time (the maximum allowable time between an aircraft de-icing and an aircraft taking-off).

The ‘with development’ models require that estimates are made of the discharge quality from the new airfield catchments. BOD concentrations will be a function of cold weather events and the rainfall/runoff that mobilises the de-icer; this cannot be predicted. For the purpose of modelling predictions, BOD in the airfield runoff has been parameterised based upon the existing monitoring data as described in the following sub-sections. The overall assumption is that the BOD loading in discharges for the existing and proposed airport catchments have a similar range and occurrence to the available monitoring data for comparable discharge locations.

The airfield plans incorporate methods of capturing some of the de-icing fluids through the use of designated de-icing pads, or de-icing at the stands where the de-icing fluids are captured for re-use or treatment. These potential measures could give rise to reduced loading of BOD in discharge flows, but have not been incorporated in the assumptions at this stage as the potential reduction in BOD that may result has not been quantified.

Eastern Balancing Reservoir The Eastern Catchment area will not be substantially altered as part of the DCO Project. Therefore, the EBR discharge flows and quality in the ‘with development’ models have not been altered. This omits any reduction in BOD that would be expected from the planned construction of a MBBR treatment plant at the EBR separate from the DCO Project. The treatment plant is expected to reduce BOD loading to the River Crane, however, there is not an estimate of how great this reduction could be. The ‘with development’ models will therefore reflect the current/baseline discharge conditions, whereas the real situation would expect a reduction in BOD due to treatment by the time of the DCO Project development.



Clockhouse Lane Pit

The Baseline Model treats the discharge from the Southern and Western Catchments as a single discharge from the CLP to Felthamhill Brook and applies the BOD concentration of the CLP Outlet. This reflects the anticipated treatment of airfield runoff to reduce peak BOD concentrations, and the attenuation of BOD as the site discharge enters and migrates through the CLP.

The discharge rate to the CLP is expected to increase as a result of the DCO Project. This could reduce the residence time for discharged water in the CLP, and therefore could reduce the potential for attenuation. To assess the effects of reduced attenuation in the CLP different model scenario runs have been applied, discussed in Section 0.

New airfield catchment discharges

The discharges from the new airfield catchments will be managed in the same way as the existing flows for the Southern Catchment: when BOD levels are below the permit limit the water will be discharged directly, when BOD levels are above the permit limit they will be sent to treatment until BOD concentrations are suitable for discharge. Therefore, for the ‘with development’ models, the discharge flows will be assumed to have the same concentration range as the Southern Catchment discharge flows.

Defining BOD quality of rivers Within the SIMCAT models the BOD solute concentrations are calculated based upon the headwater concentrations, and then additional contributions from diffuse loading, plus effluent discharges. Therefore, BOD concentrations at monitoring locations do not need to be defined as inputs, but instead are used as comparison points to assess how closely the models are replicating the measured data. It is therefore necessary to ensure that the most appropriate BOD data are used for evaluating the model output.

There are several sources of data that could potentially be used for defining the BOD of the rivers around Heathrow:

1. Environment Agency monitoring data

2. Environment Agency Thames River Basin SIMCAT models

3. Heathrow routine monitoring data

4. DCO Project baseline monitoring data

Heathrow routine monitoring of water quality includes locations on the River Crane upstream and downstream of the EBR discharge, but does not include locations on other rivers or reaches around the airport, and so has not been used for defining inputs for SIMCAT.



The baseline monitoring data collected for the DCO Project includes a wider network of monitoring locations along the rivers of interest. However, less than a year’s worth of data were available at the time of preparation of the PEIR SIMCAT models/report, and so this data has not been used.

The Environment Agency monitoring data has a wide distribution of locations, as shown in Figure 21.1.1, and extends over a number of years, and is therefore the most appropriate dataset for developing and evaluating the SIMCAT models. The Environment Agency’s BOD monitoring data for locations for around Heathrow are summarised in Table 21.1A.7Error! Reference source not found., and whilst this data provides the most comprehensive dataset available, there are limitations to the Environment Agency’s BOD data:

1. For many of the Environment Agency monitoring locations on those rivers to the west and south of Heathrow BOD analysis stopped before 2008. The exception is the Colne Above Thames location (PCNR0025), which continued BOD analysis until 2014

2. BOD analysis of the River Crane and Yeading Brook monitoring locations is more complete, having continued on to present day, with the exception of the downstream location on the River Crane (PCRR0006) which only has BOD data until 2007.

The summary data in Table 21.1A.6 shows the mean BOD concentrations are typically in the range of 1.6 to 2.1mg/l for the rivers to the west and south of Heathrow, and would be classed as at Good or High status based on the 90th percentile BOD concentrations. Mean BOD concentrations are higher in the rivers to the east of Heathrow; BOD concentrations upstream of the EBR are 2.66mg/l (PCRR0084) and 2.45mg/l (PCRR0111). Immediately downstream of the EBR the mean concentrations are higher at 4.20mg/l (PCRR0014) and then decrease downstream; 3.33mg/l at PCRR0020 and 2.55mg/l at PCRR0006.

A longer period of water quality data increases the likelihood that the BOD concentrations will be representative of the long-term conditions. The models are using flow data from 2010 to 2012 for the input and calibration / evaluation, a dry period and so is a conservative approach. It could be preferable that water quality input values would be derived for the same period as the flow data. However, this has not been possible because BOD data for the 2010 to 2012 period is not available for all locations.

For the purpose of assessing the SIMCAT model BOD output, the comparison will use the mean and 90th percentile values as presented in Table 21.1A.7. This will mean using data that covers different periods for different locations, and assumes that the baseline BOD levels in the rivers have not changed significantly since monitoring for those locations, even where monitoring stopped before 2010. There is no reason to believe that the baseline BOD concentrations have changed, and this can be assessed further once the



complete DCO Project baseline monitoring data is available. The final approach to modelling at ES will be discussed and agreed with the Environment Agency.

Conservative and non-conservative behaviour of solutes SIMCAT has several options for modelling chemical behaviour in the simulated rivers to allow for different chemical processes that may occur. These include conservative behaviour, decay of parameters, and re-aeration.

Conservative behaviour assumes that the solute behaviour is mass conservative; that other than from the defined inputs, that no further sources of input can occur, and that there are no losses of the solutes e.g. by biological decay, degassing to the atmosphere, sorption onto solid matter and deposition, or chemical conversion to other species.

A number of solutes are known to decay or be lost from river water by natural processes. This includes BOD which is consumed by biological activity. Other examples include ammoniacal nitrogen, which can be lost by degassing to the atmosphere, or can convert (by oxidation) to nitrate, or can be assimilated into plants. Another solute that may not behave conservatively is orthophosphate, which can be taken up by plants as a nutrient.

In addition, re-aeration can be applied within SIMCAT, which allows dissolved oxygen levels to increase in the rivers, to represent the presence of photosynthetic plants, or aeration of the rivers by turbulent mixing and/or passive diffusion into the river surface. These models have not included dissolved oxygen and therefore this process is not considered further.

SIMCAT models are commonly used for BOD assessment, and the decay rates for BOD are incorporated within the model and can be ‘switched on’ to allow them to be applied. Specific BOD degradation values can be applied in SIMCAT, although definition of those rates would require detailed site or reach specific information that is not currently available, and so have not been applied with the models; the models apply the standard BOD decay rate within the models, as do the original Environment Agency TRB models.

The decay / reduction of BOD is a rate function and is therefore dependent upon the length of time taken for water to pass through each reach. The flow velocity of each reach has been defined within the Environment Agency TRB models. Those flow velocities have been used as the basis for the models, but have been revised as part of the model verification/calibration process.



BASELINE SIMCAT MODEL RESULTS

Flow output This section discusses the flows as represented within the SIMCAT model results for the purpose of understanding the influences and mixing with respect to water quality, presenting a more simplified version of the flow regime and does not account for flood events or similar. Chapter 21: Water Environment should be referred to for an assessment of river flows.

The flow data from the Baseline model is presented for each of the modelled rivers in graphs in Annex C. Graphs are presented for each river for both the Mean flow and the Q95 flow. Flow data output from the model is presented at 1km intervals along the rivers, discharges, and when the reach number changes, such as for bifurcating or converging flows.

The modelled flows are summarised for the key rivers as follows:

1. River Colne: at the upstream start of the model for the River Colne, the mean flow is around 250Ml/d, reducing to 140Ml/d as the Colne bifurcates to the Wraysbury River. The Colne mean flow decreases to around 110Ml/d as the river bifurcates to the Upper Duke of Northumberland’s River, and then to around 100Ml/d after the Longford River bifurcation. Further downstream the model inputs indicate the Colne in gaining from diffuse flows, before reducing by flow reduces by around 2Ml/d from flow to the River Ash. The flow then increases to around 150Ml/d as the Wraysbury River joins the River Colne

2. Wraysbury River: The head of the Wraysbury is formed from the bifurcation from the River Colne, and the mean flow is around 115Ml/d. The mean flow in the Wraysbury reduces to around 3 Ml/d as the majority of the flow flows to the Poyle Channel. The river then gains a small proportion of flow until it reaches the confluence with the River Colne

3. Colne Brook: the mean flow is around 100Ml/d which then increases to around 185Ml/d as flow from the Poyle Channel (sourced from the Wraysbury River) enters the Colne Brook. The mean flow in the Colne Brook then increases by around 2Ml/d as the Colne Brook is joined by the smaller Horton Brook flow

4. River Crane: the mean flow at the head of the Crane (from the Yeading Brook) is around 39Ml/d. The model predicts applies around 7Ml/d of runoff discharge from the Eastern Catchment. The mean flow in the Crane is then predicted to increase by a further 30Ml/d to around 80Ml/d due to the confluence with the Upper Duke of Northumberland. Further downstream the mean flow in the Crane is then reduced to around 32Ml/d as flow bifurcates to the Lower Duke of Northumberland’s River



5. Felthamhill Brook and Portlane Brook: the mean flow in Felthamhill Brook is very small, less than 0.4Ml/d until it receives the discharge from CLP. The Baseline Model assumes the entire runoff flow from the Western and Southern Catchments discharges to CLP and then to Felthamhill Brook, which causes the mean flow to increase to just under 10Ml/d. The flow from Felthamhill Brook then joins with Portlane Brook to produce a combined mean flow of around 16Ml/d. It should be noted that the Portlane Brook catchment is relatively small and the majority of the flow contribution in Portlane Brook (mean flow around 6Ml/d) upstream of the confluence with Felthamhill Brook comes from the Kempton Waterworks discharge (mean discharge 5.6Ml/d).

Flows in the remaining rivers are relatively straightforward as there are not substantial diversions, bifurcations or other flow contributions other than from diffuse flows.

Comparison with flow gauging data The graphs of the SIMCAT Baseline Model flow output in Annex C also show the flow gauging Mean and Q95 values for the four gauging locations within the study area.

Mean and Q95 values output from the models have been compared against the flow gauging data, as shown in Table 21.1A.9. Table 21.1A.9 also presents the Relative Percentage Difference (RPD) between the modelled values and the gauging data, where the RPD is calculated as follows:

𝑅𝑅𝑅𝑅𝑅𝑅 =|(𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑓𝑓𝑓𝑓𝑀𝑀𝑓𝑓 −𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑓𝑓 𝑓𝑓𝑓𝑓𝑀𝑀𝑓𝑓)|(𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑓𝑓𝑓𝑓𝑀𝑀𝑓𝑓 + 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑓𝑓 𝑓𝑓𝑓𝑓𝑀𝑀𝑓𝑓)/2

𝑥𝑥 100

Table 21.1A.10 Baseline SIMCAT Model flow output comparison – modelled and measured

SIMCAT Reach

No.

Environment Agency flow gauging station

Mean flow data comparison Q95 flow data comparison

Monitored flow data

(Ml/d)

Modelled flow output

(Ml/d)

RPD (%)

Monitored flow data

(Ml/d)

Modelled flow output

(Ml/d) RPD (%)

804 FS Wraysbury River 43.2 35.4 19.8 19.0 15.8 18.5

795 FS Colne Brook Hythe 148 187 23.3 61.3 85.2 32.5

970 FS Crane Cranford 37.2 39.9 7.07 6.05 6.80 11.7

985 FS Crane Marsh Farm 30.2 32.7 7.89 0.86 9.7 167

The model results show relatively good agreement to the gauging data for the mean flows on the River Crane (within 10%). The proportional differences on the Wraysbury River and



on the Colne Brook are slightly higher at around 20%, and may be considered to be high. It would be possible to calibrate the model to those observed flows, however, due to the complexity of the model, those differences could be attributable to several parameters:

1. River flow entering the Colne or Colne Brook from upstream

2. The different proportional flows diverging at river bifurcations

3. Contributions from baseflow.

The first two items above could potentially explain the entirety of the differences. For instance, reduce the inflow from the upstream portion of the Colne Brook and the comparison with the Colne Brook Hythe gauging station would improve. Alternately, a lower proportion of flow from the Wraysbury River to the Colne Brook via the Poyle Channel would improve the RPD for the Wraysbury River and the Colne Brook gauging stations. SIMCAT’s gap-filling process could be used to resolve the differences. This could change a number of parameters to achieve a better fit, but there would not be a reasoned basis for the changes made. The model input values have not been adjusted to improve the fit of the flow data for the Wraysbury River or Colne Brook as there is no data to support which changes to make. Overall, the uncertainty in model flows for these reaches do not make a substantial difference to the predicted concentrations under the modelled ‘with development’ scenarios. This is because the flows in the Wraysbury River and Colne Brook are large relative to the potential discharge flows.

The agreement for the Q95 values are also broadly acceptable for the model’s purpose. The largest discrepancy in the flow comparison is for the Q95 values at Crane Marsh Farm, where the Q95 of the gauging data is 0.86Ml/d compared with 9.7Ml/d for the model output. This discrepancy is largely due to the nature of the bifurcation upstream of the flow gauging point; the bifurcation is a controlled weir that diverts the majority of the flow to the Lower Duke of Northumberland’s River under low flow conditions, and as upstream flows increase in the River Crane, more flow is allowed into the lower reach of the River Crane. This is a limitation of the model which represents divergent flows by single values, not by proportional values that change with flow rate. However, the lower reach of the River Crane does not have a substantial influence on the concentrations in the model.

The available gauging data identify some differences between the observed and modelled data. However, no assessment is possible for other rivers, such as the Colne, Upper Duke of Northumberland, or Longford Rivers.

Overall, the purpose of these models is for the assessment of water quality and solute loading. Whilst ensuring the modelled flows are reasonable is important, it is considered that the accuracy of the flows in the models is sufficient at PEIR level for the purpose of assessing BOD concentrations, and that attempting to calibrate the model to a limited number of flow gauges without further information would not be justified at this stage.



Water quality predictions

Baseline model BOD runs Having developed the flow model and determined that the predicted flows were reasonable for the purpose of the model, the next stage was to assess the BOD predictions, including comparing the mean and 90th percentile values of the calculated model outputs against the Environment Agency’s monitoring data.

The Environment Agency water quality monitoring network has more locations (13) around the Heathrow area compared to the flow gauging network, with monitoring locations on the majority of the rivers in the model area (as shown in Figure 21.1.1). This larger number of monitoring locations allows more comparison to be undertaken between the observed and modelled values, and therefore model parameters can be calibrated to achieve a better fit to the data.

To complete the calibration of the baseline model, several different model versions were run to investigate potential parameters that could improve the fit of the calculated values. The different model runs and their purposes are summarised in Table 21.1A.11.

Table 21.1A.11 Baseline Model runs

Baseline model version no. Purpose of model and parameters varied

v01

Development of reduced model developed from the Environment Agency TRB model. Included bifurcations for the Poyle Channel and Lower Duke of Northumberland. Upstream flows for River Colne, Colne Brook and River Crane defined based on the Environment Agency TRB flow results. Discharge flows from the EBR and CLP defined using .npd files based on 4R output. BOD concentrations in EBR and CLP discharge defined using mean and standard deviation values for CLP Outlet.

v02 River Crane headwater inflow adjusted to improve fit at location PCRR0084. River velocity parameters set to default values to improve fit for River Ash, Longford River and Portlane Brook.

v03 As per v02, but EBR input concentrations altered to achieve a better fit to observed data.

v04 As per v02 except the EBR input concentrations defined using .npd files.

v05 As per v03 but with CLP discharge set to zero.

v06 As per v03 but with CLP discharge set to half the 4R rate (as mean and standard deviation).

v07 As per v03 but with CLP discharge set to half the 4R rate (as .npd file).



BOD output comparison

The BOD model outputs from the Baseline v01 version are presented in Annex D. The best fit for the Baseline model was achieved in Baseline v03. Graphs of the output from that model are presented in Annex E.

In Annex D and E, graphs are presented for each of the modelled rivers for both the Mean and 90th percentile values. BOD concentrations are presented at 1 km intervals along the rivers and when the reach number changes, or where there are key features such as bifurcating flows, converging flows or discharges.

The graphs of the 90th percentile values are coloured as per the criteria for WFD status for BOD:

1. High (BOD up to 4mg/l) is coloured blue

2. Good (BOD between 4 and 5mg/l) coloured green

3. Moderate (BOD between 5 and 6.5mg/l) coloured yellow

4. Poor (BOD between 6.5 and 9mg/l) coloured orange

5. Bad (BOD greater than 9mg/l) coloured red

Mean and 90th percentile values output from the Baseline v03 model are compared against the BOD monitoring values in Table 21.1A.12. The model outputs for the different rivers and discharges are discussed in the following sub-sections.



Table 21.1A.12 Baseline v03 SIMCAT Model BOD output comparison – modelled and measured

EA location

code EA location name

Mean BOD data comparison 90th percentile BOD data comparison

Monitored BOD (mg/l)

Modelled BOD

output (mg/l)

RPD (%)

Monitored BOD (mg/l)

Modelled BOD

output (mg/l)

RPD (%)

PCNR0025 Colne Above Thames 1.89 1.90 0.44 3.05 3.09 1.3

PCNR0100 Wraysbury River Above Colne 1.66 1.85 11 2.66 3.08 15

PCNR0063 Horton Brook 2.12 2.69 24 4.29 4.23 1.4

PCNR0039 Colne Brook 1.69 1.80 6.3 2.70 2.80 3.5

PCRR0030 Duke of Northumberlands River 2.12 1.79 17 3.00 3.01 0.2

PTHR0265 Longford River at High Street, Hampton 1.79 1.58 13 2.80 2.90 3.7

PTHR0004 Ash above Ashford Common Waterworks 1.60 1.99 22 2.80 3.03 7.9

PTHR0005 Ash Above Thames 1.60 1.47 8.4 2.54 1.86 31

PTHR0054 Portlane Brook 1.60 2.02 24 2.41 3.08 24

PCRR0084 Yeading Brook at North Hydes Road 2.66 2.64 0.9 4.70 4.72 0.5

PCRR0111 Crane above Eastern Balancing Reservoir 2.45 2.49 1.6 3.97 4.38 9.8

PCRR0014 Crane Below Eastern Balancing Reservoir 4.20 4.52 7.3 7.12 7.01 1.6

PCRR0020 Crane above Duke of Northumberlands 3.33 4.45 29 6.35 6.89 8.2

PCRR0006 Crane at Northcotes Road 2.55 3.01 17 3.76 4.61 20



River Colne, Wraysbury River and Colne Brook

The predicted BOD concentrations for the River Colne and Wraysbury River are relatively consistent along the river reaches, and show good agreement with the monitoring data. For the baseline model there are no discharges into the River Colne or Wraysbury River that could substantially alter BOD, and the relatively stable concentrations in the river reflect the additional loading from diffuse flows being offset by BOD decay processes.

The model output for the Colne Brook was different, in that the Baseline v01 model output predicted a significant decrease in BOD along the reach (see Annex D) which then recovered by inflow from the Wraysbury River via the Poyle Channel. Predicted concentrations in the Cone Brook (PCNR0039) were considerably lower than the observed data. Similar rapid reductions in BOD concentrations were predicted for Felthamhill Brook, Longford River and the River Ash.

These rapid reductions arose because of the river velocity defined for those rivers within the Baseline v01 models, based on the values in the original Environment Agency TRB model. The low velocity parameters allowed degradation of BOD to dominate the BOD calculation. This was considered unrealistic based on the observed data, and so the rate of BOD decay was reduced by increasing the river flow velocity. In the absence of measured values, the flow velocity was assigned the default SIMCAT values (30km/day). As shown in Annex C, increasing the flow velocity reduced the BOD decay factor and improved the fit of the data to the observed data. This approach is conservative as it reduces the decay rate, and hence increases the predicted concentrations.

River Crane and the EBR discharge The River Crane has a complex BOD profile along the River and also has the most Environment Agency monitoring locations, as shown in the graphs in Annex D and E. The Baseline v01 model over-predicted the 90th percentile BOD concentration entering the Crane from the Yeading Brook, and this upstream inflow was reduced to improve the calibration at PCRR0084. This adjustment also improved the agreement of concentration at PCRR0014, located immediately upstream of the EBR discharge.

The effect of the EBR discharge on BOD in the River Crane is most strongly registered at PCRR0111 located immediately downstream of the EBR, with BOD concentrations increasing into the ‘Poor’ WFD status. Upstream of the EBR at PCRR0014 mean BOD concentrations were 2.5mg/l and 90th percentile concentrations were 4mg/l. Downstream of the EBR at PCRR0111 mean concentrations were 4.2mg/l and 90th percentile concentrations were 7.1mg/l, with increases resulting from the modelled EBR discharge concentrations.

The Baseline v01 model overpredicted the BOD concentration downstream of the EBR at PCRR0111. To improve the calibration at PCRR0111 it was necessary to reduce the BOD loading from the EBR discharge. In the Baseline v01 model the EBR discharge was



assigned a mean concentration of 13mg/l with a standard deviation of 40.3mg/l, based on Environment Agency monitoring data from 2000-2018. The Baseline v03 model applied a mean concentration of 12.5mg/l with a standard deviation of 40.3mg/l and achieved a closer fit. This indicates that the EBR discharge values provided a relatively close fit without needing substantial changes.

Location PCRR0020 is downstream of the EBR and immediately upstream of the confluence with the Upper Duke of Northumberland River. At this location the model overpredicts mean and 90th percentile BOD at PCRR0020, with a larger overestimation for the mean value (an RPD of 29%).

After the confluence with the Upper Duke of Northumberland the BOD concentration decreases sharply, with the Upper Duke of Northumberland contributing mean flow of 32Ml/d whilst the mean upstream Crane is estimated at 49Ml/d, including around 7Ml/d from the EBR. The dilution in the model reduces the predicted BOD concentrations to within the Good WFD status range.

Further downstream at PCRR0006 the monitoring data indicates that the WFD status for BOD would be high (<4mg/l), and that the model overpredicts the mean and 90th percentile values. This indicates that BOD dilution or attenuation is greater in the River Crane than the model predicted, or that BOD loading from diffuse flows or the Upper Duke of Northumberland are lower than defined in the models. It is not possible to discern which process is causing this discrepancy.

Overall, the Baseline v03 model shows relatively good agreement with the observed data; in the model, the upper reaches of the River Crane are at Good status with respect to BOD, the EBR discharge then increases BOD concentrations to ‘Poor’ status, and that the increase in BOD occurs for less than 1 km, until the confluence with the Upper Duke of Northumberland River. BOD concentrations in the River Crane reduce and then remain within the Good status range for the remainder of the river reach.

Felthamhill Brook, Portlane Brook and the CLP discharge

The Felthamhill Brook flow is small (mean flow of <0.2Ml/d) until the discharge from the CLP enters the brook, contributing a mean flow of over 9Ml/d. However, this high discharge rate from the CLP is based on several of the key assumptions for the model:

1. That all of the airfield runoff generated in the Southern and Western catchments is discharged to Felthamhill Brook. In reality this is not the case, as a portion of CLP water likely recharges to the surrounding aquifer. However, the rate of losses to the aquifer are not known

2. That the Feltham Relief Sewer discharges to the Felthamhill Brook less than 200m from the confluence with Portlane Brook. The precise location of the FRS discharge has still to be confirmed.



The BOD concentrations in the Baseline v01 model predicted sharp reductions in BOD in Felthamhill Brook due to the decay of BOD due to the slow river velocity. This effect was reduced by increasing the river velocity, and this improved the match of modelled to observed data.

The effect of the CLP discharge into Felthamhill Brook is to increase the BOD concentrations from the high WFD range into the Good range. Note that the CLP discharge BOD concentrations are defined using the CLP Outlet concentration, and therefore takes into account the on-site treatment and the in-situ attenuation of BOD within the CLP.

BOD concentrations in Felthamhill Brook subsequently reduce again after the confluence with Portlane Brook. This is because mean flow in Portlane Brook is modelled to be around 6 Ml/d, the majority of which is due to discharge from Kempton Water Works.

The modelled mean and 90th percentile BOD concentrations for Portlane Brook are higher than the observed data. The largest source of BOD loading into Felthamhill Brook and Portlane Brook is from the CLP, and therefore, uncertainty in the model output is likely to be attributable to uncertainties in the parameterisation of either the flows or BOD concentrations in the CLP discharge.

Baseline model v07 assessed this by only discharging half of the flow from the CLP to Felthamhill Brook. Graphs of this output are shown in Annex F. The results show better agreement for both the mean and 90th percentile concentrations. A lower rate of discharge from the CLP to Felthamhill Brook could explain the differences between the model and observed data. However, this cannot be confirmed with the available data or through modelling alone.



SIMCAT ASSESSMENT OF BOD FOR DCO PROJECT DISCHARGES

‘With development’ SIMCAT models The Baseline model has been adapted to undertake the assessment of the ‘with-development’ conditions. Two versions of the SIMCAT models have been developed for evaluation of the ‘with development’ conditions. Both models have the same river network, incorporating the planned development changes to the river system:

1. Diversion of the Colne Brook around the northwestern perimeter of the expanded airfield

2. Reducing the number of river channels to represent the two channels of the CRC, plus diversions of the rivers entering and exiting the CRC

3. The increase to runoff discharge to CLP as all runoff from the Southern and Western catchments are expected to be sent southwards to CLP

4. Adjustment to the models to account for reductions in diffuse flow due to the new river channels being lined and/or covered.

The difference between the models are the locations of the runoff discharge points, with two attenuation and treatment areas for the Preferred Approach, and a single attenuation and treatment area for the Alternative Approach.

The river network has been modelled for the operational period, post-development once all the rivers have been re-aligned. The effects of changes to the river network during construction have not been assessed at this stage.

For reaches that are lined and covered the diffuse flow is set to zero. For reaches that are lined but not covered, the diffuse flows are reduced by 50%. This is a simplifying assumption, as there is no data to support an assessment of the relative contribution from baseflow against surface water runoff. Within the models this assumption does not make a substantial difference, as the reaches affected are relatively short in comparison to the overall river lengths, and the total flows in the reaches are much larger than the diffuse flow inputs over those reach lengths.

Preferred Approach For the Preferred Approach runoff from the new airport catchments are managed at two additional attenuation and treatment areas: the northwest catchment for the airfield area to the west of the M25 will drain to the west towards a dedicated facility that will discharge to the Colne Brook. The remainder of the new airfield catchment will drain to the southwest to a location south of the new runway between the River Colne and Wraysbury River, shown in Figure 21.1.3.



The precise locations of the discharges are still to be confirmed by Heathrow. For the Northwest catchment all flow will be sent to the Colne Brook. For the Southwest attenuation and treatment area, flow could potentially be discharged into either the River Colne, or the River Wraysbury, or a combination of both. The River Colne is the larger of the two rivers, and also situated closest to the proposed treatment plant. However, it may be necessary to balance the distribution of runoff by sending a portion to the Wraysbury. Within the models, the following options have been evaluated:

1. All the 3rd runway catchment runoff is discharged to the River Colne

2. The 3rd runway catchment runoff discharge is split between the River Colne and the Wraysbury River.

Alternative Approach For the Alternative Approach all of the 3rd runway runoff is transferred via a dedicated SWOT to a single attenuation and treatment area to the north of the 3rd runway, shown in Figure 21.1.3.

The flows are expected to be discharged into the new CRC channels, but the precise locations of the discharge from the Alternative Approach are still to be confirmed for the DCO Project. Potentially, flow could be discharged into either the Colne-Wraysbury channel or the Upper Duke of Northumberland-Longford channel, or a combination of both. The Colne-Wraysbury channel will be the larger of the channels and so can accommodate a greater flow. Within the models, the following options have been evaluated:

1. All the 3rd runway catchment runoff is discharged to the Colne-Wraysbury channel.

2. The 3rd runway catchment runoff discharge is split between the Colne-Wraysbury and the Upper Duke of Northumberland-Longford Channel.

As discharge locations have not been confirmed it has been assumed that the discharges will be near the upstream edges of the CRC channel reaches. As long as the correct reaches are identified, the precise locations do not make a large difference within SIMCAT. The selected discharge locations will be incorporated into the revised models at ES.

Model scenarios The ‘with development’ models have been developed to assess the potential effects that could occur from the additional discharges to the west of the airport. Several model runs were used to evaluate the different discharge options, summarised in Table 21.1A.13.



Table 21.1A.13 SIMCAT PEIR Model runs

Drainage approach Model run. Purpose of model and parameters varied

Preferred Approach

PREF_v01

Development of the PEIR model configuration to include river diversions, reductions in diffuse flows and inclusion of Northwest Catchment discharge and 3rd runway discharge option for Preferred Approach. Discharge for the Southwest drainage area has all runoff flow discharging to the River Colne.

PREF_v02

River network configuration as per PREF_v01. Discharge from Southwest drainage area is split between the River Colne and the Wraysbury River. The discharge flow is split based on the proportion of flow in the receiving rivers, with 75% sent to the River Colne and 25% sent to the Wraysbury River.

Alternative Approach

ALT_v01 River network as per the Preferred Approach. All of the 3rd runway airfield discharge is sent to the Colne-Wraysbury Channel of the new CRC channels.

ALT_v02 As per Alternative Approach v01, except that discharge flow from the northern drainage area is split between the Colne-Wraysbury and Upper Duke of Northumberland-Longford channel. The discharge flow is split based on the proportion of flow in the receiving rivers, with 85% sent to the Colne-Wraysbury Channel, and 15% sent to the Upper Duke of Northumberland-Longford channel.

In addition to the scenarios in Table 21.1A.13, several further model scenarios have been modelled to assess different conditions for evaluating risk of effects to CLP, Felthamhill Brook and Portlane Brook. These are discussed in Section 0.

‘With development’ flow predictions The output of model PREF_v01 is shown in graphs in Annex G. Graphs are presented for the key rivers whose flows may be substantially altered by the development. These are the River Colne, the Wraysbury River, the Colne Brook and the River Crane. The model outputs also show the Environment Agency flow gauging locations for reference.

The discharge flows calculated for the total runoff from the additional airfield area indicate a mean flow of 6.7Ml/d and Q95 of 0.5Ml/d. In each scenario this flow is either sent into a single channel or split between two channels. The design of any flow apportionment between the different receiving rivers/channels have not yet been assigned. For the purpose of this study as an initial assessment, the flows have been apportioned based on the mean flow in the receiving rivers.



Table 21.1A.14 summarises the discharge flows and the flows in the receiving waters for each of the Preferred Approach and Alternative Approach model scenarios. The discharge flow to the west of the site are small relative to flows in the receiving waters at the assumed points of discharge; the mean discharge flow is 5% of the mean flow in the River Colne in ALT_v01, and the flow proportion is less than 5% in the other model scenarios.

Due to the relative proportions of discharge flow to receiving water flow (5% or less), the discharge to the west of the site do not make a substantial change to the overall flow in the receiving waters.

Table 21.1A.14 Comparison of model discharge flow with receiving rivers to the west of Heathrow

Preferred Approach Alternative Approach

River Colne

Wraysbury River Colne Brook

Colne-Wraysbury

CRC channel

Upper Duke of Northumberlands-Longford Channel

Mean discharge flow in v01 model

(Ml/d)

5.7 - 1 6.7 -

Mean receiving river

flow in v01 model (Ml/d)

105 - 103 218 -

Mean discharge flow in v02 model

(Ml/d)

4.3 1.4 1 5.7 1

Mean receiving river

flow in v02 model (Ml/d)

105 36 103 218 39

‘With development BOD predictions

The BOD model outputs from the Preferred Approach and Alternative Approach models are presented in Annex H and I respectively. The graphs are presented for the rivers that receive water from the additional discharges and therefore could be subject to a change in water quality.

The results from the models are summarised in Table 21.1A.15 and Table 21.1A.16. The tables show the Baseline Model results and the ‘with development ‘model output at locations upstream and downstream of the discharges, as well as at the downstream



Environment Agency monitoring location for each of the reaches. For the Baseline Model output the discharge locations are taken to be in the original river channel, whereas the ‘with development’ models the discharges are in new channels.

Comparison of the results in Table 21.1A.14 and Table 21.1A.15 show that the predicted water quality under each of the model scenarios do not show significant deterioration:

1. The 90th percentile concentrations do not change WFD class (all concentrations are less than 4 mg/l and are therefore within the ‘High’ WFD class).

2. The ‘with development’ BOD concentrations at the downstream Environment Agency monitoring locations are generally comparable or lower than the Baseline Model. The cause of the slight reduction in the models is mainly due to the reduction in the BOD contributions from ‘diffuse’ inputs from baseflow and runoff through the reaches that will be lined.

The models assume that the BOD concentration profile of the discharge waters for the new catchments are comparable to the existing discharge to CLP and allow for treatment of peak BOD concentrations prior to discharge. The design treatment limit for BOD for the new developments is 30 mg/l, which is lower than the current treatment limit of 40 mg/l. It is therefore likely that the majority of the time the BOD concentrations in the discharge will be lower than has been applied in the models.

Overall, the models indicate that the discharge of airfield runoff into the rivers to the west of Heathrow will not have a substantial effect on the BOD concentrations in the receiving rivers. This is largely because the discharge flows will be small (<5%) in comparison to the flows in the receiving waters (at the mean and Q95 flow rates).



Table 21.1A.15 Comparison of Baseline and Preferred Approach predicted BOD concentrations (mg/l)for rivers to the west of Heathrow

Baseline v03 Preferred Approach v01

Preferred Approach v02

Mean 90th percentile Mean 90th

percentile Mean 90th percentile

River Colne

Upstream of discharge point

1.89 3.21 1.83 3.10 1.83 3.10

Downstream of discharge point 2.08 3.58 2.02 3.38

Model value at PCRR0025 1.90 3.09 1.98 3.28 1.93 3.23

Wraysbury River


1.87 3.20 - - 1.81 3.11

Downstream of discharge point - - 1.83 3.15


Colne Brook


1.83 3.00 1.76 2.91 1.76 2.91

Downstream of discharge point 1.76 3.01 1.80 3.01

Model value at PCNR0039 1.80 2.80 1.72 2.71 1.72 2.71



Table 21.1A.16 Comparison of Baseline and Alternative Approach predicted BOD concentrations (mg/l) for rivers to the west of Heathrow

Baseline v03* Alternative Approach v01

Alternative Approach v02

Mean 90th percentile Mean 90th

percentile Mean 90th percentile

Colne-Wraysbury

CRC Channel

Upstream of discharge point*

1.89 3.26 1.87 3.22 1.87 3.22

Downstream of discharge point* 2.03 3.36 2.00 3.33

River Colne Model value at PCRR0025 1.90 3.01 1.93 3.17 1.91 3.11

Wraysbury River


Upper DoN-Longford Channel

Upstream of discharge point**

1.89 3.25 - - 1.88 3.24

Downstream of discharge point** - - 1.99 3.41

Upper DoN Model value at PCRR0030 1.79 3.00 - - 1.78 3.04

Longford River

Model value at PTHR0265 1.58 2.90 - - 1.53 2.87

*For the Baseline model values are taken from the River Colne

**For the Baseline model values are taken from the Upper Duke of Northumberland

Clockhouse Lane Pit and Portlane Brook This section presents the evaluation of the SIMCAT model output for the discharges from the Southern catchment and Western catchment airfield runoff into CLP and ultimately Portlane Brook.

SIMCAT is not designed for assessing standing bodies of water, and so CLP is not represented explicitly as a lake/pond within SIMCAT. The Feltham Relief Sewer is also not represented in the SIMCAT models. Instead, the discharge from CLP is represented in the models as a point discharge directly into Felthamhill Brook, a short distance upstream of the confluence of Portlane Brook with Felthamhill Brook.

Several model scenarios have been run to assess the changes to BOD concentrations in Portlane Brook, with different model configurations. These were run as adaptations of the Preferred Approach model to assess the sensitivity of the model to different



parameters/inputs. The output of the models that are presented and discussed are summarised in Table 21.1A.16.

Table 21.1A.16 SIMCAT PEIR Model runs for assessing Portlane Brook

PEIR model version no. Purpose of model and parameters varied

PREF_v01

Assumes a combined discharge flow from the CLP to Felthamhill Brook. Discharge flows from the CLP defined using .npd files based on 4R output. BOD concentrations for CLP discharge defined as .npd files based upon the CLP Outlet concentration profile.

PREF_v04 As per PREF v01, except the BOD concentration is defined by increasing the concentrations at the CLP Outlet by 50% (to represent a reduction in BOD attenuation).

PREF_v05 As per PREF v04, with an increase in BOD concentration of 50% at the CLP Outlet (to reflect reduced attenuation), but with CLP discharge set to half the 4R rate (as .npd file).

Graphs of the SIMCAT model output of BOD concentrations for Felthamhill Brook and Portlane Brook are shown in Annex J, and are discussed as follows:

1. The PREF v01 model output assumes that all of the discharge from the Southern and Western Catchments discharges to Felthamhill Brook with concentrations equivalent to the monitoring data for the CLP Outlet. This scenario allows for BOD treatment at Mayfield Farm and attenuation of BOD within the CLP. Under this scenario, the 90th percentile BOD concentrations show an increase to around 5 mg/l (which is a change of WFD class from High to Moderate). The increase in BOD extends from the point of discharge to the confluence with Portlane Brook, where mixing with the Portlane Brook flow (lower BOD concentration) reduces the overall concentration to around 3 mg/l, returning the flow to the High status range

2. The PREF_v04 model applies a more conservative approach; the CLP discharge quality applied at the CLP Outlet is equivalent to increasing the BOD concentration at the CLP Outlet by 50% for the entire range of BOD concentrations. This scenario has been run to assess the effects on the receiving water if:

a. there is a reduction in attenuation of BOD within the CLP

b. all of the discharge flow from the CLP enters Felthamhill Brook

The output for this scenario, as shown in Annex J, calculates a deterioration of BOD in Felthamhill Brook from High to Poor status for the short distance between the discharge and the confluence with Portlane Brook. After the



confluence the calculated effects of mixing with Portlane Brook improves the 90th percentile concentration to be in the Good status range. This is a deterioration relative to the Baseline model where status returns to High in the Portlane Brook. This scenario indicates a deterioration in class is possible, but could occur only under unfavourable conditions based on conservative assumptions.

3. The PREF_v05 model applies the same BOD concentration profile at the CLP Outlet as the PREF_v04 model, but applies the CLP discharge rate at half the discharge flow rate to simulate losses to groundwater (as per the Baseline v07 model).

The output from this scenario shows an increase in the 90th percentile BOD concentration from High to Poor status, similar to the PREF_v04 output, but the 90th percentile BOD concentration reduces to below 4 mg/l (High status) downstream of the confluence with Portlane Brook.

These model scenarios for the CLP and Portlane Brook show that the BOD concentrations are strongly dependent upon the proportion of flow that is discharged from the CLP to Portlane Brook and on the BOD concentration of the discharge, and therefore upon the attenuation of BOD within the CLP.

This is because the modelled CLP discharge to Felthamhill Brook is greater than the baseline flow in Felthamhill Brook and Portlane Brook.

Discharge flow rates to the CLP will increase as a result of the development because the option to discharge high BOD water to Spout Lane Lagoon will be removed; the discharge flow to the CLP will effectively increase by the quantity that is currently sent to Spout Lane Lagoon.

A higher flow rate to the CLP may reduce the residence time in the CLP, and therefore could reduce the attenuation of BOD within the CLP, potentially resulting in an increase in BOD at the CLP Outlet.

Overall, the PREF_v04 and PREF_v05 model scenarios are based upon conservative assumptions of potential risks, but show that higher flows and reduced attenuation could lead to increases in BOD concentrations. It is not considered likely that the BOD concentration at the CLP Outlet will increase by 50% at all times. However, a reduction in attenuation at the CLP is possible. To improve the accuracy of the models with respect to the CLP would require a better understanding hydrology and hydrogeology of the CLP, and also of the rate/controls of BOD attenuation in the CLP.

The length of reach that could be affected by the change in WFD class under the PREF_v04 and v05 scenarios have been assessed using the UKTAG recommendations on surface water classification schemes for WFD.



The Portlane Brook WFD waterbody has a length of 7.2 km. 5% of the river length is 0.36 km, and 15% of the river length is 1.08 km. Portlane Brook is at ‘High’ status with respect to BOD, and so the WFD status for BOD could be considered against the 5% or 0.5 km condition.

The SIMCAT model output, including the Baseline models, calculate that the discharge from the CLP Outlet can give rise to a change (reduction) in status with respect to BOD for a short distance between the discharge from the CLP (Feltham Relief Sewer) and the confluence with Portlane Brook. In the SIMCAT model the CLP discharge has been positioned at 1.5 km along the river, so 0.17 km upstream of the confluence with Portlane Brook. This distance is less than 0.5 km, and therefore not enough to potentially indicate a reduction in WFD status with respect to BOD across the entire waterbody. As discussed, the exact position of this discharge has yet to be confirmed (refer to the DIA).

Under the PREF_v04 scenario, there is also a reduction in WFD class from High to Good with respect to BOD for the length of Portlane Brook downstream of the confluence, and this length would be sufficient to be classed as a reduction in WFD class. BOD may influence dissolved oxygen concentrations in the water courses, and it is the dissolved oxygen levels that contribute to the assessment of the WFD ecological status. Further work will be undertaken at ES to assess BOD and the effects upon dissolved oxygen.


Annex B-1 © Heathrow Airport Limited 2019

ANNEX B CUMULATIVE PERCENTAGE PLOTS OF BOD CONCENTRATION AT DISCHARGE LOCATIONS



Graphic 21.1B.1 Cumulative percentage plots of BOD concentration for the EBR monitoring locations (Heathrow and Environment Agency data)

Graphic 21.1B.2 Cumulative percentage plots of BOD concentration for the Western Catchment discharge monitoring locations (Heathrow and Environment Agency data) and the CLP Outlet (Heathrow data). 2012-2018 data



Graphic 21.1B.3 Cumulative percentage plots of BOD concentration for the Southern Catchment discharge monitoring locations (Heathrow and Environment Agency data) and the CLP Outlet (Heathrow data). 2012-2018 data


Annex C-1 © Heathrow Airport Limited 2019

ANNEX C BASELINE SIMCAT MODEL V01, GRAPHS OF FLOW RESULTS



Graphic 21.1C.1 Baseline SIMCAT model v01: River Colne mean flow Graphic 21.1C.2 Baseline SIMCAT model v01: River Colne Q95 flow

Graphic 21.1C.3 Baseline SIMCAT model v01: Wraysbury River mean flow

Graphic 21.1C.4 Baseline SIMCAT model v01: Wraysbury River Q95 flow

Graphic 21.1C.5 Baseline SIMCAT model v01: Colne Brook mean flow

Graphic 21.1C.6 Baseline SIMCAT model v01: Colne Brook Q95 flow



Graphic 21.1C.7 Baseline SIMCAT model v01: River Crane mean flow

Graphic 21.1C.8 Baseline SIMCAT model v01: River Crane Q95 flow

Graphic 21.1C.9 Baseline SIMCAT model v01: Felthamhill Brook and Portlane Brook mean flow

Graphic 21.1C.10 Baseline SIMCAT model v01: Felthamhill Brook and Portlane Brook Q95 flow


Annex D-1 © Heathrow Airport Limited 2019

ANNEX D BASELINE SIMCAT M0DEL V01, GRAPHS OF BOD RESULTS



Graphic 21.1D.1 Baseline SIMCAT model v01: River Colne, mean BOD concentration Graphic 21.1D.2 Baseline SIMCAT model v01: River Colne, 90th percentile BOD concentration

Graphic 21.1D.3 Baseline SIMCAT model v01: Wraysbury River, mean BOD concentration

Graphic 21.1D.4 Baseline SIMCAT model v01: Wraysbury River, 90th percentile BOD concentration

Graphic 21.1D.5 Baseline SIMCAT model v01: Colne Brook, mean BOD concentration

Graphic 21.1D.6 Baseline SIMCAT model v01: Colne Brook, 90th percentile BOD concentration



Graphic 21.1D.7 Baseline SIMCAT model v01: River Crane, mean BOD concentration Graphic 21.1D.8 Baseline SIMCAT model v01: Colne Brook, 90th percentile BOD concentration

Graphic 21.1D.9 Baseline SIMCAT model v01: Felthamhill Brook and Portlane Brook, mean BOD concentration

Graphic 21.1D.10 Baseline SIMCAT model v01: Felthamhill Brook and Portlane Brook, 90th percentile BOD concentration


Annex E-1 © Heathrow Airport Limited 2019

ANNEX E BASELINE SIMCAT MODEL V03, GRAPHS OF BOD RESULTS FOR BEST FIT MODEL


Annex F-2 © Heathrow Airport Limited 2019

Graphic 21.1E.1 Baseline SIMCAT model v03: River Colne, mean BOD concentration Graphic 21.1E.2 Baseline SIMCAT model v03: River Colne, 90th percentile BOD concentration

Graphic 21.1E.3 Baseline SIMCAT model v03: Wraysbury River, mean BOD concentration

Graphic 21.1E.4 Baseline SIMCAT model v03: Wraysbury River, 90th percentile BOD concentration

Graphic 21.1E.5 Baseline SIMCAT model v03: Colne Brook, mean BOD concentration

Graphic 21.1E.6 Baseline SIMCAT model v03: Colne Brook, 90th percentile BOD concentration



Graphic 21.1E.7 Baseline SIMCAT model v03: River Crane, mean BOD concentration Graphic 21.1E.8 Baseline SIMCAT model v03: Colne Brook, 90th percentile BOD concentration

Graphic 21.1E.9 Baseline SIMCAT model v03: Felthamhill Brook and Portlane Brook, mean BOD concentration

Graphic 21.1E.10 Baseline SIMCAT model v03: Felthamhill Brook and Portlane Brook, 90th percentile BOD concentration



ANNEX F BASELINE SIMCAT MODEL V07, GRAPHS OF BOD RESULTS FOR FELTHAMHILL BROOK/PORTLANE BROOK AT REDUCED CLP DISCHARGE



Graphic 21.1F.1 Baseline SIMCAT model v07: Felthamhill Brook and Portlane Brook, mean BOD concentration

Graphic 21.1F.2 Baseline SIMCAT model v07: Felthamhill Brook and Portlane Brook, 90th percentile BOD concentration


Annex G-1 © Heathrow Airport Limited 2019

ANNEX G SIMCAT MODELS, GRAPHS OF FLOW OUTPUT FOR ‘WITH-DEVELOPMENT’ SCENARIO, PREFERRED APPROACH V01



Graphic 21.1G.1 SIMCAT model PREF_v01: River Colne mean flow Graphic 21.1G.2 SIMCAT model PREF_v01: River Colne Q95 flow

Graphic 21.1G.3 SIMCAT model PREF_v01: Wraysbury River mean flow

Graphic 21.1G.4 SIMCAT model PREF_v01: Wraysbury River Q95 flow

Graphic 21.1G.5 SIMCAT model PREF_v01: Colne Brook mean flow

Graphic 21.1G.6 SIMCAT model PREF_v01: Colne Brook Q95 flow



Graphic 21.1G.7 SIMCAT model PREF_v01: River Crane mean flow Graphic 21.1G.8 SIMCAT model PREF_v01: River Crane Q95 flow


Annex H-1 © Heathrow Airport Limited 2019

ANNEX H SIMCAT MODEL, GRAPHS OF BOD CONCENTRATION OUTPUT FOR ‘WITH-DEVELOPMENT’ SCENARIO, PREFERRED APPROACH V01


Annex H-2 © Heathrow Airport Limited 2019

Graphic 21.1H.1 SIMCAT model PREF_v01: River Colne, mean BOD concentration Graphic 21.1H.2 SIMCAT model PREF_v01: River Colne, 90th percentile BOD concentration

Graphic 21.1H.3 SIMCAT model PREF_v01: Wraysbury River, mean BOD concentration

Graphic 21.1H.4 SIMCAT model PREF_v01: Wraysbury River, 90th percentile BOD concentration

Graphic 21.1H.5 SIMCAT model PREF_v01: Colne Brook, mean BOD concentration

Graphic 21.1H.6 SIMCAT model PREF_v01: Colne Brook, 90th percentile BOD concentration


Annex I-1 © Heathrow Airport Limited 2019

ANNEX I SIMCAT MODEL, GRAPHS OF BOD CONCENTRATION OUTPUT FOR ‘WITH-DEVELOPMENT’ SCENARIO, ALTERNATIVE APPROACH V01


Annex I-2 © Heathrow Airport Limited 2019

Graphic 21.1I.1 SIMCAT model ALT_v01: River Colne, mean BOD concentration Graphic 21.1I.2 SIMCAT model ALT_v01: River Colne, 90th percentile BOD concentration

Graphic 21.1I.3 SIMCAT model ALT_v01: Wraysbury River, mean BOD concentration

Graphic 21.1I.4 SIMCAT model ALT_v01: Wraysbury River, 90th percentile BOD concentration


Annex J-1 © Heathrow Airport Limited 2019

ANNEX J PREFERRED APPROACH SIMCAT MODELS, GRAPHS OF BOD OUTPUT FOR ‘WITH-DEVELOPMENT’ SCENARIOS FOR CLOCKHOUSE LANE PIT


Annex J-2 © Heathrow Airport Limited 2019

Graphic 21.1J.1 SIMCAT model PREF_v01: Felthamhill Brook and Portlane Brook, mean BOD concentration

Graphic 21.1J.2 SIMCAT model PREF_v01: Felthamhill Brook and Portlane Brook, 90th percentile BOD concentration






Annex K-1 © Heathrow Airport Limited 2019

ANNEX K GRAPHS OF ORTHOPHOSPHATE CONCENTRATIONS IN CLOCKHOUSE LANE PIT AND PORTLANE BROOK


Annex K-2 © Heathrow Airport Limited 2019

Graphic 21.1K.1 Orthophosphate concentrations for Clockhouse Lane Pit, Heathrow monitoring data

Graphic 21.1K.2 Orthophosphate concentrations for Portlane Brook, Environment Agency monitoring data


Annex L-1 © Heathrow Airport Limited 2019

ANNEX L ENVIRONMENT AGENCY MONITORING DATA: PAH RESULTS FOR RIVERS IN THE VICINITY OF HEATHROW



Table 21.1L.1 Summary PAH values (µg/l) from Environment Agency monitoring locations on the Colne, Colne Brook, Horton Brook and Pinn

Anthracene Fluoranthene Naphthalene Benzo(a) pyrene

Benzo(b) fluoranthene

Benzo(k) fluoranthene

Benzo(ghi) perylene

Indeno(1,2,3) pyrene

AA-EQS 0.1 0.0063 2 0.00017 MQC-EQS 0.7 0.12 130 0.27 0.017 0.017 0.0082

Colne Above Thames PCNR0025

No. of analyses 51 51 51 51 51 51 Minimum 0.010* 0.010* 0.010 0.010 0.010* 0.010 Average 0.010 0.010 0.010 0.010 0.010 0.010 Maximum 0.018 0.011 0.011 0.011 0.011 0.011

Colne Brook Above Thames PCNR0039

No. of analyses 2 2 2 2 2 2 2 2 Minimum 0.010 0.010* 0.010 0.010* 0.010 0.010 0.010* 0.010 Average 0.020 0.032 0.020 0.020 0.020 0.020 0.020 0.020 Maximum 0.030 0.054 0.030 0.030 0.030 0.030 0.030 0.030

Horton Brook Above Colne Brook PCNR0063

No. of analyses 21 22 21 21 20 20 Minimum 0.00063 0.00003 0.00007 0.00003 0.00008 0.00008 Average 0.0019 0.00034 0.00037 0.00028 0.00044 0.00050 Maximum 0.0036 0.0018 0.0014 0.0022 0.0016 0.0018

Pinn above Frays

No. of analyses 19.0 20 19 20 19 19 Minimum 0.0013 0.00031 0.00037 0.00018 0.00039 0.00035 Average 0.0067 0.0024 0.0025 0.0012 0.0029 0.0027 Maximum 0.018 0.013 0.013 0.0051 0.015 0.014

Yellow cells exceed the MAC-EQS. Grey cells indicate values that exceed the EQS values, but that are biased by detection limits greater than the EQS. Values with asterisks show that the lowest detection limits are still greater than the EQS values. Results below detection limits were set at the detection limit values for calculating averages.



Table 21.1L.2 Summary PAH values (µg/l) from Environment Agency monitoring locations on the River Crane and Duke of Northumberland’s River

Anthracene Fluoranthene Naphthalene Benzo(a) pyrene

Benzo(b) fluoranthene

Benzo(k) fluoranthene

Benzo(ghi) perylene

Indeno (1,2,3)pyrene

AA-EQS 0.1 0.0063 2 0.00017 MQC-EQS 0.7 0.12 130 0.27 0.017 0.017 0.0082

Crane at Northcote Road, Isleworth

PCRR0006

No. of analyses 2 2 1 2 2 2 Minimum 0.010* 0.00082* 0.0014 0.00064 0.0013 0.0011 Average 0.012 0.0054 0.0014 0.0053 0.0056 0.0056 Maximum 0.0138 0.01 0.0014 0.01 0.01 0.01

Crane Above Duke of Northumberlands

River PCRR0020


Duke of Northumberlands

River at Kidds PCRR0025


Crane above Baber Bridge, Below Duke of

PCRR0080


Crane at Mereway Road, Twickenham

PCRR0083


Duke of Northumberlands

Below Trumans Bridge PCRR0086


Yellow cells exceed the MAC-EQS. Grey cells indicate values that exceed the EQS values, but that are biased by detection limits greater than the EQS. Values with asterisks show that the lowest detection limits are still greater than the EQS values. Results below detection limits were set at the detection limit values for calculating averages.


Annex M-1 © Heathrow Airport Limited 2019

ANNEX M DCO BASELINE MONITORING: PAH RESULTS FOR SITE WATERS


Annex M-2 © Heathrow Airport Limited 2019

Table 21.1M.1 Summary PAH concentrations (µg/l) from DCO Baseline monitoring data

Anthracene Benzo(a)

Pyrene Benzo(b)

fluoranthene Benzo(k)

fluoranthene Benzo(ghi) Perylene Fluoranthene Naphthalene

WFD MAC-EQS 0.7 0.27 0.017 0.017 0.0082 0.12 130 WFD AA-EQS 0.1 0.00017 0.0063 2

Location ID Location Round

No. µg/l µg/l µg/l µg/l µg/l µg/l µg/l

SW-94

Adjacent to SWOT discharge

to CLP

2 Jan 2018 0.020 0.030 0.030 0.030 0.030 0.11 0.030 8 Jul 2018 <0.005 <0.002 <0.005 <0.005 <0.005 0.013 0.292 9 Aug 2018 <0.005 0.0042 0.0076 <0.005 <0.005 0.023 0.0133

10 Sept 2018 <0.005 0.0046 <0.005 <0.005 <0.005 0.016 <0.01 11 Oct 2018 <0.005 <0.005 <0.005 <0.005 0.016 0.028

SW-96 EBR

Lower Pond

3 Feb 2018 0.020 <0.01 <0.01 <0.01 <0.01 0.010 0.030 8 Jul 2018 <0.005 <0.002 <0.005 <0.005 <0.005 0.014 <0.01 9 Aug 2018 <0.005 <0.002 <0.005 <0.005 <0.005 0.0086 <0.01

10 Sept 2018 0.036 <0.005 <0.005 <0.005 <0.01 11 Oct 2018 <0.005 0.0073 <0.005 <0.005 <0.005 0.027 < 0.040

SW-99

Southern Catchment Diversion Chamber

2 Jan 2018 0.030 0.050 0.050 0.040 0.040 0.18 0.040 8 Jul 2018 0.0090 <0.002 <0.005 <0.005 <0.005 0.011 <0.01 9 Aug 2018 0.010 <0.002 <0.005 <0.005 <0.005 0.019 < 0.028

10 Sept 2018 <0.005 <0.005 <0.005 <0.005 0.01 11 Oct 2018 0.0079 <0.005 <0.005 <0.005 0.030 0.0139