1MC04 Main Works - Contract Lot S2
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1MC04 Main Works - Contract Lot S2 Hydrogeological Risk Assessment (HRA) - Western Mound - Ruislip Northern Sustainable Placement S2 MDL Code: None Document no.: 1MC04-SCJ_SDH-GT-REP-SS05_SL07-000035 Revision Author Checked by Approved by Date Reason for approved revision C01 Lourds Michael Mark Gaby Christiane 19/06/2020 For Acceptance Anthonypillai / Chendorain Hof Phoebe Chappell C02 Lourds Michael Carlos Gómez Steven Anthonypillai Chendorain Bodenham RNSP WP Final Re-Submission SECURITY CLASSIFICATION: OFFICIAL Handling instructions: None Revision changes, authorisation and reason for issue records: Revision Author Date Checked by Date Approved by Date Reason for revision authored checked approved C01.1 Lourds 06/03/2020 Michael 30/03/2020 Mark Gaby / Richard 30/03/2020 First Issue Anthonypillai / Chendorain Patten Phoebe Chappell C01.2 Lourds 16/04/2020 Michael 27/04/2020 Mark Gaby / 27/04/2020 For Acceptance Anthonypillai / Chendorain Christiane Hof Phoebe Chappell C01.3 Lourds 10/06/2020 Michael 19/06/2020 Mark Gaby / 19/06/2020 RNSP - WP Anthonypillai / Chendorain Christiane Hof Phoebe Chappell C02.1 Lourds 08/02/2021 Michael 09/02/2021 Carlos Gómez / Anthonypillai Chendorain Steven Bodenham RNSP WP Final Re-Submission Document Title: Hydrogeological Risk Assessment (HRA) - Western Mound - Ruislip Northern Sustainable Placement S2 Document no.: 1MC04-SCJ_SDH-GT-REP-SS05_SL07-000035 Revision: C02 Contents 1 Introduction 2 1.1 Context 2 1.2 Report Contents 3 1.3 Associated Documents 3 2 Methodology 5 3 Data 5 3.1 Ground Investigations 5 3.2 Baseline Data Requirements 6 4 Regional Geology and Hydrogeology 6 5 Local Geology and Hydrogeology 7 5.1 Geology 7 5.2 Hydrogeology 9 6 Conceptual Hydrogeological Site Model (CSM) 10 6.1 Summary of Ruislip Northern Sustainable Placement (RNSP) Western Mound 10 6.2 Baseline Conditions 12 7 Risk Evaluation 17 8 List of Acronyms and Abbreviations 20 9 References 22 Appendix A - Drawings 24 Appendix B - Groundwater Contours and Levels 25 Appendix C - Groundwater Quality Data 26 Appendix D - Risk Methodology 27 1 Methodology 27 Appendix E - Chalk Dissolution Features - Additional Details 30 1 Summary 30 2 Additional Details 30 Template no.: HS2-HS2-IM-TEM-000-000265 Page 1 Uncontrolled when printed OFFICIAL Document Title: Hydrogeological Risk Assessment (HRA) - Western Mound - Ruislip Northern Sustainable Placement S2 Document no.: 1MC04-SCJ_SDH-GT-REP-SS05_SL07-000035 Revision: C02 1 Introduction 1.1 Context 1.1.1 The Ruislip Northern Sustainable Placement (RNSP) is located around 2.4km to the west of Ruislip, with the centre of the RNSP at an approximate location of 507104.4, 188401.5. It is part of the wider Ruislip Sustainable Placement (RSP; shown in Figure 1), which will be the onsite location for disposal of surplus excavated material to avoid causing environmental effects that would otherwise be associated with the offsite disposal of that material. The material will be derived from a single source and will consist predominately of naturally occurring Clay excavated from the Copthall Tunnel area. It will be deposited in two locations across the RNSP, known as the Western and Eastern mounds. Figure 1 - Location map showing both the Western and Eastern mounds of the Northern Sustainable Placement Area 1.1.2 This note provides a hydrogeological risk assessment (HRA) in relation to the proposed Western mound of the RNSP. It is one of a suite of documents that together will be submitted to the Environment Agency (EA) as part of an environmental permit for landfill applications for the RNSP. Applications for the Western and Eastern mounds will be submitted separately. 1.1.3 Given the permit application for the Western and Eastern mounds will now be submitted separately, a hydrogeological risk assessment will be conducted for both RNSP mounds Template no.: HS2-HS2-IM-TEM-000-000265 Page 2 Uncontrolled when printed OFFICIAL Document Title: Hydrogeological Risk Assessment (HRA) - Western Mound - Ruislip Northern Sustainable Placement S2 Document no.: 1MC04-SCJ_SDH-GT-REP-SS05_SL07-000035 Revision: C02 individually. Therefore, this report supersedes the previous HRA for the RNSP, previously known as the Northern Sustainable Placement Area (Northern SPA) [17], which considered both the Western and Eastern mounds together. 1.1.4 Nevertheless, the overall hydrogeological conceptual model and site characterisation will be the same as for Eastern mound, only the risks will be assessed in isolation, confined to the proposed Western mound boundary. 1.1.5 The intent of this document is to provide a factual presentation of available baseline data, and to evaluate and assess hydrogeological risks due to the placement of material as described in this report. This report is not intended to provide an exhaustive discussion of existing baseline conditions. 1.1.6 The term ‘S2’ included in the title of this assessment refers to Lot S2 for Phase 1 of the HS2 scheme. Lot S2 extends from the western end of the proposed Old Oak Common Station and includes tunnels, shafts, a large crossover box excavation, portal structures, cuttings and embankments. 1.2 Report Contents 1.2.1 This report is structured as follows: • Section 1: Introduction, • Section 2: Methodology, • Section 3: Data, • Section 4: Regional Geology and Hydrogeology, • Section 5: Local Geology and Hydrogeology, • Section 6: Conceptual Hydrogeological Site Model (CSM), • Section 7: Risk Evaluation. 1.3 Associated Documents 1.3.1 This report should be read in conjunction with the following documents: • Environmental Setting and Site Design Report (ESSD) [9], • Environmental Permit Application Form parts A, B2, B4, F1, • Environmental Monitoring Plan [19], • Site Operating Plan (SOP) [20], • Stability Risk Assessment [21], Template no.: HS2-HS2-IM-TEM-000-000265 Page 3 Uncontrolled when printed OFFICIAL Document Title: Hydrogeological Risk Assessment (HRA) - Western Mound - Ruislip Northern Sustainable Placement S2 Document no.: 1MC04-SCJ_SDH-GT-REP-SS05_SL07-000035 Revision: C02 • Management Systems and Procedures [22]. Template no.: HS2-HS2-IM-TEM-000-000265 Page 4 Uncontrolled when printed OFFICIAL Document Title: Hydrogeological Risk Assessment (HRA) - Western Mound - Ruislip Northern Sustainable Placement S2 Document no.: 1MC04-SCJ_SDH-GT-REP-SS05_SL07-000035 Revision: C02 2 Methodology 2.1.1 The assessment of hydrogeological risk associated with the Western mound is based on the following: • Review of available ground investigation data (including groundwater monitoring data performed at a minimum of quarterly intervals from February 2017 through December 2019), • Development of a Conceptual Hydrogeological Site Model (CSM), • Semi-quantitative assessment of risks. A semi-quantitative assessment of risk is considered appropriate given the nature of the single source waste material to be placed, and the relatively low permeability nature of both the waste material and ground conditions at the RSP (as described below). Appendix D provides a summary of the risk assessment ranking methodology. 3 Data 3.1 Ground Investigations 3.1.1 The HS2-commissioned main ground investigation within S2 is formed from several works packages. The data used in this report is primarily from the North-West Ruislip (NWR) ground investigation and the South Package B (SPB) ground investigation. The details of the factual reports issued on completion of these investigations are listed in Table 1: Report Name Date / Revision Document No. NWR Final Factual Report 12th April 2019 / P02 1G089-FES-GT-REP-000-000019 SPB Final Factual Report 26th September 2018 / P04 1G107-FES-GT-REP-000-000010 Table 1 - GI data received relevant to the RNSP Western mound 3.1.2 Association of the Geotechnical and Geoenvironmental Specialists (AGS) data and the Final Factual Report is available for the NWR package which covers Copthall Tunnel to the west of Breakspear Road South. The NWR ground investigation data has been used to assist in characterising the ground conditions and hydrogeology of the West Ruislip tunnel portal. 3.1.3 In situ tests specified for the NWR package include variable head permeability testing and groundwater monitoring. Laboratory test results available in the Final Factual Report for the NWR package include Particle Size Distribution (PSD), consolidation tests and shear strength. 3.1.4 In situ field testing and monitoring for SPB package that is considered useful for hydrogeological characterisation includes in situ falling head tests with standpipe piezometers and measurement of water pressure by vibrating wire piezometers. Template no.: HS2-HS2-IM-TEM-000-000265 Page 5 Uncontrolled when printed OFFICIAL Document Title: Hydrogeological Risk Assessment (HRA) - Western Mound - Ruislip Northern Sustainable Placement S2 Document no.: 1MC04-SCJ_SDH-GT-REP-SS05_SL07-000035 Revision: C02 3.2 Baseline Data Requirements 3.2.1 In support of the environmental permit for landfill application for the RNSP Western mound, baseline data has been compiled as follows: • Groundwater quality and water level monitoring has been performed at four installations at the limits of the RNSP (i.e. both the Western and Eastern mounds) at a minimum of quarterly intervals between February 2017 and December 2019, • Surface water monitoring has been performed at four installations between May 2017 and February 2018. 3.2.2 Ground gas baseline monitoring has not been conducted for the following reasons: • The material to be used for waste placement will be material from a single source consisting