THE MERSEY GATEWAY PROJECT CONTAMINATION OF SOI LS, SEDIMENTS AND GR OUNDWATER CHAPTER 14.0 The Mersey Gateway Project Chapter 14.0 Environmental Statement 1.0 Page 14.1 Contamination of Soils, Sediments and Groundwater CONTAMINATION OF SOILS, SEDIMENTS AND GROUNDWATER C O N T E N T S 14. CONTAMINATION OF SOILS, SEDIMENTS AND GROUNDWATER ........................ 14.3 14.1 Introduction .......................................................................................................... 14.3 14.2 Purpose of the Study ........................................................................................... 14.4 14.3 Study Area ........................................................................................................... 14.5 14.4 Relevant Legislation and Planning Policy............................................................ 14.7 14.5 Assessment Methodology.................................................................................... 14.9 14.6 Baseline and Results of Investigations.............................................................. 14.15 14.7 Effect Assessment ............................................................................................. 14.51 14.8 Mitigation, Compensation, Enhancement and Monitoring................................. 14.86 14.9 Residual Effects............................................................................................... 14.110 14.10 References....................................................................................................... 14.124 APPENDICES Appendix 14.1 Figures not included in main text Appendix 14.2 Records of Consultations Appendix 14.3 Historical Ordnance Survey maps and Envirocheck Reports Appendix 14.4 Desk top threat assessment Appendix 14.5 Geophysical investigation Appendix 14.6 Site investigation data Appendix 14.7 Chemical analysis results Appendix 14.8 Generic Assessment Criteria Appendix 14.9 Conceptual Site Model The Mersey Gateway Project Chapter 14.0 Environmental Statement 1.0 Page 14.2 Contamination of Soils, Sediments and Groundwater 14. CONTAMINATION OF SOILS, SEDIMENTS AND GROUNDWATER 14.1 Introduction 14.1.1 This Chapter of the Environmental Statement (ES) provides a summary of the detailed assessment of contamination issues in relation to soils, sediments and groundwater. This Chapter summarises those significant contamination effects identified by site investigations and technical assessment work and sets out mitigation and remedial measures where appropriate, concluding with a summary of the residual effects following mitigation. 14.1.2 This assessment draws on desk-based studies using published information relating to historical land uses, ground conditions and information from previous investigations together with information obtained from site investigations undertaken specifically for the Project. The Mersey Gateway Project Chapter 14.0 Environmental Statement 1.0 Page 14.3 Contamination of Soils, Sediments and Groundwater 14.2 Purpose of the Study 14.2.1 The work undertaken for the contamination assessment is intended to provide sufficient information to allow an assessment of ground conditions and potential effects associated with contamination as a result of the construction and operation of the Project. Potential mitigation measures to reduce the identified effects have been provided, followed by a description of residual effects and requirements for future monitoring. 14.2.2 Further investigations will ultimately be required to confirm detailed design criteria and to enable the design and any mitigation measures to be finalised. The Mersey Gateway Project Chapter 14.0 Environmental Statement 1.0 Page 14.4 Contamination of Soils, Sediments and Groundwater 14.3 Study Area 14.3.1 The study area for the contamination assessment is based on the Project area between St Michaels Golf Course in Widnes and the M56 Junction 12 in Runcorn (Figure 1.3, Chapter 1) which is located in North West England. For the purposes of this Chapter this is referred to as the Project area. Where relevant information was obtained relating to historical land uses and ground conditions outside the Project area, these were also included as part of the assessment. 14.3.2 In addition to the information obtained from the desk based studies, a series of site investigations have been undertaken. The locations of the exploratory holes and geophysical transits were chosen in order to investigate ground conditions in the Project area and areas of possible contamination highlighted from the desk studies and historical investigations, reports and plans. The site investigations have been undertaken in phases to take into account developing proposals for the Project area and to take into account information obtained on ground conditions from preceding phases. Each investigation took account of the information available at the time the investigation was designed. 14.3.3 For the purposes of assessing the results of the investigations, the Project has been divided into a number of areas (A-I). These are shown on Figure 14.1 (Appendix 14.1). These areas are consistent with those outlined in the Construction Methods Report (MG_REP_EIA_010), although they have been sub-divided further for the purposes of this assessment. The areas used in this assessment can be summarised as follows (the areas from the construction methods report (CMR) (Chapter 2, Appendix 2.1) are shown in brackets): a. Area A – Main Toll Plaza (St Michaels Golf Course) (CMR Area A); b. Areas B1 to B2 – Ditton Junction to Freight Line (CMR Area B), which includes toll plazas; c. Area C – Freight Line to St Helens Canal (CMR Area C), which includes the Widnes Loops and toll plazas; d. Area D – Mersey Gateway Bridge (CMR Area D) and approach viaducts; e. Area E – Astmoor Viaduct (CMR Area E); f. Area F – Bridgewater Junction (CMR Area F); g. Areas G1 and G2 – Central Expressway, Lodge Lane Junction and Weston Link Junction (CMR Area G); h. Area H – M56 Junction 12 (CMR Area H); and i. Areas I1 and I2 – Widnes De-linking (CMR Area I) of existing road embankments 14.3.4 A description of the construction proposals and methods with particular reference to where these require consideration of ground conditions is outlined below, a more detailed description is contained within the Construction Methods Report (Chapter 2 Appendix 2.1). A summary of the proposed foundation works which are most relevant to this topic is as follows. a. It is proposed that the ground beneath the embankments in Widnes for the new carriageways would be improved by the installation of a grid of vibro-concrete columns. The resulting grid of vibro-concrete columns would be overlain with geotextile membranes and layers of imported granular fill material; b. The construction of culvert extensions for Stewards Brook in Area A would involve some excavation below ground level; c. Towards the centre of the Widnes Loops Junction in Area C there would be two single span bridges allowing the on-slip road to pass beneath the main carriageway of the new road (Widnes Loops East Bridge) and the on-slip itself before it merges with the main carriageway (Widnes Loops Slip Road Bridge). These would be box structures with spans of approximately 20m that would eliminate the need to excavate deep foundations. The Mersey Gateway Project Chapter 14.0 Environmental Statement 1.0 Page 14.5 Contamination of Soils, Sediments and Groundwater These would be founded on ground that had been improved using the techniques described above; d. Piled foundations comprising replacement methods would be required for abutments and approach viaducts such as at the Ditton Junction, Victoria Road Viaduct and the northern and southern abutments. Piling excavation arisings and pile caps arisings would be re- used in the works where possible; e. The piled foundations for the approach viaducts in Area D would comprise groups of large diameter rotary bored cast in situ concrete piles with a stone piling platform created at each pier location in order to construct these. Temporary pile casings to support the excavation would be driven down, using vibration methods, to the level of the glacial clay or the bedrock; f. The towers in the Estuary would be supported by large diameter piles or rectangular barrettes. Barrettes are large rectangular piles formed using conventional diaphragm walling equipment and techniques, which can accommodate high horizontal forces, moments and vertical loads. These barrettes would be taken down to rockhead within a piled cofferdam; g. Piled foundations are likely to be required for structures at Bridgewater Junction and Lodge Lane Junction in Runcorn, earthworks would be needed for the formation of new embankments and the highway carriageway. A new retaining wall will be constructed on the south-east side of the existing north roundabout at M56 Junction 12 in Area H. This would involve the installation of a line of contiguous bored concrete piles over a length of 75m and 262m of inverted reinforced concrete retaining wall; and h. The existing embankment and viaduct linking to the Widnes Eastern Bypass in Area I would be removed. The material from the Widnes Eastern Bypass and Queensway embankments would be re-used in the works where possible. The Mersey Gateway Project Chapter 14.0 Environmental Statement 1.0 Page 14.6 Contamination of Soils, Sediments and Groundwater 14.4 Relevant Legislation and Planning Policy 14.4.1 A summary of relevant legislation and policies relating to contaminated land is provided in