Remediation of the Former Gasworks, High Wycombe, UK Apr-4-06 MGP Symposium, Reading UK, April 2006 Presented By Stuart Arch Apr-4-06 4/4/06 WPREpresentation1.ppt 1 Site Setting Contents of Presentation Remediation Strategy Development Stakeholder Involvement Site Characterisation and Risk Assessment Remediation Feasibility Remediation Design and Implementation 2 4/4/06 WPREpresentation1.ppt 1
1969 Aerial Photograph Current Site Boundary Pann Mill PWS River Wye 3 4/4/06 Historical Site Layout Plan 4 4/4/06 WPREpresentation1.ppt 2
Gasworks Journal 5 4/4/06 2001 Aerial Photograph Current Site Boundary GH6 GH5 6 4/4/06 WPREpresentation1.ppt 3
Strategy Development First Investigation 1997 2002 to 2003 2003 to 2004 We are now here 7 4/4/06 2006/07 Remediation Goals Mitigate the risks posed to the River Wye and Public Water Supply wells. Mitigate potential environmental liability associated with potentially mobile NAPL in the subsurface. Ensure that following remediation the site is suitable for its ongoing use. Achieve the goals above without unacceptable short-term impact. 8 4/4/06 WPREpresentation1.ppt 4
Remediation Criteria Considered: Short term risk to the environment. Destabilising a currently manageable situation Remediation Criteria have been developed based on: Site Specific Conceptual Models of Risk Groundwater Modelling Human Health Risk Assessment (CLEA) Assessment of Potentially Mobile Hydrocarbons 9 4/4/06 Stakeholder Involvement National Grid Landowner/problem holder Thames Water Utilities Ltd Provides public water supply Environment Agency Groundwater and surface water protection Remediation process licensing Other environmental protection obligations Wycombe District Council Contaminated land risks to human health Planning Permission for remediation Noise, dust, traffic and other public protection aspects 10 4/4/06 WPREpresentation1.ppt 5
Stakeholder Involvement First discussions with external stakeholders following initial investigation. Shared site data. Agreed the Remediation Strategy Development plan. Regular liaison following completion of each phase of the strategy development. Agreed specific remediation goals and detailed design. Open invitation for stakeholders to visit site. Regular progress reports. 11 4/4/06 Site Investigation 12 4/4/06 WPREpresentation1.ppt 6
Conceptual Model 13 4/4/06 NOT TO SCALE Groundwater Risk Assessment 14 4/4/06 MODFLOW and MT3DMS Deterministic Simulations Calibrated Well, however, Concerns about being over or under conservative Probabilistic Simulations Additional Information about likelihood of exceedance Additional confidence for Stakeholders Contaminants of Concern Naphthalene Benzene Xylenes Phenols Ammonium WPREpresentation1.ppt 7
Groundwater Risk Assessment 15 4/4/06 Groundwater Risk Assessment 16 4/4/06 WPREpresentation1.ppt 8
Human Health Risk Assessment UK Contaminated Land Exposure Assessment (CLEA) methodology Current and potential future uses were considered Chemicals of Concern were identified and associated remediation criteria calculated Remediation / Development measures were proposed 17 4/4/06 Remediation Criteria 18 4/4/06 WPREpresentation1.ppt 9
Contaminant Source Areas 19 4/4/06 Remediation Feasibility Study We considered Remediation Approaches to: Manage the Receptors e.g., restrict future site use or treat abstracted groundwater) Break pathway (e.g., Permeable Reactive Barriers) Remove/Treat the Sources Three Treatment Train Options Proposed Similar Approaches i.e., source removal and treatment Amount of residual contamination likely was inversely proportional to risk of creating impact during implementation High-Medium-Low Costs 20 4/4/06 WPREpresentation1.ppt 10
Cost Benefit Analysis This was not just a financial (costs) assessment Monetised Benefits included: Value of aquifer improvement Reduction in Haulage impact to environment Improvement in surrounding property value Assessment considered who benefited Remediation Option 1 gave benefit to society and landowner Partial excavation and ex-situ treatment of sources, in-situ treatment of residual, minimise disposal Remediation Option 3 mainly benefited society Removal of buried tar tanks and contaminants above water table only and long term monitoring/attenuation 21 4/4/06 Cost Benefit Analysis 22 4/4/06 WPREpresentation1.ppt 11
Cost Benefit Analysis 23 4/4/06 Implementation of Remediation A Two Stage Remediation Strategy Phase I (2005) - Enabling and Ex-Situ Remediation Diversion of Services Demolition of Buildings and Retaining Walls Installation of cofferdam containment and pumping system Soil Washing of Gravels Bioremediation of flinty clay and chalk Water Treatment Environmental Monitoring and Validation 24 4/4/06 WPREpresentation1.ppt 12
Cofferdams to Enable Excavation 25 4/4/06 Soil Washing in Action 26 4/4/06 WPREpresentation1.ppt 13
Soil Washing Performance 27 4/4/06 Bioremediation Underway 28 4/4/06 WPREpresentation1.ppt 14
Bioremediation Performance High Wycombe Gasworks - Bioremediation Cells 12000 10000 Total Petroleum Hydrocarbons C6-C40 (mg/kg) 8000 6000 4000 2000 0 A1 A2 A3 A4 A5 B1 B2 B3 B4 B5 Biopile Cells Oct-05 Jan-06 29 4/4/06 Implementation of Remediation 30 4/4/06 WPREpresentation1.ppt 15
Implementation of Remediation Currently reviewing results of Phase 1. Groundwater quality changes. Pilot studies using sodium permanganate and Fenton s Reagent have been undertaken. Updating groundwater modelling based on findings during Phase 1. Phase 2 - Proposed In-situ Remediation Extraction of remaining NAPL in the southern half of site using HVE or similar Injection of Permanganate (south end of site) Injection of Fenton s Reagent into gravels (north end of site) Ongoing groundwater monitoring potentially MNA 31 4/4/06 Acknowledgements Client: National Grid Property Holdings Ltd. Engineer: WorleyParsons Komex Ltd. Planning Supervisor: RSK ENSR Shear Ltd. Cost Consultant: Davis Langdon LLP Principal Contractor: Morrison Construction Services Ltd. Specialist Remediation Subcontractors: DEC NV Soil Washing Biogenie Ltd Bioremediation QDS Ltd. - Groundwater treatment 32 4/4/06 WPREpresentation1.ppt 16