Former Gasworks, Bristol Road, Gloucester
Site Location Code 388
Site Remediation Verification Report
Final
Report: R1461/12/4243
June 2012
Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final)
TABLE OF CONTENTS
EXECUTIVE SUMMARY 1
1.0 INTRODUCTION 3 1.1 General 3 1.2 Remediation Works Objectives and Brief 3
2.0 SITE SUMMARY 4 2.1 Site Details 4 2.2 General Site Description 4 2.3 Previous Investigation and Improvement Works 5 2.4 Site History 6 2.5 Historical Development 6 2.6 Historical Contaminative Issues within Improvement Areas 7
3.0 SITE CHARACTERISATION 14 3.1 Summary of Ground Conditions 14 3.2 Geology, Hydrogeology and Hydrology 14
4.0 IDENTIFIED CONTAMINATION AND STRUCTURES 17 4.1 Solid Contamination Distribution Summary 17 4.1.1 Area A 17 4.1.2 Area B 18 4.1.3 Area C 18 4.1.4 Area E 19 4.1.5 Area D 19 4.2 Ground and Perched Water Analysis 22
5.0 SITE ASSESSMENT 23 5.1 Proposed Development 23 5.2 Source‐Pathway‐Receptor Risk Assessment 23 5.2.1 Contamination Sources 23
6.0 DEMOLITION WORKS AND REMOVAL OF ASSOCIATED BELOW‐GROUND STRUCTURES 25 6.1 General 25 6.2 Demolition of Powerhouse Building 25 6.3 WWU Office / Depot Building 25
7.0 REMEDIATION WORKS 26 7.1 Improvement Strategy 26 7.2 Specification for Works and Parties Involved 26 7.3 Remediation Works Site Establishment 26 7.4 Service Identification 27
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7.5 Site Clearance 28 7.5.1 Badger Mitigation Works 28 7.5.2 Tree Felling and Pollarding 28 7.5.3 Removal of Japanese Knotweed 29 7.6 Installation of Monitoring Boreholes 29 7.7 Sample Validation and Procedure 30 7.7.1 General 30 7.7.2 Excavations 30 7.7.3 Chemical Validation Results 30 7.7.4 Discussion of Results Exceeding SSTLs 41 7.8 Excavation Works 43 7.9 Excavation, Remediation and Backfill Detail by Area 44 7.9.1 Area A1 44 7.9.2 Area A2 46 7.9.3 Area A3 46 7.9.4 Area B2 47 7.9.5 Area B3 48 7.9.6 Area B4 49 7.9.7 Area C 50 7.9.8 Area C Valve Compounds 52 7.9.9 Area C3 53 7.9.10 Gasholder 1 53 7.9.11 Gasholder 2 53 7.9.12 Tar Tank 55 7.9.13 Installation of Clay Low Permeability Barrier 55 7.10 Material Excavation Volumes by Area 55 7.11 Surface Cover Layer Finish, Compaction and Testing 56 7.12 Trial Pit Excavations 56
8.0 POST‐WORKS SITE REINSTATEMENT 58 8.1 General 58 8.1.1 Access Road 58 8.1.2 Tree Planting 58 8.1.3 Fencing Works 58 8.1.4 Black Ditch Clearance and Dressing Works 59 8.1.5 Tuffley Rovers Access Road Improvement 59
9.0 ENVIRONMENTAL MONITORING 60 9.1 Outline 60 9.1.1 Dust 60 9.1.2 Ambient Air/Vapour Monitoring 60 9.1.3 Noise 61 9.1.4 Ground Gas Monitoring 61
10.0 POST‐WORKS VALIDATION 64 10.1 Excavation Volumes/Mass Balance 64
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10.2 Surface Water Monitoring 64 10.2.1 Black Ditch and Black Ditch Culvert 64 10.3 Off‐Site Recycling / Disposal 66 10.3.1 Solid Waste 66 10.3.2 Liquid Disposal 66 10.3.3 Site Waste Management Plan (SWMP) 67 10.4 Quality Assessments 67
11.0 FINAL SITE CONDITIONS 68 11.1 Final Site Restrictions 68
12.0 THIRD PARTY APPROVALS 70 12.1 Prior to Works 70 12.2 During Works 70 12.3 Post Works 70
13.0 ESSENTIAL WAYLEAVES 71
14.0 WARRANTIES / CONTRACT MAINTENANCE 72
15.0 CONTACT DETAILS 73
16.0 CDM HEALTH AND SAFETY FILE 74
17.0 GLOSSARY 75
18.0 REFERENCES 76
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TABLES
Table 1: Site Details 4 Table 2: Previous Works Summary 5 Table 3: Historical Contaminative Uses/Structures 8 Table 4: Summary of Site Geology 14 Table 5: Site Hydrogeology 15 Table 6: Hydrology 15 Table 7: Summary of Identified Former Structures/Features 20 Table 8: Area A Chemical Validation Results (dataset 1) 31 Table 9: Area A Chemical Validation Results (dataset 2) 32 Table 10: Area B Chemical Validation Results 34 Table 11: Area C Chemical Validation Results 36 Table 12: Site Re‐use Material Chemical Validation Results 38 Table 13: Import Material Chemical Validation Results 39 Table 14: Soil Leachate Chemical Validation Results (dataset 1) 41 Table 15: Soil Leachate Chemical Validation Results (dataset 2) 41 Table 16: Summary of Area C Progress Events – February 2011 to January 2012 52 Table 17: Material Excavation Volumes by Area (m3) 56 Table 18: Ground Gas Monitoring Data 62 Table 19: Approximate Earthworks Mass Balance 64 Table 20: Summary of Solid Waste Off‐Site Disposal 66 Table 21: Summary of Liquid Waste Off‐Site Disposal 67
GRAPHS
Graph 1: Plot of Site Weekly Noise Monitoring Results (Peak Recorded Values) 61 Graph 2: Black Ditch Water Cyanide and Ammonium Concentrations 65
APPENDICES
APPENDIX A FIGURES
Drawing N° Title
D1338/3876/A1 Site Location Plan D1338/3876/A1 – A11 All Stage V Design Drawings D1461/4243/A1 Ground Gas Monitoring Locations D5237‐EXC‐001‐016 Site Excavations ‐ Heatmap Representation (by area) D5237‐FGL‐001‐002 As‐Built Ground Levels ‐ Topographical Survey D5237‐GH2‐001‐002 Gasholder 2 Backfill Profile D5237‐GRID‐001 Sample Grid Locations D5237‐OBSALL‐001‐006 All Site Obstructions (by area) D5237‐OGL‐001‐006 Original Ground Levels – Topographical Survey D5237‐PRO‐001‐016 Backfill Profiles (by area) D5237‐TPL‐001 Trial Pit – Japanese Knotweed – Valve Compound Locations D5237‐VAL‐001‐017 Validation Sample Locations
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D5237‐SERV‐001‐015 Services and Site Constraints
ELECTRONIC APPENDICES ON ENCLOSED CD‐ROM
APPENDIX A FIGURES
APPENDIX B PLATES
APPENDIX C ANALYTICAL RESULTS
APPENDIX D ENVIRONMENTAL MONITORING RESULTS
APPENDIX E GEOTECHNICAL TESTING RESULTS
APPENDIX F BOREHOLE & TRIAL PIT RECORDS
APPENDIX G DUTY OF CARE RECORDS
APPENDIX H DQRA ASSESSMENTS
APPENDIX I THIRD PARTY CORRESPONDENCE
APPENDIX J SITE WASTE MANAGEMENT PLAN / MATERIAL MANAGEMENT PLAN
APPENDIX K SPECIALIST ENVIRONMENTAL DATA
APPENDIX L SPECIALIST ENVIRONMENTAL DATA
Celtic, June 2012 vi National Grid Property Holdings Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final)
EXECUTIVE SUMMARY
Introduction Celtic Technologies Ltd was commissioned by National Grid Property Holdings Ltd to undertake Stage 6‐8 works on the NGPH site at Bristol Road, Gloucester in accordance with Celtic’s Remediation Design and Specification, Report No. R1338/10/3876(1). Improvement works began February 2011, and were completed March 2012. The objective was improving the site to address identified statutory liabilities, allowing also for future divestment for the purpose of open storage as part of NGPH’s ongoing Environmental Improvement Programme. The 7.94 ha site is surrounded by commercial and residential‐use properties, and located to the east of the Bristol Road A430 and south of Gloucester city. Remediation areas were identified as Areas A, B & C for the purposes of the project. These works are a culmination of 12 investigative studies since 1992, following interim improvements carried‐out in 2004. Site properties and estate beyond the scope of these improvements included National Grid’s Glevum House office, Tuffley Rovers and land under WWU ownership. This report covers the practical completion of the works. Site History The site operated as a gasworks from the 1860s until the 1970s producing mainly gas from coal and gas oil. The site also developed innovative gas production techniques and the production of carbon black in the 1950s and 60s. Known historic buildings and plant included retorts, a liquor plant, purifiers, CWG plant and settling pits. Two lagoons are believed to have existed, containing sulphide sludge, carbon black paste and ammonical tar. Background, Site geology comprises three identifiable lithological groups; Made Ground, Estuarine Alluvium Geology and and Jurassic Lower Lias Clay. The alluvium and clay deposits are classified as Secondary A and Ground unproductive aquifers respectively. Site investigations had confirmed the Made Ground to contain Contamination re‐worked natural clays, ash, coal, pipework, brick, concrete, spent oxide, foul lime, clinker, carbon black and tar. Associated contamination included cyanide, sulphate, heavy metals, PAHs and petroleum hydrocarbons to a depth of 4.7 m bgl outside of holders. The alluvium depth varied from 1 to 3.6 m bgl, with evidence of sporadic leachate staining; the Lias clay horizon is known to extend below 12 m bgl. Site The site has been assessed for proposed future open‐storage use across Areas A, B & C, with Assessment ongoing commercial use for Glevum House. A source‐pathway‐receptor risk assessment and Risk incorporating CLR8, CLR11 and DOE industry guidance has identified priority contaminants of Assessment concern to comprise TPH, BTEX, 16 priority PAHs including benzo(a)pyrene and naphthalene, VOCs and cyanide; and metals including arsenic, cadmium and mercury. Remediation The remediation strategy for the site was as proposed within Celtic’s Remediation Design and Strategy Specification Report R1338/10/3876(1). The strategy was designed such as to achieve the open storage use requirement as detailed in the projects site brief provided by NGPH. Remedial Site activities commenced within Area C (west), beginning with a 300 mm surface scrape and Works reinstatement cap, prior to establishing the site compound on a remediated section close to the entrance with Bristol Road. Site clearance under the supervision of ecological specialists Phlorum proceeded in conjunction with these works during March, extending throughout the remainder of the site. Remediation of the remaining site areas broadly followed the pre‐works programme, though variations were required to accommodate spatial constraints and working practices. Three or more areas were routinely worked at any given time, excavating a total of 86,360 m3 of hard and soft materials. Material types removed from each area were broadly in line with expectations, although volumes for WAC fail material more than doubled due to the volume of liquid tar saturated material within GH2. Additional site works included trial pit excavation and sampling within Area E and Area B1, installation of 17 ground‐gas monitoring wells, and the installation of a 10‐40 mm grade replacement cover layer to the Tuffley Rovers FC access road. Several variations to the original design remit were agreed with Gloucester City Council Contaminated Land Officer and Planning Department during the works, including a variation to retain the Area B1 WWU office building, and a concession on foul lime excavations in Areas A1 through A3 (Sections 6.3 & 7.9.1 of this report). The removal and replacement of the Black Ditch Culvert running through Area A1 was also found to be unnecessary after intensive investigations, including a water‐jet clean, and external and internal CCTV inspections confirmed the structure’s
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integrity (Section 7.9.1). All site areas were finished with a surface capping layer of minimum 300 mm, 10‐40 mm imported virgin limestone, vibration‐compacted outside those areas above live gas services. Following the finishing works, the site reinstatement was completed with the planting of 46 replacement trees, cleaning, dressing and improving the Black Ditch, and replacing and repairing internal and boundary fencing. Tarmac resurfacing of the replacement access road was completed the first week of March 2012. Final Site The site to include Areas A, B, C and D respectively have been remediated in accordance with the Conditions proposed remediation strategy(1); the NG Core Specification(2) and to the approval of statutory regulators through the initial planning process. In addition, the site has been remediated to achieve the original open storage end brief as provided by NGPH. The works have mitigated risks to controlled waters and human health, and the site is suitable for open storage end use in accordance with the limitations in the Stage V design document, and related agreements. A number of constraints remain on the site; including a PRS, high and low pressure gas mains, high voltage cables and a number of sub‐surface structures beyond the remit or scope of the improvement works. Service duct lines installed either side of the access road running west to east are intended to accommodate site service requirements. Intrusive works to accommodate or permanently install additional sub‐surface service lines are not sanctioned or approved by NGPH under the terms of the remediation. Third Party Pre‐works, the proposals were discussed with GCC’s Contaminated Land Department, Planning Approvals Department, Highways Department, the EA and Natural England. The proposed remediation strategy was submitted to GCC as part of a planning application, and consented by GCC’s Environmental Health Department and Contaminated Land Department. Copies of correspondence in this regard and the planning consent are included in Appendix I. Site visits by the EA and representatives of GCC took place during the works, and an evaluation of human health risks associated with airborne vapours completed during the works was performed (Appendix H). Concessions on material recovery in Areas A1 & A3 were agreed with the GCC CLO, and a planning application variation for the WWU building approved by GCC Planning Dept. Based on the degree of site improvement works completed, it is not proposed that any post‐ remediation monitoring will be required. It is considered that data and descriptions within this report demonstrate site conditions protective of all identified receptors, including human health and controlled waters. This final version verification report shall be submitted for regulator review in line with provided planning requirements and planning condition discharge obligations. Essential Essential wayleaves for the site are as follows: Wayleaves • minimum 6 m stand‐off from high pressure gas mains; • minimum 3 m stand‐off from the crown of the Black Ditch Culvert; • any future work requirements on the site will require further discussion with statutory regulators and site service providers (WWU). All constraints on the site have been discussed in detail throughout the report and are shown on the Constraints Drawing 5237‐SERV‐001‐015, Appendix A. Warranties / Celtic Technologies warrant the design and completion of the remediation works. Details of Post Contract warranties are included in the Deed of Appointment. The works are certified as complete. Maintenance
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1.0 INTRODUCTION
1.1 General
Celtic Technologies Ltd (Celtic) was commissioned by National Grid Property Holdings Ltd (NGPH) to undertake Stage 5 works (Pre‐Remediation Works Contract Services) on the NGPH site at Bristol Road, Gloucester GL2 5YA (Location Code 388). (Drawing, D1338/3876/A1, Appendix A.)
The specification and scope for the required works are detailed in Celtic’s Remediation Design and Specification, Report No. R1338/10/3876(1). Together with the NGPH Core Remediation Specification(2), these formed the project’s principal construction and engineering requirements . This report covers the practical completion of the works as detailed in the submitted Stage V Design Report.
1.2 Remediation Works Objectives and Brief
The brief for these works was to design a strategy and subsequent improvement works programme specifically aimed at addressing the statutory liabilities identified at the site. Furthermore, improvement works were designed based on the requirement to allow areas of the site as detailed below to be released for future divestment for the purpose of open storage.
The works were carried out in accordance with the principles and requirements of the procedures outlined within the NGPH documents Core Remediation Specification 2007 Version 1(2) and Methods for the Collection and Analysis of Samples Version 1(3) as part of NGPH’s Environmental Improvement Programme, involving the removal of principal contaminant sources.
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2.0 SITE SUMMARY
The site, its former uses, ground conditions and ground contamination are described in detail in the CELTIC Factual Site Investigation Report R860/04/2515(4), with additional details relating to the below‐ground gasholder bases presented in CELTIC Gasholder Investigation Report, October 2009(5). Additional information relating to the site is presented in previous CELTIC Stage 5 Improvement Design Supplementary Report, R860/04/2514, May 2003(6) and CELTIC Factual Validation Report, R905/05/2550(7). Information as presented in each of the above CELTIC reports is summarised in the following sections.
2.1 Site Details
Table 1: Site Details Site Location Gloucester Site Address Former Gasworks, Bristol Road, Gloucester , GL2 5YA. National Grid Reference The site is situated at National Grid Reference 381870, 215890. Location Code 388 Site Area The site occupies a total of approximately 12 ha.
The former Hempsted Gasworks site is situated to the east of Bristol Road (A430), and to the south of Gloucester City centre. The site, which is part low lying land of the River Severn Valley, is surrounded by a mixture of commercial, light industrial and residential land uses. Access to the site can be gained from Bristol Road (main site access, access to all areas) and also St Albans Road (immediate access to Area A). Access to the area of land currently occupied by Tuffley Rovers (Area D) is via Lower Tuffley Road. The location, layout and access locations to the site as detailed are shown on Drawings D1338/3876/A1, and D1338/3876/A11, Appendix A.
2.2 General Site Description
The pre‐remediation site layout and the site areas (A to E and WWU land) are shown on Drawing D1338/3876/A7 (Appendix A). The proposed improvement area covered approximately 7.94 hectares of the site and comprised sections designated Areas A, B and C. Additional investigation works were also undertaken in Area E which fell outside of the proposed planning application boundary. These investigation works were based on the requirements of Gloucester City Council (GCC) Contaminated Land Department and the Environment Agency (EA). The remainder of Area E (occupied by Glevum House) is to be retained for its current use and not subjected to the improvements identified within this report. In addition, land associated with the site owned and operated by Wales & West Utilities ((WWU) (which included a gasholder)), pressure reduction station (PRS) and ancillary buildings were outside the scope of the remedial works. With the exception of tree planting works associated with the boundary of Area D – currently occupied by Tuffley Rovers – this section of the site was also outside the scope of any significant improvement works.
Pre‐works site topography was generally flat; with Areas B, C and E being 1.0 to 1.5 m higher than Area D and the surrounding land to the east. Area A in the north of the site had a variable topography; generally 1.0 m higher than Areas B, C and E, and 2.0 m higher than Area D.
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2.3 Previous Investigation and Improvement Works
Table 2: Previous Works Summary
Report Author, Report Type and Site Summary of Findings Date and Document Investigation Works Scope (if Reference applicable) WS Atkins Boundary Survey Desk Study to Due to the presence of potentially Environment. evaluate potential environmental polluting process activities, waste October 1992 liabilities relating to ground disposal activities and storage tanks, it contamination at the boundary of was recommended that a detailed site (E5046/92/OCT/LI) the site and to assist determining boundary survey was conducted. the requirement for and likely extent of any subsequent site investigation. WS Atkins Interpretative Boundary Survey (not available for CELTIC review, Environment. comprising the excavation of 47 references used where possible from
December 1993 trial pits along the site boundary. previous reports) (E5123/93/JAN/L11) WS Atkins The initial Interpretative Site For the purposes of interpretation of the Environment. Investigation comprised 94 trial investigations, the site was split into
December 1994 pits and 16 boreholes, with a several ‘zones’. Large quantities of (E5272/94/DEC/REV2) supplementary investigation spent oxide and foul lime were comprising 6 trial trenches. encountered toward the north of the site. Much organic contamination was encountered associated with former structures such as gas holders, tar & liquor tanks and purifiers. Groundwater also shown to be significantly impacted with both organic and inorganic contaminants. WS Atkins Contamination Statement Report recommends a strategy for Environment supporting proposals to improvement. The recommendations
December 1994 redevelop the site for non‐food are based on previous site investigations (C1625/Doc015/Rev2) retail, production and distribution undertaken by WS Atkins. purposes. CELTIC Interpretative Interpretative Site Investigation Moderate to high contaminant Site Investigation Report. Comprised 14 trial pits concentrations, comprising spent oxide,
May 1998 with solid sampling and liquid PAHs, phenols, metals and inorganics (R242/97/586) sampling from existing boreholes. such as arsenic, sulphate and ammonia both in Made Ground and groundwater beneath the site. CELTIC Data Review Data Review Report. Summarises previous reports in one January 2004 Outline of previous reports document. (R860/04/2455) historical and site investigation findings.
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Table 2: continued
Report Author, Report Type and Site Summary of Findings Date and Investigation Works Scope (if Document applicable) Reference CELTIC Factual Factual Site Investigation Report. Report confirms findings of the previous Site Investigation The investigation objectives were site investigations and gives more detail of Report to supply information missing contamination delineation and updates May 2004 from previous reports. chemical data across the site. (R860/04/2515) CELTIC May 2004 Stage 5 Improvement Design Detailed design for the removal of spent (R860/04/2514) Report oxide, foul lime and carbon black in Area A. CELTIC Ground Condition Summary Summary of findings, site September 2004 Report conceptualisation and improvement works (R905/04/2559) done to date at the site. CELTIC Factual Stage 6‐8 Validation Validation Report detailing the November 2004 Report improvement works detailed within (R905/04/2550) Stage 5 Improvement Design Works (R860/04/2415). CELTIC June 2005) Interpretative Environmental Interpretative Environmental Assessment (R928/05/2742) Assessment of the 2004 CELTIC site investigation works. BRE (April 2005 – Field Validation of Cement / Report not available for review.
161259) Bentonite Slurry Cut‐off Wall CELTIC Below ground Gasholder Further delineation and characterisation Investigation Investigation Letter Report of GH1 and GH2 to determine accurate Letter Report ‐ Lt volumes of materials for improvement 002 – C1338 – works design purposes. Gloucester – NGPH – 12.10.09
2.4 Site History
2.5 Historical Development
The site operated as a gasworks from the early 1860s until its decommissioning in the early 1970s. The site had a complex industrial history including several developments of the site to produce coal gas between 1860 and the 1940s. The site was then used as an experimental station from the 1950s until closure when gas was produced predominantly from coal and gas oil. This period also included the testing of a number of innovative gas production techniques.
The earliest Ordnance Survey map dated 1890 shows the Hempsted Gasworks established in the south western corner of the improvement site and comprising two gasholders, residences and various unidentified buildings. By 1902, a further gasholder had been erected, together with kilns, tanks and a smithy along with other unidentified buildings and structures (possibly retorts and purifiers).
Between 1902 and 1940, the site included two retorts in the centre of the site, a concentrated liquor plant, benzole plant, purifiers, a Carburetted Water Gas plant (CWG) and Le Fleuve settling pits. The ground to the north of the site and the current playing field were used as coal stock areas.
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Many changes in the nature and location of plant occurred during the 1950s, during development of innovative techniques. The experimental techniques tested at the site included:
• Jones Gas (America Co.) which used a heavy fuel oil process to produce carbon black; • Water Gas Plant which used a semi‐automatic process; • Hall Plant which used heavy fuel oil and light distillate fraction (LDF) as raw materials; • Gas Integrate, (GI) involving the complete gasification of coal; • Butane Air that evaporated butane and diluted it with air; • Koppers Hasche that involved heating propane; • Micro Simplex (MS) that produced gas from LDF or Primary Flash Distillate (PFD); • Gaztechnik purifiers.
Carbon black was used in the manufacture of tyres, cosmetics and printing ink and was produced by thermal decomposition of natural gas or oil (or a mixture of both).
Two lagoons are believed to have been present at the site. The first was in the centre of the site and was used for sludge from the Liquid Purifier Plant (LPP); the second was on the southern site boundary and used for Jones Gas sludge (sulphide sludge) & carbon black paste. A further area used to dispose of the carbon black & ammonical tar is believed to have been located in north eastern corner of the site.
2.6 Historical Contaminative Issues within Improvement Areas
The historical structures, their uses and the associated contaminative processes identified as being present on or close to the current site boundaries are included in Table 3. Historic buildings and structures associated with the study area are shown on Drawings D1338/3876/A6 and D1338/3876/A7, Appendix A.
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Table 3: Historical Contaminative Uses/Structures Located Within Improvement Areas
Process / Process Description Potential Contaminants Structure Coal Stockpiling of coal for fuelling the Coal dust or carbon residues in the form if an Stocking retorts. Delivered by rail and via impacted coal carpet. Heavy metal Area Gloucester Sharpness canal. concentrations, arsenic and cadmium, sulphur compounds and potential combustibility. Firebrick and Materials storage. Commonly used catalysts at former gasworks Catalyst site include chromium oxide, cobalt oxide, iron Store oxide, nickel oxide, molybdenum oxide, uranium oxide, vanadium oxide, zinc oxide. Uranium oxide only used as a catalyst in oil gas production. No evidence for use of uranium oxide has been identified at the Gloucester site. Waste tip Deposit of by‐products of gas Spent oxide, foul lime, residual coal tar and coal manufacture. Anecdotal evidence tar by‐products. Carbon black, Jones gas suggests that condensate from pipes sludge. and carbon black being deposited in the east of this zone. Pipelines would have contained crude and refined coal gas, aqueous by‐ products and tarry by‐products. Carbon black is a powder used in the manufacture of tyres, cosmetics and printing ink, which is made by thermal decomposition of natural gas, oil or a mixture of both. Anecdotal evidence suggests that Jones gas sludge and carbon black may have been deposited along southern boundary. Jones Storage of gas. Gasholders required Contaminants may include coal tar, sulphur Gasholder water to form a gas tight seal. compounds, cyanides and BTEX compounds. Prolonged contact with the gas may Backfilled by‐products may contain a variety of have resulted in contamination with contaminants including asbestos. various compounds. Addition of a corrosion inhibitor such as zinc acetate may have occurred. Following demolition, gasholder tanks were often indiscriminately backfilled with by‐products. Jones Gas Heavy fuel oil used to produce Carbon black, heavy fuel oil. Plant carbon black. Hall Plant Gas produced using heavy fuel oil Carbon black, heavy fuel oil. and light distillate fraction (LDF). The gas was heated to between 700 – 900o C in firebrick chambers. Carbon deposited on the walls of the chambers was periodically burnt off. Gas was then passed through further oxide purification to remove the hydrogen sulphide.
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Table 3: continued
Process / Process Description Potential Contaminants Structure CWG Plant Process of passing steam through an Contaminants may include coal tars and incandescent bed of coke. The gas was clinker. then enriched by the thermal cracking of oil. The CWG was then mixed with the coal gas to trim calorific value of the coal gas prior to distribution. Boiler House Heating and steam production. Power Contaminants may include fuels such as for exhauster and pumps. heating oil, coal or lubricants. Liquor Plant No details. A highly complex mixture containing thousands of different compounds. Specific contaminants are likely to include saturated and unsaturated aliphatic and alicyclic hydrocarbons, aromatic hydrocarbons, polycyclic hydrocarbons, phenols and ammonia. Carbon Black Carbon black is a powder used in the Carbon black comprising mainly carbon and Store manufacture of tyres, cosmetics and high molecular weight PAHs. printing ink, which is made by thermal decomposition of natural gas, oil or a mixture of both. Tar and Liquor Used for the collection, storage and A highly complex mixture containing Tanks separation of tar and liquor. thousands of different compounds. Specific contaminants are likely to include saturated and unsaturated aliphatic and alicyclic hydrocarbons, aromatic hydrocarbons, polycyclic hydrocarbons, phenols and ammonia. Coal/Coke Stock piling of coal for fuelling the Coal dust or carbon residues in the form if Storage retorts. Delivered by rail. an impacted coal carpet. Heavy metal concentrations, arsenic and cadmium, sulphur compounds and potential combustibility. Gaz Integrale Method comprising the ‘complete’ The gas leaving the retorts contained crude (GI) Plant gasification of coal. coal gas, ammoniacal liquor and coal tar. Coke and clinker remained as a solid. GI Gasholder Storage of gas. Gasholders required Contaminants may include coal tar, sulphur water to form a gas tight seal. compounds, cyanides and BTEX Prolonged contact with the gas may compounds. Backfilled by‐products may have resulted in contamination with contain a variety of contaminants including various compounds. Addition of a asbestos. corrosion inhibitor such as zinc acetate may have occurred. The holder well may contain residues from settlement of gas impurities. Following demolition, gasholder tanks were often indiscriminately backfilled with by‐ products. Retort House Coal was heated in retorts in the The gas leaving the retorts contained crude No.2 absence of air at high temperatures. coal gas, ammoniacal liquor and coal tar. Horizontal Coke and clinker remained as a solid.
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Table 3: continued
Process / Process Description Potential Contaminants Structure Koppers Gas produced by heating propane. No details. Hasche Plant Gastechnik Gastechnik purifiers were first introduced to The principal contaminants may include up Purifiers the UK in 1958 as a new form of purification to 60 % free sulphur, sulphate, sulphide, No. 1&2 plant that had been developed in Germany. and up to 5 % cyanides and thiocyanate. The system consisted of a series of towers Complex cyanides may generate free containing pellets of an iron oxide mixture. cyanide under strong alkaline conditions or The pellets fed into the top of the towers during combustion of complex cyanides. and were extracted from the base Hydrogen cyanide may be generated from intermittently and the rate at which the free cyanide. pellets travelled through the tower determined the volume of crude gas that could be purified. In order to remove impurities such as hydrogen sulphide, carbon dioxide and hydrogen cyanide from the gas, the gas was passed through pellets of ferric oxide. Initially, fouled pellets containing up to 25‐30 % sulphur were transported to Gloucester from other sites around the UK for treatment to recover the sulphur. Koppers Storage of gas. Gasholders required water Contaminants may include coal tar, Hasche to form a gas tight seal. Prolonged contact sulphur compounds, cyanides and BTEX Gasholder with the gas may have resulted in compounds. Backfilled by‐products may contamination with various compounds. contain a variety of contaminants including Addition of a corrosion inhibitor such as zinc asbestos. acetate may have occurred. The holder well may contain residues from settlement of gas impurities. Following demolition, gasholder tanks were often indiscriminately backfilled with by‐products. Tar liquor Tar and liquor separation prior to storage A highly complex mixture containing and and processing. thousands of different compounds. Effluent Specific contaminants are likely to include Plant saturated and unsaturated aliphatic and alicyclic hydrocarbons, aromatic hydrocarbons, polycyclic hydrocarbons, phenols and ammonia. Liquor Used for the collection, storage and A highly complex mixture containing Tanks separation of tar and liquor. thousands of different compounds. Specific contaminants are likely to include saturated and unsaturated aliphatic and alicyclic hydrocarbons, aromatic hydrocarbons, polycyclic hydrocarbons, phenols and ammonia.
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Table 3: continued
Process / Process Description Potential Contaminants Structure Primary PFD (also known as Light Distillate Hydrocarbons. Flash Fraction) is a very odorous, flammable Distillate and volatile liquid produced from the oil (PFD) reforming process, capable of being Tanks catalytically reformed into town gas completely interchangeable with coke oven gas. Oil Tanks Above ground tanks. Used for collection, Petroleum, diesel, fuel and lubricant oils. storage and separation of process by‐ products and waste, but also storage of fuels for the process. Le Fleuve No details. No details. conveyor and settling pits Workshop Maintenance and fabrication May have contained fuel, oils, and paints. Coal Store Stockpiling of coal for fuelling the retorts. Coal dust or carbon residues in the form of Delivered by rail. an impacted coal carpet. Heavy metal concentrations, arsenic, cadmium, and sulphur compounds and potential combustibility. Lagoon This lagoon is suspected to have been The principal contaminants may include up used to dispose liquid purifier plant to 60% free sulphur, sulphate, sulphide, and sludge. In order to remove impurities up to 5 % cyanides and thiocyanate. such as hydrogen sulphide, carbon Complex cyanides may generate free dioxide and hydrogen cyanide from the cyanide under strong alkaline conditions or gas, the gas was passed through the during combustion of complex cyanides. slaked lime or ferric oxide. By‐products Hydrogen cyanide may be generated from (foul lime and 'Spent' oxide) were often free cyanide. revivified on the ground surface or disposed of on‐site, such as in the voids of redundant below ground gasholders. Town Used originally for the storage of gas from Contaminants may include coal tar, sulphur Gasholder the vertical retort house and later by the compounds, cyanides and BTEX compounds. No. 1 (also Hall Plant. Gasholders required water to Backfilled by‐products may contain a variety known as form a gas tight seal. Prolonged contact of contaminants including asbestos. CWG with the gas may have resulted in Holder) contamination with various compounds. Addition of a corrosion inhibitor such as zinc acetate may have occurred. The holder well may contain residues from settlement of gas impurities. Following demolition, gasholder tanks were often indiscriminately backfilled with by‐ products.
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Table 3: continued
Process / Process Description Potential Contaminants Structure Town Used for the storage of gas from the Contaminants may include coal tar, sulphur Gasholder horizontal retort house and later compounds, cyanides and BTEX compounds. No. 2 referred to as the ‘mixing holder’. Backfilled by‐products may contain a variety Gasholders required water to form a of contaminants including asbestos. gas tight seal. Prolonged contact with the gas may have resulted in contamination with various compounds. Addition of a corrosion inhibitor such as zinc acetate may have occurred. The holder well may contain residues from settlement of gas impurities. Following demolition, gasholder tanks were often indiscriminately backfilled with by‐ products. Purifiers In order to remove impurities such as The principal contaminants may include up to Liquid hydrogen sulphide, carbon dioxide and 60% free sulphur, sulphate, sulphide, and up Purifiers hydrogen cyanide from the gas, the gas to 5% cyanides and thiocyanate. Complex was passed through the slaked lime or cyanides may generate free cyanide under ferric oxide. By‐products (foul lime and strong alkaline conditions or during 'Spent' oxide) were often revivified on combustion of complex cyanides. Hydrogen the ground surface or disposed of on‐ cyanide may be generated from free cyanide. site, such as in the voids of redundant below ground gasholders. Retort Coal was heated in retorts in the The gas leaving the retorts contained crude House No. 1 absence of air at high temperatures. coal gas, ammoniacal liquor and coal tar. (vertical) Coke and clinker remained as a solid. Continuous Vertical Retort Benzole Crude Benzole was removed together Range of compounds (benzene, toluene, Plant with naphthalene as part of gas xylene, naphtha and phenols) and process purification process and by distilling chemicals including sulphuric acids and coal tars in other processes. The sodium hydroxide. benzole was further refined by washing with acids and alkalis and further distillation to produce motor fuel (benzole) and feeds for benzene, toluene, xylene and naphtha production. Condensers, In order to clean the gas, the gas Contaminants may include those associated Washers, passed through a condenser and with coal tar, phenols, ammonia, sulphate, scrubber to reduce the tar, liquor, sulphide, hydrogen sulphide, hydrogen Detarrers ammonia, hydrogen sulphide and cyanide and acids. hydrogen cyanide concentration before passing through the purifiers. Resultant tar and liquor were often stored or re‐circulated within tar and liquor tanks.
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Table 3: continued
Liquor/Tar Used for the collection, storage and A highly complex mixture containing Well separation of tar and liquor. thousands of different compounds. Specific contaminants are likely to include saturated and unsaturated aliphatic and alicyclic hydrocarbons, aromatic hydrocarbons, polycyclic hydrocarbons, phenols and ammonia. Lagoon Disposal site of liquid Purifier Plant The principal contaminants may include up to sludge. In order to remove impurities 60% free sulphur, sulphate, sulphide, and up such as hydrogen sulphide, carbon to 5% cyanides and thiocyanate. Complex
dioxide and hydrogen cyanide from the cyanides may generate free cyanide under gas, the gas was passed through the strong alkaline conditions or during slaked lime or ferric oxide. By‐products combustion of complex cyanides. Hydrogen (foul lime and 'Spent' oxide) were often cyanide may be generated from free cyanide. revivified on the ground surface or disposed of on‐site, such as in the voids of redundant below ground gasholders.
Note: Coal tars are complex mixtures containing thousands of different compounds. Specific constituents may include saturated/unsaturated aliphatic and alicyclic hydrocarbons, aromatic hydrocarbons including benzene, toluene, ethylbenzene and xylene compounds, polycyclic aromatic hydrocarbons, phenols and ammonia.
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3.0 SITE CHARACTERISATION
3.1 Summary of Ground Conditions
The following sections summarise the ground conditions encountered within the improvement areas during investigations and previous improvement works as described in the Celtic Report R1338/10/3876(1).
3.2 Geology, Hydrogeology and Hydrology
Interpretation of the site geology prior to works commencement was based on information obtained from the published 1:50,000 British Geological Survey (BGS) geological mapping sheet 235, Solid and Drift Editions. The geology and unit thickness beneath the site were consistent with this interpretation, and presented in Table 4 below.
Table 4: Summary of Site Geology
Geological Unit Lithology Approximate Thickness Highly variable but comprising of grey‐ Made Ground highly brown sandy clay with demolition rubble variable in nature and and gasworks waste such as ash and coal, depth but encountered Made Ground pipe fragments, brick, concrete foundation between depths of and fragments, spent oxide/lime, clinker 0.2 m bgl and 4.7 m bgl and tar. outside holder bases. Firm, grey‐brown orange clay with Discontinuous horizon, occasional localised gravel and sand lenses. generally encountered Estuarine Alluvium Generally dry with no pockets of water below the Made Ground encountered. Alluvium was discoloured in from approximately several locations by leachate from the 1.0 m bgl to a maximum overlying Made Ground. depth of 3.6 m bgl. Stiff green‐grey mottled clay, laminated in Encountered below the Jurassic Age Lower places, occasional gypsum crystals and trace alluvium to a proven Lias Clay fossil shells. maximum depth of 12.2 m bgl in BH4 from the previous WS Atkins Site Investigation (25).
The hydrogeology of the site is presented in Table 5, and is based on the Environment Agency (EA) information, together with an assessment of other available hydrogeological information.
General hydrogeological conditions as detailed in Table 5 below are summarised in the following section. The Lower Lias Clay is generally considered to be an unproductive aquifer and it may be assumed that the majority of the near surface groundwater is likely to flow within the natural and man‐made overburden soils. Alluvium encountered at the site is also considered for the purposes of this report as a non‐aquifer (Secondary ‘A’ Environment Agency classification) due to its clayey nature and low permeability.
The local catchment area rises to the east toward Robins Wood Hill and comprises a series of east to west trending streams that form tributaries of the River Severn. The site lies between two such streams and is closer to the more northerly stream. Several of these streams have been culverted, especially in the vicinity of the site, including the one to the north.
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Table 5: Site Hydrogeology
Geological Aquifer Aquifer Characteristics Source Groundwater Unit Classification Protection Abstractions Zone * Made Not Classified Intermediate variable permeability N/A N/A Ground and porosity. Perched water may be present with variable flow direction. Soils of intermediate leaching potential. Soils which have a moderate ability to attenuate diffuse source pollutants or in which it is possible that some non‐absorbed diffuse source pollutants and liquid discharges could penetrate the soil layer.* Estuarine Secondary A Likely to be of very limited N/A No known Alluvium significance directly under the site abstractions due to its discontinuous nature. from within 2 km of the site Jurassic Age Unproductive Generally regarded as containing N/A No known Lower Lias Aquifer insignificant quantities of abstractions Clay (negligibly groundwater. from within permeable) 2 km of the site Notes: * Based on information obtained from the Environment Agency and Envirocheck.
The hydrology of the site is presented in Table 6 and is based primarily on visual observations and information held by the Environment Agency.
Table 6: Hydrology
Feature Distance Flow Classification Abstraction* Discharge* Surface N/A Due to underlying geology, N/A N/A N/A Runoff surface runoff is known to be a problem at the site. Black Ditch 0‐100 Partially culverted stream N/A ‐ 4 previously used to drain the north of the site. Gloucester 140 <0.31 cumecs. Known to Grade A 6 5 and be of high amenity value. Sharpness Canal River 1 km Known to be of high Class 1B None within 1 5 Severn amenity value. km of the site. (Estuary) Notes:* Based on information obtained from the Environment Agency, Envirocheck and previous reports. The Chemical Grading for Rivers and Canals (GQA) comprises six hierarchical classes, ‘A’ indicating very good quality to ‘F’ indicating poor quality.
In addition to the main east to west trending streams as detailed above, there are some further minor surface water features present at the site. Of particular relevance is the “Black Ditch”. This is considered to have originated as a natural stream and was most likely a
Celtic, June 2012 15 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) tributary of the now culverted stream to the north, but its route has been modified over the history of the site, probably to assist site drainage. The Black Ditch now discharges to the stream to the north, a little way to the northwest of the site. Although overall directions of groundwater flow within the catchment may be expected to be east to west, the Black Ditch and other minor streams to the east of the site intercept the majority of groundwater flow from the site.
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4.0 IDENTIFIED CONTAMINATION AND STRUCTURES
The summary of identified contamination and structures described in this section are taken from the CELTIC reports as completed during investigations and previous improvement works described in Celtic Report R1338/10/3876(1), carried out during 2004, 2005, 2006, and 2009.
4.1 Solid Contamination Distribution Summary
Based on previous investigation and improvement works completed at the site, it had been identified that contamination was, in general, associated with Made Ground present as a result of historic gasworks processes and structures.
The Made Ground identified at the site was considered highly heterogeneous, comprising grey‐brown sandy clay with demolition rubble and a variety of common gasworks wastes such as ash, coal, pipe fragments, brick, concrete foundation and fragments, spent oxide, foul lime, clinker and tar with associated odours.
The nature of the contamination identified within the Made Ground across the site was closely related with historical gasworks processes and structures located in all areas of the site, except the northern half of the site (Area A) where ad‐hoc disposal of spent oxide, foul lime, carbon black and gas condensate was undertaken.
The following sections summarise the main types of Made Ground encountered in each of the improvement areas during the investigative works.
4.1.1 Area A
The ground condition in Area A could be largely defined by four material types:
• Spent Oxide and Foul Lime impacted materials; • Carbon Black/Rubble; • Imported/reworked material; • Clean cover materials.
Spent Oxide and Foul Lime Impacted Materials
These materials were identified as being present in the northern & western areas of Area A (Area A1 / A2 Statutory) and locally present in the eastern area. These areas were also known to be have been used for the deposition of gas condensate, which is a by‐product of cleaning gas pipework.
Approximately 12,000 tonnes of these materials were removed during improvement works undertaken during 2004 (CELTIC Report: R905/04/2550(7)). Based on a review of previous investigative data it was estimated that approximately 9,550 m3 of material impacted by spent oxide remained in Area A.
Known materials comprised mainly spent oxide impacted material and pure spent oxide mixed with demolition rubble and reworked materials. Localised areas of carbon black and foul lime were also present. Foul lime, possibly used in the construction of the rail access to the site, was known to be present to a depth of a least 2.0 m bgl along the western boundary of Area A.
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Analysis of these materials had identified highly variable pH levels and elevated concentrations of cyanide, sulphate, ammonium, PAHs, TPH, total phenols & metals (lead, mercury, copper, nickel and boron).
Two lined bentonite cells constructed as part of feasibility trials undertaken by the Building Research Establishment (BRE) during 1996 remained in the northwest of the site, and were known to contain spent oxide material. Each of the two cells were expected to be approximately 100 m2 in area and 5 m deep; constructed directly onto the Lias Clay. The BRE excavated the cell structures during March 2005 to assess material performance, but the contents of the cells remained on‐site.
Carbon Black/Rubble
The majority of the southern part of Area A (approximately 26,000 m3) comprised demolition rubble mixed with reworked materials and carbon black. Localised areas of spent oxide were also present.
Analysis of these materials had recorded elevated concentrations of sulphate, high molecular weight PAHs associated with the carbon black, and metals (lead, mercury, copper, nickel). Localised elevated concentrations of cyanide and TPH were also identified.
Imported/reworked Material
Apart from the spent oxide‐containing cells, the north‐western corner of the site comprised mainly reworked clays imported from adjacent land in the 1980s. Similar materials were also found present along the eastern boundary of Area A. Analysis of these materials had recorded elevated concentrations of metals.
4.1.2 Area B
The analyses of Made Ground samples obtained from Area B indicated elevated concentrations of PAHs, TPH, VOCs, ammonium, sulphate, arsenic and metals (cadmium, chromium, lead, mercury, copper, zinc and boron). Localised elevated concentrations of phenols, cyanide and elemental sulphur were also identified, as were localised sulphate concentrations within the underlying Alluvium and Lias Clay. Organic contamination (PAHs, TPH and VOCs) was identified in a small area in the eastern part of the area, close to the existing WWU PRS.
These areas of contamination were assessed as being likely associated with former tanks and process areas in the west, and a suspected gasholder in the east. Full investigation of this area had been limited due to the presence of live high pressure mains.
On the southern boundary between Areas B and C, foul lime, spent oxide and what was suspected to be purifier sludge was encountered. This area of contamination overlapped a similar area of contamination in Area C.
4.1.3 Area C
Several bases & foundations of former purifier sheds, a retort house, railway hoppers, gasholders and a tar tank had been identified within Area C. The analysis of samples from Area C indicated elevated concentrations of PAHs, TPH, VOCs, cyanide, sulphate, sulphur, ammonium, arsenic and metals (lead, boron) present in the Made Ground.
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The majority of the Made Ground samples indicated elevated concentrations of cyanide present in the eastern half of Area C, where spent oxide was identified close to the former purifiers. The majority of the elevated organic contamination (TPH, PAH and VOCs) was identified within and surrounding the two former gasholders (GH1 and GH2), the benzole plant in the western half of Area C and more recently the below ground tar tank situated immediately to the south west of the former benzole plant. Elevated ammonium concentrations were also identified across Area C.
4.1.4 Area E
Area E and the area upon which Glevum House is currently located had been previously investigated by CELTIC during works completed in 2004. It was noted from historical information that the area was previously the location of the Jones Gas Plant and associated buildings, structures and process areas. As such and based on the proximity of Area E to Area B it was considered that contaminants in this area would be similar to those encountered elsewhere across the site and include determinands associated with spent oxide, tar / hydrocarbon impacted materials and carbon black.
Table 7 presents details of structures identified during site investigation works. Due to the complex nature and number of redevelopments that had been undertaken at the site, significant below ground obstructions and foundations were expected to be present across Areas B, C and locally within Area A.
4.1.5 Area D
Previous site investigation information, analysis and observed ground conditions for Area D (Tuffley Rovers), indicates no evidence of contamination.
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Table 7: Summary of Identified Former Structures/Features
Structure Location Description Identified onsite Horizontal Area B, Under the tarmac and hardcore/road stone loose brick rubble including Retort inside yellow fire bricks and large (up to 1 m3) reinforced concrete with steel House fenced off girders were encountered between 0.4 m bgl and 3.7 m bgl where a car park solid brick floor was encountered. Perched water was encountered at area. 1.0 m bgl with occasional tarry globules and oily residues. CWG/Hall South CBHC16 drilled in the centre of the gasholder structure. Demolition Gasholder western rubble of bricks, concrete and wood encountered to 0.8 m bgl. Tarry corner of oily stained perched water encountered from 1.1 m bgl becoming Area C thicker, more odorous and viscous with depth. Small circular Gastechnik (adjacent to iron oxide pellets encountered at 8.6 m bgl. Black‐grey soft sticky tar No. 1 Town stained Clay encountered at 9.6 m bgl. Dry, stiff grey silty clay with Gasholder). mudstone bands between 9.7 and 10.3 m bgl. No obvious solid gasholder base, gasholder presumed to be built on top of the Lower Lias Clay. CTPC23 dug on the edge of the gasholder structure and encountered a brick built circular structure coming away from the main gasholder, assumed to be holder well. Oily perched water with strong tarry odours encountered at 0.3 m bgl. Unable to excavate fully due to live services and fence line. CBHC11, CBHC12 drilled outside gasholder, oily tarry stained Made Ground between 1.7 and 3.0 m bgl and tarry/oily perched water. CBH203 installed in August 2009 was advanced to a depth of 10 m bgl. Materials encountered during installation included heavily tar / hydrocarbon impacted clay and gravels from a depth of 1.6 m bgl to approximately 9 m bgl respectively. No definitive base was encountered with approximately 1 m of stiff grey clay being recorded between 9 m bgl and 10 m bgl respectively. Perched water was observed at approximately 0.6 m bgl. Vertical Middle of Brick and concrete rubble encountered just below site surface until a Retort Area C. concrete base was encountered at 1.2 m bgl. Perched water level at House 0.4 m bgl with a slightly oily sheen. Gaz Southern Concrete base of gasholder identified at approximately 0.6 m bgl and Integrale edge of approximately 0.3 m thick with a brown soft silty plastic clay under the Gasholder Area A. base. No evidence of an underground structure. Purifiers Across south Several concrete and brick bases were identified within the south eastern half eastern half of Area C. Blue spent oxide staining was evident on several of Area C. of the bases and localised areas of spent oxide were identified within the Made Ground in these areas. No. 2 Town South CTPC13 drilled in the centre of the gasholder structure, demolition Gasholder western rubble, coal and ash encountered with impenetrable concrete corner of base/obstruction at 5.7 m bgl. CTPC14 also drilled in the centre of the Area C gasholder structure with demolition rubble becoming tarry at 3.0 m bgl. (adjacent to Concrete base/obstruction encountered at 7.7 m bgl. CBHC15 and CWG/Hall CBHC15A drilled inside the outer edge of the gasholder structure. Gasholder). CBHC15 abandoned after encountering an obstruction at 4.0 m bgl. CBHC15A encountered demolition rubble becoming slightly tarry at 9.3 m bgl and perched water from 0.55 m bgl. Solid red brick base encountered at 11.4 m bgl with dark grey silty Lias Clay to 11.6 m bgl. CBHC8 and CBHC9 drilled outside gasholder structure with slightly tarry and oily stained Made Ground between 1.2 and 2.4 m bgl.
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Table 7: continued
Structure Location Description Identified onsite No. 2 Town South CBHC10 drilled between the two gasholders, oily Made Ground with Gasholder western diesel odour between 1.2 and 3.8 m bgl. CBH201 and 202 were installed corner of during works undertaken in August 2009. CBH201 was extended to a Area C depth of 8 m bgl. CBH201 was recorded as consisting of grey clay with (adjacent to angular gravels and cobbles to a depth of 7 m bgl at which stiff grey CWG/Hall alluvial clay was encountered. CBH202 was advanced to a depth of Gasholder). 9.5 m bgl with materials as per CBH01 encountered. Perched water was encountered at a depth of approximately 0.6 m bgl. Benzole Plant Adjacent of A series of various sized concrete and brick bases were encountered in & former CTPC32 and CTPC33. A variety of steel and cast iron pipes were Condensers, gasworks identified between the structures the majority of which contained oily washers and cottages in and tarry, very odorous water. detarrer western half of Area C. Tar well Western The area of the tar well was investigated with a number of trial pits; half however no distinct structure could be identified. The location of the Area C. tank is covered by a number of mass concrete slabs associated with the adjacent benzole plant and large pieces of demolition rubble. Oily perched water was identified at a depth of 1.2 m bgl. From historical records, it is anticipated that the tank is approximately 15 m in diameter and likely to contain heavily tarred material. The depth of the tank is not known. Purifier Crossing Blue, green white and sometimes grey spent oxide/foul lime was Sludge between encountered in several trial pits (CTPB19, 21, 27, 28; CTPC6, 9, 11, 13‐ Lagoon the north 20). The material typically was very soft, silty fine sand and eastern encountered within the Made Ground above and below the perched boundary water table. The material was very odorous and encountered in a of Area C horizon between 0.5 and 1.5 m thick. Localised areas of shallow spent and into oxide and foul lime were also identified in the south eastern corner of the Area C but likely to be associated with the former purifiers. southern half of Area B. Tar and Area B, Trial pit CTPB22 identified a large reinforced concrete base structure. Liquor Plant under small The trial pit was excavated along the side of the concrete base. Oily grassed tarry perched water encountered at 1.5 m bgl with tarry and oily football stained sandy clay with demolition rubble of concrete and bricks. pitch. Additional trial pits identified concrete and brick bases and structures, typically 300 mm thick. The majority of these structures are below a 400 mm topsoil and grass cover. Tarry Made Ground surrounds several of the concrete bases. Railway Evidence of Evidence of the railway tracks is noticeable on the surface of the site Tracks the former within the east of Area B and across large areas of Area C. Concrete railway hopper structures identified within Area C infilled with general identified demolition rubble. Anecdotal evidence suggests that the railway across the sidings along the north western site boundary were built on a stockpile whole site of foul lime deposits. Across the whole site, the railway tracks were (Areas A‐C) associated with coal, ash and slag within the Made Ground.
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Table 7: continued
Structure Location Description Identified onsite Tank North eastern A small (approximately 3 x 4 m), concrete rectangular tank was corner of identified in CTPB15 and CTPB17. The structure was approximately Area B. 1.0 m deep and infilled with general demolition rubble, brushes, wood and some oily residue on the perched water and tarry odours. Disused Area A. Comprises of a brick arch structure approximately 1 m below the Black current ground surface. The structure has never been fully exposed Ditch but where encountered significant volumes of heavily contaminated Culvert perched is present. Historical plans indicate the structure is approximately 95 m long.
4.2 Ground and Perched Water Analysis
The dissolved phase contaminants found in the perched groundwater related closely to contaminants identified within the Made Ground.
The contaminants of concern identified within the perched groundwater at the site included cyanide, sulphate, PAHs, phenols and metals; and a highly variable pH was also noted.
Contour plots developed to measured average groundwater conditions/elevations on site suggested the site to be largely isolated from the ingress of external groundwater. A local groundwater high had been identified located to the west of the centre of the site, with predicted perched groundwater flow from the main site area being to the east & north towards the Black Ditch and to the west towards tributaries of the Severn.
Groundwater elevations in the western area of the site were lower than the level of the nearby canal, with the groundwater contours indicating flow to the west. This suggested that the canal was not leaking significant water to the extent that groundwater was mounded to canal level. Given the elevation of the canal above groundwater, this provided indication that the canal was not a potential receptor with respect to groundwater discharging to the canal.
The measured groundwater contours also suggested a component of flow to the south, further suggesting some drainage feature existing to the south of the site; possibly one of the sewers.
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5.0 SITE ASSESSMENT
In order to assess the extent of improvement requirements and to provide the basis for the risk based improvement design in consideration of the proposed future use of the site, qualitative and quantitative risk assessments have been undertaken. The assessments undertaken are commensurate with the requirements of CLR‐11(8) and current legislation.
5.1 Proposed Development
The site has been assessed for the following proposed future uses:
• Open storage use across Areas A, B, C, D • Ongoing current commercial use – Area of Glevum House (remainder of Area E not defined above and in Drawing D1338/3876/A1, Appendix A) to continue as per current operations with respect to National Grid.
5.2 Source‐Pathway‐Receptor Risk Assessment
In assessing potential environmental risk, three main factors contribute to the risk associated with contaminated land, these are:
(i) Source ‐ the presence of potential contamination sources (these may be individual contaminants, structures or specific areas of the site); (ii) Pathway ‐ the presence of pathways linking contamination sources to receptors; (iii) Receptor ‐ the presence of sensitive receptors (principally human health and the environment).
An in‐depth and detailed qualitative and quantitative assessment of all risk factors associated with the site, commensurate to the proposed end‐use criteria, is considered in Section 6 of Celtic Stage V Report R1338‐10‐3876. These assessments considered data obtained during previous site investigation works and reports, professional experience of gasworks sites, CLR‐ 8(9) guidance and published DOE industry profiles. A summary of the relevant factors are presented below.
5.2.1 Contamination Sources
Contaminants of Concern
The following priority substances of concern associated with former gasworks and brownfield sites were considered by the risk assessments: • Total petroleum hydrocarbons (TPH): risk banded aliphatic & aromatic carbon fractions C8‐C40; • BTEX compounds: benzene, toluene, ethylbenzene, total xylenes; • 16 priority PAHs: naphthalene, acenaphthene, acenaphthylene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo‐a‐anthracene, chrysene, benzo‐b‐fluoranthene, benzo‐k‐fluoranthene, benzo‐a‐pyrene (BaP), dibenzo‐a,h‐ anthracene, benzo‐g,h,i‐perylene, indeno‐1,2,3‐c,d‐pyrene; • Phenols; • VOCs: Trimethylbenzenes, dichloromethane; • Metals (including metals, semi‐metals and non‐metals): arsenic, cadmium, chromium, copper, lead, inorganic mercury, nickel, selenium, zinc;
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• Easily liberatable & total cyanide.
Contamination Sources
Contamination on the site is detailed in Section 4.0 of the Celtic Stage V report; the investigations undertaken having identified the following principal sources of contaminants of concern:
• Known below ground tanks that contain tar and heavily tarred demolition rubble; • Subterranean structures & pipework associated with tankage & former processes; • Spent oxide, foul lime and carbon black mainly present in Made Ground in Area A, with localised areas of impacted Made Ground also present in Areas B & C; • Hydrocarbon and tar impacted Made Ground associated with tar, liquor and benzole processing in Areas B & C; • Historical process pipe work and drainage (Areas A, B and C); • Residual contamination present in Made Ground across Areas A, B and C; • Contaminated perched groundwater present in overburden above the Lias Clay, especially in Area A adjacent to the Black Ditch Culvert.
Celtic, June 2012 24 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final)
6.0 DEMOLITION WORKS AND REMOVAL OF ASSOCIATED BELOW‐GROUND STRUCTURES
6.1 General
The presence of numerous and significant historical brick or concrete hard structures across the site had been identified prior to works commencement as outlined in previous sections. A total of 8,576 m3 of hard materials were recovered during the works; the vast majority of which was re‐used as site backfill following screen and crush processing. Table 17; Section 7.10 presents a breakdown of hard material volumes recovered from each improvement area.
In accordance with the requirements of the project design, where practicable, hard materials were excavated to the depth of the soft materials being recovered, or otherwise, to a minimum of 0.3 m below pre‐existing ground levels. A notable exception to this requirement occurred in the eastern section of Area C, where pile caps associated with the construction of the former gas purifiers were removed to a minimum elevation of 0.250 m below pre‐existing ground levels. This decision was arrived at through consultation between the Celtic project design team, which balanced the net material benefits achievable, against the additional time and resource efforts required. Location drawings outlining the position of both removed structures and those left in situ are included in Appendix A.
6.2 Demolition of Powerhouse Building
Prior to excavation of the central section of Area B4, works were carried out to demolish the existing powerhouse building and associated below‐ground structures. A Type‐3 (demolition) asbestos survey was performed during August 2010 in preparation for this work. A small amount of asbestos was identified within the building’s roof section, all of which was removed under controlled duty‐of‐care conditions by Erith prior to commencement of the demolition proper. The asbestos survey report can be found in Appendix N of Celtic Report R1338/10/3876. Demolition of the main building structure was completed during a three‐ week period between the end of July and early August 2011.
6.3 WWU Office / Depot Building
Under the terms of the original planning consent for the site agreed with GCC, provision had been made for demolition of the existing WWU office / depot building. Part of the planning requirement stipulated that no permanent above‐ground structures should remain on the site upon project completion. As the project progressed however, it became increasingly apparent that significant environmental and cost benefits could be achieved by retaining this structure for future use. Following consultations with GCC Contaminated Land Officer (CLO), and in conjunction with data obtained from surrounding soil chemical results and atmospheric monitoring, information was provided to GCC Planning Department requesting a variation to the original planning consent in order to allow for the building’s retention. Permission to vary the planning consent to accommodate this request was forthcoming January 2012. Correspondence relating to the retention request is included in Appendix I, along with a copy of the revised planning notice.
Celtic, June 2012 25 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final)
7.0 REMEDIATION WORKS
7.1 Improvement Strategy
The remedial design strategy for the site was adopted following a period of extensive site investigation works, consultation with the Client, the relevant external regulatory bodies and existing stakeholders. Details outlining the statutory and non‐statutory obligations, objectives and targets for each area – including the respective reinstatement requirements – are provided in Sections 6.5 to 6.10.5 of Celtic Report R1338/10/3876(1). The remedial targets agreed were principally driven by risks to controlled waters with respect to the ‘Black Ditch’ surface watercourse, which had been identified as the key receptor. The relevant pollutant linkages to the Black Ditch were considered to be leaching, direct migration of contaminants to shallow groundwater and entry into the Black Ditch of contaminant sources directly through defects that might be present in the existing culvert construction.
Improvement of the site to risk based targets calculated to be protective of controlled waters was also deemed sufficient to mitigate risks to future site users for the purposes of open storage activities. The completed finished surface quality was a requirement to provide a uniform granular platform for the purposes of open storage. Agreed final site levels (elevation) were based on the Environment Agency requirement that there be no loss of flood capacity due to Area C being located within a potential flood risk zone.
7.2 Specification for Works and Parties Involved
The final version of the Remediation Design and Specification, Report No. R1338/10/3876(1), was issued in October 2010. Together with the NGPH Core Remediation Specification(2), these formed the main design and construction specifications for the proposed works.
Erith Contractors Ltd was appointed by NGPH as Principal Contractor for the remediation works. Celtic Ltd provided continued remediation design support, project management and representation for the Client (NGPH). Davis Langdon was appointed as Project Cost Consultant; 3cRSK Ltd undertook the CDM‐Coordinator role. The works were carried out during the 54 week period between February 2011 and March 2012. Key personnel contact details are provided in Section 15 of this report.
7.3 Remediation Works Site Establishment
The site establishment was designed and managed with regard to reducing environmental impacts. Activities identified as having a potential risk of impact upon the environment were recognised and the necessary precautions taken to ensure negative impacts were controlled to a satisfactory and practicable level.
The main site compound set‐up comprised a number of office, security, storage, decontamination and welfare cabins located within Area C, close to the main NG Hempsted site entrance at Bristol Road. This set‐up was permanently established during the last week of March 2011, having been previously located in a nearby temporary location whilst remedial work could progress within the western section of Area C. The site was arranged such that all personnel entered the site into a designated clean area, with site workers required to sign a daily attendance register held within the main site welfare cabin. Visitors were required to report to the site offices for registration and/or induction as required. Car parking sections were made available within the compound area, with the main site security deterrent provided by the installation of a site‐wide monitored CCTV camera system, which
Celtic, June 2012 26 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) incorporated an audible intruder warning function. Additional vehicular traffic security to the site was provided by 24 hour remotely operated security barriers controlled from NG’s Glevum House offices. Electrical power for the compound was provided by a portable generator, with water and sanitation provided through connection to the existing water and sewerage systems.
The clean area was separated from the contaminated (dirty) working areas of the site by a combination of pedestrian and Heras fencing, with pedestrian access to the dirty area facilitated via the decontamination unit. Personnel moving between areas were under strict instructions to only do so through the decontamination unit. Emergency showers, hand washing facilities and changing areas were also provided within this unit. The decontamination unit was retained until no contaminated material remained on site, and all intrusive site works had been completed.
Compulsory personal protective equipment (PPE) was provided for all site workers and visitors. Minimum site requirements comprised of coveralls, safety boots, hard hat, gloves, safety glasses and high visibility clothing; with dust masks made available on request. Vapour masks were available as a contingency in the event that concentrations of hydrocarbon vapour in the work areas exceeded occupational exposure limits or at the request of the Site Manager or staff. Ear protection was also made available for potentially noisy works such as breaking out, cutting and backfill compaction works.
A heavy‐duty wheel wash was located on the site access road at the exit of Area B3. Upon exiting the site, vehicle drivers were instructed to compulsorily use this facility. To further minimise the risk of contaminant transfer to external roadways, a road sweeper was routinely deployed to all adjacent on and off‐site roads as required. When necessary, nominated members of site staff managed the completion and retention of waste transfer documentation.
One internally‐bunded fuel bowser was maintained on‐site with a number of spill kits on hand at all times. Waste skips used were either covered or contained and appropriately labelled. Fire extinguishers were located on‐site for emergency use.
Dust, vapour and odour monitoring for the duration of the works was undertaken as detailed in Section 9.
JBP Public Relations were appointed by NGPH to manage communication and public relations requirements. Prior to and throughout the duration of the site works, JBP continued an ongoing dialogue with local residents, businesses and Glevum House staff through a combination of targeted mail‐shots, telephone calls and regular liaison meetings with council ward officials and residents groups.
7.4 Service Identification
Prior to the commencement of any intrusive works, a phase of services identification was undertaken. Service drawings, historical plans and data gained from past site investigations were used to identify services within the excavation areas. Major service identification work was undertaken by a specialist company appointed by Erith, whilst localised service identification was performed by suitably qualified personnel with the use of a cable avoidance tool (CAT) and signal generator. Where identified, all service locations were physically proved by vacuum excavation or hand‐dug excavation. All proven service runs were surveyed and recorded and where appropriate physically marked with pegs/pins, warning signs and/or
Celtic, June 2012 27 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) marker paint. Drawing 5237‐SERV‐001‐015 as presented in Appendix A outlines the finalised services and site constraints.
7.5 Site Clearance
At the start of site works, several areas were continuing to be used by WWU Depot maintenance operatives and civil engineering contractor Morrisons. These included the WWU Building and adjoining area, a materials storage area near the soon to be demolished Powerhouse Building (Area B4), and a container and vehicle storage area on Area B3. Dispensation was provided by the NGPH Commercial Property Manager overseeing the works to allow continued access and use of these areas whilst the early part of the works proceeded. WWU were subsequently allowed to relocate in a completed section of Area C along the site’s eastern boundary. Site clearance therefore was achieved during two separate periods; the first during March 2011, when the site was cleared of general waste and vegetation, and the second which was undertaken during June in those areas detailed above. All clearance work was performed following an ecological assessment by specialist ecological contractor Phlorum, with due regard to environmental sensitivities. Phlorum letter PB/3901/N_S/GP, Appendix K refers to assessment works and findings with respect to the site.
Vegetation was prevalent across most of the site, and was stockpiled and disposed of to landfill under appropriate duty‐of‐care conditions. Other items of general waste comprised discarded plastics materials, and metals and rubble arising from the removal and clearance of minor surface structures such as chain‐link fencing and brick walls. Hazardous waste arising from surface clearance and shallow excavation comprised Japanese knotweed, which is reported in more detail in Section 7.5.3 below.
All wastes were segregated into separate stockpiles, tested where appropriate and removed offsite by a suitable contractor to appropriately licensed facilities. Details of all waste removed can be reviewed in the site disposal log Appendix G, together with the relevant waste tickets and contractors’ details.
7.5.1 Badger Mitigation Works
Previous site investigations had revealed the presence of two badger setts, one located within Area A and the other Area C3. Following on‐site assessment and monitoring by Phlorum, it had been determined that the setts previously located within Area A were no longer active. Information with respect to survey and assessment works completed in relation to badger activity at the site is presented in Appendix H of Celtic report R1338/10/3876.
The removal of both setts was undertaken during the initial site clearance works in March 2011 under the supervision of Phlorum. The works were performed according to the requirements of National Grid’s Core Specification; and under licence from Natural England ‐ Licence WLM/2010/1384. Both setts were confirmed by the supervising contractor to be redundant prior to their removal, and no badgers either dead or alive were observed during this or any subsequent site works.
7.5.2 Tree Felling and Pollarding
Planning consent for the works allowed for the felling of two rows of trees, located between Areas E and B; and within Area A3 along the boundary with Betjemin Close. Whilst the trees had previously been subjected to tree preservation orders, these had been revoked by the Gloucester City Council Tree Officer during the consultation process in lieu of replacement
Celtic, June 2012 28 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) planting during the restoration phase of the works. In addition, pollarding of trees located off‐ site within the rear gardens of the Betjemin Close properties bordering the site had also been agreed at the request of their residents.
Due consideration was given to the presence of nesting avians and other wildlife prior to the commencement of any tree removal works, the result of which postponed the felling and pollarding of trees within A3 due to activity evidence of corvid species. The trees along the Area E – B boundary were felled during late March 2011, the removal of which later included full extraction of all root systems during the Areas B2 and B3 remediation work. With the exception of two Poplar trees directly adjacent to number 24 Betjemen Close, the trees within Area A were felled early October 2011. The root systems of these trees were not however removed, but were instead prevented from future re‐generation by having their stumps substantially cross‐cut.
Pollarding of the remaining trees in and adjacent to Area A3 occurred during two separate events. Trees within the rear gardens of the Betjemen Close properties were pollarded during early September 2011, and the two remaining trees adjacent to 24 Betjemen Close were pollarded / reduced at the beginning of November 2011. The delay in pollarding the trees at 24 Betjemen Close resulted from the requirement to assess any potential adverse effects that either their removal or reduction might have on the long‐term stability of adjacent properties. The assessment process included the commissioning of an independent structural investigation and risk assessment, and geotechnical analyses of soil samples recovered from the vicinity. The Quad‐Consult report ‘Assessment of Risks Associated with the Removal of Trees Adjacent to 24 Betjemen Close, Gloucester’ and relevant soil sample data are included in Appendix K.
Following a consideration of the available information, including discussions between the property’s owners / residents, the Client, and the site project team; and mindful also of the stated desire by the residents to either remove, or as a minimum, to at least significantly reduce the size of the trees, the decision was arrived at to reduce the height of the trees to 3 m above ground.
All of the tree surgery works undertaken on site were performed by appropriately qualified sub‐contractors under the direct supervision of Phlorum.
7.5.3 Removal of Japanese Knotweed
The presence of Japanese knotweed had been confirmed at four separate locations across the site. The first at the western end of Area C, two locations in Area A3 adjacent to a tarmac paved area near the Black Ditch, and the fourth along the northern boundary of A4 near the site access gates off St Albans road. During March 2011, all locations were excavated to the full extent of their affected areas under direct supervision by Phlorum. Removed material was quarantined under sheeted stockpiles until disposal off‐site under duty‐of‐care conditions. A follow‐up visit was performed by Phlorum during October 2011 to assess the extent of any knotweed recurrence, during which none was observed. A second follow‐up visit has been scheduled for June / July 2012. Correspondence and a location plan of the knotweed removal work supervision are included in Appendix K.
7.6 Installation of Monitoring Boreholes
Fourteen additional ground‐gas monitoring wells were installed during the period of the works to assist the provision of ongoing ground gas data collection. The additional wells brought the
Celtic, June 2012 29 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) number available for use to a total of seventeen. The locations of these boreholes can be viewed on Drawing D1461/4243/A1, Appendix A.
7.7 Sample Validation and Procedure
7.7.1 General
Samples of excavated surfaces, stockpiled soils, processed hard and import materials were recovered in accordance with the procedures outlined in NG’s Methods for the Collection and Analysis of Samples from National Grid Sites, Version 1(3). Sampling frequency for stockpiled and import material followed the requirements of Section B720, and Table 19 of Celtic Report R1338/10/3876. Depending on individual or specific requirements, the samples were submitted to accredited laboratories for chemical and/or geotechnical analyses, and, where applicable, asbestos identification. Where asbestos was identified, the mass of asbestos present was quantified. Full and complete laboratory reported datasets of all analytical results are included in Appendix C; summary tables of validated chemical sample results are produced in Section 7.7.3 below.
7.7.2 Excavations
Notwithstanding exceptions outlined under their relevant sections, where practicable, excavations were generally continued to natural ground, and soils with visual or olfactoral signs of contamination were removed. Upon completion to depth, the excavations were inspected by the Celtic Resident Engineer or deputy, and where confirmed that all recoverable contaminated materials had been removed, further validation works were undertaken. These comprised the photographic recording of excavations, recovery of soil samples for chemical analyses and a full topographical survey. Locations were only backfilled when chemical, and where appropriate, supplementary validation criteria were found to be met to the satisfaction of the Resident Engineer. Pictorial tableaux of the works performed within each area are included in Appendix B; and heatmap representations of excavation depth profiles throughout the site are presented in Drawing 5237‐EXC‐001‐016, Appendix A. The locations of all soil samples collected for validation are presented on Drawing 5237‐VAL‐001‐017, Appendix A.
With the exception of Area C, soil validation samples were obtained from the margins and bases of each excavation on a 10 m x 10 m sectioning grid (Drawing 5237‐GRID‐001, Appendix A). Within Area C, a larger grid of 20 m x 20 m was applied as it had been determined from previous exploratory works that the majority of the materials present within this area were relatively shallow and lightly impacted.
7.7.3 Chemical Validation Results
All material was sampled in accordance with the procedures outlined above. Tabulated chemical validation result datasets for soil and leachate samples obtained from excavated surfaces, re‐used material stockpiles and import material are presented in Tables 8 to 15 below. The results have been screened to provide statistical analyses for maximum, mean and upper confidence limit (UCL) mean concentrations for those analytes for which site specific target levels (SSTLs) have been set. The summary datasets represent relevant samples reflecting completed site excavations, backfill and material conditions; and therefore excludes those results from areas and materials for which additional excavation and validation resampling was required. Results relating to pre‐planned trial pit excavations within Area E and Area B; and stockpiled material failing SSTLs and subsequently disposed of off‐site are also excluded. Due to the high number of leachate and Area A soil samples taken, each of these
Celtic, June 2012 30 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) datasets are presented within two tables. Copies of all laboratory results are included in Appendix C. In addition to laboratory issue results as included within Appendix C, a compiled results database that has been specifically developed to present area specific (Area A – D) data has been included for review within Appendix C.
Table 8: Area A Chemical Validation Results (dataset 1)
Soil Excavation Face Validation Samples Area A Remedial No. of Analyte Mean UCL Mean Max Target Samples Concentration Concentration Concentration (mg/kg) (mg/kg) (mg/kg) (mg/kg) Phenol SGV 1,000 196 0.056 0.063 1 Naphthalene 37 196 0.678 0.875 13 Acenaphthylene 850 196 0.243 0.292 3 Acenaphthene 850 196 0.299 0.431 14 Fluorene 850 196 0.387 0.549 13 Phenanthrene 1,000 196 1.7 2.3 39 Anthracene 850 196 0.473 0.624 10 Fluoranthene 850 196 2.0 2.6 40 Pyrene 850 196 1.9 2.6 40 Benzo‐a‐anthracene 90 196 0.915 1.2 20 Chrysene 140 196 1.0 1.4 21 Benzo‐b‐fluoranthene 100 196 0.976 1.3 23 Benzo‐k‐fluoranthene 140 196 0.650 0.854 13 Benzo‐a‐pyrene 14 196 0.968 1.3 19 Indeno‐123‐cd‐pyrene 60 196 0.857 1.2 22 Di‐benz(a,h,)anthracene 13 196 0.198 0.245 3 Benzo‐ghi‐perylene 650 196 1.3 1.8 49 Benzene 28 196 < DL
Celtic, June 2012 31 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final)
Table 8: (continued)
Soil Excavation Face Validation Samples Area A Remedial No. of Mean UCL Mean Max Analyte Target Samples Concentration Concentration Concentration (mg/kg) (mg/kg) (mg/kg) (mg/kg) Aromatic TPH C12‐16 1,000 196 5.2 8.9 350 Aromatic TPH C16‐21 1,000 196 11 17 570 Aromatic TPH C21‐35 1,000 196 9.8 14 250 Aromatic TPH C35‐44 1,000 196 0.326 0.515 16 Total Aliphatic / Aromatic 2,500 196 61 98 2,800
Table 9: Area A Chemical Validation Results (dataset 2)
Soil Excavation Face Validation Samples Area A Remedial No. of Analyte Mean UCL Mean Max Target Samples Concentration Concentration Concentration (mg/kg) (mg/kg) (mg/kg) (mg/kg) Phenol SGV 1,000 9 < LDL Celtic, June 2012 32 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 9: (continued) Soil Excavation Face Validation Samples Area A Remedial No. of Mean UCL Mean Max Analyte Target Samples Concentration Concentration Concentration (mg/kg) (mg/kg) (mg/kg) (mg/kg) Aliphatic TPH C6‐8 1,000 9 0.7 0.979 1 Aliphatic TPH C8‐10 1,000 9 0.7 0.979 1 Aliphatic TPH C10‐12 1,000 9 0.7 0.979 1 Aliphatic TPH C12‐16 1,000 9 0.7 0.979 1 Aliphatic TPH C16‐35 1,000 9 0.7 0.979 1 Aliphatic TPH C35‐44 1,000 9 0.7 0.979 1 Aromatic TPH C5‐7 1,000 9 0.7 0.979 1 Aromatic TPH C7‐8 1,000 9 0.7 0.979 1 Aromatic TPH C8‐10 940 9 0.7 0.979 1 Aromatic TPH C10‐12 1,000 9 0.7 0.979 1 Aromatic TPH C12‐16 1,000 9 1.5 2.4 4 Aromatic TPH C16‐21 1,000 9 3.7 6.3 12 Aromatic TPH C21‐35 1,000 9 7.2 13 24 Aromatic TPH C35‐44 1,000 9 0.7 0.979 1 Total Aliphatic / Aromatic 2,500 9 15 23 40 Celtic, June 2012 33 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 10: Area B Chemical Validation Results Soil Excavation Face Validation Samples Area B Remedial No. of Analyte Mean UCL Mean Max Target Samples Concentration Concentration Concentration (mg/kg) (mg/kg) (mg/kg) (mg/kg) Phenol SGV 1,000 171 0.051 0.052 0 Naphthalene 37 171 0.867 1.1 12 Acenaphthylene 850 171 0.257 0.336 8 Acenaphthene 850 171 0.282 0.348 4 Fluorene 850 171 0.342 0.48 13 Phenanthrene 1,000 171 1.2 1.7 39 Anthracene 850 171 0.362 0.502 13 Fluoranthene 850 171 1.3 1.7 29 Pyrene 850 171 1 1.3 20 Benzo‐a‐anthracene 90 171 0.57 0.766 12 Chrysene 140 171 0.573 0.769 12 Benzo‐b‐fluoranthene 100 171 0.512 0.685 9 Benzo‐k‐fluoranthene 140 171 0.365 0.48 6 Benzo‐a‐pyrene 14 171 0.583 0.8 13 Indeno‐123‐cd‐pyrene 60 171 0.436 0.601 13 Di‐benz(a,h,)anthracene 13 171 0.157 0.197 3.5 Benzo‐ghi‐perylene 650 171 0.513 0.7 14 Benzene 28 168 0.004 0.007 0 Toluene 870 168 0.003 0.005 0.3 Ethylbenzene 520 168 0.015 0.034 1.9 Sum Xylene 475 168 0.01 0.013 0.2 Sum BTEX 1,000 168 0.032 0.057 2.5 Easily Liberatable Cyanide 32 171 0.509 0.520 1.6 Total Cyanide 150 171 13 18 270 Arsenic 640 171 18 22 370 Cadmium SGV 240 171 0.147 0.166 1 Chromium III 1100 171 34 37 140 Chromium VI 35 168 0.498 0.502 1 Lead 750 171 34 26 610 Inorganic Mercury 50 171 0.213 0.292 8 Selenium 1,000 171 0.328 0.4 8 Copper 1,000 171 34 42 690 Nickel 1,800 171 33 36 180 Celtic, June 2012 34 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 10: (continued) Soil Excavation Face Validation Samples Area B Remedial No. of Mean UCL Mean Max Analyte Target Samples Concentration Concentration Concentration (mg/kg) (mg/kg) (mg/kg) (mg/kg) Aliphatic TPH C6‐8 1,000 171 0.332 0.381 1 Aliphatic TPH C8‐10 1,000 171 0.688 1.3 61 Aliphatic TPH C10‐12 1,000 171 2.1 4 200 Aliphatic TPH C12‐16 1,000 171 2.9 6.2 340 Aliphatic TPH C16‐21 1,000 171 1.5 2.2 79 Aliphatic TPH C21‐35 1,000 171 2.1 3.7 160 Aliphatic TPH C35‐44 1,000 171 0.905 0.94 1 Aromatic TPH C5‐7 1,000 171 0.332 0.381 1 Aromatic TPH C7‐8 1,000 171 0.332 0.381 1 Aromatic TPH C8‐10 940 171 0.529 0.84 32 Aromatic TPH C10‐12 1,000 171 2.3 4.4 220 Aromatic TPH C12‐16 1,000 171 4.3 9.2 510 Aromatic TPH C16‐21 1,000 171 3 4.9 200 Aromatic TPH C21‐35 1,000 171 3.1 4.2 96 Aromatic TPH C35‐44 1,000 171 0.911 0.947 2 Total Aliphatic / Aromatic 2,500 171 22 41 1,900 Celtic, June 2012 35 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 11: Area C Chemical Validation Results Soil Excavation Face Validation Samples Area C Remedial No. of Analyte Mean UCL Mean Max Target Samples Concentration Concentration Concentration (mg/kg) (mg/kg) (mg/kg) (mg/kg) Phenol SGV 1,000 82 0.276 0.47 6 Naphthalene 37 82 1.2 1.7 15 Acenaphthylene 850 82 0.542 0.736 6 Acenaphthene 850 82 0.449 0.687 9 Fluorene 850 82 0.712 1 9 Phenanthrene 1,000 82 2.7 3.6 26 Anthracene 850 82 0.636 0.865 8 Fluoranthene 850 82 2.5 3.6 43 Pyrene 850 82 2.4 3.3 36 Benzo‐a‐anthracene 90 82 1.2 1.8 20 Chrysene 140 82 1.3 1.9 21 Benzo‐b‐fluoranthene 100 82 1.2 1.7 18 Benzo‐k‐fluoranthene 140 82 0.665 1 12 Benzo‐a‐pyrene 14 82 1.4 2.1 23 Indeno‐123‐cd‐pyrene 60 82 1.1 1.6 21 Di‐benz(a,h,)anthracene 13 82 0.281 0.407 4.4 Benzo‐ghi‐perylene 650 82 1.2 1.7 18 Benzene 28 82 0.255 0.464 7 Toluene 870 82 0.025 0.04 0.5 Ethylbenzene 520 82 0.017 0.026 0.3 Sum Xylene 475 82 0.06 0.093 0.9 Sum BTEX 1,000 82 0.356 0.598 8.9 Easily Liberatable Cyanide 32 82 0.781 1 11 Total Cyanide 150 82 6.7 9.1 82 Arsenic 640 82 18 21 130 Cadmium SGV 240 82 0.167 0.198 1 Chromium III 1,100 82 32 35 110 Celtic, June 2012 36 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 11: (continued) Soil Excavation Face Validation Samples Area C Remedial No. of Mean UCL Mean Max Analyte Target Samples Concentration Concentration Concentration (mg/kg) (mg/kg) (mg/kg) (mg/kg) Chromium VI 35 60 0.500 0.500 1 Lead 750 82 85 30 4,200 Inorganic Mercury 50 82 0.223 0.327 5 Selenium 1,000 82 0.291 0.333 2 Copper 1,000 82 26 28 81 Nickel 1,800 82 34 37 80 Zinc 1,500 82 74 82 280 Aliphatic TPH C5‐6 1,000 82 0.122 0.147 1 Aliphatic TPH C6‐8 1,000 82 0.122 0.147 1 Aliphatic TPH C8‐10 1,000 82 0.224 0.383 8 Aliphatic TPH C10‐12 1,000 82 1.9 4.7 140 Aliphatic TPH C12‐16 1,000 82 12 31 900 Aliphatic TPH C16‐21 1,000 82 19 40 1,000 Aliphatic TPH C21‐35 1,000 82 88 223 6,700 Aliphatic TPH C35‐44 1,000 82 2.5 6.3 190 Aromatic TPH C5‐7 1,000 82 0.122 0.147 1 Aromatic TPH C7‐8 1,000 82 0.133 0.164 1 Aromatic TPH C8‐10 940 82 0.368 0.531 6 Aromatic TPH C10‐12 1,000 82 3.1 5.5 110 Aromatic TPH C12‐16 1,000 82 13 24 460 Aromatic TPH C16‐21 1,000 82 15 23 260 Aromatic TPH C21‐35 1,000 82 21 38 840 Aromatic TPH C35‐44 1,000 82 0.187 0.242 2 Total Aliphatic / Aromatic 2,500 82 177 350 8,100 Celtic, June 2012 37 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 12: Site Re‐use Material Chemical Validation Results Soil Site‐Won Re‐Use Material Validation Samples Remedial No. of Analyte Mean UCL Mean Max Target Samples Concentration Concentration Concentration (mg/kg) (mg/kg) (mg/kg) (mg/kg) Phenol 1,000 174 0.224 0.336 8 Naphthalene 37 176 2.9 3.6 35 Acenaphthylene 850 176 2.4 2.9 27 Acenaphthene 850 176 1.4 1.8 38 Fluorene 850 176 2.2 2.9 42 Phenanthrene 1,000 176 13 16 300 Anthracene 850 176 4.3 5.4 94 Fluoranthene 850 176 18 21 320 Pyrene 850 176 17 21 230 Benzo(a)anthracene 90 176 7.6 9 100 Chrysene 140 176 8.3 9.8 110 Benzo(b)fluoranthene 100 176 7.4 8.5 54 Benzo(k)fluoranthene 140 176 4.8 5.8 83 Benzo(a)pyrene 14 176 7.9 9.1 80 Indeno(1,2,3‐cd)pyrene 60 176 6.5 7.5 54 Di‐benz(a,h,)anthracene 13 176 1.3 1.5 13 Benzo(g,h,i)perylene 650 176 10 12 110 Benzene 28 174 0.013 0.025 1 Toluene 870 174 0.009 0.015 0.6 Ethylbenzene 520 174 0.018 0.038 2.1 Celtic, June 2012 38 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 12: (continued) Soil Site‐Won Re‐Use Material Validation Samples Remedial No. of Mean UCL Mean Max Analyte Target Samples Concentration Concentration Concentration (mg/kg) (mg/kg) (mg/kg) (mg/kg) Sum Xylene 475 174 0.027 0.043 1.4 Total BTEX 1,000 174 0.067 0.119 5.3 Easily‐liberatable Cyanide 32 174 0.795 0.861 3 Total Cyanide 150 174 66 76 380 Arsenic 640 174 39 44 200 Cadmium 240 174 0.385 0.442 5 Chromium III 1,100 174 28 32 320 Chromium VI 35 118 0.504 0.511 1 Lead 750 177 150 80 3,000 Mercury 50 174 0.624 0.814 13 Selenium 1,000 174 0.455 0.527 6 Copper 1,000 174 49 58 830 Nickel 1,800 174 39 49 980 Zinc 1,500 174 124 135 420 Aliphatics >C5‐C6 1,000 174 0.240 0.281 1 Aliphatics >C6‐C8 1,000 174 0.240 0.281 1 Aliphatics >C8‐C10 1,000 174 0.619 0.856 17 Aliphatics >C10‐C12 1,000 174 2.7 3.7 64 Aliphatics >C12‐C16 1,000 174 22 31 650 Aliphatics >C16‐C21 1,000 174 34 48 880 Aliphatics >C21‐C35 1,000 174 25 34 660 Aliphatics >C35‐C44 1,000 174 0.861 1.5 70 Aromatics >C5‐C7 1,000 174 0.241 0.282 1 Aromatics >C7‐C8 1,000 174 0.240 0.281 1 Aromatics >C8‐C10 940 174 2.1 3 43 Aromatics >C10‐C12 1,000 174 5.1 6.9 120 Aromatics >C12‐C16 1,000 174 27 35 390 Aromatics >C16‐C21 1,000 174 63 79 940 Aromatics >C21‐C35 1,000 174 90 108 780 Aromatics >C35‐C44 1,000 174 2.6 4.0 140 Total Aliphatic / Aromatic 2,500 174 273 341 3,200 Table 13: Import Material Chemical Validation Results Soil Import Fill Validation Samples Remedial No. of Analyte Mean UCL Mean Max Target Samples Concentration Concentration Concentration (mg/kg) (mg/kg) (mg/kg) (mg/kg) Phenol SGV 1,000 55 0.188 0.387 7 Naphthalene 37 55 0.120 0.154 1 Acenaphthylene 850 55 0.1 0.1 0 Acenaphthene 850 55 0.103 0.107 0 Fluorene 850 55 0.1 0.1 0 Phenanthrene 1,000 55 0.217 0.308 2 Anthracene 850 55 0.115 0.13 0 Fluoranthene 850 55 0.176 0.226 1 Pyrene 850 55 0.156 0.193 1 Celtic, June 2012 39 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 13: (continued) Soil Import Fill Validation Samples Remedial No. of Mean UCL Mean Max Analyte Target Samples Concentration Concentration Concentration (mg/kg) (mg/kg) (mg/kg) (mg/kg) Benzo‐a‐anthracene 90 55 0.111 0.12 0 Chrysene 140 55 0.115 0.127 0 Benzo‐b‐fluoranthene 100 55 0.105 0.113 0 Benzo‐k‐fluoranthene 140 55 0.101 0.102 0 Benzo‐a‐pyrene 14 55 0.102 0.104 0 Indeno‐123‐cd‐pyrene 60 55 0.101 0.104 0 Di‐benz(a,h,)anthracene 13 55 0.101 0.101 0.13 Benzo‐ghi‐perylene 650 55 0.102 0.106 0.23 Benzene 28 55 0.001 0.001 0 Toluene 870 55 0.001 0.001 0 Ethylbenzene 520 55 0.001 0.001 0 Sum Xylene 475 55 0.002 0.002 0 Sum BTEX 1,000 55 0.004 0.004 0 Total Cyanide 150 55 0.587 0.644 1.7 Arsenic 640 55 24 29 87 Cadmium SGV 240 55 0.372 0.524 4 Chromium III 1,100 55 7.6 8.1 13 Chromium VI 35 51 0.5 0.5 1 Lead 750 55 20 14 190 Inorganic Mercury 50 55 0.1 0.1 0 Selenium 1,000 55 0.284 0.328 1 Copper 1,000 55 5.1 5.2 8 Nickel 1,800 55 5.8 6.3 17 Zinc 1,500 55 61 92 820 Aliphatic TPH C5‐6 1,000 55 0.1 0.1 0 Aliphatic TPH C6‐8 1,000 55 0.1 0.1 0 Aliphatic TPH C8‐10 1,000 55 0.1 0.1 0 Aliphatic TPH C10‐12 1,000 55 0.771 0.861 1 Aliphatic TPH C12‐16 1,000 55 0.771 0.861 1 Aliphatic TPH C16‐21 1,000 55 0.771 0.861 1 Aliphatic TPH C21‐35 1,000 55 0.771 0.861 1 Aliphatic TPH C35‐44 1,000 55 0.771 0.861 1 Aromatic TPH C5‐7 1,000 55 0.1 0.1 0 Aromatic TPH C7‐8 1,000 55 0.1 0.1 0 Aromatic TPH C8‐10 940 55 0.1 0.1 0 Aromatic TPH C10‐12 1,000 55 0.771 0.861 1 Aromatic TPH C12‐16 1,000 55 0.785 0.871 1 Aromatic TPH C16‐21 1,000 55 0.828 0.924 2 Aromatic TPH C21‐35 1,000 55 0.849 0.977 4 Aromatic TPH C35‐44 1,000 55 0.771 0.861 1 Total Aliphatic / Aromatic 2,500 55 8.0 8.8 10 Chromium VI 35 51 0.5 0.5 1 Celtic, June 2012 40 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 14: Soil Leachate Chemical Validation Results (dataset 1) Soil Excavation Face and Re‐use Validation Samples Leachate No. of Analyte Mean UCL Mean Max Target Samples Concentration Concentration Concentration (mg/L) (mg/kg) (mg/kg) (mg/kg) Phenol SGV 52 174 0.015 0.027 1 Naphthalene 2.3 174 0.004 0.007 0 Phenanthrene 8.2 174 0.003 0.004 0 Total Cyanide 2.5 174 0.437 0.529 5.6 Chromium 0.18 174 0.006 0.007 0 Lead 0.18 177 0.003 0.002 0 Copper 0.18 174 0.006 0.008 0 Nickel 1.8 174 0.008 0.01 0 Zinc 1.8 174 0.038 0.049 1 Aromatics >C8‐C10 47 151 0.005 0.007 0 Aromatics >C10‐C12 49 151 0.014 0.022 1 Aromatics >C12‐C16 79 151 0.063 0.117 5 Aromatics >C16‐C21 580 151 0.665 1.7 92 Ammonia (free) 19 164 0.672 1 20 Table 15: Soil Leachate Chemical Validation Results (dataset 2) Soil Excavation Face and Re‐use Validation Samples Leachate No. of Analyte Mean UCL Mean Max Target Samples Concentration Concentration Concentration (mg/L) (mg/kg) (mg/kg) (mg/kg) Phenol 52 81 0.012 0.02 0 Naphthalene 2.3 81 0.012 0.026 1 Phenanthrene 8.2 81 0.001 0.002 0 Total Cyanide 2.5 81 0.261 0.341 2.8 Chromium 0.18 81 0.037 0.068 1 Lead 0.18 81 0.006 0.002 0 Copper 0.18 81 0.006 0.009 0 Nickel 1.8 81 0.012 0.021 0 Zinc 1.8 81 0.043 0.066 1 Aromatics >C8‐C10 47 79 0.020 0.035 1 Aromatics >C10‐C12 49 79 0.054 0.102 2 Aromatics >C12‐C16 79 79 0.05 0.081 1 Aromatics >C16‐C21 580 79 0.03 0.052 1 Ammonia (free) 19 79 0.344 0.442 3 Notes – LDL – Less Than Detection Limit 7.7.4 Discussion of Results Exceeding SSTLs The mean and UCL mean values for all samples affecting final site conditions remained well below their respective site target levels. Several exceedances for maximum SSTLs are however reported, and as such are qualified accordingly below. Area A Total cyanide at 720 mg/kg. This sample – number 870 – was an excavation face sample obtained at 1.76 m bgl from cell N3 close to the northern site boundary and at the geographical limit of the excavation. No additional material excavation was possible due to Celtic, June 2012 41 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) the possibility of undermining the stability of the permanent site‐boundary fencing and adjoining structures. All leachate values for this sample remained within SSTLs. Total cyanide at 690 mg/kg. This peat sample from cell N2 was reviewed in consultation with the GCC CLO and permission granted to leave affected material in situ (discussed in detail in Section 7.9.2). An additional sample from the same cell analysed for leachate (sample number 542) provided additional confidence on the limitations of the contaminant’s mobility. Aliphatic C16‐C21 at 1,500 mg/kg, and total aliphatic at 2,800 at mg/kg. Sample number 704 was obtained from 1.5 m bgl, and formed one of eleven samples (samples 704 to 714) obtained from material in and around the Black Ditch Culvert as part of an evaluation of the material conditions. Apart from one additional sample having a C16‐C21 aliphatic range concentration of 1,200 mg/kg, all remaining concentrations – including all leachate values – remained within SSTLs. Copper at 1,300 mg/kg, sample number 1281. Isolated outlier – single exceedance for Area A. Zinc at 1,600 mg/kg, sample number 508. Isolated outlier – single exceedance for Area A. Area B Three samples – 1799, 1801 and 1802 – exceeded the SSTL for total cyanide to a maximum value of 270 mg/kg. The samples were obtained from cell R13 at a depth of 1.5 m bgl. All samples returned leachate values below SSTLs for all analytes. Area C Two Area C samples for material that remains in‐situ returned high values for TPH; samples 136 (cell I10) and 1143 (cell D9C). Sample 136 also had aliphatic‐range C21‐35 value of 6,700 mg/kg. Both samples are from site boundary face excavations extended to their geographical excavation limits. The soil leachate results for sample 136 returned values within SSTLs for all analytes; no leachate data is available for sample 1143. Site‐won Re‐use Material In order to minimise the environmental impact associated with avoidable off‐site material disposal, several stockpiles with total cyanide values exceeding SSTLs up to a maximum of 380 mg/kg were re‐used on site as fill at depth below 5 m within Gasholder 2. Soil leachate results for all material re‐used were well within SSTLs. Sample 339 from stockpile 31 exceeded SSTLs for benzo(a)pyrene and benzo(a)anthracene at 80 and 100 mg/kg respectively. As two additional sample sets (samples 340 and 341) from the same stockpile both returned results below 1 mg/kg, and also the leachate values for sample 339 were all below SSTLs, this material was re‐used as site backfill at depths below 0.5 m. The TPH value of 3,200 mg/kg was in sample 1345, one of three samples (1343, 1344) taken of stockpile 60. Both additional soil samples and all leachate values were well within SSTLs, and on this basis the material was declared suitable for re‐use as site backfill below 1 m. Sample 2008 from stockpile 98, also one of three (2006 and 2007) had a TPH value of 2,900 mg/kg. Similarly, both additional samples and all leachate values were within SSTLs. Whilst a proportion of this material was re‐used as backfill below 1 m, almost two‐thirds was deemed geotechnically unsuitable and sentenced for off‐site disposal. Celtic, June 2012 42 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) The 3,000 mg/kg value for lead is an anomalous result from stockpile 57 (sample 1183). Three additional samples, 1243, 1244 and 1245, were obtained and re‐tested for lead; all of which returned values below 10 mg/kg. Leachate concentrations were also below detection limits, and on these bases the material was classified as suitable for site re‐use. Cross‐sectional profiles of all backfill areas and the material stockpiles used are provided on Drawing 5237‐GH2‐001‐016, Appendix A Soil Leachate Most soil leachate results for material retained on site fell well within site target levels, with very few samples returning elevated concentrations. The 5.6 mg/L value for total cyanide in Table 14 was the original leachate value for the N2 peat sample discussed above. The total cyanide value of 2.8 mg/L in Table 15 was a minor exceedance for excavation face soil sample 1797 (cell S13), for which all other values remained well within target. The minor exceedance for free ammonia at 20 mg/L was from cell D9C3 (sample 1142); concentrations for all other analytes within the soil sample met SSTLs. No additional exceedances for free ammonia within site‐retained materials were received. 7.8 Excavation Works Excavations commenced late February 2011 within the western section of Area C (subsequent location of site compound), including also shallow remediation work to Gasholder 1. Initially extending eastward and beyond throughout the remaining site areas, the excavation work broadly followed the chronological sequence outlined within the main construction programme. The brief provided to all site workers involved in excavations across the site was to ensure that materials were segregated and stockpiled on a like‐for‐like basis to ensure geochemical and geophysical consistency. Efforts were made throughout the project to efficiently focus resources given the large geographical area of the site. Where necessary however, variations in start and/or completion dates for specific areas were employed in order to accommodate site spatial and operating constraints, or other imperatives as they emerged. This often required simultaneous and contiguous operations to progress at three or more site locations during any given period. The order of each area worked broadly followed the following sequence: • GH1, and Area C • Area A1, A3, Tar Tank, Area C, Area E (low permeability barrier) • Area A2, A3, GH2, Area C • Area A3, Area B3, Area C3 • Area B2, B3, B4 • Area B2, B3, B4, Area C For the purposes of simplification, details of the remedial works attributed to each area or location reported in the following sections are presented in alphabetical and numerical order. References to ‘x’ m bgl refers to depth below ground level, specifically either pre‐existing survey elevations or completed survey elevations according to context. In addition to excavations of the remedial areas outlined above, exploratory trial pits were excavated within Area E (Glevum House Car Park), and within and adjacent to the former WWU Building. Details of these works are covered in Section 7.12. Celtic, June 2012 43 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) The following drawings are included in Appendix A and should be reviewed in conjunction with the descriptions of the excavation areas: D1338/3876/A1 Site Location Plan D5237‐EXC‐001‐016 Site Excavations ‐ Heatmap Representation (by area) D5237‐FGL‐001‐002 As‐Built Ground Levels ‐ Topographical Survey D5237‐GRID‐001 Sample Grid Locations D5237‐OBSALL‐001‐006 All Site Obstructions (by area) D5237‐OGL‐001‐006 Original Ground Levels – Topographical Survey D5237‐TPL‐001 Trial Pit – Japanese Knotweed – Valve Compound Locations D5237‐VAL‐001‐017 Validation Sample Locations D5237‐SERV‐001‐015 Services and Site Constraints 7.9 Excavation, Remediation and Backfill Detail by Area 7.9.1 Area A1 Outline and Context Located in the north‐western corner of the site, this area was known to contain extensive deposits of spent oxide and foul lime. Excavations began end of May 2011, and completed at the end of July 2011. Existing surface material across the whole of Areas A1, A2 and A3 prior to remediation works commencing, comprised a surface capping layer of 50‐70 mm limestone above a geotextile fabric. These materials had been installed to create a dust‐break layer following previous remediation works carried‐out by Celtic during 2004. Stripping and stockpiling of this cover layer for subsequent re‐use preceded any significant remedial activities across all of the northern site ‘A’ areas. Other important features to be found within A1 included a tarmac surfaced roadway running north to south connecting the site to St Albans Road through the site’s northern access gates, and the Black Ditch culvert, which flowed east to west towards the western site boundary and beneath and beyond the adjoining premises. Details of excavation and investigative works relating to the Black Ditch are contained in the following section. An additional notable structure to be found within Area 1 was the remnant of a previous Black Ditch culvert which had become abandoned upon construction of the replacement structure. The distribution of material requiring removal across A1 was expected to cover an area of 6,300 m2. Re‐profiling works for Area A were completed to provide a level working platform at a final elevation of 11.36 m AOD. In order to restrict the migration of perched water, and to continue the utilisation of the haulage road for the purposes of off‐site material disposal for as long as possible, this area was excavated in segregated sections. Excavation Works Excavations began in the north‐western corner, continuing along the western boundary to a maximum depth of 4 m bgl. The extensive foul lime deposit extended to a 0.3 m thick layer of natural dark brown peat. An analysis of a sample obtained from this material returned a Total Cyanide concentration which exceeded the SSTL for this compound. The Easily Liberatable Cyanide value however was less than 5 % of the agreed SSTL which indicated that the contaminant remained tightly bound within complexed compounds with limited mobility. A Celtic, June 2012 44 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) leachate test was performed on material from the same location, and following consultations with the CLO which considered the material’s limited range, leaching potential, existing ground conditions, environmental receptors, material depth and the site’s proposed end‐use, it was agreed that the excavation could terminate at this level with no further material removal necessary. The sampling location for this material can be found on Drawing numbers 5237‐GRID‐001 and 5237‐VAL‐001, Appendix A (cell N2); relevant correspondence relating to this issue is provided at Appendix I. As the excavation extended eastward and southward, the prevalence of ashy or otherwise carboniferous material containing highly visible blue‐green spent oxide deposits emerged. The extent to which the majority of excavations were required to proceed throughout the remainder of A1 varied between 1 and 3 m bgl; with a mean cut level throughout the area of 2.2 m. With the exception of the material in and adjacent to cell N2, most visibly contaminated material was excavated to natural alluvium or clay. In total, 13,600 m3 of material was excavated from Area A1, which also included 380 m3 of concrete and hard material associated with the remnants of the old Black Ditch culvert. Excavation depths throughout the dig area remained above the known groundwater level of 4.2 m bgl, although significant volumes of perched water were encountered at the interface between the clay‐rich natural materials and made ground. Having reviewed the data from a sample of water from this location, a variation to the existing consent to discharge was granted by Severn Trent Water allowing the Contractor to discharge accumulated surface water to foul sewer with no additional treatment requirement. Correspondence confirming this concession is presented at Appendix K. Validation results for Area A are presented in Tables 8 and 9 above. Black Ditch Culvert Included within the original design plans for Area A1 was the removal and replacement of the Black Ditch culvert, which had previously been considered to be in a poor physical condition. On the basis of extrapolated information from previous investigation work, it was also believed likely that grossly contaminated material to be found in this area extended beneath the structure, further supporting that its removal and replacement was a necessary requirement. Following its extensive exposure during the course of the work, visible inspections of the access chambers and the external concrete encasing the pipework indicated that the structure remained in a generally good state of repair. In addition, underlying natural material did not appear to be impacted by contamination sources; confirmed by material samples obtained for chemical analyses, all of which showed the surrounding material to be below SSTLs. A subsequent jet‐clean and CCTV survey of the entire structure revealed one minor defect where a rootlet had entered through the collar on a pipework joint. As a result of a series of discussions amongst the Client and Project Team, it was concluded that due to the satisfactory condition of the surrounding materials, and the generally very good condition of the culvert throughout its length, that replacement of the structure was unnecessary. The minor repair required at the pipe joint was completed by removing fully the plant and its rootlet, and grouting the small void. An additional concrete and shutter repair was made to a small section of defective concrete casing at one location. Copies of the CCTV report and DVD are included at Appendix K. Celtic, June 2012 45 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Backfill Works Backfill materials were deposited in accordance with the instructions outlined in Paragraph 7.5.4 of the Stage V Celtic Design Document. The material used for reinstatement of the formation layer was a combination of site‐won material comprising screened soils, and crushed brick and concrete to grade 6F2. The basal layer comprised vibration‐rolled 6F2 of minimum 300 mm depth, over which were deposited screened soils in 300 mm layers, compacted with a vibrating roller at each interleave. Surfacing comprised locally sourced <40 mm limestone scalpings over geotextile fabric. As required across the whole of Areas A, completed reinstatement elevation was established at 11.36 AOD. Cross‐sectional backfill profiles, including details of stockpile materials used are provided on Drawings 5237‐PRO‐003 and 5237‐PRO‐004, Appendix A. 7.9.2 Area A2 Area A2 was located at the north‐western boundary of the site, forming a strip of approximately W14.5 x L100 m (1,477 m2), south of and adjoining Area A1. Previous SI data had indicated an estimated remediation volume of 4,300 m3 of mainly carbon black and hydrocarbon impacted material. Excavation and reinstatement work within A2 was completed over a seven week period between late August and Early October 2011. No major structures or obstacles were encountered through the dig area, although to the south‐west, the need emerged to chase isolated shallow veins of contamination beyond the extent of the original A2 boundary. At the western extreme adjacent to the site boundary, a strip of foul lime was uncovered, which when further investigated, was found to extend southward into the corresponding section of Area A3 to a maximum depth of 2.7 m bgl. When delineated by trenching, the affected area was established to be an approximate rectangle of dimensions W20 x L40 m. In common with the similar material retained in Area A1, consultations were held with the CLO with a view to allowing the majority of the material to remain in‐situ. The application was supported by analytical data demonstrating that the material met all SSTLs. Agreement was reached to reduce the excavation formation level in this part of the site to 1 m below the final completion levels (0.7 m plus 0.3 m capping layer over geotextile). Correspondence relating to this application is included at Appendix I. A total of 3,380 m3 of material was excavated from A2 Backfilling of A2 was completed to the same specification as Area A1. A cross‐sectional backfill profile, including details of the stockpile materials used is provided on Drawing 5237‐PRO‐005, Appendix A. 7.9.3 Area A3 This section formed the majority of the northern site section by area, comprising in total 5,700 m2. The design remit for this area required the removal of the cover layer and reduction or fill, as appropriate, to a pre‐completion elevation of 11.06 m AOD prior to validation and completion to finished elevation. Significant pre‐planned excavations within A3 included the reduction of a 500 m2 rectangular area of uncontaminated land at the far north‐western site corner boundary, and removal of a Japanese knotweed cluster located to the south, close to the demarcation of Area B2 (cells T9 and T10 of Drawing 5237‐GRID‐001, Appendix A). Celtic, June 2012 46 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) During the shallow excavation and validation process, several further areas were identified as requiring further remediation due to the presence of spent oxide. In addition, isolated deposits of asbestos were also uncovered, mainly located in the east‐central section. All identified hotspots were excavated to their full extent and validated prior to backfilling. The removal, stockpiling and off‐site disposal of asbestos and asbestos containing material was performed under appropriate duty‐of‐care. Excavation of the Japanese knotweed was performed under the supervision of the specialist contractor as described in Section 7.5.3. Several significant hard obstructions were also removed, including reinforced concrete bases and platforms up to a thickness of 0.8 m, and a small tarmac access road which extended northwards from the operational PRS. With the exception of a limited number of isolated hotspots, the vast majority of excavations through A3 occurred within the surface 1 m. In total, 17,450 m3 of material was excavated, which included 680 m3 of concrete. The final surface cover comprised the installation of a 300 mm capping layer of imported virgin material over geotextile fabric. Within those areas requiring additional fill due to pre‐ existing topographical variation, and those areas excavated to remove contamination hot‐ spots, formation layers comprising site‐won materials were installed to the specification described in Area A1. A cross‐sectional backfill profile, including details of the stockpile materials used is provided on Drawing 5237‐PRO‐006, Appendix A. 7.9.4 Area B2 The initial works within B2 were undertaken between April and May 2011, with surface obstruction clearance and the stripping and stockpiling of topsoil. Between May and September 2011, the area was used for the stockpiling of site won material arising from Area A. Full remedial excavation work within B2 began in October 2011, commencing with the initial stripping to 1 m bgl of the material, leading to the breaking‐out and removal of sub‐ surface structures including reinforced concrete foundations. In addition to the concrete structures found throughout the section, further remnant structures removed included a small steel tank and a range of process pipework of varying diameters. With the exception of the bisecting access road spur, which was completed during February / March 2012, Area B2 works were completed by mid‐December 2012. To the west of the access road spur, much of the material type was largely consistent with reworked natural clays and alluvium, with isolated clinker, demolition rubble and ashy soil deposits. Directly adjacent to the boundary with Area B1, and extending eastward towards the boundary with the road spur, the material varied to consist of a combination of black ashy low density soils, clinker and occasional spent oxide deposits. Much of the material removed from this area was established as being hydrocarbon impacted, with numerous localised areas of tarry contamination. All sections of pipework discovered within the main dig area were removed in their entirety. In instances where pipework extended beyond the B2 boundary – to the west beneath the western boundary with Area E and the northern area boundary with Area B1 – these pipes were sealed with low permeability clay and cement mortar prior to backfilling. Most excavation depths within the western section of B2 were between 1.5 to 3 m bgl, but extended to below 3 m bgl in several locations. To the east of the road spur, the material types ranged between reworked alluvium, ash and carbon deposits, and soils affected with spent oxide and oxidised iron deposits. With the exception of a narrow strip adjacent to the southern boundary of Area A3, the excavation depth throughout this section was relatively consistent, with natural materials encountered at between 1.8 and 2.2 m bgl. Excavations Celtic, June 2012 47 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) within the eastern section continued to within 2 m of the PRS. Due to the need to continue using the access road for as long as possible, work to remediate the segment of B2 that extended beneath this section was dealt with last. Break‐out and excavation continued through January 2012, before full reinstatement during February / March 2012. A total of 10,740 m3 of material, which included 1,180 m3 of concrete, was excavated from Area B2. Backfilling of B2 was completed to the specification outlined in Paragraph 7.5.4 of the Stage V Celtic Design Document, with surface elevations reinstated to pre‐works’ original ground levels (OGLs). 52 % of the material used for the formation layers was site‐won; the remainder comprising imported virgin crushed limestone. Cross‐sectional backfill profiles, including details of stockpile materials used are provided on Drawings 5237‐PRO‐007 and5237‐PRO‐007, Appendix A. 7.9.5 Area B3 Area B3 had continued to be used as a service base for civil engineering contractor Morrisons during the early stages of the works. Full clearance of the surface area began in early June 2011 following their demobilisation. In conjunction with Area B2, B3 was used as a location for site‐excavated material stockpiling until the Autumn of 2011. During the period February 2011 to January 2012, the access road running through Areas B and connecting the site entrance to the Glevum car park was used as the main access and egress route for site‐service and WWU traffic. It was also used for the siting of the site wheel‐ wash. The requirement to maintain the access route meant it was necessary to remediate B3 in three separate stages. Remediation work began early September with the removal of the surface cover which comprised mainly 100 mm tarmac and occasional concrete slabbing. The first stage of works began with the excavation of the strip adjacent to the access road to the south of the section, latterly extending northwards into Area B2, and culminating with the excavation and reinstatement of the replacement access road. Numerous and significant concrete and brickwork structures were encountered throughout most of B3. These included heavy‐gauge foundations, slabs, pile caps and a 6 m length oval‐ shaped concrete tank within the southern section. Centrally, the area was traversed by two major brick walls running NW‐SE, between which was recovered a large volume of red brick and demolition rubble fill. Also located centrally at a depth of 2.5 m bgl were the remains of a 0.9 m diameter brick culvert, running NW beneath the Glevum car park. All structures were broken‐out or removed to a minimum 1 m bgl or the relevant excavation cut level; whichever was deeper. The range of material recovered was variable. To the east of the dig area, the northern and western extent of a shallow foul‐lime deposit extending into Areas C and B4 was removed. Centrally and northerly, the made ground comprised dark ashy and/or clayey soils, often containing a high percentage mix of brick or rubble fragments. Isolated pockets of heavily hydrocarbon‐impacted materials were removed, most of which were associated with remnant pipework. The mean vertical extent to which the excavations were required to proceed was generally shallower than the pre‐project estimate of 3 m bgl. With the exception of the brick‐filled central section, the majority of the cut levels through this area extended to between 1.5 and 2.5 m bgl. Several sections of metal pipework which progressed beneath the adjoining Area E Celtic, June 2012 48 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) to the west were cut at the area boundary, plugged with low permeability clay and sealed with mortar. The brick culvert was similarly plugged and sealed prior to backfilling. Excavation of the access road proceeded during January / February 2012. Whilst the works were completed within the overall programme time‐frame, the progress through this section of the site was impeded due to the need to deploy a 5‐tonne excavator to work around numerous active services and infrastructure, including manhole chambers connected to the main sewerage line running through the site, and a live high voltage cable which ran the road length, extending westward through Area E and eastward through Area B4. 7,850 m3 of material was excavated from B3, 22 % of which was brick and concrete. The main B3 dig area was backfilled and reinstated in accordance with the specification outlined in Paragraph 7.5.4 of the Stage V Celtic Design Document, with surface elevations reinstated to pre‐works OGLs. Cross‐sectional backfill profiles, including details of the stockpile materials used are provided on Drawings 5237‐PRO‐009 and 5237‐PRO‐009, Appendix A. 7.9.6 Area B4 Following the demolition of the Powerhouse Building (see Section 6.2), extensive major reinforced concrete structures were revealed throughout this area. These consisted of substantial foundations and piles up to 0.8 m diameter, and extensive concrete slabs of up to 1 m thick. In accordance with the practices agreed for the site, structures were removed to either a minimum of 0.3 m bgl or the excavation cut level, whichever the deeper. The local geology and material contamination type within this section was expected to be similar to that encountered in Area B3, although areas of peat were encountered towards to the south western sector at around 1.8 m bgl. Also, much of the made ground excavated from beneath the Powerhouse Building comprised a loose brown soil containing varying degrees of building rubble. Much of this recovered material was found to be chemically suitable for re‐ use. A secondary major constraint in B4 was the numerous services running north to south in close proximity to the PRS and access road. These included medium and high pressure gas mains, a high voltage electric cable, water hydrant and pipework, and access chambers connected to the sewerage system. These services restricted the amount of deeper excavation work that could be performed in some sections, limiting the dig to 0.3 m bgl for the section of B4 south of the PRS at its boundary with Area C, and a six‐metre strip to the PRS front (west). In order to provide a qualitative record, several trial pits were excavated within these areas, and soil and (where present) water samples were analysed. Where possible, throughout the remaining area of B4, visibly contaminated material was excavated to its full extent, though this was not a practical option in the southern section adjoining Area C and Area B3 due to a HV cable and water pipes. In these areas, material was chased back around each service line as far as practicably achievable, validated and backfilled as appropriate. The remaining area of B4 was unaffected by service restrictions, excavations continued to natural materials. Process pipework removed was mainly remnant cast‐iron lines connected with historical gas ‐ workings. Due to their orientation and proximity to the PRS, all pipework was drilled and tapped by WWU prior to their removal and disposal. A total of 6,250 m3 of material was excavated from B4, of which 2, 440 m3 was concrete and hard structures. Celtic, June 2012 49 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 1,730 m3 of site‐won re‐use material was used as backfill, the balance made up with imported crushed limestone. Materials were layered and compacted in accordance with the specification outlined in Paragraph 7.5.4 of the Stage V Celtic Design Document, with surface elevations reinstated to pre‐works OGLs. A cross‐sectional backfill profile, including details of the stockpile material used, is provided on Drawing 5237‐PRO‐011, Appendix A. Most of the work within Area B4 was completed by the first week of December 2011. Works associated with the installation of the ducting channels and the access road reinstatement, which extended eastward into B4 from Area B3, were completed first week of March 2012. Validation analysis results for Area B are presented in Table 10 above. 7.9.7 Area C Area C at 28,755 m2 comprised the largest individual remedial area, identified as a rectangular strip orientated across the southern site section, and bounded by Bristol Road to the west, Pearce Way to the south and Tuffley Rovers / Shakespeare Avenue to the east. Located within Area C were four additional sub‐areas identified as Gasholder 1 (GH1), Gasholder 2 (GH2), Tar Tank and Area C3. The remediation work undertaken in each of these sub‐areas is further discussed below. Historical investigation data had indicated that the majority of contaminated materials comprised of shallow surface deposits containing elevated cyanide within the eastern half of Area C, and spent oxide identified close to the former purifiers in the eastern section, extending northwards into Area B4. The majority of the elevated organic contamination (TPH, PAH and VOCs) was identified as being within and surrounding the two former gasholders (GH1 and GH2), and the benzole plant in the western half of Area C. Due to the expected shallow depth‐range of the contaminated materials, the remedial objective for Area C required excavation of surface materials to a depth of 300 mm below existing site levels prior to validation testing and backfilling. Work began in the western section of Area C at the end of February 2011 with site clearance and surface 300 mm scrape and reinstatement in preparation for the site compound set‐up (described in Section 7.3 above). This work included a shallow excavation of GH1. In accordance with the completion design specification for Area C, chemically validated and backfilled areas were covered with a geotensile mat over a geotextile membrane, and reinstated to pre‐works’ topographical levels with imported granular fill. This specification was repeated throughout all of Area C reinstatement, including sub‐areas. In order to comply with a Client request to provide a temporary alternative compound area for the use of WWU, remediation of the eastern section where it formed a boundary with Tuffley Rovers to the east and Area B4 to the north was undertaken next. Due to the presence of major gas service lines, the excavation works in this area, as within all areas where gas services existed, was performed using a 5‐tonne mini excavator. In localised dig areas close to service runs, where the need existed to either chase contaminated materials below 300 mm, or to break‐out extensive hard structures, these works were completed under the supervision of a WWU engineer. Approximately 70 % of Area C excavations remained within the surface 0.5 m, although two areas required more extensive excavation to depths up to 1 m bgl and 2 m bgl respectively. The first of these involved the removal of foul lime and spent oxide deposits found in the eastern section bordering areas B3 and B4; and the second the excavation of a concrete channel construction running parallel to the access road close to the boundary with Area B3. Celtic, June 2012 50 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Additional isolated hotspots were also removed; including the recovery of cast‐iron process pipework and associated hydrocarbon‐impacted materials in the western section, most of which were adjacent to the Tar Tank, GH2 and C3 remediation areas. Drawing 5237‐EXC‐001 provides a heatmap representation of the excavation depths across the site. The remaining Area C works were prioritised according to the site constraints and operations. These included the requirement to set aside the central section for the purposes of stockpiling site‐won materials and the maintenance of viable access routes for plant, machinery and haulage vehicles. As a result, most of the major work undertaken throughout this area was performed during two periods near the beginning and end of the project. Table 16 below provides a summary of all Area C progress events between February 2011 and January 2012. Validation analysis for Area C is presented in Table 11 above. Celtic, June 2012 51 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 16: Summary of Area C Progress Events – February 2011 to January 2012 Period Duration Description of Works Feb – Mar 4 weeks Removal of surface vegetation, rubbish and debris. Demolition of 2011 superficial surface structures, surface excavation to western section, capping and site compound set up. Mar – Apr 2 weeks Continued excavations to Area C west. Excavation to Area C east. 2011 Mar – Apr 4 weeks Live service investigation (extended during this period throughout the 2011 whole site). Apr 2011 2 weeks Backfilling Area C east and Area C west. Mar – May 8 weeks Excavation over live gas services along southern and eastern 2011 boundaries. Apr – Jun 9 weeks Capping layer fill over southern and eastern boundary gas services. 2011 Nov 2011 – 8 weeks Concrete break‐out and excavation of central section. Jan 2012 Dec 2011 – 3 weeks Backfilling and capping of central section. Jan 2012 A total of 9,600 m3 of material was excavated from Area C, of which 24 % was concrete and hard structures. Much of the excavated material from Area C was deemed suitable for site re‐use as backfill in other areas. The vast majority of the material type used for reinstatement of Area C therefore was imported quarried limestone. 7.9.8 Area C Valve Compounds Two open compounds containing gas service and valve connections were present within the central and western sections of Area C at the beginning of the project. The first compound located west‐of‐central at the site’s southern boundary with Pearce Way was designated by WWU as a live service facility to be retained. Following several visits by WWU engineers designed to improve the serviceability of the installation, the area within the compound was cleaned of rubbish and debris, and a surface improvement finish using the imported surface capping material was laid in and around the individual services. Prior to site demobilisation, the temporary high visibility net fencing previously used to highlight the compound’s location was replaced with metal pedestrian barriers. The second valve compound was located near the Area C western site boundary close to Bristol Road. In order to properly investigate the extent and range of its valve and pipe network, the area immediately in and around the compound was fully excavated using a 5‐tonne mini excavator. Following an inspection by WWU engineers, whereby the installation was declared dead, the area was backfilled and compacted, then reinstated with a finished cover layer. Correspondence relating to the inspection of the compound is included in Appendix I. Its location is indicated on Drawing 5237‐TPL‐001, Appendix A. The location of the remaining live service compound is provided on Drawing 5237‐FGL‐001, Appendix A. Celtic, June 2012 52 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 7.9.9 Area C3 The C3 remediation area comprised a rectangular section located within the west‐central section of Area C, in close proximity to the Tar Tank and Gasholder 2. This section of the site included the benzole plant, condenser works and detarrer units. Area C3 was excavated and backfilled in three stages during a nine‐week period between early August and late September 2011. The remediation work through this area was uncomplicated, with most of the excavated area falling within the pre‐works design location. Obstructions included a near‐surface concrete platform to the north‐west which was removed in its entirety, and a rectangular reinforced concrete tank extending to 2.5 m bgl, the walls of which were demolished and the base broken‐out to evaluate the soils beneath. Several further reinforced concrete structures were also removed. Numerous sections of heavy‐duty process pipework, some of which extended towards the tar tank and Gasholder 2, were also removed during the works. Most of the contaminated materials had been impacted, but had also included an isolated 10 m x 10 m x 1 m section in the northern half which contained water and red diesel. This section was drained to the tar tank prior to continuing the excavation. Apart from the concrete tank referred to above, the depth to which most of the deeper excavations extended throughout this section was generally close to 2 m bgl; equating to approximately 40 % by area. Much of the remaining dig was between 1 and 2 m bgl. The total volume of all materials removed from C3 amounted to 3,300 m3. With the exception of one stockpile batch – SP78 – deposited at 1.8 m bgl to the south of C3, all of the material used for backfill was validated site‐won fill having chemical concentrations consistent with site re‐use criteria. The batch referred to as SP78 contained a benzo(a)pyrene exceedance, but was considered acceptable for re‐use at depth due to its very low leachate concentration of <0.0001 mg/L. 7.9.10 Gasholder 1 Shallow excavation to a depth of 0.6 m bgl was undertaken within GH1 during the early preliminary works carried‐out to facilitate the site compound set‐up. An assessment of the material concurred with the findings of previous SI information, that no additional remediation work was required. In accordance with the Celtic Stage V design remit, the excavation was terminated and reinstated with a finishing cover layer of 300 mm imported quarried virgin stone. 7.9.11 Gasholder 2 Gasholder 2 (GH2) comprised a cylindrical brick structure of 40 m diameter by 11 m depth. The remediation of GH2 began the first week of May 2011, commencing with the excavation and stockpiling of the upper 1 m layer. Following chemical analyses, this material was within the site re‐use criteria and reused as backfill material. GH2 Odour Control Pre‐works investigation data had suggested that up to 10,000 m3 of the remaining material was expected to be tar and hydrocarbon impacted. As the excavation progressed however, it became apparent that the material had been even more heavily impacted by tar than Celtic, June 2012 53 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) previously anticipated, the removal of which ultimately required considerable efforts to minimise odours. Initial control and management procedures for the odours combined ongoing real‐time vapour monitoring using a photo‐ionisation detector (PID) instrument at the excavation boundary, supplemented with retrospective time weighted average (TWA) monitoring by use of vapour adsorption tubes at selected monitoring points across the site. In addition, sorption monitoring inside Glevum House was also carried‐out. Physical suppression of odours at the excavation face was provided by two aerosol diffusion units supplying a constant stream of diffused water containing an odour suppressant and neutralising agent. By week six of the GH2 excavation works, several complaints had been received from both off‐ site sources and site occupants, requiring a technical review of procedures by the site management team. This resulted in the deployment of additional odour suppressant measures, a change in working practices to more efficiently manage the stockpiling and prompt disposal of materials; and the maintenance of an ongoing dialogue with GCC CLO. These measures continued until the excavation of the gasholder was completed in the second week of August 2011. Excavation and Material Management The integrity of the GH2 structure was physically sound, resulting in the presence of perched water and liquids at approximately 1.2 m bgl. Extraction of the liquids within the gasholder was accomplished continuously as the excavation progressed to completion. The adjacent tar tank was utilised as a holding tank to receive the pumped liquids prior to onward transfer to a product separator and water treatment system. Treated water was discharged to foul sewer under licenced consent, a copy of which can be viewed at Appendix K. All dewatering and water treatment provision for this section of the works was performed by an Erith‐nominated sub‐contractor. The material was grossly contaminated throughout the gasholder depth profile. Chemical analyses of the excavated material established that none of the soft materials present below 1 m bgl were either recoverable for further site re‐use or off‐site disposal as hazardous waste; with all submitted samples failing the waste acceptance criteria (WAC) for disposal to landfill. As a result, all soft materials were sent for treatment at the Augean waste recycling and treatment centre in Peterborough. Much of the holder contents contained liquid tar, necessitating additional stabilisation prior to its removal. This was achieved by introducing and mixing dry foul lime and spent oxide material already sentenced for off‐site disposal. Due to their highly odorous character, stockpiles were managed by limiting their size to an approximation consistent with planned daily off‐site disposal vehicle haulage capacity. Where stockpiles were required to be left overnight, these were covered with polyethylene sheeting. Similarly, exposed surfaces within the gasholder excavation were covered and contained by introducing a capping layer of disposable materials prior to daily cease‐work. The excavation continued to the full extent of the gasholder’s depth, the bottom of which comprised a convex clay base; the deepest point being 11.2 m bgl at the structure’s outer rim. Total material volumes removed were 12,200 m3. Backfilling works began immediately after the structure had been emptied of its contents, beginning with the introduction of a site‐won 6F2 basal starter layer, followed by additional site‐won materials compacted in 300 mm layers. The backfill proceeded during two separate fortnightly periods in August and October 2011, corresponding with material availability. To Celtic, June 2012 54 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) minimise off‐site disposal, a number of stockpiles containing chemical concentrations with minor exceedances of the site re‐use criteria were used for fill at a depth at or lower than 5 m below finished ground elevation. For this material, re‐use criteria were limited to their chemical content containing no more than two exceedances from the fifty five for which maximum values had been established, and that the materials passed the corresponding leachate criteria. Drawing 5237‐GH2‐001, Appendix A outlines in cross‐section the backfill profile for this section of the site, complete with the relative stockpile identifiers. 7.9.12 Tar Tank Works to empty the tar tank contents began on May 6th 2011, and were completed within one week. The materials beneath 1 m bgl were heavily impacted with tar. The dimensions of the Tar Tank were 9.8 m (diam) x 5.6 m (depth). A total of 427 m3 of material was excavated. All material recovered from below 1 m failed WAC and site re‐use criteria, requiring off‐site disposal to the Churngold recycling facility at Bristol. As described in Section 7.9.11, the Tar Tank, when empty of its contents, was used as a liquids holding and transfer tank to facilitate the remediation of GH2. The storage of liquids within this facility continued until all treatment and disposal requirements had been discharged; with the backfilling begun and completed during the second week of November 2011. 7.9.13 Installation of Clay Low Permeability Barrier In order to mitigate future migration of potential contamination from Area E to the remediated Areas B2 and B3, until such time as improvement works were completed in Area E, a low permeability clay barrier was installed along the Area E boundary. The barrier comprised a wall of 0.6 m thickness at the northern and eastern perimeter of Area E, installed at sufficient depth to interface with the underlying natural Lias Clay (approximate mean depth of 2.25 m bgl). The installation proceeded during April 2011. Due to the presence of a number of narrow‐ gauge metal pipes and hard structures, installation of several small sections of the wall at the southern end of the excavation trench was postponed until these obstructions could be properly investigated and removed during and following the Areas B2 and B3 remediation work. Completion to the final stages of the clay barrier was achieved in early December 2011. 7.10 Material Excavation Volumes by Area A summary of the material excavation volumes from each area, excluding trial pit locations, ispresented in Table 17 below. Celtic, June 2012 55 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 17: Material Excavation Volumes by Area (m3) Area Soils or Soft Concrete Total Materials Requiring Break‐ out A1 13,220 381 13,601 A2 3,383 0 3,383 A3 16,771 678 17,449 B2 9,561 1,179 10,740 B3 6,126 1,724 7,850 B4 3,808 2,443 6,251 C 7,752 1,860 9,612 C3 3,009 292 3,301 GH2 12,196 0 12,196 Tar Tank 426 0 426 Total 76,252 8,557 84,809 7.11 Surface Cover Layer Finish, Compaction and Testing The minimum final surface cover requirement across the whole site comprised the installation of a 300 mm capping layer of imported virgin material. Within all of Area C, including sub‐ areas, this minimum requirement was supplemented with the installation of a Tensar mechanical ground stabilisation geogrid over the geotextile fabric. In accordance with the Stage V design requirements, where possible, all reinstated finished cover layers were vibration rolled to achieve a minimum 5 % CBR compaction rate. This was achieved using rollers with a minimum specification of 2.7 tonnes. Exceptions to this occurred in areas of the site reinstated over live gas services, which for precautionary reasons, were instead dead rolled. Compaction verification testing was performed upon completion of final surface cover grading and compaction in each area. Again, an exception to this rule was applied over the live gas service lines where a minimum stand‐off requirement of 6 m was required. All tested areas exceeded the minimum 5 % CBR compaction requirement. Test results can be viewed in Appendix E; test locations are presented on Drawing 5237‐VAL‐001, Appendix A. 7.12 Trial Pit Excavations In accordance with the requirements outlined under Sections 6.7.1 and 6.9.2 of the Celtic Stage V design document, a programme of trial pit works across Areas E and B1 were scheduled to be excavated during works. These investigations were required to evaluate and determine the extent of any remaining contamination outside of the remediated areas. Information gained from the B1 trial pit analytical data was considered during the proposed planning variation for retention of the former WWU building. The trial pits were located following a thorough review of known site constraints and service drawings. Each location was scanned using a cable avoidance tool, and was supervised by a Celtic, June 2012 56 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) dedicated banksman and Celtic Engineer. Each trial pit was logged and photographed, and material samples obtained from a range of depths according to the type of material encountered and at the discretion of the Celtic Engineer. Area E A total of thirty‐nine trial pits were excavated within the Glevum House car park (Area E), of which six were excavated in the access road adjacent to the fence‐line demarcating the road with site Area C. The types of materials recovered were consistent with the heterogeneous range of gasworks waste material expected, including foul lime to the east and hydrocarbon‐ impacted soils to the north. B1 Twenty three trial pits were excavated within Area B1, including two within the former WWU building. Most of the material type encountered throughout the area comprised Made Ground combinations of ashy sands and gravels, reworked silty clays, rubble, brick and brick fragments. Of the forty seven samples obtained for analysis, fifteen exceeded site threshold targets for determinands including PAHs, TPH, total cyanide and a range of heavy metals. Analytical datasets and logs for all trial pits are included in Appendix F; locations are indicated on Drawing 5237‐TPL‐001, Appendix A. Celtic, June 2012 57 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 8.0 POST‐WORKS SITE REINSTATEMENT 8.1 General Completed site conditions required the reinstatement of important landscaping, security and screening features, including a replacement access road, replacement trees, border fencing, additional drainage and improvements to the Black Ditch and its environment. Details of these works are outlined below. 8.1.1 Access Road The access road was reinstated to coordinates approximate to the original structure, notwithstanding minor variations required to facilitate operational and service constraints. The backfilling and reinstatement specification for the replacement access road was provided within Quad Consult Report 2010‐11‐26 10067, Revision D (Appendix L). In brief, this comprised a two‐stage formation layer consisting of a crushed stone capping layer and sub‐ base, overlain with 90 mm coarse bitumen, a 60 mm bitumen binding course and 25 mm wearing course. In order to facilitate future service installations, two sets of service ducts were installed either side of the access road. Each service run has three 125 mm channels, with each channel having a set of three heavy duty nylon draw strings. Access to ducting location is via two access chambers. Identifier labels for allocated service runs for water, electricity and telecoms were also installed. Supplementary drawings outlining the construction detail are provided in Appendix L. Road reinstatement completion was achieved during the first week of March 2012. 8.1.2 Tree Planting During the second week in December, a total of forty‐six saplings were planted at three locations to replace trees removed during the project. These comprised twenty six Rowan to the rear of Glevum house between the Area E car park and Areas B2 and B3, ten Field Maple along the eastern site boundary with Betjamin Close, and ten Silver Birch at the eastern boundary of Area D (Tuffley Rovers). Tree planting was supervised by Phlorum, and undertaken in full consultation with the Gloucester City Council Tree Officer. Tree planting locations and correspondence are presented in Appendix K. 8.1.3 Fencing Works Fence repair and replacement works were carried‐out across the site prior to completion. These comprised the installation of a wire‐strung 2 m‐high cranked chain‐link fence along the eastern site boundary adjacent to Betjemin Close and the Black Ditch, and a 1.2 m‐high chain‐ link fence between Area E and Areas B2 and B3. An additional section of chain‐link fence was also installed along the western boundary of Area C as a like‐for‐like replacement for an existing damaged section. Repairs to additional sections of damaged fencing were also made, including a repair to a residential property at 44 Shakespeare Avenue, and a damaged section of wooden fencing to the rear of a Betjemin Close residence. These repairs were carried‐out as part of NG community initiatives. A site fence‐line at Betjemin Close was installed to the west of the Maple Trees, creating a 4 m stand‐off from the boundary fences of the adjoining residential properties. This served to demarcate the extent of the operational areas of the north‐eastern parcel of Area A and provide additional security along this boundary. Celtic, June 2012 58 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Two sets of access gates were installed as part of the works. These included a pair of purpose‐ made galvanised steel gates across the access road entrance adjoining Area E and the car park, and a galvanised steel pedestrian gate installed within the Betjemin Close site boundary fence to enable future maintenance access to the trees. All fencing works were carried‐out by a suitably experienced sub‐contractor. 8.1.4 Black Ditch Clearance and Dressing Works In order to improve flow and aesthetics improvements were carried‐out along the open section of the Black Ditch adjacent to Area A3. This included the clearance and removal of debris, vegetation and accumulated silt from the ditch base, and the removal of general rubbish and fly‐tip waste in the surrounding area. Saplings and silt obstructing the mouth of the culvert inlet were also removed. Once cleared, the embankment was dressed and battered, and edging boards installed to create a tangible border between the ditch and the site proper. 8.1.5 Tuffley Rovers Access Road Improvement As part of NG’s on‐going commitment to foster and maintain good relations with site neighbours and the local community, works for Tuffley Rovers Football Club to improve the club’s access road were completed during late July 2011. This involved the clearance, levelling and preparation of the existing surface structure, followed by the laying and rolling of a <40 mm imported quarried‐stone cover layer. Celtic, June 2012 59 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 9.0 ENVIRONMENTAL MONITORING 9.1 Outline During the remediation works, a programme of environmental monitoring was undertaken to help minimise the environmental impact of the works, guide environmental impact mitigation measures and assess risks to the health and safety of both site workers and neighbours. The monitoring was also carried out as a condition of National Grid’s Core Specification(2) and the remediation strategy(1). The environmental monitoring carried out included measurements of dust, vapour and noise. The monitoring locations used at the site are indicated on Drawing D1338/3876/A8, Appendix A; the results of relevant appropriate chemical analyses and recorded daily datasets are presented in Appendix C. 9.1.1 Dust Dust levels were monitored visually on a daily basis, with quantification of dust deposition rates gained through the use of Frisbee‐type deposition dust gauges collected for analyses weekly from thirteen monitoring station locations. Samples collected from the gauges were then sent to an accredited laboratory each week for quantification of total solids collected, while sticky pads were also used to determine deposition and nuisance values relative to prevailing wind direction. The dust deposition rate threshold set for the site was 220 mg/m2/day; results are included in Appendix D. Most results were consistently below the site threshold levels, although results from one location (Monitoring Station 12), located on the western Area C site boundary between Bristol Road and the site compound, exceeded the threshold value on twelve occasions. During the early stages of the works, a number of complaints were received from Glevum House occupants regarding dust settlement on cars within the parking area. This was a combined consequence of wind conditions, and a period of high site traffic activity which included the movement, crushing and stockpiling of concrete and hard materials. The immediate problem was mitigated by closer scrutiny and management of these activities, including wetting and spraying of the crushing operations, a reduction and strict enforcement of the site speed limit, and a sustained and focused effort to ensure continuous wetting of site access ways using a dedicated water bowser. Apart from the one incident, no further dust‐related complaints were received for the remainder of the works. 9.1.2 Ambient Air/Vapour Monitoring Total hydrocarbon vapour levels were monitored daily using a photo‐ionisation detector (PID) meter. The PID monitoring indicated total vapour levels were consistently less than 0.1 ppm which is less than the occupational exposure limit (OEL) for benzene of 1.0 ppm(10). Validation of the PID monitoring was carried out by deploying passive diffusion tubes at thirteen monitoring locations across the site (increased to fourteen during June 2011), five locations within Glevum House, and with effect from September 2011, the former WWU Building located within Area B1. Tubes were submitted for analyses every fortnight for units deployed across the site, and every twenty eight days for the units located within the buildings. The tubes were sent to an accredited laboratory for the quantification of BTEX, naphthalene and phenol compounds. Celtic, June 2012 60 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) In order to provide risk‐based validation of airborne vapours with respect to human health, further assessment was undertaken. The findings of the report confirmed that no significant risk to both on‐site and off‐site receptors was generated during the duration of the works. The report is included in Appendix H. Passive tube analysis results and daily monitoring dataset records are included in Appendix D. 9.1.3 Noise Noise levels were monitored on a daily basis using a hand held noise meter; monitoring records are included in Appendix D. Records of the maximum values for each week at any given monitoring station shows a mean value of 70.7 dB through the duration of the works; with a peak maximum at a very modest 79.2 dB. No complaints regarding noise were received during the works. Weekly maximum levels are plotted on Graph 1 below. Graph 1: Plot of Site Weekly Noise Monitoring Results (Peak Recorded Values) 9.1.4 Ground Gas Monitoring As outlined in Section 7.6, a total of seventeen boreholes have been used to monitor ground gas concentrations both during and post works. Table 18 below presents the monitoring data between the dates 05/07/2011 to 05/04/2012. The data shows notable variations between locations for gaseous oxygen, carbon dioxide and methane. Whilst these variations exist, the required design remit for suitability as open storage final end‐use is sustained by the stated usage stipulations of air breaks beneath temporary structures and no intrusive service installation. Should development in future go beyond open storage, then a full ground‐gas assessment in accordance with publication Ciria C665(11) should be undertaken in conjunction with consultation with the regulatory authorities. Celtic, June 2012 61 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester – Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 18: Ground Gas Monitoring Data Ground Gas Monitoring Ground Gas Monitoring Ground Gas Monitoring 05/07/11 27/09/11 21/12/11 (% by volume) (% by volume) (% by volume) Borehole Flow Flow Flow 02 C02 CH4 Rate 02 C02 CH4 Rate 02 C02 CH4 Rate (l/h) (l/h) (l/h) GG1 1.3 12.9 <0.1 <0.1 7.6 10.9 <0.1 0.4 11.8 0.1 <0.1 6.6 GG2 5.2 11.4 <0.1 <0.1 4.6 4.7 <0.1 <0.1 17.3 <0.1 <0.1 3 GG3 15.4 0.7 <0.1 <0.1 8.6 4.4 0.1 <0.1 8.2 3.7 <0.1 0.1 GG4 11.5 10.7 <0.1 <0.1 4.5 16 <0.1 0.3 14 3.9 0.1 <0.1 GG5 4.1 11.7 <0.1 <0.1 3.5 10.3 0.1 <0.1 15.2 2 <0.1 <0.1 GG6 20.3 0.7 <0.1 <0.1 17.9 1 0.1 <0.1 17 0.2 <0.1 <0.1 GG7 15.3 3.9 <0.1 <0.1 16.1 4.2 0.1 <0.1 17.6 4 0.1 <0.1 GG8 16.9 4.4 <0.1 <0.1 20.5 0.2 0.1 <0.1 21.3 0.2 0.1 <0.1 GG9 12.6 6.3 <0.1 <0.1 11.3 9.2 0.1 <0.1 14.9 4.9 0.1 <0.1 GG10 20.7 <0.1 <0.1 <0.1 20.6 <0.1 0.1 <0.1 20.8 0.3 0.1 0.2 GG11 N/I N/I N/I N/I N/I N/I N/I N/I 18.6 0.2 <0.1 <0.1 GG12 N/I N/I N/I N/I N/I N/I N/I N/I 17.2 2.7 <0.1 <0.1 Note: N/I denotes borehole not installed at time of monitoring Table 18: (continued) Ground Gas Monitoring Ground Gas Monitoring Ground Gas Monitoring 11/01/12 02/03/12 07/03/12 (% by volume) (% by volume) (% by volume) Borehole Flow Flow Flow 02 C02 CH4 Rate 02 C02 CH4 Rate 02 C02 CH4 Rate (l/h) (l/h) (l/h) GG1 6.6 0.5 <0.1 0.2 4.3 2.7 0.1 1.6 2.6 1.9 0.1 <0.1 GG2 20 <0.1 <0.1 <0.1 17.4 0.01 0.1 1.6 17.2 0 0.1 <0.1 GG3 9.5 3.4 <0.1 0.1 14.7 3.1 0.1 1.6 13.3 3.4 0.1 <0.1 GG4 11.8 2.3 0.1 <0.1 1 4.6 0.1 0.1 10 3.2 0.1 <0.1 GG5 20.6 <0.1 <0.1 0.1 18.3 0.7 0.1 1.6 2.1 2.6 0.1 <0.1 GG6 ND ND ND ND 21.9 0.01 0.1 <0.1 0.5 5.2 0.1 <0.1 GG7 19.1 4 0.1 <0.1 19 3.8 0.2 <0.1 17.7 3.1 0.1 <0.1 GG8 21.3 <0.1 0.1 <0.1 21.4 0.1 0.1 1.4 21.5 0.1 0.1 <0.1 GG9 17.4 4.7 0.1 <0.1 10.2 7 0.1 1.4 9.8 6.8 0.1 1.8 GG10 20.5 <0.1 0.1 <0.1 0.1 1 24.9 1.4 1.7 1.1 22 <0.1 GG11 16.9 0.2 <0.1 <0.1 18.7 0.4 0.1 1.6 19.5 0.5 0.1 <0.1 GG12 19.3 1.4 <0.1 <0.1 11.3 2.5 0.1 1.5 14.9 2.4 0.1 <0.1 GG13 N/I N/I N/I N/I 7.9 8.2 0.8 1.6 2.5 6.5 0.1 <0.1 GG14 N/I N/I N/I N/I 7.2 0.1 0.008 1.5 2.5 <0.1 0.5 <0.1 GG15 N/I N/I N/I N/I 18.2 <0.1 0.1 1.5 15.8 <0.1 0.1 <0.1 GG16 N/I N/I N/I N/I 16.6 <0.1 0.1 1.4 18.5 3 0.1 <0.1 GG17 N/I N/I N/I N/I 21.7 <0.1 0.1 0.1 15.4 <0.1 0.3 <0.1 Celtic, June 2012 62 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 18: (continued) Ground Gas Monitoring Ground Gas Monitoring Ground Gas Monitoring 28/03/12 05/04/12 16/05/12 (% by volume) (% by volume) (% by volume) Borehole Flow Flow Flow 02 C02 CH4 Rate 02 C02 CH4 Rate 02 C02 CH4 Rate (l/h) (l/h) (l/h) GG1 10.7 2.4 2 1 17.9 2.4 <0.1 <0.1 17.8 0.3 0.1 <0.1 GG2 14.7 <0.1 0.2 1 13.8 <0.1 <0.1 <0.1 16.4 <0.1 0.1 <0.1 GG3 13.9 3.2 0.2 1 15.4 3.6 <0.1 <0.1 11.5 3.5 <0.1 0.3 GG4 10.7 2.9 0.2 0.9 0.6 8 <0.1 <0.1 20.6 0.4 0.1 <0.1 GG5 7.4 1.9 0.2 1 11.8 3.1 <0.1 <0.1 11.1 1.1 0.1 <0.1 GG6 1.6 6 0.2 0.9 8.9 4 <0.1 <0.1 18.4 0.4 0.1 <0.1 GG7 18 1.6 0.2 <0.1 17.1 2.3 <0.1 <0.1 18.7 2.4 <0.1 <0.1 GG8 20.4 <0.1 0.1 0.8 21.9 <0.1 <0.1 <0.1 21.4 0.1 0.1 <0.1 GG9 9.2 7.4 0.1 0.6 8.4 9.2 <0.1 <0.1 20.5 0.9 <0.1 <0.1 GG10 0.2 1 16.2 0.6 6.7 0.7 3.2 <0.1 21.1 <0.1 <0.1 <0.1 GG11 18.2 0.5 0.2 0.9 15 1.1 <0.1 <0.1 19.3 0.2 0.1 <0.1 GG12 5.5 4.9 0.2 0.9 8.1 5.4 <0.1 <0.1 16.6 2.8 <0.1 <0.1 GG13 3.2 7.2 0.2 0.9 1.3 9.6 <0.1 <0.1 16.3 2 0.1 0.2 GG14 4.9 <0.1 0.3 0.9 13.2 0.1 <0.1 <0.1 21.1 <0.1 0.1 0.1 GG15 9.4 <0.1 0.2 0.9 6.9 <0.1 <0.1 <0.1 9.2 <0.1 0.1 <0.1 GG16 8.2 <0.1 0.2 <0.1 3.5 <0.1 <0.1 <0.1 13.1 <0.1 0.1 0.1 GG17 10.6 <0.1 3 0.9 11.8 0.1 <0.1 <0.1 5.1 0.4 0.1 <0.1 Celtic, June 2012 63 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 10.0 POST‐WORKS VALIDATION 10.1 Excavation Volumes/Mass Balance An approximate materials mass balance for the remediation works is detailed in Table 19 below. Table 19: Approximate Earthworks Mass Balance Excavated Soils/Oversized Materials Volume (m3) Contaminated and uncontaminated soils 76,518 Concrete, brickwork and other obstructions 8,576 Tarmac 1,270 Total Excavation Volume 86,364* Backfill – Re‐used and Imported Material Volume (m3) Crushed brick / concrete 18,118 Reused site won validated soils 40,342 Imported 10‐40 mm quarried granular cover fill 25,786 Imported 6F5‐grade quarried stone 3,977 Imported Type 1‐grade quarried stone 9,582 Class 7J clay 203 Total Backfill Volume 98,008** Disposal of Excavated Soils Volume (m3) Non‐hazardous waste 15,859 Hazardous classed waste 1,890 WAC failing classed waste 12,744 Total Disposal Volume 30,493 * The higher value for backfill volume versus excavation volume is accounted for by excavated soil and processed concrete & hard bulking factors; and the volume estimates applying to bulk imports prior to deposition compaction. ** The discrepancy between excavated and backfilled concrete and brick materials is accounted for by a) the inclusion of above‐ground building and structures’ demolition materials in ‘Backfill – Re‐used’ figures, and b) bulking factor volume increase for processed hard and concrete. The mass balance indicates that a total of 52 % of site won soils were re‐used, the total volume of re‐used site‐won materials increasing to 69 % when including hard and concrete re‐ use volumes. 10.2 Surface Water Monitoring 10.2.1 Black Ditch and Black Ditch Culvert Notwithstanding the variations outlined in Section 7.9.1, the site improvement strategy considered protection of water quality within the Black Ditch. These included selected remedial targets principally driven by risks identifying the Black Ditch as a key receptor via the relevant pollutant linkages of leaching and direct migration of contaminants to shallow groundwater. Improvement of the site to risk based targets was also calculated as sufficient to mitigate risks to future site users for the purposes of open storage activities. The relevant contamination improvement strategy proposed is outlined in Celtic’s Stage V report R1338/10/3876(1). Celtic, June 2012 64 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Monitoring of the Black Ditch water quality was performed during the works in accordance with the requirements set‐out in the Celtic Stage V design document. When possible, fortnightly water samples were obtained from two Black Ditch locations and submitted for chemical analyses in accordance with NG Core Specification requirements. Between the period July to December 2011 however, this was not achievable due to the lack of sufficient volume or flow within the ditch to enable a representative sample to be obtained. The available results are therefore split between two batches representing the early site works period of March – June 2011, and the period of January – February 2012; the project’s final two full months when remedial works were nearing completion. A total of thirteen individual samples were recovered from the Black Ditch, from locations at the culvert’s inlet and Manhole 1 at the point nearest to where the Black Ditch Culvert continues its westward flow direction and exits the site. Graph 2: Black Ditch Water Cyanide and Ammonium Concentrations Of the fifteen contaminants of concern for which maximum threshold targets had been developed, two compounds – total cyanide and total ammonium were recorded at concentrations exceeding their targets (2.5 and 19 mg/L) during the early stages of the works. Upon resumption of sampling in January 2012, concentrations for both had reduced to below target thresholds and were sustained for the following two samples. Similar, though less prominent, reductions for most other determinands including sulphate and heavy metals were also observed over the same period. Laboratory data including the Black Ditch and other water sample results are presented in Appendix C. Whilst these results form only one measure of the localised environment and are not therefore conclusive; the downward trend may be interpreted as a positive indicator towards realising improvements to the site conditions resulting from the works, supporting one of the project’s key objectives of reducing risk to the Black Ditch and other sensitive receptors. Graph 2 above presents the combined cyanide and ammonium data. Celtic, June 2012 65 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 10.3 Off‐Site Recycling / Disposal 10.3.1 Solid Waste Full and complete records of the volume of solid material recycled or disposed of off‐site are included in the Final Disposal Log, Appendix G. Relevant disposal site licences and registered waste carriers certificates are recorded in Section H of the Site Waste Management Plan (SWMP), Appendix J. A summary of the volumes and destination of materials disposed of or recycled off‐site is detailed in Table 20 below. Table 20: Summary of Solid Waste Off‐Site Disposal Waste Type Destination Quantity (t) Non‐Hazardous classified waste Cory Environmental, Gloucester GL2 5FR (includes foul lime, spent oxide, 35,599 knotweed, vegetation, tarmac) Hazardous classified waste (tar‐ Churngold Recycling, St Andrews House, St impacted material from Tar Tank) Andrews Road, Avonmouth, Bristol, Avon, 881 BS11 9DQ Hazardous classified waste Cory Environmental, Himley Quarry Landfill, (hydrocarbon‐impacted soils) Oak Lane, Kingswinford, 1,868 Dudley DY6 7JS Hazardous classified waste Hills Waste Solutions, Purton Landfill Site, New (asbestos and asbestos‐impacted Road, Purton, Swindon 234 soils) SN5 9HE Hazardous classified waste Augean Thornhaugh Landfill, Leicester Road, (asbestos and asbestos‐impacted Thornhaugh, 724 soils) Wansford, Peterborough PE8 6NH WAC fail classified material (GH2 Augean Landfill, East Northants Resource and wider site) Management Facility, Stamford Road, Kings 22,736 Cliffe, Peterborough, PE8 6XX Total 62,042 10.3.2 Liquid Disposal Most contaminated liquid waste produced on site comprised tar and other hydrocarbon‐range impacted perched water recovered from GH2 and the Tar Tank. These included liquid tars and lighter non‐aqueous‐phase liquids separated or quarantined during the water treatment process. Full and complete records of the volume of liquid material recycled or disposed of off‐site are included in the Final Disposal Log, Appendix G. Disposal site licences and registered waste carriers’ certificates are recorded in Section H of the SWMP, Appendix J. A summary of the volumes and destination of materials disposed of or recycled off‐site is detailed in Table 21 below. Celtic, June 2012 66 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) Table 21: Summary of Liquid Waste Off‐Site Disposal Waste Type Destination Quantity (t) No. Wagon Loads Tar contaminated Augean Avonmouth, Waste Incinerator, 77 3 waters Smoke Lane, Avonmouth, Bristol BS11 0YA Tradebe ltd, The Recycle Centre, Liquid tar Todwick Road Industrial Estate, Bookers 88 4 Way, Dinnington Sheffield, S25 3SH Tredebe Fawley Thermal Treatment Tar contaminated Centre, Charleston Road, Hardley, 36.5 2 waters Hythe, Southampton SO45 3NX Envirosol Ltd, Unit 28 Thornleigh Trading Tar contaminated Estate, Blowers Green Rd, 2.4 1 waters Dudley DY2 8UB Total 204 10 Of the 204 t of untreatable liquids removed from site, the 88 t of tar disposed of to Tradebe, Sheffield was recycled for use as furnace fuel. The total volume of water successfully treated and discharged to foul sewer under licence was 1,912 m3. This represents a 94 % treatment rate for this aspect of the works. 10.3.3 Site Waste Management Plan (SWMP) The SWMP was updated by Erith regularly during the works. This was independently audited by the Qualified Person. The final signed SWMP is included in Appendix J, along with details of relevant audits and reviews. 10.4 Quality Assessments Quality assessment was undertaken throughout the duration of works via the validation procedures described in the previous sections. In addition, a Celtic Resident Engineer inspected the works regularly, keeping records and photographs. A progress/review meeting was held generally every four weeks throughout the works with the full project team including the NGPH Regeneration Manager, Supervising Engineer, CDM‐ Coordinator, Cost Consultants and Public Relations Consultants. Celtic, June 2012 67 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 11.0 FINAL SITE CONDITIONS The site has been remediated in accordance with the proposed remediation strategy(1), National Grid Core Specification(2) and the original open storage end use brief as provided by NGPH. Prior to the works, the proposed remedial strategy was approved by the Statutory Regulators through the planning process. The works have mitigated risks to controlled waters and human health, and the site will be suitable for open storage end use in accordance with the limitations observed in the Stage V design document listed below, and related agreements. A number of constraints remain on the site; including a WWU‐operated pressure reducing station (PRS), high and low pressure gas mains, high voltage cables and a number of sub‐surface structures beyond the remit or scope of the improvement works. Current site constraints and the locations of remnant below ground structures are shown on Drawing 5237‐SERV‐001‐015, Appendix A. Notwithstanding the minor variations outlined in Section 6.1, all below‐ground structures were removed to either 0.3 m below the original proposed remediation levels, or extended to the soft material excavation level whichever the deeper. Remaining obstructions below these depths are marked on Drawing 5237‐OBSALL‐001‐016, Appendix A. Service duct lines installed either side of the access road running west to east are intended to accommodate future service requirements. Intrusive works to accommodate or permanently install additional sub‐surface service lines are not sanctioned or approved by NGPH under the terms of the remediation accomplishments or objectives. Any actions, whether intentional or otherwise, is beyond the remit of the site’s intended end use, and should not proceed without referral to the landlord. 11.1 Final Site Restrictions In order to maintain the suitability of the site for open storage in the long term and meet the planning requirements for the remediation, limitations to the site’s use are detailed below. Any proposed use over and above that of open storage as described within this report and the limitations herein, will need to be discussed and agreed with Gloucester City Council/Environment Agency and any alternatives may be subject to further restriction or require further planning permission. Limitations on Open Storage • Appropriate vapour protection measures (such as elevated temporary office / storage structures to provide air gap) must be incorporated into the design of any occupied structures e.g. cabins, stores, temporary offices etc. • The surface cover layer and underlying geotextile must not be removed or penetrated. The surface cover layer should be inspected on a regular basis (annually as a minimum) and be repaired where required (e.g. depressions and ruts). The improvement works are based on a future design life of 12 years, however, with regular maintenance it is anticipated that the design life will extend beyond this. • Limitations may be placed by Wales and West Utilities (WWU) on land use adjacent to their plant and live gas mains. Future site use in the vicinity of gas equipment should be agreed with WWU. Celtic, June 2012 68 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) • Manholes associated with the Black Ditch must remain accessible at all times as the Environment Agency may require access to the Black Ditch to undertake sampling.] • No discharges should be made to the site or the Black Ditch without prior consent of the Environment Agency. • With the exception of the services ducts which run either side of the access road, no below ground services should be installed at the site. Limitations on End Uses Other than Open Storage Including Service Installation • In addition to the limitations detailed above, should development more sensitive than open storage as described in this report be proposed, further assessment and Environment Agency / Gloucester City Council agreement may be required. Further assessments, which should be in line with best practice and guidance at the time of completion, may include but not be limited to the following: • Human health and groundwater assessment based on the scope of any future development proposals; • Ground gas assessment including Volatile Organic Hydrocarbons and radon; • If below ground services are considered, they should take full account of the contents of this report and meet best practice for service installation on brownfield sites. The specification construction of such services should be agreed with the utility provider and Gloucester City Council as required. • Material generated from any excavation should be assessed for disposal under appropriate duty of care or be assessed appropriate for retention on site in any final development. • Obstructions may remain in some areas below the finished site levels. The location of these should be taken into account as part of any future design/development. • The design does not take account of any flood assessment or geotechnical issues other than those detailed in this report for an open storage end use. Celtic, June 2012 69 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 12.0 THIRD PARTY APPROVALS 12.1 Prior to Works Prior to the remediation works, the proposals were discussed with GCC’s Contaminated Land Department, Planning Department, Highways Department, the Environment Agency and Natural England. The proposed remediation strategy(1) was submitted to GCC as part of the planning application. As part of the planning process, GCC formally consulted their Environmental Health Department and the Environment Agency. The proposed remediation strategy(1) was approved by GCC’s Environmental Health Department and the Environment Agency, and subsequently through the planning process, planning consent for the works was then granted. Copies of correspondence in this regard and the planning consent are included in Appendix I. 12.2 During Works During the works, the EA, Health Protection Agency (HPA) and representatives of GCC visited the site. Increased scrutiny of collected environmental monitoring data and closer liaison regarding ongoing work procedures were requested by GCC during June due to perceived concerns with vapours during the GH2 remediation. An evaluation of human health risks associated with airborne vapours completed during the works is provided at Appendix H. During the works, a concession was allowed by GCC CLO on the presence of foul lime deposits within Areas A1 and A3. During the works, a variation was made to the original planning conditions for the works by GCC’s planning department to allow for the retention of the former WWU building. In addition, during works no written correspondence specifically relating to works or future site requirements were provided by WWU respectively. Copies of all correspondence detailing the above are included in Appendix I. 12.3 Post Works Based on the degree of site improvement works completed, it is not proposed that any post‐ remediation monitoring will be required. It is considered that data and descriptions within this report demonstrate site conditions protective of all identified receptors, including human health and controlled waters. In addition it is considered that upon completion of the works detailed above all areas of the site meet the original brief such as to allow future open storage end use activities. Celtic, June 2012 70 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 13.0 ESSENTIAL WAYLEAVES Essential wayleaves for the site are as follows: • Minimum 6 m stand‐off from high pressure gas mains; • Minimum 3 m stand‐off from the crown of the Black Ditch Culvert; • Any future work requirements on the site will require further discussion with statutory regulators and site service providers (WWU). All constraints on the site have been discussed in detail throughout the report and are shown on the Constraints Drawing 5237‐SERV‐001‐015, Appendix A. Celtic, June 2012 71 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 14.0 WARRANTIES / CONTRACT MAINTENANCE Celtic Technologies warrant the design and completion of the remediation works. Details of warranties are included in the Deed of Appointment. The remediation works were carried out under ICE Conditions of Contract 7th Edition with National Grid Property Holdings Standard Amendments. Erith Group Ltd. successfully tendered for the works as part of their contractors term framework agreement. The contract documents were compiled in accordance with the term framework agreement between NGPH and their term contractors. The works are to be certified as complete. This will be detailed in the addendum to this report. Celtic, June 2012 72 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 15.0 CONTACT DETAILS National Grid Property Holdings Ltd. CELTIC Designer/Engineer CELTIC Project Auditor Scott Lewis Stephen Kidley Martin Holmes Regeneration Manager Columbus House Columbus House St Catherine's Lodge, Green Meadow Springs Green Meadow Springs Bearwood Road, Tongwynlais Tongwynlais Wokingham, Cardiff Cardiff Berkshire. CF15 7NE CF15 7NE RG41 5BN. Tel: 02920 368636 Tel: 02920 368636 Tel: 01256 308803 Mobile: 07985 836241 Mobile: 07985 836220 Mobile: 07795666320 Fax: 02920 368637 Fax: 02920 368637 Email: [email protected] Email: stephen.kidley@Celtic‐ltd.com Email: martin.holmes@Celtic‐ltd.com CELTIC Resident Engineer NGPH Cost Consultant Planning Supervisor Michael Smith David Carter Tim Waters Columbus House Davis Langdon LLP Planning Perspectives LLP Green Meadow Springs 4 Pierhead Street 24 Bruton Place Tongwynlais Capital Waterside London, W1J,6NE Cardiff Cardiff CF10 4QP Tel: 020 7529 1505 CF15 7NE Mobile: 07798 826988 Tel: 02920 368636 Tel: 02920497497 Email: [email protected] Mobile: 07810 392114 Email: Fax: 02920 368637 [email protected] Email: michael.smith@Celtic‐ltd.com NGPH Contractor Gloucester City Council Communications Centre Simon Frost Contaminated Land Officer JBP Public Relations The Erith Group Stephen Moreby Nicola Beech Erith House Gloucester City Council, The White House Queen Street North Warehouse, 6 Whiteladies Rd Erith The Docks, Clifton, Bristol Kent Gloucester. GL1 2EP. BS8 1PD DA8 1RP Tel: 01452 396312 Tel: 0870 950 8800 [email protected] Tel: 0117 9073400 Fax: 0870 950 8808 Fax: 0117 907 3417 [email protected] Email: [email protected] Environment Agency Wales and West Utilities Gas Emergency Steve Brown Ashley Griffiths Area Hydrogeologist Asset Officer (Environment) Tel: 0800 999111 Contaminated Land Wales & West House Riversmeet House Spooner Close Newtown Industrial Estate Celtic Springs Northway Lane Coedkernew Tewkesbury Newport Gloucestershire NP10 8FZ GL20 8JG T 02920 278686 External: 01684 864433 M 07792 621255 Internal: 722 4433 E [email protected] Fax: 01684 293593 Mobile: 07710903289 steve.brown@environment‐ agency.gov.uk Celtic, June 2012 73 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 16.0 CDM HEALTH AND SAFETY FILE The CDM Co‐ordinators, RSK, have not submitted a CDM Health and Safety File for the works. Contact details for RSK are provided in Section 15.0 of this report. No accidents or incidents reportable to the Health and Safety Executive occurred during the works. Celtic, June 2012 74 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 17.0 GLOSSARY AOD Above Ordnance Datum bgl Below Ground Level BGS British Geological Survey BH Borehole BOD Biochemical Oxygen Demand BTEX Benzene, Toluene, Ethylbenzene, Xylene COD Chemical Oxygen Demand COSHH Control of Substances Hazardous to Health EA Environment Agency HSE Health and Safety Executive NAPL Non Aqueous Phase Liquid NGR National Grid Reference OGL Original Ground Level OS Ordnance Survey PAHs Polycyclic aromatic Hydrocarbons PID Photo‐Ionisation Detector PRS Pressure Reduction Station TP Trial Pit TPH Total Petroleum Hydrocarbons VOCs Volatile Organic Compounds EA Environment Agency GCC Gloucester City Council CDM Construction Design and Management Regulations HPA Health Protection Agency Celtic, June 2012 75 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) 18.0 REFERENCES 1. Celtic Technologies Stage 5 Remediation Design, October 2010 – Report No. R1338/10/3876. 2. National Grid Property Holdings (2007). Core Remediation Specification, Version 1.0. 3. National Grid Property Holdings (2006). Methods for the Collection and Analysis of Samples Version 1 4. CELTIC Factual Site Investigation Report, R860/04/2515 – Bristol Road, Gloucester. 5. CELTIC Gasholder Investigation Report, October 2009 – Bristol Road, Gloucester. 6. CELTIC Stage 5 Improvement Design Supplementary Report, R860/04/2514, May 2003 – Bristol Road, Gloucester. 7. CELTIC Factual Validation Report, R905/05/2550 – Bristol Road, Gloucester. 8. DEFRA/Environment Agency, Model Procedures for the Management of Contaminated Land. Contaminated Land Report 11 (CLR‐11). 9. DEFRA and the Environment Agency (2002), The Contaminated Land Exposure Model (CLEA): Key Reports. CLR 7 to 10. 10. Health and Safety Executive, EH40/2005 Workplace Exposure Limits, HSE Books, Crown Copyright 2005. 11. Ciria C665 (2007), Assessing Risks Posed by Hazardous Ground Gases – London 2007, Ciria Publication. Celtic, June 2012 76 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) APPENDICES Celtic, June 2012 National Grid Property Holdings Ltd Former Gasworks, Bristol Road, Gloucester Site Remediation Verification Report ‐ Report No. R1461/12/4243 (Final) ELECTRONIC APPENDICES ON ENCLOSED CD‐ROM APPENDIX A – Figures (All) APPENDIX B – Plates APPENDIX C – Analytical Results APPENDIX D – Environmental Monitoring Results APPENDIX E – Geotechnical Testing Results APPENDIX F – Borehole & Trial Pit Records APPENDIX G – Duty of Care Records APPENDIX H – DQRA Assessments APPENDIX I – Third Party Correspondence APPENDIX J – Site Waste Management Plan & Materials Management Plan APPENDIX K – Specialist Environmental Data APPENDIX L – Reinstatement Civil Designs Celtic, June 2012 National Grid Property Holdings Ltd