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Geological and Hydrogeological Review Hinkley Point C Power Station

January 2009 Draft Report v1.0 9S4862

A COMPANY OF

Document title Geological and Hydrogeological Review, Hinkley Point Power C Station

Status Draft Report v1.0 Date January 2009 Project name British Energy - Hinkley point C Project number 9S4862 Reference 9S4862/18/R003/Rachel Brown/PBor

Rightwell House

Bretton

Peterborough PE3 8DW United Kingdom

+44 (0)1733 334455 Telephone 0191 211 1313 Fax [email protected] E-mail www.royalhaskoning.com Internet

HASKONING UK LTD.

ENVIRONMENT

Drafted by Rachel Brown

Checked by Mengfang Chen

Date/initials check …………………. ………………….

Approved by Lyall Seale

Date/initials approval …………………. ………………….

Geological and Hydrogeological Review 9S4862/18/R003/Rachel Brown/PBor Draft Report v1.0 -i- January 2009

CONTENTS Page

1 INTRODUCTION 2

2 SOURCES OF INFORMATION 3

3 ENVIRONMENTAL SETTING 4 3.1 Site Description 4 3.2 Surrounding Land Uses 4 3.3 Hydrology 4 3.4 Groundwater Abstractions 9 3.5 Site Walkover 9

4 GEOLOGY 10 4.1 Introduction 10 4.2 Drift Deposits 11 4.3 Solid Geology 11 4.4 Structural Features 12

5 HYDROGEOLOGY 16 5.1 Hydrostratigraphical Units 16 5.2 Aquifer Properties of the Jurassic Strata 16 5.3 Aquifer Properties of Triassic Strata 17 5.4 Groundwater Levels 18 5.5 Recharge and Groundwater Flow 18 5.6 Groundwater Response to Tidal Variation 19 5.7 Summary of Hydraulic Characteristic 20

6 HYDROGEOLOGICAL CONCEPTUAL MODEL 21 6.1 Development Scenarios 21 6.2 Summary of Impact 21

7 ENVIRONMENTAL IMPACT ASSESSMENT 23 7.1 Assessment Methodology 23 7.2 Potential Impacts due to Dewatering 23 7.3 Potential Impacts to Aquifer Recharge 26 7.4 Potential Impacts from Off Site Disposal of Excavated Material 26 7.5 Potential Impacts to Groundwater Quality 26 7.6 Potential Impacts to Designated Areas 26

8 CONCLUSIONS AND RECOMMENDATIONS 28

Geological and Hydrogeological Review 9S4862/18/R003/Rachel Brown/PBor Draft Report v1.0- January 2009 1

LIMITATIONS This report has been prepared by Royal Haskoning (RH) on behalf of, and for the sole benefit of, British Energy (BE). This report has been prepared by RH with all reasonable skill and care, within the terms of the contract with BE. The conclusions and recommendations in this report are professional opinions, based solely on previous works including site investigations and pumping test reports and proposed development. Both the finite data on which they are based and the proposed works to which they are addressed direct the assessments and judgements given in this report. The acquisition of data is constrained by both physical and economic factors and, by definition, is subject to the limitations imposed by the methods of investigations employed. In this instance, the data have been obtained from tests from mechanically drilled boreholes and mechanically excavated trial pits, which by their nature only provide information about small discrete volumes of soil. They can not provide data on every section of the ground / groundwater beneath the site but the data are taken to be spatially representative of the groundwater regime beneath the site and the zones of materials between exploratory borehole locations. Conditions at the site will change over time due to natural variations and may be affected by human activities. In particular, groundwater, surface water and soil gas conditions should be anticipated to change with diurnal, seasonal and meteorological variations. Soil and water chemistry may change due to the actions of groundwater flows and microbiological activity etc. The likely variations in the data with time can be assessed following extended periods of measurement and statistical analyses. Unless specifically discussed in the text, such extended measurement and analysis have not been carried out and the data collected are taken to be representative. This document has been prepared for the titled project and should not be relied upon or used for any other project. This document is confidential and has been prepared for the sole benefit of BE to support the implementation of an Environmental Impact Assessment. Royal Haskoning accepts no responsibility or liability for the consequences of this document being used for a purpose other than that purpose for which it was commissioned. The assessments and judgements contained herein should not be relied upon as legal opinion. The findings and opinions are relevant to the dates of the site work and should not be relied upon to represent conditions at substantially later dates. The opinions included herein are based on the information obtained from the investigations undertaken at the site and from our experience. If additional information becomes available which might impact our conclusions, we request the opportunity to review the information, reassess the potential concerns, and modify our opinion, if warranted.


1 INTRODUCTION

Royal Haskoning is working on behalf of British Energy to undertake a comprehensive and robust Environmental Impact Assessment (EIA) for the development of a new nuclear power station, known as Hinkley Point C (the site). To support the EIA, Royal Haskoning was commissioned by British Energy to collate and review available geological and hydrogeological data at the site. The proposed new nuclear power station is to be located adjacent to two existing nuclear power stations, Hinkley Point A and B, to the northwest of Bridgwater in Somerset. Hinkley Point A power station was operational for 35 years until electricity generation ceased in 1999. Hinkley Point B power station has been in operation since 1976 and is likely to remain operational until 2016. The development of the Hinkley Point C Power Station will include two generation units up to 1650 MW each, administration, storage, maintenance and parking facilities as well as temporary facilities to enable the construction work. The main structures of concern at this site with regards to the impact to groundwater are the buildings with deep foundations. These buildings include the cooling water pumphouse with foundation depth to 34 m below ground level (bgl), the reactor building with foundation depth to 13.6 m bgl and the turbine hall, with foundations ranging in depth from 10 m to 15 m bgl.

This study has been undertaken to provide necessary geological and hydrogeological information to understand the groundwater flow beneath the proposed site and identify the potential effects of dewatering works on the groundwater beneath the site and surrounding areas, e.g. adjacent Severn Estuary candidate Special Area of Conservation (cSAC) and Ramsar Site, and the Bridgwater Bay Special Protection Area (SPA), National Nature Reserve (NNR) and Site of Special Scientific Interest (SSSI). The principal aim of the study is to identify any changes to the baseline hydrogeology as a result of the proposed Hinkley Point C Power Station. The main components of the work for this review are to:

• Describe the baseline superficial and bedrock geology and hydrogeological conditions beneath the site

• Collate relevant data from boreholes drilled from previous investigations and

publicly available sources and identify whether additional investigations would be required to characterise geology and hydrogeology of the site.

• Define hydrostratigraphical units with reference to their hydrogeological

characteristics such as hydraulic conductivity, thickness, and the extent of interactions with surface water.

• Discuss the likely variation in the level of the water table and potentiometric

surface throughout a typical annual cycle.

• Produce hydrogeological conceptual model for the proposed Hinkley Point C Power Station.

• Analyse the potential impact to the groundwater regime from the proposed

earthworks.

• Present conclusions and recommendations with regard to the requirements of the EIA carried out to support the planning application.


2 SOURCES OF INFORMATION

The following sources of information have been reviewed for this study:

• Aspinwall & Company, Analysis of Groundwater Conditions at Hinkley Power Station, March 1996.

• Borehole records obtained from the British Geological Survey at Wallingford.

2008

• British Geological Survey, British Regional Geology, South-West England (Fourth Edition), HMSO, 1975

• British Geological Survey, Geology Sheet 279, Weston-Super-Mare (Solid and

Drift Edition), 1:50,000 series, 1980

• British Geological Survey, Geology Sheet ST24NW (Solid and Drift Edition), 1:10,560 series, 1980

• Jones, H.K., Morris, B.L., Cheney, C.S., Brewerton, L.J., Merrin, P.D., Lewis,

M.A., MacDonald, A.M., Coleby, L.M., Talbot, J.C., McKenzie, A.A., Bird, M.J., Cunningham, J., Robinson, V.K., The physical properties of minor aquifers in England and Wales, British Geological Survey and Environment Agency, Technical Report WD/00/04 R&D Publication 68. 2000.

• Centre for Ecology and Hydrology and British Geological Survey: Hydrological

data UK: Hydrometric register and statistics, 1996 – 2000.

• Environment Agency, Policy and Practice for the Protection of Groundwater, Groundwater Vulnerability, Sheet 42 Somerset Coast, 1:100,000 series, HMSO, 1996

• Rendel, Palmer & Tritton, Hinkley Point – Report on Site Investigations 1978/9,

Volume 1, Characteristics of the Hinkley Point Area, April 1980

• WS Atkins Environmental for Nuclear Electric plc, Environmental Due Diligence Audit Main Report, July 1996

• Serco Assurance for British Energy, Review of Potentially Contaminated Land

Interest, Hinkley Point B, January 2002

• Hinkley Point Seismic Hazard Assessment Volume 2A Appendices 1 – 8, F1037/3, CEGB Barnwood, 1987

• Hinkley Point Seismic Hazard Assessment Volume, Supplement The Hinkley

Point Fault, 754/1037, CEGB Barnwood, June 1988


3 ENVIRONMENTAL SETTING

3.1 Site Description

The proposed Hinkley Point C Power Station is located on the Severn Estuary in Somerset, approximately 12km north west of Bridgwater, centered on National Grid Reference (NGR) ST 205 460, as shown in Figure 3.1. The site of the proposed power station comprises an open field to the west of Hinkley Point A Power Station (Figure 3.2), as well as hardstanding areas, comprising a car park and a visitors centre for Hinkley Point B. This site slopes from an elevation of 27 m above ordnance datum (AOD) in the south to approximately 15 m AOD in the north (as indicated on Figure 3.2). There are two areas of raised ground at the site, which are a result of waste materials excavated during the construction of Hinkley Point A and B being deposited in mounds. These spoil heaps can be noted from the topography in Figure 3.2.

3.2 Surrounding Land Uses

The land to the north, east and southeast of the power station site falls within the Bridgwater Bay SPA, SSSI and NNR, and that to the north of the site, falls within the Severn Estuary cSAC, SPA and Ramsar site. Full details on these designations can be found in the Proposed Nuclear Development at Hinkley Point: Environmental Scoping Report (Royal Haskoning, 2008) and a brief summary is provided here. Bridgwater Bay comprises a succession of habitats ranging through extensive intertidal mudflats, saltmarsh, shingle beach and grazing marsh intersected by a complex network of freshwater and brackish ditches. It supports internationally and nationally important numbers of over-wintering and passage migrant waders and waterfowl and as such forms an integral part of the Severn Estuary SPA and Ramsar system. This area is also ecologically linked to the Somerset Levels which provide alternative winter feeding grounds for the waders and wildfowl. Furthermore, the ditches and ponds in the grazing marsh (wetland) to the east and southeast of the power station site contain a diverse invertebrate fauna including a number of nationally rare and scarce species. Hinkley Point A and B are located to the east of the site. Hinkley Point A is a Magnox power station commissioned in the 1960s and Hinkley Point B is an Advanced Gas Cooled Reactor power station commissioned in the late 1970s. To the south and west of the site, grassland has been set aside for grazing. This land is now owned / leased by EdF Energy.

3.3 Hydrology

Surface water drains to the north and the east towards the estuary. There is a minor stream or ditch which flows along the western boundary of the site in a northerly direction. Although there is no flow data available for this stream, it is reported to be groundwater fed (Rendel, Palmer & Tritton, May 1986 as referenced in Aspinwall & Company, March 1996). There is also a series of ditches and unnamed streams located in the wetland to the east and southeast of the site. Hydrological data, including rainfall and evapotranspiration data, from the nearest hydrometric station in the catchment of the Currypool Stream are summarised in Table


3.1. These data have been referenced from Centre for Ecology and Hydrology (CEH) and British Geological Survey: Hydrological data UK: Hydrometric register and statistics (1996 – 2000). Table 3.1 Rainfall and Evaporation Data

River Name

Station Name

NGR Record Period

Mean Annual Rainfall (mm)

Mean Annual Runoff (mm)

Mean Annual Loss (mm)

Mean Flow (m3.s-1)

Base Flow Index

10th Percentile (m3.s-1)

95th Percentile (m3.s-1)

Currypool Stream

Currypool Farm ST 221382 1971 – 2000 947 434 513 0.22 0.71 0.4 0.06

CEH report the mean annual runoff in the Currypool Stream catchment to be 434 mm. This parameter is described as being a notional depth of water in millimetres over the catchment equivalent to the mean annual flow measured at the catchment’s gauging station. This value will therefore include a proportion of water derived as groundwater flow. The mean annual loss is reported by CEH to be 513 mm, which is the difference between mean annual catchment rainfall and the mean annual catchment runoff and it provides a reasonable estimation of the average annual evaporative losses from the catchment. In the previous study by Aspinwall & Company (1996), rainfall evaporation data obtained from the Meteorological Office have been analysed to estimate a value for effective rainfall (i.e. rainfall minus evapotranspiration). This value was estimated to be 200 mm/yr. CEH report the Base Flow Index (BFI) to be 0.71, which represents the proportion of stream flow that is derived from stored sources, i.e. groundwater during low flow periods. Rivers draining impervious clay catchments typically have BFIs of 0.15 to 0.35, whereas rivers draining chalk catchments have BFIs greater than 0.9, therefore, the value of 0.71 for the Currypool Stream catchment represents a relatively high groundwater component in the river discharge. Whereas the data in this report represent average values taken over a long period of time (1971 to 2000), the Aspinwall report calculated effective rainfall using monthly rainfall and evaporation data collected over a one year period during 1996, which happened to be a drought year. The base flow index has been estimated as 0.71, indicating that groundwater contributes to over two thirds of the flow in the Currypool Stream. Tidal data referenced from the Admiralty Charts Volume 1 (2008) are summarised in Table 3.2. The data indicate that the tidal range at Hinkley Point is approximately 10.7 m. This is considered to be a large tidal range; the Severn Estuary having the second highest tidal range in the world. Given this tidal range, it is surprising that no evidence of groundwater response to tidal variation was noted during previous investigations. Table 3.2 Summary of Tidal Information

Location Mean High Water Spring (MHWS)

Mean High Water Neap (MHWN)

Mean Low Water Neap (MLWN)

Mean Low Water Spring (MLWS)

Hinkley Point 5.0 m OD 1.9 m OD -2.9 m OD -5.7 m OD

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3.1

Location of Proposed Developmentof Hinkley Point C

Hinkley Point Power StationNew Nuclear Development

British Energy

December 2008 1:50,000

Source: Based upon Ordnance Survey maps byBritish Energy PLC by permission of OrdnanceSurvey on behalf of H.M.S.O. Controller, Crowncopyright British Energy PLC Licence 100019324

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Approximate Line of Cross Section(Figure 6.1 Hinkley Point C - Cinceptual Model)

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3.2

Proposed Development Area


British Energy

December 2008 1:10,000


PS Approximate Location of Power Station

CL Contractors Laydown Area

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SAND/MUD/ROCKY OUTCROPS/SHINGLE CLOSER TO CLIFF

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Sites of Special Scientific Interest

Special Protection Areas

Special Areas of Conservation

RAMSAR Sites

3.3

Surrounding Land Use AroundHinkley Power Station


British Energy

March 2008 1:60,000


Sand/Mud/Rocky Outcrops/Shingle Closer to Cliff

Wetland

Potential Extent of PermanentWorks


3.4 Groundwater Abstractions

The North Wessex area office of the South West region of the Environment Agency has stated that there are no licenced groundwater abstractions within 2 km of the site.

3.5 Site Walkover

A site walkover was undertaken by two Environmental Scientists from Royal Haskoning on Monday 15 December 2008. The walkover was undertaken for two main reasons: to identify any potential areas of contamination, the findings of which are presented in the Contaminated Land Phase 1 Desk Study (Royal Haskoning, 2008); and to assess the general site setting with regards to surface water features, the lie of the land, surrounding land uses and any outcrops of rock. Photographs taken during the walkover, which help describe the geology and hydrogeology characteristics of the site, are included in Appendix A of this report. For other general photographs of the site, reference should be made to Contaminated Land Phase 1 Desk Study, Royal Haskoning (2008). The principal observations from the walkover comprised:

• The site was very wet underfoot and in some low-lying areas small ponds were forming. This indicates that the land is not free-draining. The ponding probably occurs in areas where the drift comprises silty clay deposits.

• The drainage ditch or small stream forming the western boundary of the site was stagnant.

• There are two mounds on site as described in previous reports. • The south east of the site comprises a car park used for staff working at Hinkley

Point A and Hinkley Point B. • Hinkley Point B visitors centre is located in the eastern part of the site, in front of

which is a large area of hardstanding, which would have been part of the temporary contractors area during construction of Hinkley Point B.

• At the time of the walkover, there were site investigations being undertaken to the west of the site on land owned / leased by EdF Energy.


4 GEOLOGY

4.1 Introduction

This section summarises the drift and solid geology of the area as well as the structural features beneath the site. This information has been referenced from published reports, BGS mapping and BGS borehole logs (Appendix B) as well as data contained in previous reports by Aspinwall & Company (1996) and Rendel, Palmer and Tritton (1980). The geology of this site and surrounding area is shown in Figure 4.1 and Figure 4.2 and the geological succession is summarised in Table 4.1. Table 4.1 Summary of Geological Succession at Hinkley Point C (referenced from BGS borehole logs, Appendix B)

Age Group and Formation

Lithology Mean

Thickness (m)

Depth to Base of Formation

(mbgl)

Elevation of Base of

Formation (m OD)

Recent Made Ground

Brown and greyish brown clay or very silty clay with fragments of mudstone and limestone. Occasional larger cobbles and boulders of limestone and mudstone

5 4.1 to 9.55 8.65 to 12.45

Jurassic Blue Lias Interbedded limestones and mudstones

55 40.95 to 66 -21 to -50

Penarth Group, comprising Cotham Beds and Westbury Beds (previously known as Rhaetic)

Limestones and mudstones with pyrite deposits

11 56.17 to 77 -33 to -61

Triassic Mercia Mudstone Group (previously known as Grey Marl and Tea Green Marl and Keuper Marl)

Red, green and grey mudstones and siltstones with halite deposits

Not fully penetrated


4.2 Drift Deposits

The British Geological Survey (BGS) 1:10,560 series mapping indicates that the topsoil at the site comprises heavy grey and brown clay soil with limestone fragments. The BGS 1:50,000 series mapping indicates that the site is directly underlain by solid geology, i.e. there are no drift deposits. However, drift deposits, comprising clay and silty clay with fragments, cobbles and boulders of limestone and mudstone have been identified in seven of the fourteen boreholes drilled during previous investigations, obtained from the BGS. It should be noted that the seven boreholes which do not identify drift deposits are deep rotary drilled boreholes, which were drilled for core examination. The logs for these boreholes begin at the depth when coring commenced, between 2.0 m and 3.6 m, so do not provide any details of strata at the shallower depth. It is likely that this drift material is Made Ground, comprising weathered Lias mudstone and limestone which was excavated during the construction of Hinkley Point A and B (Aspinwall & Company, 1996). Its thickness ranges from approximately 2 m up to a maximum of 9 m where the material has been placed as two large spoil heaps (Rendel, Palmer and Titton, 1980). The borehole logs obtained from BGS indicate that the Made Ground is at an average thickness of 5 m beneath the site. The BGS 1:10,560 series mapping indicates that the wetland situated to the south east of the site is underlain by Marine and Estuarine Alluvium. These deposits have been described in the Aspinwall (1996) report as being up to 5 m in thickness and consisting of soft to firm organic clays. It has also been reported by Aspinwall (1996) that Fluvial Glacial Sands, consisting of sandy gravel to sandy silty clay and ranging in thickness from 2.4 m to 5.2 m, occur beneath the estuarine alluvium.

4.3 Solid Geology

The Made Ground is underlain by Jurassic age rocks comprising interbedded mudstones and thin limestones of the Blue Lias Formation. The Blue Lias Formation is the basal, more permeable zone of the Lower Lias (Jones et al, 2000). The BGS 1:10,560 scale mapping indicates that the Blue Lias dips to the north at about 10 degrees and is up to 116 m thick in this area. However, the BGS borehole logs report that the mean thickness of the Blue Lias is only approximately 55 m at this site. The 1:10,560 scale mapping indicates that the Blue Lias Formation outcrops along the foreshore. There is also shingle material along the foreshore, mainly concentrated in areas closer to the cliff. Reference should be made to photographs taken during the site walkover, illustrating the geology of the foreshore (Appendix A). Four lithostratigraphic subdivisions exist within the Blue Lias beneath this site (Aspinwall & Company, 1996). The thicknesses and percentages of the limestone bands within these subdivisions have been estimated during previous investigations and are detailed in Table 4.2. The limestone bands are generally around 0.25 m thick, but have been reported to be up to 1 m thick (Aspinwall & Company, 1980). The mudstone bands within this sequence are reported to increase in thickness with depth.


Table 4.2 Summary of thicknesses and percentages of limestone bands in the Lower Lias (taken from Aspinwall Report)

Formation Lithostratigraphic Subdivision

Description % limestone bands

Typical mean thickness

Angulata Zone Interbedded limestones and mudstones

14 40

Liasicus Zone (Upper)

Interbedded limestones, mudstones and fissile mudstones

18 9.4

Liasicus Zone (Lower)

Calcareous mudstones and fissile mudstone with occasional limestone bands

0 21.3

Blue Lias of the Jurassic Period

Planorbis Zone Calcareous and often bituminous interbedded limestones and mudstones

23 12.7

At this site the Blue Lias Formation is underlain by Triassic rocks of the Penarth Group, which out crops approximately 240 m south of the site. The Penarth Group is made up of the Lilstock Formation (also known as Cotham Beds) and the Westbury Formation. It is described as comprising fine grained limestones, calcareous mudstones and siltstone, with pyrite occurring throughout. The BGS 1:10,560 mapping illustrates that the Penarth Group is approximately 12 m thick and this is confirmed by the BGS borehole logs, where the mean thickness was noted to be 11 m. This Penarth Group extends downwards into the Triassic Mercia Mudstone Group, which is subdivided into the Blue Anchor Formation and Red Marl. The Mercia Mudstone Group comprises red, green and grey mudstones and siltstones, with halite deposits. Details from previous investigations at this site have described the Mercia Mudstone as dolomitic mudstones and siltstones, with pyrite being locally present and gypsum present throughout. In the previous investigation, gypsum and anhydrite bands have been encountered near the base of the Mercia Mudstone Group. The BGS borehole logs at this site did not fully penetrate the Mercia Mudstone Group, however, the BGS 1:50,000 scale mapping indicates that it is up to 484 m in thickness.

4.4 Structural Features

Rendel, Palmer and Tritton (1980) report that there is an east - west trending fault with a downthrow to the north located in the south of this site. There is also a major fault located approximately 400 m east of the site, beneath Hinkley Point B, orientated north east to south west (Figure 4.2). The downthrow to the west of this fault is estimated to be between 10 m and 75 m and the horizontal displacement is estimated to be between 50 m and 250 m. There is another fault located along the western boundary of the land owned by British Energy. This fault is expressed at surface as a valley (Rendel, Palmer & Tritton, 1980). Vertical to subvertical joints are present mainly within the limestone bands orientated either east-west; north-south; or south-west. The frequency of the joints is high at shallow depths, where it is estimated that there are approximately 3 to 4 joints per metre. The joints become tight with depth and are often infilled with calcite or gypsum deposits.


The zone of weathering at this site is considered to be confined to the upper 5 to 10 m of the bedrock (Rendel, Palmer and Tritton, 1980).


Figure 4.1 Geological Map (Solid and Drift Edition, 1:50 000)

Indicative boundary of Hinkley Point C Power Station Indicative boundary of Hinkley Point C Contractors Laydown area


Figure 4.2 Geological Map (Solid and drift 1: 10 000)

Indicative boundary of Hinkley Point C Power Station Indicative boundary of Hinkley Point C Contractors Laydown area

Geological and Hydrogeological Review 9S4862/18/R003/Rachel Brown/PBor Draft Report v1.0 January 2009 16

5 HYDROGEOLOGY

5.1 Hydrostratigraphical Units

This section describes the hydrogeological characteristics of the geological units beneath the site and these characteristics have been summarised in Table 5.1. Table 5.1 Hydrogeological characteristics of the main geological units

Age Group and Formation

Nature Characteristics

Recent Made Ground

Minor aquifer / aquitard

Perched groundwater may be present if there are significant gravel layers. Where there is significant clay or silty clay, recharge to the underlying minor aquifer will be reduced.

Jurassic Blue Lias Minor aquifer / aquitard

Limestone beds are important for groundwater flow, mudstone horizons act as aquitards

Penarth Group

Minor aquifer / aquitard

Limestone beds are important for groundwater flow, mudstone horizons act as aquitards

Triassic Mercia Mudstone Group

Aquiclude Forms an impermeable base to the aquifer

5.2 Aquifer Properties of the Jurassic Strata

The Environment Agency has designated the Blue Lias as a Minor Aquifer (Secondary Aquifer), i.e. one which does not produce large quantities of water for abstraction, although it is important for local supplies and in supplying base flow for rivers. As detailed in Section 4 of this report, the Blue Lias is a multilayered aquifer comprising relatively low permeability mudstone interbedded with permeable limestone and subordinate sandstone bands (Aspinwall & Company, 1996). Groundwater movement and storage occurs in the joints or fractures in the limestone and as such, the Blue Lias is considered to have high secondary permeability. However, as the permeable bands are relatively thin, transmissivity values are generally low. The BGS physical properties of minor aquifers in England and Wales reports that six analyses from outcrop samples at Hinkley Point gave porosities ranging from 1.9 % to 3.1 %. Aspinwall & Company (1996) report that permeability tests have been undertaken in boreholes at Hinkley Point during the investigation by Soil Mechanics (1990) and by Norwest Holst (1983/84). A range of tests have been carried out, including packer tests, falling and rising head tests as well as pumping tests. These tests were carried out for different depth ranges. The average hydraulic conductivity (K) values calculated from the results of this test ranged from 10-6 m/s to 10-5 m/s. Two previous investigations have been undertaken by Allott, Atkins and Mouchel (1988) and Rendel, Palmer and Tritton (1986) and are described in report by Aspinwall (1996). Both of these investigations produced hydrogeological models for the site. Allott, Atkins and Mouchel considered the aquifer properties beneath the site in five separate layers:


made ground, sands and gravels, angulata zone, lower liasicus zone and Penarth Group. Whereas the Rendel, Palmer, Tritton model considered only two layers in their model, namely an upper weathered zone and an unweathered zone. The hydraulic conductivities (K) used in these models are summarised in Table 5.1. It should be noted that the values of K range from 10-9 to 10-3 m/s, but are on average 10-5 m/s. Table 5.1 Summary of Hydraulic Conductivities used in Previous Studies (derived from field measurements)

Model Layer K (m/s) Kh1 (m/s) Kh2 (m/s) Kv (m/s)

Upper weathered zone 1x10-5 to 2x10-3 Rendel, Palmer, Tritton Unweathered zone 8x10-9 to 1x10-5

Made ground 10-5 10-5 10-5

Sands and gravels 10-3 10-3 10-3

Angulata Zone 2x10-5 4x10-5 10-6

Lower Liasicus Zone 5x10-7 1x10-5 5x10-7

Allott, Atkins and Mouchel

Penarth Group 2x10-6 10-6 10-7

Note: Kh1 = horizontal hydraulic conductivity (north-south); Kh2 = horizontal hydraulic conductivity; Kv = vertical hydraulic conductivity

As vertical groundwater movement is restricted by the lower permeability mudstones and shales, the aquifer is considered to be under semi-confined to confined conditions. As such, the aquifer has low storage coefficients ranging from 0.001 to 0.00008, which are reported to be typical of semi-confined to confined conditions (Aspinwall & Company, 1996). It is reported by Rendel, Palmer & Tritton (1980) that, typically, following pumping for a few hours during a pumping test, both yields and water levels fall significantly. The aquifer is described as being compartmentalised due to faulting and this, coupled with the low storage coefficients, means that pumping tests generally do not reach equilibrium without being affected by boundary conditions. Previous pumping and recovery tests indicate that drawdown during pumping would induce limited impact on the water level outside excavation (Rendel, Palmer and Tritton, 1980).

5.3 Aquifer Properties of Triassic Strata

Underlying the Jurassic Blue Lias are the Triassic aquifers comprising the Penarth Group followed by the Mercia Mudstone Group. The Penarth Group comprises limestone horizons and is therefore considered to be similar to the Blue Lias at the site. In contrast to the Blue Lias and the Penarth Group aquifers, the Mercia Mudstone Group has low permeability and is considered to be a poor aquifer. It has been classified by the Environment Agency as a non-aquifer. Non-aquifers, also known as ‘unproductive aquifers’, are defined as having negligible permeability and are generally not considered as containing water in exploitable quantities. Any groundwater within the Mercia Mudstone Group will be present within the siltstones and sandstones which are interbedded within impermeable mudstones. These sandstone and siltstone horizons may contain and transmit limited quantities of groundwater through fractures. These


bands are low yielding and are considered to act as an aquitard for groundwater flow at this site.

5.4 Groundwater Levels

Rendel, Palmer and Tritton (1980) report that the groundwater levels in the Blue Lias Formation aquifer follow the natural ground surface, approximately 1 and 3 m bgl. Water strike details have been provided on several of the BGS borehole logs, which indicate that the groundwater level beneath the site is between 6.35 m and 10 m AOD. Groundwater level contours have not been plotted due to the lack of groundwater level data. Much data would be required to be able to plot the groundwater level contours due to the complexity of this multilayered aquifer. The site is further complicated by the tidal influence and as such, many monitoring boreholes would be required, drilled to different depths and all monitored using data loggers at regular intervals in order to obtain sufficient data to construct contour mapping. Artesian groundwater was encountered in the southwest of the site, as well as to the west of the site during the investigation reported in Rendel, Palmer and Tritton, 1980. Artesian conditions in this area of the site are the result of groundwater in limestone bands and open-jointed calcareous mudstones, recharged at higher levels, being overlain by less permeable mudstones. Water levels were monitored as part of the Aspinwall & Company (1996) report and the following observations were made:

• Groundwater levels in some limestone horizons were sub-artesian (i.e. groundwater level was measured above the top of the limestone)

• Positive head gradients exist at some borehole locations (i.e. there is a downward vertical flow);

• Negative head gradients exist at some borehole locations (i.e. there is upward vertical flow); and

• Artesian heads (of up to 1 m) were recorded in boreholes near to the coast. Although there is no groundwater level data available for the drift deposits, it is considered likely that the groundwater level in the wetland to the east of the site is not connected to the groundwater level in the Blue Lias Formation. The wetland sits on Marine and Estuarine Alluvium and it is likely that this is fed by the groundwater in the underlying Fluvial Glacial Sands and is an aquifer unit, not in hydraulic continuity with the Blue Lias Aquifer.

5.5 Recharge and Groundwater Flow

Where the Blue Lias Formation outcrops, the aquifer responds rapidly to recharge by precipitation. This is explained by the fact that the aquifer has a low storage capacity (Rendel, Palmer and Tritton, 1980). As discussed in Section 4.2, there is uncertainty to where drift deposits are located at the site, as such it has been assumed that the drift deposits, which are likely to be Made Ground only, extend as far as the site boundary and that the limestone outcrops to the south of the site. In addition to rainfall recharge, there may be further recharge from lateral flow from areas outside site boundary (Rendel, Palmer and Tritton, 1980).


Although the data from CEH indicates that the mean annual rainfall in this area is 947 mm/year, the recharge to the aquifer is considerably lower than this, due to evapotranspiration, run-off and crop uptake etc. Recharge to the aquifer is also dependent upon the vertical hydraulic conductivity (K) and the Transmissivity (T) of the drift deposits. As the values of K and T of the drift deposits are relatively low at this site, on the basis of a rule of thumb, the maximum effective recharge to the aquifer at outcrop has been estimated to be one third of the mean annual rainfall, which is 315 mm/year. In areas where the site is underlain by clay and silty clay to a depth of around 5 m, the effective recharge to the aquifer will be significantly lower than that of the outcrop area. Again, based on a rule of thumb, the recharge in areas of drift deposits is estimated to be one tenth of the mean annual rainfall, which is 95 mm/year. In the previous modelling studies by Allott, Atkins and Mouchel (1988) and Aspinwall & Company (1996), recharge was calculated as a percentage of rainfall, depending on landuse or presence of drift deposits. A value of 50 mm/year for effective recharge was used by Aspinwall & Company (1996), i.e. a quarter of the value of effective rainfall used in their study. However, it is considered that this value is low, especially in the area of outcrop. Groundwater flows through the Blue Lias Formation aquifer northwards to the estuary, where it discharges. The hydraulic gradient is considered to range from 0.01 to 0.02 (Aspinwall & Company, 1996). Currently we have no groundwater flow data for the Marine and Estuarine Alluvium, however, it is considered likely that groundwater in these drift deposits will be in hydraulic continuity with the streams and ditches (unnamed) within the wetland. Groundwater movement in this aquifer is complex due to the interbedded mudstones and limestones. It is considered that the mudstone bands are aquitards, acting as boundaries to groundwater flow, whereas, groundwater flow is possible between limestone bands of the Blue Lias and Penarth Group. Aspinwall & Company (1996) report that vertical head differences exist between different horizons in the Blue Lias and Penarth Group caused by the interbedded mudstones acting as aquitards. For purposes of this study, it is assumed that groundwater flow is limited to the upper layers of the Blue Lias. Values of hydraulic conductivity (K) at shallow depths range from 10-6 to 10-4m/s, whereas below this zone the aquifer is described as being only slightly permeable, with K values reported to range from 10-8 to 10-7m/s. As flow at depth is restricted, discharge to the sea occurs by upward vertical movement through mudstone horizons via fractures (Aspinwall & Company, 1996).

5.6 Groundwater Response to Tidal Variation

Although there is a very large tidal range in the Severn Estuary at Hinkley Point, tidal responses in groundwater have not been observed in the monitoring boreholes. Perhaps this is due to the low permeability of the strata and the depth to which the boreholes have been drilled. Tidal responses of up to 1 m were noted in an investigation carried out by the BGS at Hinkley Point B, however, these boreholes were shallow and were monitoring weathered and permeable horizons (Aspinwall & Company, 1996). Rendel, Palmer and Tritton (1980) report that water levels monitored during three separate tidal cycles were found to be relatively constant and Foundation Engineering Ltd also did not find significant variation in water level over a tidal cycle.


5.7 Summary of Hydraulic Characteristic

Table 5.2 summarises the important properties of the Blue Lias aquifer underlying this site. These hydraulic characteristics will form the basis of the conceptual model for the site. Table 5.2 Summary of the Aquifer Properties

Characteristic Reported Range

Direction of groundwater flow North

Hydraulic gradient 0.01 to 0.02

Hydraulic conductivity (K) 10-6 to 10-4 m/s

Effective thickness of aquifer (Based on the fact that hydraulic conductivity of the aquifer decreases with depth. This value is based on pumping test data carried out during previous studies)

15 m

Transmissivity (T) 1.5 X10-5 to 1.5X10-3 m2/s

Storage coefficient 0.001 to 0.00008

Porosity 1.9 to 3.1%

Effective porosity 3% (conservative value on the basis of porosity)

Recharge to the aquifer (outcrop) 315 mm/year

Recharge to the aquifer (where drift is present) 95 mm/year


6 HYDROGEOLOGICAL CONCEPTUAL MODEL

In order to assess the potential environment impacts, a hydrogeological conceptual model for the proposed construction and operational activities associated with the site development was established. The conceptual model is a qualitative evaluation of hydrostratigraphical units and their interactions, aquifer properties including recharge and hydraulic properties in relation to the construction of the Hinkley Point C. In particular, the conceptual model will highlight any potential changes to groundwater levels, from which the associated environmental impacts will be assessed. The hydrogeological conceptual model, as illustrated in Figure 6.1 has been used as the basis for qualitatively assessing the environmental impacts in Section 7.

6.1 Development Scenarios

Table 6.1 Details of Proposed Structures at Hinkley Point C

Structure Area (m) Final Ground Level (m OD)

Foundation Level (m OD)

Thickness from top of Blue Lias Aquifer to base of excavation (m)

Cooling Water Pumphouse

130 x 50 +15 -19 29

Reactor Building 90 x 100 +15 +1.4 8.6

Turbine Hall 115 x 60 +15 +5 5

The following key construction activities have been identified for the construction of Hinkley Point C:

• dewatering to the excavation base to allow construction to be undertaken in dry conditions. The foundation depths of the cooling water pumphouse, the nuclear island and the turbine hall are likely to be -19 mOD, +1.4 mOD and +5 mOD, respectively (Table 6.1); and

• permanent placement of surplus excavated material on land to the south of the power station site (refer to Figure 3.2).

6.2 Summary of Impact

Based on the hydrogeological conceptual model and construction scenarios, the potential environmental impacts have been identified to be mainly associated with:

• reduced recharge to the aquifer beneath the proposed contractors laydown area due to placement of excavated material during the construction of the Hinkley Point C;

• construction of structures with deep foundations, which may act as a barrier to flow;

• creation of pathways for contamination to impact aquifer during construction phase; and

• the impact of dewatering on the groundwater levels in the Blue Lias Formation. • The potential impacts are qualitatively assessed in Section 7.


7 ENVIRONMENTAL IMPACT ASSESSMENT

7.1 Assessment Methodology

On the basis of the conceptual model (Figure 6.1) and details of proposed structures, potential environmental impacts to groundwater have been identified. These potential environmental impacts will be assessed on the basis of a qualitative analysis of hydrogeological system.

7.2 Potential Impacts due to Dewatering

Currently the groundwater levels range between 6.35 m and 10 m AOD. The proposed foundation levels of the deep structures at site are well below these groundwater levels (Table 6.1). As such, groundwater levels will have to be significantly reduced through dewatering to allow construction to take place in dry conditions. Therefore, the potential environmental impacts as a result of the reduction in groundwater level include the impact to nearby sensitive receptors, particularly the wetland to the east and southeast of the site. Prior to qualitatively assessing the potential impacts, the volume of dewatering required was calculated. The volume of groundwater which will require removal has been estimated to be a maximum of 6,300 m3 within the Blue Lias aquifer to maintain a dry surface to construct the cooling water pumphouse, the reactor building and the turbine hall. Periodic pumping may be required to prevent groundwater build-up due to effective recharge in unpaved area during the operation of the site. Assuming that 10 wells were used at rate of 1 l/s, it would require up to 8 hours for groundwater levels to fall below the proposed base of excavation at -19 mOD for construction of the cooling water pumphouse (Table 7.1). Whereas, it would take 6 hours for groundwater levels to fall below the proposed base of excavation at +1.4 mOD for construction of the reactor building (Table 7.2) and approximately 2.4 hours for groundwater levels to fall below the proposed base of excavation of +5 mOD for the construction of the turbine hall (Table 7.3).


Table 7.1 Estimation of Volume of Groundwater and Duration of Dewatering for the Cooling Water Pumphouse

Parameters Value Unit

Volume Calculation

Length of the cooling water pumphouse 50 m

Width of the cooling water pumphouse 130 m

Area 6,500 m2

Effective porosity 0.03 -

Volume of Water to the base (-19 mOD) 2,925 m3

Duration of Dewatering

Pumping Rate 1 l/s

No. of Wells 10 -

Duration of Pumping 8 Hours

Table 7.2 Estimation of Volume of Groundwater and Duration of Dewatering for the Reactor Building


Volume Calculation

Length of the reactor building 90 m

Width of the reactor building 100 m

Area 9,000 m2


Volume of water to the base (+1.4 mOD) 2322 m3


Pumping Rate 1 l/s

No. of Wells 10 -

Duration of Pumping 6 Hours


Table 7.3 Estimation of Volume of Groundwater and Duration of Dewatering for the Turbine Hall


Volume Calculation

Length of the turbine hall 60 m

Width of the turbine hall 115 m

Area 6,900 m2


Volume of Water to the base (+5 mOD) 1035 m3


Pumping Rate 1 l/s

No. of Wells 10 -

Duration of Pumping 2.4 Hours

It should be noted that the above estimates of volume and duration of the pumping are based on:

• an assumption of the size of the proposed structures (provided by BE). These values are only indicative at this stage;

• an effective aquifer thickness of 15 m (assuming that very little groundwater flow

occurs below this depth);

• the top of the Blue Lias Formation being at 10 mAOD;

• for simplicity of the calculation it is assumed that perched water is not present;

• value of 3% as a conservative estimate of effective porosity; and

• no effective recharge during dewatering. Temporary control measures, such as the installation of sheet piled cofferdam around the perimeter of the proposed deep structures, during the construction is considered necessary to allow dewatering to be maintained effectively. On the basis of these calculations, it is considered that the groundwater levels surrounding the deep structures on site will be lowered as a result of dewatering activities. However, the extent to how much the water level will be lowered is unknown. The aquifer has a relatively low storage coefficient and observations made during previous pumping tests (Rendel, Palmer & Trittion, 1980) indicate that groundwater levels do not reach equilibrium without being affected by boundary conditions. Further investigations and pumping tests are recommended to determine the potential impact to groundwater levels and flow regime beneath the site.


7.3 Potential Impacts to Aquifer Recharge

It is expected that surplus excavated material from the construction of Hinkley Point C will be placed on the land to the south of the development area in the contractor’s laydown area. Although it is expected that the laydown area will be restored to its current land use, i.e. arable fields, following construction of Hinkley Point C, the placement of the surplus excavated material may reduce effective recharge to the aquifer in this area. With the exception of a relatively small area in the north which is underlain by Alluvium, the Blue Lias Formation outcrops beneath the majority of the contractor’s laydown area. Although the Alluvium is likely to be in hydraulic continuity with the wetland, as it covers a relatively small proportion of the contractor’s laydown area, any reduction in recharge as a result of the placement of surplus material is not considered to present a significant impact to the wetland.

7.4 Potential Impacts from Off Site Disposal of Excavated Material

There is the possibility that the surplus material from the power station area is contaminated. Before the surplus material can be placed within the contractor’s laydown area of the site, there will be a requirement for it to be characterised by testing for potential pollutants and assessing the risk to human health and the environment, in particular the risk of potential impact to Blue Lias aquifer. Providing certain requirements are met according to the CL:AIRE Code of Practice the surplus material will not be considered waste and can be reused on site without an environmental permit. If the material is considered waste, it may still be used on site, however, its use will be subject to the requirements of an environmental permit, or a suitable registered exemption from permitting.

7.5 Potential Impacts to Groundwater Quality

Given that the site is underlain by interbedded permeable limestones and lower permeability mudstones, the potential also exists for a pathway for vertical groundwater flow to be created during construction of the deep structures. This pathway may result in vertical groundwater flow between limestone horizons altering the groundwater flow regime beneath the site. This pathway may in turn allow for potential contaminants to enter the aquifer if, for example, there are spills or leaks of pollutants during construction phase of the project. The assessment of such potential impacts is outside the scope of this report but would be considered in the Environmental Impact Assessment.

7.6 Potential Impacts to Designated Areas

Once the deep structures, which extend below the watertable, have been constructed they will act as a barrier to groundwater flow. To assess the significance of the effects of dewatering during operation, it is necessary to consider receptors which may be impacted as a result of this lowering of groundwater levels. As there are no reported groundwater abstractions within 2 km of the site, the potential receptors include the designated areas surrounding the site, such as the wetland to the east and southeast and the cSAC, SPA and SSSI to the north. As the dewatering is likely to result in a reduction in groundwater levels beneath the site and therefore a change in the groundwater flow, the potential exists for alteration in


saltwater freshwater interface. There are two main considerations of this change in the saltwater – freshwater interface:

1. The construction of the power station will result in an increase in groundwater levels upgradient of the structures, resulting in an increase to the hydraulic gradient of the groundwater. This is likely to result in the groundwater being diverted around the deep structures at the site. However, if this was to occur, it is considered unlikely that there will be any detriment to the habitat on the foreshore as the intertidal habitat of the cSAC, SPA and SSSI is likely to be able to endure changes in saltwater – freshwater composition.

2. The construction of the power station will result in saltwater intrusion into the

aquifer, which is likely to be most significant during spring tides. There is particular concern of this occurring during construction of the cooling water pumphouse as it is closest to the foreshore and has relatively deep foundations. Although this is the case, the potential impact to the habitats within the cSAC, SPA and SSSI are not considered to be significant as groundwater level reduction will be small compared to the tidal range in this area.

3. There is the potential that the changes to the groundwater flow regime may

result in saltwater intrusion to the wetland, however, as the wetland is underlain by Marine and Estuarine Alluvium followed by Fluvial Glacial Sands, it is considered unlikely that it is in hydraulic continuity with the Blue Lias Formation aquifer and as such the risk is considered to be low. As there is currently no monitoring data available as evidence for this assumption it is recommended that monitoring be carried out in this area to confirm that the groundwater level in the Marine and Estuarine Alluvium is not contiguous with the groundwater level in the Blue Lias Formation.


8 CONCLUSIONS AND RECOMMENDATIONS

Royal Haskoning was commissioned by British Energy to collate and review available geological and hydrogeological data for the proposed power station site at Hinkley Point C, Somerset. This geological and hydrogeological assessment has been based on a review of readily available data from public sources and previous reports. Data included BGS borehole logs; published geological mapping; and previous groundwater modelling studies. A conceptual model has been formulated based on the analysis of available data for the proposed Hinkley Point C Power Station. The potential structures have been identified and subsequently assessed in a qualitative manner and the potential environmental impacts have been assessed qualitatively. The primary conclusions and recommendations of the hydrogeological review are summarised below:

• It is considered that the main geological and hydrogeological characteristics of the strata are well established at the Hinkley Point C power station site. However, the groundwater flow regime within the Blue Lias Formation aquifer and its connection with the Marine and Estuarine Alluvium beneath the wetland is not well established. It is therefore recommended that further hydrogeological investigation is undertaken to establish baseline conditions and the connection between the aquifer and the wetland.

• It is likely that the groundwater levels surrounding the deep structures will be

lowered as a result of dewatering activities. An additional investigation is recommended to fully assess the impact to the groundwater level from dewatering. Alternatively, if original data from previous investigations were to be made available, these data could be reviewed, potentially negating the requirement for further investigations.

• The dewatering and construction of the power station may result in a change to

the groundwater flow regime at the site, potentially changing the freshwater – saltwater interface. This in turn could potentially result in saline intrusion to the wetland to the southeast of the site if there is connectivity. However, as connectivity with the Blue Lias Formation aquifer is unknown, an investigation is required to determine whether it is likely to be impacted as a result of dewatering the Blue Lias Formation aquifer.

• The volume of groundwater to be dewatered to maintain a dry surface during the

construction of the cooling water pumphouse, the reactor building and turbine hall is estimated to be up to 4,200 m3. Additional periodic pumping may be required to prevent groundwater build-up following rainfall events. Assuming using 10 pumping wells located around the deep structures and a rate of 1 l/s, it would take a maximum of approximately 1 day to reach the proposed base of excavations for the cooling water pumphouse, the nuclear island and the turbine hall at -19 m OD; +1.4 mOD and +5 mOD, respectively.

• The placement of excavated material is likely to result in a reduction of effective

recharge to the Blue Lias Formation aquifer. However, the potential impact from


this reduction in recharge is not considered to be significant. Given that there is the potential for the excavated material to contain contaminants, it is recommended that WAC testing and an assessment of risks for potential pollutants to impact Blue Lias aquifer within the contractor’s laydown area is undertaken.

Appendix A Photographs taken during Site Walkover

Photo 1 Example of Blue Lias Strata on Foreshore (north of Hinkley Point A)

Photo 2 Foreshore (north of Hinkley Point B)

Photo 3 Northern boundary of Hinkley Point A (looking west towards Hinkley Point C)

Photo 4 Exposed weathered Blue Lias (Made Ground?) on Hinkley Point site

Appendix B BGS Borehole Logs

Created by Rod Bowie on 5/6/2008 3:12:00 PM N:\scans\CONDITIONS OF USE FORM.doc

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