stowe hill quarry proposed extension, clearwell, gloucestershire …r+230214... · hyg145stowe hill...

Stowe Hill Quarry Proposed Extension,

Clearwell,

Gloucestershire

Hydrogeological Impacts Review

On Behalf of

Newland Parish Council

HYG122Page’s Lane Groundwater Review ii August 2014

Quality Management

Prepared by: Chris Betts MSc, BSc, CGeol, FGS

Reviewed by: Mike Willis MSc, BSc, FGS

Authorised by: Chris Betts

Date: 23/02/2015

Revision: DRAFT

Project Number: HYG145

Document Reference: HYG145 R 150214 CB Stowe Hill Quarry

Document File Path: P:\HYG145 Stowe Hill Quarry\Reports\Draft\HYG145 R 150214 CB Stowe Hill Quarry.docx

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HYG122Page’s Lane Groundwater Review iii August 2014

Contents

Quality Management ............................................................................................... ii

Contents .................................................................................................................. iii

1 Introduction ......................................................................................................... 1

1.1 Background ......................................................................................................... 1

1.2 Objectives ........................................................................................................... 1

1.3 Report Author ..................................................................................................... 1

2 Background and Key Issues .............................................................................. 3

2.1 Site Geology and Hydrogeology ........................................................................ 3

2.2 Karst and Groundwater ...................................................................................... 5

2.3 Slade Brook Site of Special Scientific Interest (SSSI) ...................................... 8

3 Baseline Monitoring Review ............................................................................. 10

3.1 Groundwater level monitoring ......................................................................... 10

3.2 Water Quality Monitoring ................................................................................. 10

3.3 Continuous Monitoring .................................................................................... 11

4 Quarrying Impacts ............................................................................................ 13

4.1 Removal of Epikarst and Solution Features ................................................... 13

4.2 Slade Brook Catchment ................................................................................... 14

4.3 Sub-Water Table Working ................................................................................ 16

4.4 Mitigation measures ......................................................................................... 19

4.5 Expected Changes to the Groundwater Conceptual Model – Operational Phase

........................................................................................................................... 20

5 Restoration Impacts .......................................................................................... 22

5.1 Inert Waste ........................................................................................................ 22

6 Conclusions ....................................................................................................... 23

HYG122Page’s Lane Groundwater Review iv August 2014

Tables

Figures

Figure 1 - Site Geology............................................................................................................... 3

Figure 2 - Slade Brook Location ................................................................................................. 4

Figure 3 - Stowe Hill Quarry – Looking to the East, February 2015. ........................................... 4

Figure 4 - Karst Cross-Section .................................................................................................. 6

Figure 5 - Solution Cavity in the Base of Stowe Hill Quarry (circa 2001) ..................................... 6

Figure 6 - Solution Features inside Extension ............................................................................ 8

Figure 7 - Slade Brook SSSI Tufa Dams .................................................................................... 9

Figure 8 pH Recorded in Slade Brook ...................................................................................... 11

Figure 9 - Maximum Indicative Recharge Area from Table 3.2 Based on Hydrograph Separation

.......................................................................................................................................... 15

Figure 10 Maximum Indicative Recharge Area from Table 3.1 Based on Measured Baseflow

(Low Flow)......................................................................................................................... 15

Figure 11 Cross Sections through the site (taken from ES Hydrogeology Chapter) NTS .......... 18

HYG145Stowe Hill Quarry Hydrogeology Review 1 February 2015

1 Introduction

1.1 Background

Hydrogeo Limited (Hydrogeo) were commissioned by Newland Parish Council (the

client) to undertake a review of the potential groundwater relatedimpactsfrom

theproposed extension of the quarry workings at Stowe Hill Quarry.

This hydrogeological impacts review includes a technical appraisal of the Hydrogeology

Chapter (RPS, 2014) and supporting technical appendices submitted as part of the

Environmental Statement Chapter for the site. Throughout this document the RPS

Hydrogeology Chapter and Appendices is referred to as ‘the report’.

An independent and unbiased technical review of the proposal has been undertaken

including the author’s knowledge of the geology/hydrogeology of the site and the

surrounding area.

1.2 Objectives

The objectives of this study are to review pre-existing information, data and reports

relating to the geology and groundwater in the areaplus a visual inspection of the area

surrounding the site to identify any potential hydrogeological issues associated with the

potential extension of the limestone quarry.

The review concludes with an opinion on whether the quarry is suitable for large scale

expansion, or not, based on the identifiedpotential impacts on groundwater resources,

and habitats that are dependent on groundwater near the site.

The Environmental Statement and Technical Appendices provided are comprehensive

and highly technical documents. This report addresses technical issues but also aims to

summarise the key points in a non-technical manner to aid understanding and decision

making for the wider audience.

1.3 Report Author

The site walkover, desk study and report has been undertaken by Chris Betts (BSc,

MSc, CGeol, FGS) a Chartered Geologist with eighteen years’ experience as a

Professional Hydrogeologist in the UK. Chris’s experience as a hydrogeologist is wide

ranging working on hydrogeological investigations and risk assessments for landfill sites,

contaminated land, water resources and mineral extraction projects.


Chris is a Director of Hydrogeo Limited, a specialist scientific environmental consultancy

established in 2006 which provides expertise in the water environment, geology and

land quality.


2 Background and Key Issues

2.1 Site Geology and Hydrogeology

Stowe Hill Quarry is located on the eastern limb of the Forest of Dean Coalfield syncline

which is oriented north-south. The geological map (Figure 1) indicates that the site is

situated on the Limestone Beds within Lower Limestone Shale, which forms the basal

unit of the Carboniferous Limestone Series. An outlier of the overlying Lower Limestone

Shale is shown to exist in the southern part of the extension area associated with the

rise in topography towards Bears Common and Orles Wood.

The existing quarry (Figure 3) is working the Limestone Beds down to a shale layer

(visible in the distance in the photo). The proposed extension is to continue to quarry the

Limestone to the east and south-east, and to deepen the workings beyond their current

depth.

Figure 1- Site Geology

The Carboniferous Limestone in the area is a well-fissured karstic limestone, with

numerous solution features.

The Limestone in the Lower Limestone Shale Unit is underlain by the Tintern Sandstone

Group and basal Quartz Conglomerate of the Upper Old Red Sandstone.

Limestone/Shale

Limestone

Proposed

Extension

Existing

Quarries

Slade Brook Main Spring


The boundary between the Lower Limestone Shale and the underlying Tintern

Sandstone Group is typically marked by an increase in topography and a line of springs

or resurgences, emerging at the base of the Limestone.

Numerous springs are marked along the length of Slade Brook. The elevation of the

spring providing flow to Slade Brook, which is closest to the site is at approximately

135mAOD.

Figure 2- Slade Brook Location

The water flowing in Slade Brook supports a series of tufa 'travertine dams', classified as

a Site of Special Scientific Interest (SSSI), described in detail in Section 2.3.

Figure 3 - Stowe Hill Quarry – Looking to the East, February 2015.

Main Slade Brook Spring

Tufa Dams - SSSI

Proposed

Extension


2.2 Karst and Groundwater

The term ‘karst’ refers to a type of landscape that develops from the dissolving action of

water on soluble bedrock, primarily limestone.

Karst features and the 3-dimensional landscape (Figure 4) are the result of a highly

complex interrelationship between geology, climate, topography, hydrology, soil and

biological factors over a very long period of time.

The formation of Karst landscapes in limestone involves carbon dioxide dissolving into

rainfall and into water infiltrating the soil zone, forming a weak carbonic acid. This slightly

acidic water then infiltrates joints and fissures in the bedrock slowly dissolving the

limestone and creating cavities and larger openings. The water in the underground

system is commonly ‘supersaturated’ with calcium, when this water loses carbon dioxide

either in the underground cave network or on emergence at ground surface, the calcium

carbonate is precipitated out forming ‘tufa’.

Epikarst is the zone of solutionally enlarged openings or fractures that extends from the

ground surface (the exokarst) down as much as 10–30 metres below the surface to the

underlying endokarst. The endokarst describes all deeper components of the

underground karst landscape, including the smallest cavities, cave speleothems, cave

sediments, and cave passages. The epikarst zone therefore plays a critical role in the

karst system, allowing water, air, and other materials (sediment, organic debris,

andnutrients) to be readily transferred from the surface to the subsurface.

Theepikarst layer will be removed by quarrying in the extension area. Water will

therefore drain directly into the endokarst (larger cavities) and will not have the

opportunity to become supersaturated with calcium carbonate.


Figure 4 - Karst Cross-Section

Image from: Karst Geomorphology, Hydrology, and Management, Chapter 11

An example of a solution cavity encountered in the base of the Stow Hill Quarry is shown

in Figure 5.

Figure 5- Solution Cavity in the Base of Stowe Hill Quarry (circa 2001)


Orles Wood located to the south of the existing Stowe Hill Quarry is underlain by a

Lower Limestone Shale ‘outlier’ which overlies and is surrounded by the Limestone

bedrock. The Limestone bedrock also contains shale beds within it. Numerous sinkhole

features have formed at the boundary of the Lower Limestone Shale, where surface

runoff and soil seepages from the shale meet the edge of the Limestone, focussing

infiltration and forming solution features.

The report emphasises the fact that the most continuous line of sink holes in the vicinity

of the site is along the southern edge of Bearse Common/Orles Wood. However, the

following observations are made regarding the land inside the proposed excavation area

as shown on Figure 6;

1. The largest sinkhole feature in the catchment is the Longley Farm Doline and

sink line located to the west of the farm buildings. This significant solution feature

was studied in 2003 by RPS; A tracer test was undertaken in the Longley Farm

swallow hole in 2003 by RPS using flourcecent dye. The test proved a positive

link between the Longley Farm sinkhole and Slade Brook. This indicates a travel

time of between 70 and 96 hours. During the current extension (2005) the

Longley Farm Doline was left untouched and outside of the permitted excavation

area due to the proven link with Slade Brook. This large solution feature would

now be removed by the proposed extension, a point which does not seem to be

mentioned in detail in the reports.

2. The RPS Flood Risk Assessment (Para 3.1.5) states ‘There is some qualitative

evidence of near surface integrated down slope drainage and potential

(infilled) dolines on the Site to the south east of Longley Farm from crop marks

on aerial imagery’. This area is shown on Figure 6, and is at the edge of the

Shale/Limestone boundary. It is likely that any solution features inthis area would

have been infilled by landowners many years ago to enable the field to be

farmed. This observation is not mentioned at all in the Hydrogeology Chapter and

Report.

Geophysics and / or LiDAR surveys are frequently used to identify and map shallow

subsurface features such as infilled solution features. It is surprising that given the

highly sensitive nature of the SSSI that more resources have not been committed to

identifying solution features across the proposed extension area. The Scoping Opinion

from Gloucester County Council did request a geophysical survey and trial pitting

across 2% of the extension area as part of the archaeological survey. These


investigative techniques, if undertaken, would have also aided the identification of near

surface solution features. The results of these surveys have not been presented or

referred to in the reports reviewed.

Figure 6 - Solution Features inside Extension

2.3 Slade Brook Site of Special Scientific Interest (SSSI)

‘Slade Brook is nationally important for its active tufa-forming stream system. The

stream supports a series of tufa dams (with associated plunge pools and

connecting stream sections) that are a result of the combination of a series of

complex physical and chemical processes within the stream. This system of tufa

dams forms the longest series of such structures representing this feature in

Britain’ (Natural England SSSI citation, 2003).

The 'travertine dams' are present intermittently for a stretch of approximately 700m of

Slade Brook.

Tufa or 'travertine' dams are generally believed to be formed by the loss of carbon

dioxide due to cooling, evaporation or the presence of algae.

Research undertaken by Pentecost et al (2000) on the formation of the travertine dams

at Slade Brook, involved a monitoring programme of water sampling and discharge

estimates once a month for a period of 14 months. The research concluded that the

higher than normal carbon dioxide concentrations in the water are likely to be a

consequence of the partially wooded nature of the assumed catchment, resulting in

high soil respired carbon dioxide. It was also suggested that the groundwater is close

to equilibrium with the limestone on emergence at springs, and the chemical

Possible infilled

circular sinkholes

Longley Farm

Sinkhole /

Doline

Proposed edge of

extension

Orles Wood


composition of the main springs and side streams suggest similar groundwater

sources. In terms of catchment area, the report suggested that Orles Wood to the north

of Slade Brook, which contains a line of sinkholes, could be a major source of

groundwater for Slade Brook.

Earlier work (Viles and Pentecost, 1999), on the tufa deposits of Nash Brook, South

Wales, indicated that 'degassing of carbon dioxide in turbulent water may encourage

calcium carbonate deposition, as may the biological uptake of carbon dioxide'. This

research also pointed out the importance of fallen trees and other woody debris in the

development of tufa dams via a barrage building method.

Calcium carbonate is still being actively deposited in Slade Brook, with sediment

observed on roots and branches submerged in the brook. An example of the tufa dams

is shown in Figure 7.

Figure 7- Slade Brook SSSI Tufa Dams


3 Baseline Monitoring Review

3.1 Groundwater level monitoring

The groundwater level across the site is critical as the proposals are to avoid working

below the groundwater level to avoid dewatering the limestone aquifer and associated

impacts.

A clearly defined continuous ‘groundwater level’ typically does not exist in karst

limestone. It is entirely possible to drill a ‘dry’ borehole in one position that does not

encounter any water bearing fissures, and then drill another a few meters away which

encounters water. This is complicated further by the presence of shale bands at the site

which can result in ‘perched groundwater’. The groundwater units at the site are divided

in the report to the ‘shallow’ and ‘deep’ limestone, separated by a shale band. The

perched or shallow groundwater should not be discounted as unimportant, as it may be

draining slowly through the shale, or over the top of the shale towards solution features.

The epikarst has developed in the shallow limestone and may be providing significant

storage for slowly infiltrating water.

The report does not refer to an old groundwater borehole located in the base of the early

south western part of Stowe Hill Quarry, which was used as a water supply in 2001; a

pumping test was undertaken in this boreholein 2001 as part of the MSc study which

indicated a continuous groundwater level in that part of the site at approximately 168-

170mAOD.

Catchment Pond E in the west of the current working area was formed by a ‘trial

deepening’ of the quarry floor by the former quarry operator, and formed a permanent

water feature. This pond now receives surface water runoff from the working quarry. The

water level in this pond is shown to be approximately 169.90mAOD on the site survey. It

is therefore possible that the water level in Catchment Pond E is representative of the

groundwater level beneath this part of the site.

3.2 Water Quality Monitoring

The aim of the monitoring program established in 2005 was to derive control and trigger

levels for key water quality parameters in order to enable the identification of any change

in the water chemistry, which could impact on the active formation of the tufa dams at

Slade Brook.


A detailed review of trends in water quality has been undertaken by RPS, and compared

to flow in the Slade Brook. This has developed a basic understanding of how the water

chemistry and flow in Slade Brook varies following rainfall and during dry periods.

No trigger values or control levels have been set as originally intended.

pH is recorded monthly in water samples sent to a laboratory. There was a slight fall in

the general pH of water samples during 2010/2011 as shown in Figure 8. This fall in pH

is commented on in the report, but is not explained or explored further. The conclusion

that the general water chemistry in Slade Brook is unchanged over the monitoring period

is therefore questionable.

Figure 8 pH Recorded in Slade Brook

The elevated sulphate and strontium concentrations in water samples from Slade Brook

during low flows may be linked to water derived from seepage or storage within the

shale layers, supporting the importance of the water flowing over and through these

layers in the water chemistry.

3.3 Continuous Monitoring

The monitoring scheme established for the 2005 planning permission originally included

a ‘tipping bucket’ raingauge installed at the quarry office. These instruments are used to

measure the intensity and timing of rainfall events at a location. This was considered

critical to studying the baseline ‘time to peak’ from peak rainfall at the quarry to peak flow

in Slade Brook. This could then be used to assess if there were any long term changes

to the ‘time to peak’ as the quarry was extended. This information is important as the

removal of the upper limestone (epikarst) could remove storage and attenuation of

Fall in pH


rainfall thereby resulting in flows becoming more ‘flashy’. If stream flow following rainfall

becomes more ‘flashy’ then the increased energy in the watercourse could potentially

damage the fragile tufa deposits.

On-site rainfall data is not reported in the study or compared against the Slade Brook

hydrographs of steam flow over time. The baseline ‘time to peak’ has therefore not been

presented and any impacts on Slade Brook due to the previous extension have not been

studied as originally intended. Statements on the ‘resilience’ of the catchment to

quarrying are therefore not fully validated in the assessments.

It is accepted that Karst drainage is highly unpredictable and highly variable both

laterally and vertically, this characteristic presents difficulties in presenting definitive

statements on baseline conditions. The water chemistry will also depend on the point of

the hydrograph when the sample is collected (eg. before or after a high flow event).

However, after nearly 10 years of monitoring there is still no real definition of the typical

range of chemistry indicators in the water quality samples at the site. The original

intention to derive control and trigger levels for key water chemistry indicators has not

been undertaken.Given the numerous uncertainties listed in the report and the lack of

site ‘event-based’ rainfall data no clear baseline has been established in Slade Brook.

There is uncertainty in the construction of the monitoring borehole installed in the

extension area with statements in the table ‘not yet know which unit slotted section

completed in’. Borehole installation details are critical in providing a robust conceptual

model, however this information has not been presented within the report.

Given the number of uncertainties cited in the report it is questionable whether the

baseline conditions at the site have been adequately defined.


4 Quarrying Impacts

4.1 Removal of Epikarst and Solution Features

The proposed quarry extension will remove a major solution feature (Longley Farm

Sinkhole) and will also remove the land directly north and east of the woodland at

Bearse Common/Orles Wood. There is evidence of the presence of infilled solution

featuresdirectly east of the woodland as cited by the supporting hydrological report.Major

solution features are frequently not visible on the ground surface.

The report summarises the interpretation of the aquifer system at the site as follows;

“The layered limestone aquifer system with an uncertain flow direction does not

reflect the known karstic hydrogeology that connects surface sink holes /

swallow holes to strong spring resurgence to the south and southwest. The

observation of deep saturated groundwater within the limestone aquifer and

known karstic behaviour demonstrates that the hydrogeological behaviour of

this catchment area is likely to be the product of at least two distinct

hydrogeological systems / regimes that involve (1) a background interconnected

fracture porosity in the limestone and (2) rapid, high flow karstic porosity system.”

Following a review of the analysis of the Slade Brook hydrographs it is hypothesised that

water entering the deeper limestone karst system then recharges into the smaller

network of fractures at depth. This network of fractures then stores and slowly releases

the water over an extended period, maintaining the flow to the springs.

The aquifer system described in the report is unusual for a typical karst catchment, and

our understanding is that without evidence to suggest otherwise ashallower water

storage system compared to a deeper storage system is more likely. The network of

fissures are unlikely to be as well developed and continuous in the deep endokarst zone

compared to the epikarst and the shallow limestone above the shale bands. It is

therefore more likely that the long term baseflow to the karst system and springs to

Slade Brook are maintained by storage and slower seepage from the soil zone and

shales at OrlesWood which feed laterally into the epikarst layer.

The system of the ‘background interconnected fracture porosity’ will be present in the

upper section of limestone at the site and the epikarst. This layer will be removed by the

quarrying process; if it is this layer that provides the supersaturated water stored and

slowly released to the main karst system (critical for tufa formation) then the removal by


quarrying will shorten residence contact time within the limestone and have a direct

impact on water chemistry and active tufa formation.

Different methods have been applied in the report to estimate the catchment area for the

springs at Slade Brook. These indicate that the catchments are likely to extend beneath

the extension area at least in part, therefore the assessment accepts that groundwater

discharging at Slade Brook is derived in part from the extension area.

The report states ‘the 2007 extension to the quarry has shown no evidence of

having any measurable effect on water quality and carbonate chemistry of Slade

Brook (where active tufa formation is ongoing) despite being located closer to the

SSSI relative to the proposed extension area. Taken together these observations

imply a degree of natural resilience of the chemistry of groundwater discharged at

Slade Brook’.

The ‘natural resilience’ of the system to quarrying is stated a number of times in the

report, however it may be that the amount of limestone removed from the catchmentto

date has had a small but not clearly measurable effect. As more limestone is removed

the effect must ultimately become measurable and critical to water chemistry and

subsequent tufa formation.

The proposed ‘epikarst recreation and recontouring’ for the permitted quarry area has

not been undertaken over most of the site and a large quarry open area is apparent; this

lack of restoration across the site is not fully explained and is at odds to the agreed

restoration and quarry management plan. The practicality and effectiveness of the

proposed restoration methodology for the extension area has therefore not been fully

proven.

4.2 Slade Brook Catchment

The report has undertaken calculations of the indicative recharge areas of the springs at

Slade Brook, using measured baseflow (low flow) and hydrograph separation. The

maximum recharge estimates are shown inFigure 9 and Figure 10 below for a

hemispherical recharge radius.

These show that a large proportion of the Slade Brook catchment has been quarried

already, and the extension would increase this significantly.


Figure 9- Maximum Indicative Recharge Area from Table 3.2 Based on Hydrograph

Separation

Figure 10 Maximum Indicative Recharge Area from Table 3.1 Based on Measured

Baseflow (Low Flow)


4.3 Sub-Water Table Working

The Environment Agency requested a Hydrogeological Impact Assessment (HIA) to be

undertaken to address potential impacts from quarry dewatering. This has not been

undertaken to the level of detail required; a ‘Hydrogeological Conceptual Framework’

report has been prepared however this describes the interpretation of the

hydrogeological conceptual model in detail but falls short of undertaking any impact

assessment based on this model.

The existing quarry has been worked down to a major shale band which forms the base

of the current workings. It is understood that it was agreed with Natural England and

LPA to work down to a shallower depth than originally intended, to a maximum level of

176 mAOD, working mainly above the shale layer in order to ensure that the quarry did

not extend below the groundwater level, perched or otherwise.

See Point 2.24 Revised Working Plan

http://glostext.gloucestershire.gov.uk/Data/Planning%20Committee/20040917/Agenda/S

towe%20and%20Clearwell%20Quarries,%20Agenda%20Item%205%20-

%20attachment%201.pdf

Working below the shale bands, which support a shallow groundwater level (which may

or may not be continuous) will ‘short circuit’ the natural route of shallow water flow,

whereby water which would previously either percolate sideways from Orles Wood or by

direct rainfall and move more slowly above the shale layer (and ultimately downwards

via solution features) will now infiltrate rapidly into the lower limestone at depth. This will

change the chemistry of this water and would also shorten the ‘time to peak’ from rainfall

to aquifer recharge/discharge.

It is understood that the proposed depth of the workings is based on the expected

groundwater level in the deeper limestone and not the shallow limestone, however the

working levels proposed to not maintain any significant unsaturated zone beneath the

base of the quarry. The ‘epikarst’ develops in the unsaturated zone therefore the

proposed ‘epikarst recreation’ is unlikely to be effective, resulting in a reduction of

carbon dioxide and calcium carbonate dissolution, which will change the groundwater

geochemistry.

The report states that sub-water table working will not be undertaken to prevent

dewatering impacts.


Cross sections in Drawing 215K-10-04show the geology and conceptual hydrogeological

model of the site in its current condition. Cross sections in Drawing 215K-10-05 shows

the site in its proposed future conditions with the extent and depth of the proposed

workings clearly marked.

Extracts from these drawings are presented in Figure 11 for comparison purposes; these

clearly show the workings extending to and below the groundwater level in the

limestone, and a significant distance below the perched groundwater level in the upper

limestone.

This implies that workings will extend well below the water level in the ‘shallow

limestone’ and partially below the water level in the ‘deeper’ limestone. A full HIA is

therefore required to satisfy the requirements of the Environment Agency.


Figure 11 Cross Sections through the site (taken from ES Hydrogeology Chapter) NTS

Sub-watertable

working implied

Workings extend well

below the upper water

level (labelled A)

recorded in shallow

limestone

HYG145Stowe Hill Quarry Hydrogeology Review February 2015

4.4 Mitigation measures

The following mitigation measures are detailed in the report;

Incorporated Mitigation Measure 1: Above groundwater table / saturated zone

excavation only;

The proposals as they stand appear to involve working below the ‘shallow groundwater

level’ and into the deeper groundwater. This depends on the interpretation of the

‘groundwater table’ in the Limestone, which is highly subjective. It is considered that

there are insufficient monitoring data/points at the site to clearly justify the assumption of

a discontinuous shallow groundwater level.

Incorporated Mitigation Measure 2: Defensive groundwater monitoring

strategy for the Avon Group limestone aquifer along the southern (presumed

down hydraulic gradient) site boundary;

Incorporated Mitigation Measure 3: Slade Brook Monitoring Strategy (to be

tied in with the Defensive monitoring strategy); and

The impact assessment states that there will be a ‘moderate adverse’ impact on the

Slade Brook SSSI. The residual impact is then reduced to ‘minor adverse’ following the

implementation of ‘defensive monitoring’. Defensive monitoring will only provide a means

of identifying an impact once it has happened, therefore defensive monitoring cannot be

used as a mitigation measure. In addition to this the practicality of installing a network of

boreholes around the site in a karst aquifer that will provide a representative site-wide

means of monitoring groundwater quality and flow is questionable, given the uncertainty

in encountering connected water bearing fractures in the Limestone.

The report mentions ‘trigger levels’ for water quality in the groundwater boreholes to

enable any changes to be identified, however after nearly 10 years of monitoring in

Slade Brook no clear trigger levels have been defined as was the original intention.

The report states that “rapid changes(in geochemistry) are not anticipated thus a

defensive monitoring strategy shall enable any effects on groundwater to be

identified at an early stage should they occur, thus allowing effective actions to

implemented before any changes are observed at the key environmental receptor

associated with the proposed development (i.e. Slade Brook SSSI)”. Travel times


between sink holes and Slade Brook are rapid in the well-developed karst conduits,

therefore it cannot be assumed that changes will be gradual and easy to stop/reverse.

No clear contingency measures are detailed in the report. i.e what action would be

undertaken in the event that the a change in the groundwater geochemistry is observed

in the defensive monitoring boreholes. This is important as there must be a high degree

of confidence that any contingency measures are capable of halting or reversing any

impact observed.

The potential impact of the extension on Slade Brook SSSI should therefore remain as

‘moderate adverse’. The impact could even be defined as ‘major adverse’ if there

arelong-term / irreversible effects on the carbonate geochemistry or flow regime at Slade

Brook, resulting in the loss of tufa forming potential and/or degradation of the site. Given

the unique nature of the SSSI a potential level of moderate to major adverse impact is

unacceptable.

Incorporated Mitigation Measure 4:Epikarst Recreation and Re-contouring.

The effectiveness of this technique is yet to be proven as to date this has not been

undertaken across the majority of the existing quarry.

4.5 Expected Changes to the Groundwater Conceptual Model –

Operational Phase

Slade Brook tufa dams exist as a result of a number of complex processes in the

catchment soil/geology and hydrogeology combined with the right conditions in the

watercourse environment, all creating the right conditions for tufa dam formation. A

change in any one of the number of processes could result in the sudden cessation of

active tufa formation. Cessation may not be a ‘gradual’ process as the karst catchment

will be highly sensitive to changes in water drainage pathways. For example, the water

draining sideways in the soil subsurface from Orles Wood may be critical in providing

water with carbon dioxide from the soil zone. The extension will remove the ‘critical

edge’ just beyond the woodland where water naturally infiltrates to the limestone. This

could potentially change the flow characteristics and chemistry of the water entering the

conduits feeding Slade Brook.

Once the groundwater chemistry has changed at the site it will be impossible to ‘reverse’

this as the water chemistry is dependent on a number of complex individual factors

which would be very difficult if not impossible to engineer once changed.


The County Council Gloucester Structural Plan states the following;

Section 11 (Minerals): Policy M.3 (Environment) - In making provision for the

supply of minerals, and taking into account national and regional guidance,

the appropriate degree of protection must be afforded to:

• internationally, nationally, regionally and locally important areas of landscape,

nature conservation, archaeological interest, and

• important natural resources including agricultural land and the water-based

environment

Forest of Dean District Council Local Development Framework States;

Policy CSP.1 Design and Environmental Protection (strategic objective:

providing quality environments) - The design and construction of new

development must take into account important characteristics of the

environment and conserve, preserve or otherwise respect them in a manner

that maintains or enhances their contribution to the environment, including their

wider context.

Based on the evidence provided within the report there is insufficient protection provided

to the Slade Brook SSSI from the proposed quarrying and restoration activities at Stowe

Hill Quarry given the rapid direct link between the two and the dependence of the

actively forming tufa dams on the natural processes inside the Brook catchment which

includes the extension area.


5 Restoration Impacts

5.1 Inert Waste

The proposed restoration scheme includes the importation of inert soils to the quarry

void to achieve the required profile.

No Hydrogeological Risk Assessment (HRA) has been undertaken for the importation of

Inert Waste Materials to the site; a HRA is required for all inert sites (recovery and

landfill) where the site is in a sensitive groundwater environment. The quarry is situated

in a highly sensitive groundwater environment on a Principal aquifer, with a shallow

water table beneath the quarry floor and over a karst aquifer with well-developed rapid

pathways. Typically a HRA would include a ‘rogue load’ assessment representing the

importation of material outside the definition of inert waste.

As far as the author is aware the agreement for the existing quarry at Stowe Hill was for

the restoration to comprise site derived limestone waste with a soil covering. Importing

‘locally derived’ inert fill as part of the proposed restoration scheme for the extension

increases the risk of poor subsurface drainage (if clay rich soils are used), and also

contaminating groundwater in the karst aquifer and Slade Brook in the event that a

‘rouge load’ is deposited at the site. The soil zone in Slade Brook catchment is critical in

maintaining the water chemistry of the water recharging the limestone; importing

material with a slightly different mineralogy (albeit technically meeting inert criteria) could

potentially change the chemistry of the water and impact on the tufa formation in Slade

Brook.

This has not been addressed adequately in the assessment.


6 Summary and Conclusions

This report has undertaken a review of the potential impacts on groundwater dependent

receptors which would result from the proposed extension to the limestone quarry at

Stowe Hill. The main source of data for the review is the Hydrogeology Chapter (2014)

and supporting appendices submitted in the Environmental Statement. This study has

been supported by a walkover of publicly accessible areas around the site and Slade

Brook and is supported by information available in the public domain.

The existing quarry and the proposed extension are located inside the recharge area for

the Slade Brook Site of Special Scientific Interest (SSSI); Slade Brook is designated a

SSSI due to the best example of actively forming tufa dams in Britain. The focus of the

review is on the potential impact of the quarry extension on the SSSI, as the actively

forming tufa are entirely dependent on the flow and water chemistry in the water

emerging from springs at the base of the limestone.

The active tufa dam formation at Slade Brook is unique to the UK in its extent and is

highly dependent on the complex interrelationship between a number of factors (climate,

geology, topography, vegetation in the catchment and along the brook, plus other

biological factors). The disturbance or slight change in any one of these factors can lead

to the cessation of active tufa formation.

The Karst groundwater system is typically highly complex; the report separates the

Limestone into a shallow and deep aquifer, separated by a shale layer. The recharge

and storage of water into the interconnected fissures and non-karstic storage in the deep

aquifer are suggested as providing the supersaturated (with calcium and magnesium)

baseflow to Slade Brook. The workings are not intended to extend beneath the

groundwater level in the deeper aquifer. It is considered more likely that the majority of

the water storage will be in the soil and shales at Orles Wood and the epikarst layer

across the Limestone, which then gradually feed into the deeper aquifer to maintain

flows to the springs at Slade Brook. This upper epikarst layer will be removed by the

proposed quarry.

One major solution feature (Longley Farm sinkhole) and a line of potentially infilled

solution features will be removed by the proposed extension. There is a proven link

between the water discharging to Longley Farm sinkhole and the spring at Slade Brook.

The removal of these solution features and the land draining towards the features from


Orles Wood has the potential to change the flow and chemical characteristic of the water

entering the Limestone aquifer and ultimately discharging to Slade Brook.

It is proposed that the quarry will not be worked below the groundwater table; however

the working depths stated in the report and shown on the cross sections do extend

below the groundwater level recorded in the shallow limestone and are shown to work

down to and slightly below the water level in the deep limestone.

Defensive monitoring is proposed as a mitigation measure; however this would only

measure the impact and would not stop or prevent any impact (such as a change in

water chemistry). No contingency measures are detailed in the documents. It would be

virtually impossible to reverse any change in the natural hydrochemistry once impacted.

The many uncertainties in the karst aquifer are recognised in the ES Chapter and in this

report, however in order to undertake an impact assessment and to mitigate impacts by

monitoring it is critical to establish baseline conditions. To date the baseline conditions

in the Slade Brook and the Limestone aquifer have not been established to the extent

required to define trigger or control levels for monitoring.

The restoration proposals include the importation of locally derived inert waste; this

could result in a change in the chemistry of the water infiltrating the soil zone and there

is a risk from a ‘rouge load’ of non-inert waste leading to pollution in the aquifer. A

detailed hydrogeological risk assessment is therefore required given the highly sensitive

setting of the site.

In summary the proposed extension to the Limestone quarry at Stowe Hill is considered

to be unsustainable due to potential moderate to major impacts on the Slade Brook

SSSI.

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