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.
HYG145Stowe Hill Quarry Hydrogeology Review 2 February 2015
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.
HYG145Stowe Hill Quarry Hydrogeology Review 3 February 2015
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
HYG145Stowe Hill Quarry Hydrogeology Review 4 February 2015
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
HYG145Stowe Hill Quarry Hydrogeology Review 5 February 2015
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.
HYG145Stowe Hill Quarry Hydrogeology Review 6 February 2015
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)
HYG145Stowe Hill Quarry Hydrogeology Review 7 February 2015
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
HYG145Stowe Hill Quarry Hydrogeology Review 8 February 2015
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
HYG145Stowe Hill Quarry Hydrogeology Review 9 February 2015
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
HYG145Stowe Hill Quarry Hydrogeology Review 10 February 2015
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.
HYG145Stowe Hill Quarry Hydrogeology Review 11 February 2015
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
HYG145Stowe Hill Quarry Hydrogeology Review 12 February 2015
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.
HYG145Stowe Hill Quarry Hydrogeology Review 13 February 2015
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
HYG145Stowe Hill Quarry Hydrogeology Review 14 February 2015
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.
HYG145Stowe Hill Quarry Hydrogeology Review 15 February 2015
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)
HYG145Stowe Hill Quarry Hydrogeology Review 16 February 2015
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.
HYG145Stowe Hill Quarry Hydrogeology Review 17 February 2015
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.
HYG145Stowe Hill Quarry Hydrogeology Review 18 February 2015
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
HYG145Stowe Hill Quarry Hydrogeology Review February 2015
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.
HYG145Stowe Hill Quarry Hydrogeology Review February 2015
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.
HYG145Stowe Hill Quarry Hydrogeology Review February 2015
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.
HYG145Stowe Hill Quarry Hydrogeology Review February 2015
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
HYG145Stowe Hill Quarry Hydrogeology Review February 2015
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.