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1
Hydrology, Water Quality and Condition
of Pant-y-Llyn, Wales’ only Turlough
Farr G, Hatton-Ellis T, Jones DA,
Lambourne C, Bevan J., Naughton O.
CCW Staff Science Report No. 12/8/1
This investigation is a collaborative project between hydrogeologists from the Environment
Agency Wales (EAW) and ecologists from the Countryside Council for Wales (CCW). The
investigation was undertaken to fulfil requirements detailed by CCW in their Special Sites
Actions Database, which is a list of actions required to brings protected sites (SACs and
SSSIs) into favourable status.
You may reproduce this document free of charge in any format or medium, provided that you
do so accurately, acknowledging both the source and copyright, and do not use it in a
misleading context.
© CCGC/CCW and Environment Agency Wales 2012
2
Report series: CCW Staff Science Report
Report number: 12/8/1
Publication date: January 2012
Title: Hydrology, Water Quality and Condition of Pant-y-
Llyn, Wales’ only Turlough
Author(s): Gareth Farr
1
, Tristan Hatton-Ellis
2
David Jones
1
, Caleb
Lambourne
1
, Jamie Bevan
3
and Owen Naughton
4
1
Environment Agency, Ty Cambria, 29 Newport Road, Cardiff CF24 0TP
2
Countryside Council for Wales, Maes-y-Ffynnon, Ffordd Penrhos, Bangor,
Gwynedd LL57 2DW
3
Countryside Council for Wales, Maes Newydd, Llandarcy, Neath SA10
6JQ
4
Department of Civil, Structural and Environmental Engineering, Trinity
College Dublin, Dublin 2, Ireland
Restrictions: None
Distribution list (core):
CCW HQ Library, Bangor 2
National Library of Wales 1
British Library 1
Welsh Assembly Government Library 1
Scottish Natural Heritage Library 1
Natural England Library (Electronic Only) 1
Distribution list (others):
Charlotte Gjerløv, CCW
Peter Jones, CCW
Simon Neale, Environment Agency Wales
Isabel Macho, Carmarthenshire CBC
Nigel Ajax Lewis, Wildlife Trusts Wales
Lizzie Wilberforce, WTSWW
Tony Waterman, NIEA
Ken Irvine, Trinity College Dublin
Steve Ormerod, Cardiff University
Fred Slater, Cardiff University
Jeremy Biggs, Pond Conservation
National Parks & Wildlife Service, Dublin
Northern Ireland Environment Agency, Belfast
Recommended citation for this volume:
Farr G, Hatton-Ellis TW, Jones DA, Lambourne C, Bevan J., Naughton, O. 2011. Hydrology,
Water Quality and Condition of Pant-y-Llyn, Wales’ only Turlough. CCW Staff Science
Report No 12/8/1, 64pp, CCW, Bangor.
CCW Staff Science Report 12/8/1
iii
CONTENTS
CONTENTS III
LIST OF FIGURES V
LIST OF TABLES V
CRYNODEB GWEITHREDOL VI
EXECUTIVE SUMMARY VIII
1
INTRODUCTION 10
1.1
The Turlough Environment 10
1.2
Hydrology and Hydrogeology of Pant-y-Llyn 11
2
WATER LEVEL 14
2.1
Collection of water level data 14
2.2
Pant-y-Llyn Hydrograph 14
2.2.1
General Hydrological Characteristics of Pant-y-Llyn 15
2.2.2
Time Lags between Rainfall and Recharge 20
2.2.3
Filling Cycle 21
2.2.4
Emptying Cycle and Recession Constants 22
2.2.5
Community Zonation in relation to Water Level 22
2.3
Groundwater Level Data 24
2.4
Turlough Hydrological Modelling 28
2.4.1
Digital Terrain Modelling 28
2.4.2
Hydrological Modelling 29
2.4.3
Modelling Results 31
2.4.4
Flood Duration 33
2.4.5
Model Limitations 34
2.5
Comparison with Irish Turloughs 34
3
WATER QUALITY 37
3.1
pH and Alkalinity 38
3.2
Conductivity 38
3.3
Dissolved Oxygen 39
3.4
Temperature 39
3.5
Nutrients and chlorophyll-a 39
3.6
Ammonia 41
3.7
Other determinands 41
4
SCRUB ENCROACHMENT 43
CCW Staff Science Report 12/8/1
iv
5
RISKS TO THE ECOLOGICAL FUNCTIONING OF PANT-Y-LLYN 45
5.1
Hydrology 45
5.2
Nutrients and other Pollution 45
6
SUGGESTED WATER QUALITY LIMITS FOR PANT-Y-LLYN 47
6.1
pH and Conductivity 47
6.2
Chlorophyll and Nutrients 47
7
REVISED PERFORMANCE INDICATORS AND CONDITION ASSESSMENT
FOR PANT-Y-LLYN 50
7.1
Condition Assessment 54
8
DISCUSSION AND RECOMMENDATIONS 57
8.1
Hydrology 57
8.1.1
General Hydrological Findings 57
8.1.2
Defining the Groundwater Catchment 57
8.2
Water Quality 59
8.2.1
General Water Chemistry 59
8.2.2
Nutrients 59
8.2.3
Water Quality Targets 59
8.3
Biological Monitoring 60
8.4
Site Management 60
9
ACKNOWLEDGEMENTS 60
10
REFERENCES 60
APPENDIX 1: ORIGINAL PERFORMANCE INDICATORS FOR THE TURLOUGH
FEATURE (CCW, 2008) 63
APPENDIX 2: BOREHOLE LOGS 65
APPENDIX 3: LRG (2006) TOPOLOGICAL SURVEY MAP 69
APPENDIX 4: WATER CHEMISTRY DATA 71
APPENDIX 5: 3D VIEW OF SURFACE WATER CATCHMENT 73
APPENDIX 5: DATA ARCHIVE APPENDIX 73
CCW Staff Science Report 12/8/1
v
LIST OF FIGURES
Figure 1 Geological cross section showing relationship of Pant-y-Llyn to the adjacent Glangwenlais quarry and
also generalised hydrogeological conditions (Hardwick & Gunn 1992).................................................................... 12
Figure 2 Proposed surface water catchment for Pant-y-Llyn (after LRG, 1998). OS base maps reproduced with
permission of HMSO. Crown copyright reserved. CCW licence No. 100018813.......................................................13
Figure 3 Pant y Llyn and estavelle (Dry) 17
th
Aug. 2010 (view NE towards road). Photograph Dave Jones EAW... 17
Figure 4 Pant y Llyn and estavelle (submerged) after only a few days heavy rainfall. 25
th
Aug. 2010 (View West from
road). Photograph Gareth Farr EAW.........................................................................................................................17
Figure 5 Pant-y-Llyn hydrograph and temperature (red line represents base of estavelle at 157.47mAOD).
Temperature fluctuations become very large when the water level falls below the height of the sensor....................18
Figure 6 Pant-y-Llyn Hydrograph and rainfall..........................................................................................................19
Figure 7 Plot of water level and cumulative rainfall for two filling phases................................................................21
Figure 8 Pant-y-Llyn and groundwater level hydrographs........................................................................................ 27
Figure 9. (a) Historic topographic survey and (b) derived shaded contour map ....................................................... 28
Figure 10. Stage-volume and stage-area plot for Pant-y-Llyn Turlough.................................................................... 29
Figure 11. Plot of net flow and rainfall for Pant-y-Llyn Turlough.............................................................................30
Figure 12. Plot of net outflow for Pant-y-Llyn Turlough............................................................................................30
Figure 13. Plot of recorded and modelled stage for Pant-y-Llyn Turlough. ..............................................................32
Figure 14. Plot of long-term modelled stage for Pant-y-Llyn Turlough..................................................................... 33
Figure 15. Flood duration curves for Pant-y-Llyn Turlough......................................................................................34
Figure 16. Maximum depth and volume/area ratio at maximum depth for Pant-y-Llyn and recorded Irish turloughs
(adapted from Naughton et. al., 2012)........................................................................................................................ 35
Figure 17. Comparison of Pant-y-Llyn hydrograph with Turloughmore (Co. Clare, Ireland) and Coolcam (Co.
Galway, Ireland) (adapted from Naughton, 2011)......................................................................................................36
Figure 18. Recession duration comparison (adapted from Naughton et al., 2012)...................................................37
Figure 19 TP concentrations (solid line) and chlorophyll concentrations (dotted line) in Pant-y-Llyn. The blue
shaded area indicates the lower limit of detection for the TP analytical method used. All chl-a values were above the
detection limit..............................................................................................................................................................40
Figure 20 Nitrate (dashed line) and Total nitrogen (solid line) concentrations in Pant-y-Llyn................................. 41
Figure 21 Piper diagram showing water quality in Pant y Llyn Turlough and Environment Agency monitoring
boreholes in the Cernydd Carmel SAC.......................................................................................................................42
Figure 22 Pant-y-Llyn from the air, photographed on 11/9/2009. Aerial photograph © Infoterra Ltd, reproduced
under CCW Licence.................................................................................................................................................... 43
Figure 23 Pant-y-Llyn from the air, photographed on 20-22/7/2000. The blue line indicates the outline of the
turlough in September 2009. Aerial photograph © Getmapping Ltd, reproduced under CCW Licence. ................... 44
Figure 24 Pant-y-Llyn from the air, photographed on 5/4/2006. The blue line indicates the outline of the turlough in
September 2009. Aerial photograph © COWI-Vexcel Ltd, reproduced under CCW Licence..................................... 44
Figure 25 Location of mains sewers (red) and areas served by them (yellow) in the Cernydd Carmel area. The
surface water catchment of the turlough is shown by a heavy blue line. The green line is the SAC boundary. OS base
maps reproduced with permission of HMSO. Crown copyright reserved. CCW licence No. 100018813 ..................46
Figure 26 ‘Catchment Visualisation Diagram’ showing Pant-y-Llyn within the Cernydd Carmel SAC. LRG/CCW
tracer tests and other key features such as quarries, and Environment Agency Wales boreholes are annotated. .....58
LIST OF TABLES
Table 1. Summary of hydrological data from Pant-y-Llyn data logger __________________________________ 15
Table 2. Time lags from peak rainfall events during the filling cycle. ___________________________________ 20
Table 3. Vegetation and flood depth_____________________________________________________________ 23
Table 4. Summary of Environment Agency monitoring boreholes.______________________________________ 24
Table 5. Summary (arithmetic mean) of main nutrients in Pant-y-Llyn Turlough and EAW boreholes. _________ 38
Table 6. Suggested nutrient limits for Pant-y-Llyn. Determinands in bold are required; determinands in plain text
are recommended.___________________________________________________________________________ 49
Table 7. Revised Performance Indicators for the Turlough feature of Cernydd Carmel SAC._________________ 50
Table 8.Condition assessment for the Turlough feature of Cernydd Carmel SAC, based on the revised performance
indicators in Table 7. ________________________________________________________________________ 54
CCW Staff Science Report 12/8/1
vi
CRYNODEB GWEITHREDOL
Pant-y-Llyn yw'r unig lyn diflannol sydd wedi'i gofnodi yng Nghymru a Phrydain. Mae'n brif
nodwedd Safle o Ddiddordeb Gwyddonol Arbennig (SoDdGA) ac Ardal Cadwraeth Arbennig
(ACA) Cernydd Carmel ac yn ecosystem ddaearol sy'n dibynnu ar ddŵr daear ( GWDTE) sydd
wedi cael ei glustnodi o fod mewn perygl o wynebu niwed sylweddol yn sgil mewnbwn dŵr
daear wedi'i gyfoethogi.
Mae'r ymchwiliad hwn yn brosiect ar y cyd rhwng hydroddaearegwyr o Asiantaeth yr
Amgylchedd Cymru ac ecolegwyr o Gyngor Cefn Gwlad Cymru. Cynhaliwyd yr ymchwiliad er
mwyn cyflawni'r gofynion a glustnodwyd gan Gyngor Cefn Gwlad Cymru yn eu Bas Data
Gweithrediadau Safleoedd Arbennig, sydd yn rhestr o weithrediadau sydd eu hangen er mwyn
dod â safleoedd wedi'u diogelu (SoDdGA ac ACA) i statws boddhaol. Prif amcanion yr
ymchwiliad yw:
● Gwella ein dealltwriaeth gysyniadol a hydrolegol o'r safle yng nghyd-destun
hydroddaeareg yr ardal o amgylch;
● Adolygu cemeg dŵr Pant-y-Llyn a'r dŵr daear o amgylch, gan glustnodi unrhyw faterion
parthed maetholion posibl;
● Ymgorffori data diweddar ar gyfer ansawdd a mesur dŵr i gynllun rheoli ac amcanion
craidd Cyngor Cefn Gwlad Cymru;
Gan ddefnyddio cyfarpar monitro parhaus, mae hydroleg y safle wedi'i fesur. Oherwydd
materion parthed diogelwch offer, ni ellid cofnodi hydroleg 1.5m isaf y llyn diflannol. Rydym
hefyd wedi casglu holl ddata ansawdd dŵr perthnasol ar gyfer y safle. Byddai data lefel dŵr
wedi'i fodelu yn arwain at wneud rhagolygon hydrolegol mwy manwl y gellid eu cysylltu gyda'r
strwythur llystyfiant.
Yn 2005, nododd Cyngor Cefn Gwlad Cymru mewn asesiad fod Pant-y-Llyn mewn cyflwr
"anffafriol - heb ei ddosbarthu", yn seiliedig ar ddata cyfyngedig am ansawdd y dŵr. Mae'r
Gyfarwyddeb Cynefinoedd yn mynnu fod pob safle a ddynodwyd fel Ardal Cadwraeth Arbennig
i fod mewn cyflwr boddhaol erbyn 2015. Yma rydym yn diweddaru'r asesiad cyflwr 2005 gyda
data hydrolegol manwl a data ansawdd dŵr llawer gwell.
Mae perthynas glir rhwng dŵr glaw, lefelau dŵr daear a thoniannau lefel dŵr o fewn Pant-y-
Llyn. Mae'r lefel llenwi mwyaf ychydig dros 3m o ddyfnder uwchben yr estavelle. Mae oediad
amser ail-lenwi yn amrywio o dri diwrnod i bythefnos. Mae ail-lenwi yn gyflymach yn ystod
misoedd gwlyb pan fydd y tabl dŵr yn uwch. Mae cylch gwacáu a llenwi Pant-y-Llyn hefyd yn
cael ei adlewyrchu yn nosbarthiad a math y llystyfiant.
Mae data olrhain blaenorol yn awgrymu fod y cyflenwad dŵr daear yn dod o ddalgylch bloc
calchfaen carstig ffawt-ffinedig o fewn cyffiniau agos Pant-y-Llyn yn hytrach nag o’r brif system
carst. Er hynny mae'r hydrograffau dŵr daear o fewn ardal carst Glanwenlais yn debyg iawn i
lefelau dŵr ym Mhant-y-Llyn. Argymhellir gwaith manwl pellach er mwyn diffinio dalgylch
dŵr daear gwell ar gyfer y llyn diflannol, fel bod modd gwneud penderfyniadau rheoli ar sail y
wybodaeth orau posibl.
Mae cemeg dŵr ym Mhant-y-Llyn yn arferol i system alcalinedd uchel. Mae'r dŵr ym Mhant-y-
Llyn yn cynnwys llai o fwynau na'r dŵr daear yn yr ardal, gan adlewyrchu dŵr wyneb neu
fewnbynnau dŵr daear cyflym. Dylid ystyried pwysigrwydd dalgylch y dŵr wyneb. Y risg
mwyaf arwyddocaol i ansawdd dŵr o fewn y dalgylch dŵr wyneb yw'r ffordd gyfagos y gallai
dŵr wyneb ddraenio ohoni.
Mae lefelau maetholion ym Mhant-y-Llyn yn isel, ond mae ambell gynnydd mewn maetholion a
chloroffyl heb ei esbonio sydd angen ymchwiliad pellach yng nghyd-destun lefel dŵr a / neu
ddata glawiad. Prin oedd y ffosffad ar y safle ar bob cyfnod samplo. Yn seiliedig ar y set data
CCW Staff Science Report 12/8/1
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diwygedig rydym wedi cynnig targedau cemeg dŵr diwygiedig ar gyfer y llyn diflannol. Mae'r
monitro biolegol cyfredol yn ddigonol ac mae angen arolygon infertebratau a llystyfiant.
Yn hydrolegol, ystyrir bod Pant-y-Llyn yn cyflawni dangosyddion perfformiad cyffredinol o ran
gwacáu a llenwi. Mae cemeg y dŵr yn achosi peth pryder ond mae angen monitro pellach er
mwyn gweld a oes cysylltiad rhwng ansawdd dŵr gwael ysbeidiol â llygredd neu amrywiad
naturiol. Nid oes pryderon ynghylch cylchfaoedd llystyfiant er bod data cyfredol wedi'u cyfyngu
i ffotograffiaeth awyr.
Yn seiliedig ar yr uchod, rydym o'r farn bod llyn diflannol Pant-y-Llyn mewn cyflwr boddhaol,
gyda hyder isel. Mae yna nifer o ffactorau risg allai effeithio ar hyfywdra tymor hir y safle yn
cynnwys ailgychwyn chwarela a'i effeithiau ar hydroleg; effeithiau draenio ffyrdd; cynnydd
mewn prysgwydd; llygru dŵr daear yn sgil triniaeth carthion domestig a newid hinsawdd.
CCW Staff Science Report 12/8/1
viii
EXECUTIVE SUMMARY
Pant-y Llyn is Wales’ and Britain’s only recorded Turlough. It is a key feature of Cernydd
Carmel SSSI and SAC and a groundwater dependent terrestrial ecosystem (GWDTE) that has
been identified as being at risk of significant damage from enriched groundwater input.
This investigation is a collaborative project between hydrogeologists from Environment Agency
Wales (EAW) and ecologists from the Countryside Council for Wales (CCW). The investigation
was undertaken to fulfil requirements detailed by CCW in their Special Sites Actions Database,
which is a list of actions required to bring protected sites (SACs and SSSIs) into favourable
status. The main aims of the investigation are to:
• Improve our conceptual and hydrological understanding of the site in the context of the
hydrogeology of the surrounding area;
• Review the water chemistry of Pant-y-Llyn and the surrounding groundwater, identifying
any potential nutrient issues;
• Incorporate recent data for water quality and quantity into CCW core management plan
and objectives;
Using continuous monitoring equipment, the hydrology of the site has been measured. Due to
instrument security issues the hydrology of the bottom 1.5m of the turlough could not be
recorded. We have also compiled all relevant water quality data for the site. In 2005 CCW
assessed Pant-y-Llyn as in “unfavourable – unclassified” condition, based on limited water
quality data. The Habitats Directive requires all SAC designated sites to be in favourable
condition by 2015. Here we update the 2005 condition assessment with detailed hydrological
data and much improved water quality data.
There is a clear relationship between rainfall, groundwater levels and water level fluctuations
within Pant-y-Llyn.
The maximum fill level is just over 3m depth above the estavelle. Water
level peaks lagged by between 1 and 8 days behind heavy rainfall events.
Recharge is more rapid
during wetter months where the water table is higher. The empty and filling cycle of Pant-y-Llyn
is also reflected in the distribution and type of vegetation.
Previous tracer data suggests that the groundwater supply comes from a fault-bounded karstic
limestone block catchment within the immediate vicinity of Pant-y-Llyn rather than the main
karst system. However groundwater hydrographs within the Glanwenlais karst area are very
similar to water levels at Pant-y-Llyn. Further detailed work is recommended to define a better
groundwater catchment for the turlough, so that future management decisions can be taken from
the best possible knowledge base.
A hydrological model capable of predicting water level from rainfall and evapotranspiration
records was developed for Pant-y-Llyn. The hydrological response of the turlough, represented
as a reservoir within the model, was controlled by inflow and outflow relationships derived from
analysis of the turlough water budget. Long-term modelling of Pant-y-Llyn water levels
highlighted the control the man-made overflow pipe exerts on the hydrological regime of the
turlough.
The hydrological behaviour of Pant-y-Llyn was compared with that of recorded Irish turloughs.
This found that Pant-y-Llyn was a relatively small, shallow turlough that lay towards the end of
the turlough flooding continuum characterised by uni-modal, long-duration flooding.
Water chemistry at Pant-y-Llyn is typical of a high alkalinity system. The water in Pant-y-Llyn
is less mineralised than groundwater in the area, reflecting surface water or rapid groundwater
inputs. The importance of the surface water catchment should be considered. The most
significant water quality risk within the surface water catchment is the adjacent road which may
contribute surface water drainage.
CCW Staff Science Report 12/8/1
ix
Nutrient levels at Pant-y-Llyn are low, but there are unexplained spikes in nutrients and
chlorophyll that require further investigation in combination with water level and / or rainfall
data. The site was phosphate limited on all sampling occasions. Based on the revised dataset we
have proposed revised water chemistry targets for the turlough. Current biological monitoring is
inadequate and both invertebrate and vegetation surveys are required.
Hydrologically, Pant-y-Llyn is considered to be meeting existing performance indicators for
emptying and filling. Water chemistry gives some cause for concern but further monitoring is
required to establish whether episodic poor water quality is linked to pollution or to natural
variation. There are no concerns with regard to vegetation zonation although current data is
limited to aerial photography.
Based on the above we conclude that Pant-y-Llyn turlough is in favourable condition, with low
confidence. There are several risk factors that might affect the long-term viability of the site
including a resumption of quarrying and its effects on hydrology; the effects of road drainage;
scrub invasion; pollution of groundwater from domestic sewage treatment, and climate change.
CCW Staff Science Report 12/8/1
10
1 INTRODUCTION
The primary aim of this report is to summarise surface water and groundwater quality data that is
available for Pant-y-Llyn turlough, Cernydd Carmel SAC. The water quality data will be
discussed first and is followed by discussion on the hydraulic functioning of the site, including
data that shows a ‘complete’ fill and empty cycle of Pant-y-Llyn. Turloughs are an Annex 1
priority habitat and are the primary reason for selection of this site, Cernydd Carmel as a SAC.
Pant-y-Llyn is the only known turlough in Britain.
Turloughs are an unusual aquatic habitat in that they are temporary water bodies that support
unique animal and plant communities. The most recent definition (Tynan et al. 2007) defines a
turlough as:
‘A topographic depression in karst which is intermittently inundated on an annual basis, mainly
from groundwater, and which has a substrate and/or ecological communities characteristic of
wetlands’
Under the Water Framework Directive (WFD) a turlough is classified as a groundwater
dependent terrestrial ecosystem (GWDTE). The Water Framework Directive introduces a 2015
deadline for achieving restoration to favourable conservation status in Natura 2000 Protected
Areas (water dependent SACs and SPAs). Outcome 21 of the Wales Environment Strategy sets
a target that 95% of internationally protected sites should be in Favourable Condition by 2010,
and all sites (including SACs and SSSIs) by 2026.
The ecology of Pant-y-Llyn was well studied in the early 1990s, but has been neglected since.
Blackstock et al. (1993) described the vegetation zonation of the site. The invertebrate
community is described by Rundle & Ormerod (1991) and Rundle (1993). However, the
hydrological functioning of the site is less well understood, and the water chemistry is also
poorly described. These deficiencies have hampered the setting of adequate management targets
for the site.
In 2005 CCW assessed Pant-y-Llyn as in “unfavourable – unclassified” condition. This
assessment was based on limited water quality data. The Habitats Directive requires all SAC
designated sites to be in favourable condition by 2015.
The report presents the first ‘complete’ empty and fill cycle water level data collected from Pant-
y-Llyn. This empty and fill cycle is compared to the groundwater level data and rainfall data
within the Cernydd Carmel SAC area. An improved conceptual understanding of the functioning
of Pant-y-Llyn and the current risks to the site are discussed. We propose revised performance
indicators for the turlough Annex I habitat feature, and identify priorities for future monitoring
and research.
1.1 The Turlough Environment
Turloughs are very variable in morphology, periods of flooding and emptying, and area but share
several characteristic features. As they occur only on karstic limestone, they are typically base-
rich, alkaline systems. The regular cycle of flooding and emptying means that they contain
distinctive organisms and communities that often show strong zonation. At their lowest point,
turloughs contain an estavelle or open conduit. Estavelles are the pathway by which turloughs
drain during dry periods when groundwater levels are falling and reverse to fill the turlough
when groundwater levels rise. Finally, because they are primarily groundwater fed, turloughs
have clear water and very low silt loading. The quality of water entering any turlough will
depend upon the baseline chemistry of the groundwater body that feeds the turlough.
As a habitat, turloughs are primarily restricted to Ireland, where about 300 examples have been
documented (Sheehy Skeffington et al. 2006) with only three examples in Northern Ireland.
CCW Staff Science Report 12/8/1
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Although they share a clear geomorphological description, biologically turloughs are extremely
variable, with their communities being strongly influenced by the empty and fill cycle (Sheehy
Skeffington et al. 2006; Porst & Irvine 2009) and prevailing grazing regime (Sheehy Skeffington
et al. 2006).
The role of nutrients is less clear; Sheehy Skeffington et al. (2006) considered nutrient
enrichment to be a significant risk factor, in common with most freshwater and terrestrial
environments. However, Porst & Irvine (2009) found no relationship between nutrient
concentration and invertebrate community in a short-term study of eight different turloughs. Due
to the karstic limestone aquifers that supply turloughs, nutrient levels are likely to vary more
than in other standing waters and some of the symptoms commonly seen in eutrophied lakes,
such as deoxygenation of the water column, are very unlikely. Moreover, the regular emptying of
turloughs may provide an effective mechanism by which nutrients can be flushed out of the
system. Nevertheless, sustained nutrient enrichment is likely to result in multiple impacts
including shifts in plant or invertebrate species competition and increased algal dominance, and
CCW (2008) accordingly included nutrient concentrations as performance indicators.
Experience from Ireland has so far failed to identify generic performance indicators for
turloughs. Instead, several investigators advocate the use of site-specific targets to conserve these
habitats (Sheehy Skeffington et al. 2006, Porst & Irvine 2009; Williams & Gormally 2009).
1.2 Hydrology and Hydrogeology of Pant-y-Llyn
Pant-y-Llyn was officially recognised as a turlough by Campbell et al. (1992). Subsequent
investigations, most notably by the former Limestone Research Group (LRG) at the University
of Huddersfield were undertaken in 1998, 2005 and 2006 (CCW, 1998, LRG, 2007). Professor
John Gunn and Dr Paul Hardwick undertook the majority of field work and reporting for the
LRG at Pant-y-Llyn. Hardwick & Gunn (1992) carried out tracer test that are incorporated into
the conceptual model. They provide information on groundwater flow direction within the
Cernydd Carmel SAC, mostly discharging at the Nant Gwenlais Spring.
The 2006 LRG investigation (LRG, 2007) was limited due to the theft of instruments used to
collect water level data from Pant-y-Llyn.
Tracer Tests suggest the turlough is not simply fed by discrete, well defined conduits or along
the strike of the limestone (Hardwick & Gunn, 1992). Instead, Pant-y-Llyn seems to gain and
lose its water locally as the piezometric head rises and falls.
Hardwick & Gunn (1992) note that the changing levels in Pant-y-Llyn reflect seasonal variations
in the piezometric surface of the local groundwater body. They suggest the Bettws fault may
impede drainage eastwards towards the Devonian strata. The Bettws fault may also offer a
fracture zone, allowing water to pass southwards along or near to the strike of the Bettws fault.
During Hardwick & Gunns early work the EA boreholes had not been installed and so
comparison of groundwater data with Pant-y-Llyn was not possible. Based on the above,
Hardwick & Gunn (1992) suggest the majority of the recharge comes from the west (Figure 1),
flowing down dip and then along strike towards Pant-y-Llyn, with further tracers showing
discharge to the River Gwenlais (possibly via the Bettws fault).
Since the LRG work, water level data from the boreholes and from Pant-y-Llyn has become
available, as has more detailed water quality data. This will help us to further understand the
groundwater and surface water catchments for Pant-y-Llyn.
CCW Staff Science Report 12/8/1
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Figure 1 Geological cross section showing relationship of Pant-y-Llyn to the adjacent Glangwenlais quarry and also generalised hydrogeological conditions (Hardwick & Gunn
1992).
CCW Staff Science Report 12/8/1
13
It will be useful to consider both the immediate surface water catchment and the wider
groundwater catchment for Pant-y-Llyn. Hydrographs from boreholes and Pant-y-Llyn show that
precipitation can cause rapid recharge, reflecting the effect of the surface water catchment
providing rapid recharge to Pant-y-Llyn. Water quality data also confirms that water in Pant-y-
Llyn is less mineralised with a lower conductivity, suggesting a shorter residence time than, for
example, waters from Gorswen borehole. This is further suggestion that some shorter flow paths,
perhaps within the surface water catchment are important.
The surface catchment of Pant-y-Llyn is likely to be principally influenced by the steep sided
topography within the immediate area. LRG (1998) proposed a surface water catchment (Figure
2), with which we are in agreement. Recharge falling in this area will reach Pant-y-Llyn rapidly.
This will be instant in the case of direct rainfall, and may take only days via through flow in soils
and shallow groundwater pathways after heavy rainfall events.
Figure 2 Proposed surface water catchment for Pant-y-Llyn (after LRG, 1998). OS base maps reproduced with
permission of HMSO. Crown copyright reserved. CCW licence No. 100018813
Drains under the adjacent road will also channel surface water runoff from the east directly into
the lake. It is therefore important that land use within the immediate surface water catchment is
well managed, bearing in mind that any activities (or incidents) in this area could have a rapid
effect upon the water quality or quantity at Pant-y-Llyn.
Short time lags between rainfall events and increasing levels in Pant-y-Llyn highlights a fairly
rapid response and suggests there may well be an element of more rapid conduit flow. Work in
Ireland shows turloughs fed by conduit systems usually respond very rapidly (within hours or
days) to rainfall, whereas those fed by shallow, more diffuse ‘epikarst’ fissuring respond much
more slowly.
CCW Staff Science Report 12/8/1
14
The groundwater catchment of Pant-y-Llyn is harder to define. The eastern boundary may be
marked by the Bettws fault (Figure 1), which brings the Devonian strata into contact with the
Carboniferous Limestone, and seems to act as a barrier to groundwater. LRG (1998) suggest that
there may be a possibility of flow along or near to the Bettws fault, south to the Nant Gwenlais
brook. This is certainly possible, but the fault brings Devonian strata into juxtaposition with the
Carboniferous Limestone and this effect is likely to act as a barrier to groundwater flow, due to
the contrasting hydraulic properties of the two stratigraphic units.
LRG (1998) suggest that the groundwater catchment should be considered to be the area of the
Gwenlais impounded karst until further research is carried out. In general groundwater could be
assumed to flow downdip and from the west. Tracer tests also carried out by LRG show that
most groundwater in the catchment discharges at the Nant Gwenlais Spring. One tracer test from
Pant-y-Llyn was traced to the base of Glanwenlais quarry.
At present the area of the Gwenlais impounded karst is considered to be the potential
groundwater catchment. This position was adopted on the recommendation of LRG as a
precautionary approach and reflects uncertainty regarding the real extent of the groundwater
catchment.
2 WATER LEVEL
2.1 Collection of water level data
On 25/8/2010 EAW installed a Solinst Diver water level logger in the southern end of Pant-y-
Llyn to monitor the change of water depth and temperature. This data is compensated with
barometric pressure data from a logger installed in the Environment Agency monitoring borehole
at Glangwenlais Quarry. Ideally a logger would have been installed near the groundwater inflow
and estavelle in the deeper, northern part of the lake, but previous attempts to monitor levels here
were affected by the theft of loggers (LRG, 2007). There were also health and safety issues with
accessing the lowest part of the turlough where LRG previously installed a pipe and logger.
Water level data was recorded by LRG between 22/02/2006 and 6/08/2006. Although the
southern end of the lake is not ideal it gives the best chance of actually recording data and
keeping the equipment safe for extended periods of time.
A wooden stake was sunk into the ground and a Solinst 10m range logger attached. A 30m direct
read cable connected to the logger has been buried 2’’ underground, and runs to the bank of the
lake, adjacent to the road. The connection end of the direct read cable is under a stone at the base
of a tree.
The position of the logger was surveyed using GPS by Rhys Heath (EAW), providing a datum
point for the top of the stake (159.36mAOD) and ground level (158.925mAOD). This was
corrected to give an actual datum point for the logger (159.023mAOD).
2.2 Pant-y-Llyn Hydrograph
The data presented is based on only one empty and fill cycle for the shallow southern end of the
lake, where the logger was installed.
The potential location of the logger was discussed in detail prior to installation. The current
location is not at the deepest part of the lake, which is in the north, near the estavelle, and
therefore the initial part of the fill and latter part of the empty cycle are not recorded. The
advantages of the southern location are that it is safe to access and the logger is hidden from the
public. It was still possible to obtain a good amount of usable data from the logger, which
provides the basis for the discussions contained here.
CCW Staff Science Report 12/8/1
15
The maximum level of Pant-y-Llyn is controlled naturally by the steep sided topography of the
depression in which it sits. However a man-made overflow pipe, installed in the southern end of
the lake in 1940/41 also maintains present day maximum level. Although the invert of the
overflow pipe has not been recently surveyed, LRG did commission a detailed survey of Pant-y-
Llyn. On this survey a coloured dot is made in the area of the outflow pipe (however it is not
labeled) and it occurs between the 160mAOD and 161mAOD contours.
Prior to the installation of the overflow pipe levels in Pant-y-Llyn may have reached 4m or more
above the estavelle. This is a general assumption based on knowledge of historic flooding of the
adjacent road which is approximately 1m above the current maximum fill level.
2.2.1 General Hydrological Characteristics of Pant-y-Llyn
The general hydrological characteristics of Pant-y-Llyn are summarized in Table 1, and are
discussed in more detail below.
Dry period Intermittently during August 2010, and late August 2011
Start of filling cycle ~late August 2010
Start of emptying cycle ~ Feb 2011
Flood duration ~late August 2010 – ~ late August 2011
Recession Duration (days) 84 days
Max stage mAOD 160.507mAOD
Minimum stage mAOD 157.47mAOD (estavelle)
Maximum depth above
estevelle (m) 3.03m
Maximum Volume (m
3
) 9695 m
3
Karstic flow system Shallow epikarst
Source of data Environment Agency Wales Solinst Logger
Table 1. Summary of hydrological data from Pant-y-Llyn data logger
CCWs site management plan, performance indicator indicates an upper limit (or depth) of 3.5m
above the estavelle and a lower limit when the lake is dry. The data in this report shows the
maximum depth of Pant-y-Llyn is in fact just over 3m and this performance indicator has been
changed to reflect the maximum depth of 3m.
During the dry summer of 2010 the levels fell steadily from c.1.5m depth on 17/06/10 to c.0.5m
by 01/07/10. There was one significant rainfall event after 1/07/10 which resulted in the water
level being maintained at c.0.5m on 7/07/2010, (Jamie Bevan pers. obs) and was dry by the 17th
August 2010 (Figure 3), which allowed access and installation of the data logger.
Pant-y-Llyn was totally dry only for a very short period of the year, during August 2010. Even
during these periods the ground near the estavelle was noticeably saturated (Figure 3). Heavy
rain after the 17/08/10 caused the lake to recharge and a pool formed in the depression near the
estavelle in the northern end of the lake, 25 August 2010 (Figure 4). The filling cycle
commenced in August/September 2010 with water reaching the southern part of the lake (and the
data logger) by the 13
th
September 2010.
During the current study period Pant-y-Llyn remained either partially or fully flooded for the
majority of the year, from late August 2010 to August 2011. Even when the majority of water is
lost from the lake, small amounts remain in the lower depression near the estavelle and in the
western side.
CCW Staff Science Report 12/8/1
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The maximum depth above the estavelle is 3m (c.160.5 mAOD). The majority of Pant-y-Llyn
(where ground elevation is higher) reaches a depth of 1 -1.5m. This is especially true for the
middle and southern part of the lake, where the ground level is elevated. There are some areas of
the lake that are deeper than the estevelle and a depth greater than 3m will be recorded here. The
outflow pipe is located between the 160 and 161mAOD contours which correlates well with the
maximum level of the lake recorded at 160.5mAOD. The filling cycle happens in several stages
and is clearly related to precipitation events (Figure 6) and groundwater levels in the local area
(Section 2.3). Decreasing levels at Pant-y-Llyn correspond to periods of limited recharge and
falling groundwater tables. Recharge events are also reflected in both groundwater and Pant-y-
Llyn water level data.
Winter 2010 was especially dry with limited precipitation between November 2010-January
2011 (Figure 5). It was also very cold (a record of -18
o
C in Powys on 28
th
November 2010) in
the latter part of November with any potential recharge being frozen on the ground. December
was the coldest December in 100 years (Met Office) with temperatures 5
o
C lower than average.
It was also very dry with only a third of the normal precipitation.
As the cold spell was broken, after the first week of Jan 2011 and temperatures and precipitation
returned to normal, recharge was made available to groundwater and Pant-y-Llyn. This is clearly
seen in the rapid rise between 9
th
-18
th
Jan where the lake reached is maximum level (Figure 6).
CCW Staff Science Report 12/8/1
17
Figure 3 Pant y Llyn and estavelle (Dry) 17
th
Aug. 2010 (view NE towards road). Photograph Dave Jones EAW.
Figure 4 Pant y Llyn and estavelle (submerged) after only a few days heavy rainfall. 25
th
Aug. 2010 (View West
from road). Photograph Gareth Farr EAW.
CCW Staff Science Report 12/8/1
18
157.0
157.5
158.0
158.5
159.0
159.5
160.0
160.5
161.0
21/06/2010
10/08/2010
29/09/2010
18/11/2010
07/01/2011
26/02/2011
17/04/2011
06/06/2011
26/07/2011
D epth m AO D
-5
0
5
10
15
20
25
30
35
40
temp eratu re oC
Pant-y-Llyn level
Temperature oC
Estavelle datum
Figure 5 Pant-y-Llyn hydrograph and temperature (red line represents base of estavelle at 157.47mAOD). Temperature fluctuations become very large when the water level falls
below the height of the sensor.
CCW Staff Science Report 12/8/1
19
157.0
157.5
158.0
158.5
159.0
159.5
160.0
160.5
161.0
21/06/2010
10/08/2010
29/09/2010
18/11/2010
07/01/2011
26/02/2011
17/04/2011
06/06/2011
26/07/2011
14/09/2011
mAOD
0
10
20
30
40
50
60
70
rainfall mm
Pant-y-Llyn level
Gorslas Rainfall mm
Estavelle datum
Figure 6 Pant-y-Llyn Hydrograph and rainfall
CCW Staff Science Report 12/8/1
20
2.2.2 Time Lags between Rainfall and Recharge
The water level data for Pant-y-Llyn (Figure 6) shows that rainfall events recorded at
Gorslas (rainfall station) have a very rapid recharge effect at Pant-y-Llyn. The peak of
the rainfall events almost match the start of the increase in water level at Pant-y-Llyn
reflecting some very rapid initial recharge. The bulk of the recharge arrives later and
can be assessed by comparing the time between the peak rainfall event and peak water
level at Pant-y-Llyn.
Rainfall
Peak Pant-y-Llyn
Peak
Pant-y-Llyn
Time Lag
(days)
Gorswen BH
Peak
Groundwater
Time Lag
(days)
30/09/2010 14/10/2010 14 14/10/2010 14
17/11/2010 20/11/2010 3 25/11/2010 8
12/01/2011 16/01/2011 4 25/01/2011 13
12/02/2011 13/02/2011 1 21/02/2011 9
25/02/2011 26/02/2011 1 2/03/2011 5
Table 2. Time lags from peak rainfall events during the filling cycle.
Peak rainfall events were correlated with peak water levels at Pant-y-Llyn and
groundwater levels in Gorswen borehole (Table 2). The time lags between peak
rainfall events and peak levels at Pant-y-Llyn range from 14 days to just 1 day.
The longest recharge event and lag occurred in September/October when Pant-y-Llyn
was only just filling up after being dry in August 2010 and where the aquifer was still
recharging (and there was more available storage in the aquifer). As the groundwater
levels increase and Pant-y-Llyn beings to fill (and groundwater storage is high) the
time lags between peak rainfall and peak levels in the lake reduce between 4 and 1
day.
While the longest lag followed the peak rainfall event on 30/9/2010, there was
substantial additional rainfall following this date which would influence the peak
water level timing within the turlough. To further examine the relationship between
rainfall and peak water levels, cumulative rainfall plots were generated and compared
to the corresponding turlough water levels for this and a second major filling event in
January (Figure 7 (a) and (b)). During the first filling cycle the water in the turlough
continues to rise for a significant amount of time after rainfall has ceased (Figure 7
(a)), peaking 8 days after the last major rainfall event. In contrast, at higher stages the
water level in the turlough begins to fall almost immediately after rainfall ceases
(Figure 7 (b)).
CCW Staff Science Report 12/8/1
21
Figure 7 Plot of water level and cumulative rainfall for two filling phases
2.2.3 Filling Cycle
During the filling cycle Pant-y-Llyn receives some very rapid recharge directly after
rainfall events, most likely from the immediate surface water catchment. The
influence of the surface water catchment may have important implications for the
protection and management of water quality and levels at Pant-y-Llyn.
Peak water levels at Gorswen EA borehole lag slightly behind those seen at Pant-y-
Llyn, with the borehole water level continuing to rise after water levels in the turlough
had begun to recede. The peaks are approximately coincident at lower stages but a lag
exists at higher water levels of up to 9 days. This is no surprise as depth to water table
at Gorswen can be c.45m below ground level, and any recharge would need more
time to reach this depth. The man-made overflow in Pant-y-Llyn may also contribute
to the increase in lag time at higher stages. The additional drainage capacity provided
by the outflow would damp the turlough response to rainfall events, causing it to
begin to recede earlier and thus increase the lag between turlough and borehole.
CCW Staff Science Report 12/8/1
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2.2.4 Emptying Cycle
There are two major recessions during the monitoring period, both showing a smooth,
gradual fall in water levels relatively unaffected by recharge. The first occurred
between the 17
th
November and 7
th
January, with the turlough dropping by 0.7 m over
this period. The second and main emptying cycle started on the 1/03/2011 and is
recorded until 6/05/2011 (where the level drops below that of the data logger). In
reality the recession is still continued (just not being recorded) as there is still water in
the deeper end of Pant-y-Llyn around the estavelle.
Corresponding recessions are found in the groundwater hydrograph from the Gorswen
borehole. However, as mentioned in section 2.2.3, the onset of recession in Gorswen
lags behind Pant-y-Llyn by approximately 6 days. There was also a difference in
response of the two hydrographs to rainfall of 50mm which fell between the 29
th
March and the 5
th
of April, i.e. during the main recession period. Following this
rainfall event the water level ceased to fall in the turlough, and there was a brief phase
of net inflow into the turlough before recession resumed at a similar rate as before. In
contrast, groundwater levels in the Gorswen borehole continued to fall following the
rainfall event, but at a markedly slower rate.
2.2.5 Community Zonation in relation to Water Level
The main focus of the report is to assess hydrology and water quality. However, in
this section we match flooding depths with the relevant plant communities to provide
an ecological context for the changes in level.
Blackstock et al. (1993) described the vegetation zonation and invertebrate fauna at
Pant-y-Llyn. The vegetation zones (Table 3) are considered to still be representative
of present day vegetation distribution (J. Bevan, pers obs) although no recent NVC
(National Vegetation Classification) mapping has been undertaken. If vegetation
zones have not changed we could infer that hydrological conditions and water quality
have been broadly consistent since 1993.
Blackstock et al (1993) recognised that the plant communities are consistent with an
annual cycle of winter flooding and summer drainage, although they had no detailed
hydrological data to work with, other than a maximum fill level and general
observations of fluctuations in level. EPA (2007) does much to correlate the
hydrological characteristics of Irish turloughs and the associated vegetation.
CCW Staff Science Report 12/8/1
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Plant community
(Blackstock et al
1993)
Flood depth
for inundation
to occur (m)
Comments
Fontinalis antipyretica
mat / Drepanocladus
aduncus mat 0.5 Aquatic bryophytes within immediate
vicinity of estavelle
Bare ground with
Persicaria spp. and
Agrostis stolonifera 1 Short, sparse amphibious vegetation.
Potentilla anserina
community 1.5 Short amphibious vegetation.
Carex vesicaria
swamp
Equistum fluviatile
swamp
Eleocharis palustris
swamp
1.7-2.7
1.6
1.5
Swamp, requiring constantly moist roots
and tolerant of both flooding and
drought. C. vesicaria community
dominates Pant-y-Llyn.
Salix cinerea – Galium
palustre woodland 2 Narrow band of damp woodland around
edge.
Phalaris arundinacea
swamp 2.7 Dominates northern end of Pant-y-llyn
Fraxinus – Acer –
Mercurialis woodland 3 Dry woodland, intolerant of prolonged
flooding. At very edges.
Table 3. Vegetation and flood depth
From the data it is clear that plant communities and distribution are related to both the
duration of flooding and the depth of flooding within Pant-y-Llyn.
Blackstock et al (1993) also sampled the invertebrate fauna of Pant-y-Llyn. They
found an assemblage typical of seasonal standing water bodies, including many
species also found in Irish turloughs. In particular, invertebrates adapted to survive
dry periods including the snail Lymnaea palustris, and a wide range of cladocerans,
ostracods and copepods. A resurvey of invertebrates is planned for 2012.
Pant-y-Llyn is also renowned for its amphibian populations, including large numbers
of common toad which spawn there each spring (Slater 1993). In this respect it may
well be unique, as there are no common toads in Ireland. The fluctuating, water levels
will exclude fish and many other predators, thereby providing very good conditions
for amphibians.
CCW Staff Science Report 12/8/1
24
2.3 Groundwater Level Data
Groundwater levels are recorded at three Environment Agency Wales boreholes (see
Table 4 below). The location of these boreholes is shown on Figure . Data from these
monitoring boreholes is plotted on Figure and Figure 8.
Table 4. Summary of Environment Agency monitoring boreholes.
A short summary of each borehole is provided below. The complete borehole logs are
included within the Appendix of this report.
Groundwater levels recorded in Gorswen borehole (Figure 9) show most similarity to
levels in Pant-y-Llyn. Although the hydrographs are similar in general shape, the
amplitude of change in Pant-y-Llyn is just 3m, whereas at Gorswen it is over 10m.
Groundwater levels in the Dyllgoed Isaf borehole show much smaller changes than
Gorswen, but however it does react rapidly to recharge events (Figure 8). This
borehole is furthest west in the SAC and therefore furthest from Pant-y-Llyn.
Glangwenlais Quarry borehole has artesian over flowing conditions during times of
high groundwater levels and as a result has been very difficult to monitor. No water
level data were collected for a number of years after construction. In 2010 new above
ground headworks were installed. This has enabled the EA to install water level
recording equipment however the data is still to be corrected to a known datum at the
time of writing. Consequently it could not be included in the analysis.
Carmel borehole is often dry (June 2010) to its total depth about 50m and as such no
groundwater quality or level data has been collected. Water level data was manually
recorded at 22.7m b.g.l. (19/01/2011). This borehole shows very large and possibly
rapid changes in groundwater level, which may indicate it is well connected to the
conduit system in the limestone aquifer. Currently there is no water level recording
equipment installed at this site.
Borehole NGR Level data Quality Data Number of
samples
Dyllgoed Isaf
SN564155
2001-Current 2001-2010 12
Gorswen SN578159
2001-Current 2001-2010 9
Carmel SN588161
Dry (June 2010) – No
level equipment
installed
No samples -
dry 0
Glangwenlais
Quarry SN605164
Artesian-Equipment
installed 2010 2010 1
CCW Staff Science Report 12/8/1
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Figure 8. Location of Environment Agency Wales boreholes (turquoise points). The SAC boundary is shown in red; the estimated surface water catchment of Pant y Llyn in
dark blue, and the maximum extent of the turlough in light blue. OS base maps reproduced with permission of HMSO. Crown copyright reserved. CCW licence No.
100018813
CCW Staff Science Report 12/8/1
26
180
185
190
195
200
205
210
21/06/2010
10/08/2010
29/09/2010
18/11/2010
07/01/2011
26/02/2011
17/04/2011
06/06/2011
26/07/2011
14/09/2011
m AOD
0
10
20
30
40
50
60
70
rain fall m m
Gorswen EA borehole
Gorslas Rainfall mm
Figure 9. Gorswen borehole groundwater level hydrograph and rainfall
CCW Staff Science Report 12/8/1
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Figure 8 Pant-y-Llyn and groundwater level hydrographs
CCW Staff Science Report 12/8/1
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2.4 Turlough Hydrological Modelling
The objective of this part of the study was to develop a hydrological model capable of predicting
the water level in Pant-y-Llyn turlough from rainfall and evapotranspiration records. This
required:
• The development of a digital terrain model (DTM)
• The derivation of characteristic relationships governing the turloughs hydrodynamics
• The combination of these relationships into a hydrological model
• The application of this model to long-term rainfall records
In addition, a comparison of the hydrological behaviour of Pant-y-Llyn was made with turlough
hydrological data from Ireland to put Pant-y-Llyn in context with the overall turlough habitat.
2.4.1 Digital Terrain Modelling
The nature of inflow and outflow within turlough basins makes direct measurement or estimation
of flow rates extremely difficult, as the flow itself is often diffuse in nature and/or flow locations
are typically submerged during the period of inundation. Relevant hydrological quantities (such
as volume and net flow) must therefore be calculated indirectly using digital terrain modelling
(DTM). Data from an historic topographic survey (Figure 9 (a)) was used as a basis for the Pant-
y-Llyn DTM. As the survey data itself was not available, the survey image was first geo-
referenced to the appropriate scale using the grid lines shown (100m between lines). The
coordinates of the survey points were then manually extracted and collated with their
corresponding elevations to give a topographic dataset. A DTM was created using this dataset in
Surfer® version 8.6 (Figure 9 (b)).
Figure 9. (a) Historic topographic survey and (b) derived shaded contour map
CCW Staff Science Report 12/8/1
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Turlough volume and area were calculated at 20 mm stage intervals across the range of flooding
to produce stage-volume and stage-area curves (Figure 10). The curves were applied to the water
level time series to produce hourly volume and area time series, which were then aggregated to
daily average values for use in the water balance and hydrological modelling.
Figure 10. Stage-volume and stage-area plot for Pant-y-Llyn Turlough
2.4.2 Hydrological Modelling
Reservoir (“storage-release”) modelling was used to develop a hydrological model capable of
predicting turlough volume, and associated water level, based upon daily rainfall and
evapotranspiration records. This technique has previously been used in the modelling of Irish
turloughs displaying a range of flooding regimes (Naughton, 2011). In this approach the turlough
is conceptualised as a reservoir with the same physical characteristics as the turlough being
modelled (stage-volume-area relationships). The hydrological response of the reservoir is
controlled by inflow and outflow relationships derived from analysis of the turlough water
budget. The simplified water budget equation for turloughs is:
t
V
QQ oi ∆
∆
±=−
Where Q
i
is the sum of all inflows, Q
o
is the sum of all outflows, and ∆V/∆t is the change in
storage within the basin. Given the difficulties in directly measuring turlough flows, the only
measureable parameter in the water budget is the net change in volume (or net flow) and so the
inflow and outflow parameters must be estimated from the behaviour of the net flow time series.
Average daily net flow was calculated from the turlough volume time series (Figure 11). A
comparison of the net flow hydrograph with the rainfall record of the same period shows a strong
relationship between rainfall and flood dynamics. Each filling event corresponds to a period of
intense, prolonged rainfall while the recession limbs all occur during a period of little or no
rainfall. The net inflow peaks were also substantially greater than the maximum net outflow,
highlighting the control the outflow capacity exerts on the turlough’s behaviour.
CCW Staff Science Report 12/8/1
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Figure 11. Plot of net flow and rainfall for Pant-y-Llyn Turlough
The nature of the turlough recession was more complex than inflow. A stage-discharge curve for
the turlough (Figure 12), defining the turlough’s drainage capacity across the flooding range, did
not show a connection between water level in the turlough and net outflow. Instead, following
the cessation of rainfall the rate of net outflow increased over a period of weeks towards a
maximum value of approximately 180m
3
/day (≈2 l/s). This pattern of slowly increasing net
outflow over time is also clear in the net flow plot (Figure 11) during the two main recession
periods (November-December 2010 and February-May 2011). Following this analysis flow
behaviour and rainfall response of the turlough, mathematical relationships capable of replicating
this flow behaviour were formulated.
Figure 12. Plot of net outflow for Pant-y-Llyn Turlough
Turlough Inflow
Turlough inflow was derived from rainfall on two notional contributing areas defined within the
model: the proximal (local) catchment and the groundwater catchment. Here the proximal
catchment is defined as the area of the turlough itself and its immediate surroundings. Given the
relatively small volume of the turlough (<10,000 m
3
), precipitation directly onto the turlough
CCW Staff Science Report 12/8/1
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surface and immediate surrounding area represents a substantial element of the water budget. It
is assumed that precipitation within this area enters the turlough within a day of the rainfall event
with no losses.
The second notional catchment area, the groundwater catchment, is assumed to contribute water
to the turlough more slowly and over a longer period of time than the proximal catchment, and
also control the rate at which the turlough recedes. Inflow from this catchment was explicitly
related to the exponential outflow relationship (described below) and infiltration, rather than
rainfall. A simple soil moisture deficit model was used to transform precipitation into infiltration
(Fleury et al., 2007; Tritz et al., 2011). Infiltration was calculated at each time step using a linear
reservoir, with the changes in level in the reservoir and infiltration (I) dependent on the reservoir
inputs (rainfall) and outputs (potential evapotranspiration (PET)). Monthly average PET values
were used, with a scaling factor applied to the PET to allow for the relatively high proportion of
rainfall that can become infiltration in karst areas and any overestimation of PET. Where
infiltration was of sufficient quantity it produced a net inflow into the turlough. Otherwise the
contribution from the groundwater catchment reduced the rate of net outflow and prolonged the
onset of outflow at full capacity.
Turlough Outflow
The increase in outflow with time towards the maximum of 180m
3
/day was approximated using
an exponential relationship. Outflow Qo at time t was defined as:
∑
=
−
−
×−=
30
0
)(180
i
ki
ito
eIQ
t
where I is infiltration and k is the outflow recession coefficient. Therefore, Qo
t
is the maximum
outflow capacity less the cumulative effects of any antecedent rainfall events. The longest effect
of rainfall was estimated from the net flow hydrograph as 30 days, as following the cessation of
rainfall net outflow approached the maximum capacity after approximately 30 days. The rate at
which outflow increased was controlled by k, with higher values of k representing a more rapid
recession rate. k was limited to 0.07 – 0.09 within the model optimisation process as values in
this range fitted the outflow behaviour of the turlough reasonably well during the two main
recession periods. Given that the turlough contains a residual pond throughout most of the year,
it is likely that the outflow capacity of the turlough drops significantly below the 180 m
3
/day
threshold at stages below the logger level. However, given the lack of data it was assumed this
relationship applied across the entire range of flooding.
In addition, an overflow pipe installed in the southern end of the turlough provides additional
outflow capacity at high water levels and thus limits the extent of flooding. A second outflow
relationship was therefore needed. Using a point marked on the original topographic map and the
turlough DTM, the elevation of the overflow pipe was estimated at 160.4 mAOD. In the absence
of any available flow data for this discharge, this outflow was estimated as proportional to the
volume above the overflow level. The overflow coefficient was a fitted model parameter.
2.4.3 Modelling Results
The Nash-Sutcliffe efficiency (or R
2
) was used to assess model accuracy and to select the best
model parameters. Model efficiency ranges from -∞ to 1, where an efficiency of 1 corresponding
to a perfect fit, and is defined as:
CCW Staff Science Report 12/8/1
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Where V
0
is observed volume, V
M
is modelled volume and is V
0
is the mean observed
volume. As the diver was located above the base of the turlough, efficiency was only calculated
over the period during which the water level was above the measurement threshold (12/9/2010 –
4/5/2011). Five model parameters were defined within the model: k (the outflow recession
coefficient), the direct catchment, the groundwater catchment, the overflow coefficient and PET.
Optimisation of the model was initially carried out manually on an iterative basis, and then
optimised using Solver within Microsoft Excel. Upper and lower bounds were assigned to each
variable, and a number of optimisation runs were carried out from different initial conditions to
check the parameters converged on similar values for each run. The final model, shown in Figure
13, achieved efficiencies for volume and stage of 97.4% and 96.7% respectively, with the timing
of the initial onset of flooding, flood peaks and the final recession within the monitoring range
all showing a good fit.
Figure 13. Plot of recorded and modelled stage for Pant-y-Llyn Turlough.
Long term modelling of Pant-y-Llyn was carried out for the period 1/9/2003 to 4/8/2011 (Figure
14), which represented the longest available daily rainfall and average monthly PET records. The
annual maximum flood level remained reasonably constant throughout this time with the
exception of the winter of 2007/2008, where the rainfall between September and April was low
at only 790mm compared to an average of 1050mm. This consistency shows the control the
overflow pipe exerts on the hydrological regime of Pant-y-Llyn turlough. While the initial
installation of this overflow would have had a severe impact on the turlough’s hydrology and
therefore its ecology, sufficient time has since passed such that any characteristic ecology would
have adapted to this anthropogenic change and so the maintenance of the overflow may now
represent a supporting condition for the habitat.
CCW Staff Science Report 12/8/1
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Figure 14. Plot of long-term modelled stage for Pant-y-Llyn Turlough.
2.4.4 Flood Duration
The duration of inundation strongly influences the distribution and composition of ecological
communities within turloughs with flood duration primarily controlling plant species survival
(Casanova and Brock, 2000; Sheehy Skeffington et al., 2006). Duration curves provide a way to
represent the amount of time a given level is equalled or exceeded over a defined interval. In the
context of turlough hydrology, duration curves present a means of quantifying the flooding effect
or disturbance experienced by ecological communities at any point within a turlough basin.
Flood duration curves were generated for the recorded and modelled data over the monitoring
period, and for the long term modelled data from September 2003 to June 2011 (Figure 18). As
would be expected given the high efficiency of the model, the duration curves of the recorded
and modelled data showed a very close match. The long term modelled curve showed lower
flood duration across the mid- to high-range of flooding, as these included the less extensive
flood events of 2005/2006 and 2007/2008. As there was no data available for the turlough
response at low water levels (<159 mAOD), durations below this threshold are uncertain.
However, given the field observations reporting the turlough retaining water throughout most of
the year, it is likely that the duration curves drop off more gently below 159 mAOD than those
shown in Figure 15.
CCW Staff Science Report 12/8/1
34
Figure 15. Flood duration curves for Pant-y-Llyn Turlough.
2.4.5 Model Limitations
The relatively short water level records available, combined with the siting of the water level
monitoring equipment above the base of the turlough, represented a significant constraint in the
validation of the turlough hydrological model. Ideally a second year of monitoring will be
available to validate the model behaviour. Given that the turlough is reported to retain a residual
pond for most of the year it is likely that outflow capacity drops significantly at some level
below 159 mAOD, the current lowest recordable level, and so the duration of flooding below this
level is substantially higher than the model shows. Additional monitoring data would therefore
be required improve model performance at low water levels. Also, in the current model design
the upper limit of flooding is heavily dependent on the outflow capacity of the overflow pipe.
This may in fact be the case but further details, such as the upper limit of any historical floods,
hydraulic information for the outflow or direct flow measurement, could ascertain the validity of
long term water level predictions.
2.5 Comparison with Irish Turloughs
The hydrological typology of turloughs does not naturally group into distinct types. Instead
turlough flooding regimes formed a continuum, ranging from short-duration flooding in basins
with a rapid response to rainfall events, to long-duration flooding in response to longer term
precipitation patterns (Naughton
et al
., 2012). The hydrological behaviour of Pant-y-Llyn was
compared to that of 22 Irish turloughs investigated as part of a recent study into turlough
hydrology (Naughton
et al
., 2012). The turloughs in the Irish study were selected to reflect the
continuum of turlough hydrological regimes. All sites in this study were all at least an order of
magnitude larger than Pant-y-Llyn in terms of area and volume, and so a direct comparison of
water balance parameters was not carried out. Instead less scale dependent parameters such as
depth, ratio of volume to area at peak water level (average depth), recession duration and general
hydrograph behaviour were used.
In Ireland over 90% of karst is located in lowland areas of less than 150m AOD, with the vast
majority of turloughs found in these lowland karst areas (Drew, 2008). At 157m above sea level,
CCW Staff Science Report 12/8/1
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Pant-y-Llyn is above the upper end of this altitudinal range. In the Irish study, flood depths
ranged from 3.0 to 15.4m, and so the 3m for Pant-y-Llyn would put it at the lower end of this
scale (Figure 16). The average depth for Pant-y-Llyn of 1.43m is again at the lower end of the
recorded values, indicating that in geomorphological terms the flooded area represents a
relatively flat, shallow basin.
Figure 16. Maximum depth and volume/area ratio at maximum depth for Pant-y-Llyn and recorded Irish turloughs
(adapted from Naughton et. al., 2012).
Water level hydrographs were available for three years from late 2006 to mid-2009 with which
to compare Pant-y-Llyn. In order to select the most appropriate year for a direct comparison,
Irish rainfall records over this period were compared with the Gorslas record for 2010/2011.
Cumulative rainfall time series from the start of July to the following May were correlated and
rainfall totals over this period were compared. The 2008/2009 year showed the closest
relationship to the Pant-y-Llyn data and so was used in the hydrograph comparison.
Figure 17 shows hydrographs for the end members of the turlough flooding continuum and the
corresponding behaviour of Pant-y-Llyn. At one end of the continuum are sites such as
Turloughmore, Co. Clare (Figure 17 (top)) which displayed a multimodal flooding pattern
consisting of a series of rapid filling and emptying events. At the other end of the spectrum are
sites such as Coolcam, Co. Galway, which showed a single long duration flood event with an
extended recession within each hydrological year (Figure 17 (bottom)). The regime of Pant-y-
Llyn lies towards the end the flooding spectrum characterised by uni-modal, long duration
flooding. This was supported by correlation analysis of hourly Pant-y-Llyn water levels with
those of the 22 Irish sites. Pant-y-Llyn showed the greatest similarity with turloughs showing a
relatively long duration of flooding such as Brierfield (Co. Roscommon), Croaghill and Coolcam
(Co. Galway).
CCW Staff Science Report 12/8/1
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Figure 17. Comparison of Pant-y-Llyn hydrograph with Turloughmore (Co. Clare, Ireland) and Coolcam (Co.
Galway, Ireland) (adapted from Naughton, 2011).
While the recession behaviour of turloughs can be complex, it was found that an indicative
hydrological indicator could be obtained by linearly approximating sections of the volume
hydrograph (Naughton
et al
., 2012). The slope of this linear approximation represents an average
drainage capacity for the turlough, and by dividing the maximum recorded volume by this
capacity it provided an indicator for recession duration and overall behaviour. This figure
represents a notional minimum time it would take for the turlough to drain from full. Three
sections of the Pant-y-Llyn recession were linearly approximated, and an average drainage
capacity of 110 m
3
/day and a recession duration of 84 days. When compared to the recession
duration of Irish turloughs (Figure 18), this again shows Pant-y-Llyn as a turlough with a
relatively slow recession and associated long duration of flooding.
CCW Staff Science Report 12/8/1
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Figure 18. Recession duration comparison (adapted from Naughton et al., 2012).
3 WATER QUALITY
This section discusses water quality data from Pant-y-Llyn turlough, and groundwater within
Cernydd Carmel SAC. Water quality samples were taken monthly through 2008 and early 2009
by Mike Jenkins (EAW on secondment to CCW). Further samples have been taken in August
2010, January 2011 and March 2011. In total 14 water samples are available over this period. pH
values for groundwater (from EAW boreholes) were analysed in the field using hand held
meters, and pH values for Pant-y-Llyn were analysed in the laboratory.
Table 5 summarises the main nutrients, for both Pant-y-Llyn and nearby Environment Agency
Wales boreholes. Full datasets are provided in Appendix 4. Water quality statistics are reported
as arithmetic means for all the samples for which we have a result. Not all determinands were
measured on all occasions, so the number of results varies. For the Dyllgoed Isaf borehole, there
are major inconsistencies in the concentration of major ions prior to 2007 and after that date, so
data for 2007 onwards has not been used.
In contrast to many standing water systems, water chemistry in Pant-y-Llyn is likely to be
influenced by fluctuating water levels. In particular, abnormally high solute concentrations are
more likely during low water levels due to evaporation. For this reason it is greatly preferable for
water chemistry and water level data to be available simultaneously, so that abnormal values can
be placed in context.
CCW Staff Science Report 12/8/1
38
Turlough Groundwater Boreholes
Determinand Unit Pant y Llyn Dyllgoed Isaf Gorswen
Ammonia (as N) mg/l 0.065
0.03
0.02
N Oxidised mg/l No data
1.74
0.81
Nitrate-N mg/l 0.29
1.72
0.8
Nitrite-N mg/l <0.004
0.02
<0.004
Nitrogen total N mg/l 0.69
No data
no data
Potassium K mg/l 0.706
0.96
1.47
Orthophosphate µg/l 21
12
10
Phosphate µg/l 35
42
20
Table 5. Summary (arithmetic mean) of main nutrients in Pant-y-Llyn Turlough and EAW boreholes.
Regional groundwater quality can be compared to baseline data in Fahrner
et al
. (2008). Further
reference will be made to other limited nutrient data from Irish turloughs (EPA, 2007).
3.1 pH and Alkalinity
Pant-y-Llyn has an average pH of 7.78. Recent data from Irish turloughs (EPA, 2007) show them
to have a high pH (ranging 8-8.4), while groundwater pH (in the Pant-y-Llyn area) averages pH
7.5. The majority of results for both EAW boreholes tend to be stable, however two higher
results of 9.7 were collected from Dyllgoed Isaf borehole in 2007 and 2008.
As would be expected from its Carboniferous Limestone geology, Pant-y-Llyn is a hard water
site with calcium concentrations (42.81± 8.71 mg/l, N=14). The peak Ca
2+
value was 69.3 mg/l
on 2 June 2008, which likely reflects evaporation and low water levels, as concentrations of
other major ions were also elevated on this date. Winter values are typically just below the
40mg/l mark.
Despite high calcium, concentrations of other major ions were not especially high (Mg
2+
= 2.3 ±
0.2 mg/l; Na
+
= 8.0 ± 1.1 mg/l; K
+
= 0.72 ± 0.26 mg/l; SO
42-
= 12.3 ± 3.93 mg/l; Cl
-
= 14.0 ±
1.79 mg/l).
In the past acid deposition has had widespread impacts across Wales. Due to its very well
buffered geology, Pant-y-Llyn is considered unlikely to be acid sensitive. To verify this, acid
neutralising capacity (ANC) was calculated for 18/1/2011, when a full suite of determinands was
available and during the winter when sites are most likely to be impacted by acidification. ANC
was 1878
µ
eq/l, well above the minimum threshold of 100
µ
eq/l at which calcium-rich sites
could be impacted by acidificiation (Monteith & Simpson 2007).
Alkalinity as CaCO
3
averages 107 mg/l (2140
µ
eq/l) in Pant-y-Llyn. This would place it in the
‘high alkalinity’ bracket in WFD UK Lake typology terms. In Ireland the turlough waters are
also high in CaCO
3
. High calcium levels restrict nutrient availability, resulting in some Irish
turloughs having oligotrophic conditions (EPA, 2007). Groundwater alkalinity was 176mg/l at
Dyllgoed Isaf and 256mg/l at Gorswen. Reduced alkalinity may reflect either dilution or a
shorter residence time from recharge to discharge at the estavelle.
3.2 Conductivity
Conductivity measured at 20
o
C averages 232 µs/cm and 440 µs/cm in Pant-y-Llyn and
groundwater and respectively. Lower conductivities in Pant-y-Llyn may reflect shorter residence
times from recharge and also the effect of some direct rainfall recharge. It is usual for there to be
higher conductivities in groundwater than surface water bodies.
CCW Staff Science Report 12/8/1
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3.3 Dissolved Oxygen
Dissolved Oxygen (%) averages 44% in groundwater with lower readings in Dyllgoed Isaf
(average 31%) compared to Gorswen (average 59%). The dissolved oxygen levels are fairly low,
especially at Dyllgoed Isaf, and this may be related to the uptake of oxygen by chemical or
biological processes. No dissolved oxygen readings have been collected from Pant-y-Llyn. It
would be expected that DO levels in Pant-y-Llyn would be high compared to groundwater due to
exchange with the atmosphere. Monitoring of DO is not appropriate for Pant-y-Llyn because the
shallow nature of the site means that it is unlikely to suffer from DO sags, and because low DO
may be related to influx of groundwater rather than being an indicator of ecological problems.
3.4 Temperature
Temperature in Pant-y-Llyn shows an expected seasonal variation, which is further amplified
during lower water levels. The data logger is not situated near the estavelle which would reflect
the influence of a more consistent groundwater temperature. Groundwater temperatures in the
study area average 11
o
C, with little variation. This is also typical of groundwater from the
Carboniferous Limestone in Wales.
Temperatures at the southern end of the turlough vary seasonally between 3.7-16.4
o
C with an
average of 8.1
o
C. There is an interesting period between 27
th
November 2010 and 12
th
January
2011 where the temperature remains very low (~4.5
o
C) and stays relatively stable. This
correlates with the extremely cold winter weather over this period, when most of the lake froze,
apart from a small area near the estavelle. The stable temperature during this period perhaps also
reflects the lack of recharge bringing in warmer groundwater. The lack of ice over the estavelle
emphasises the local influence of groundwater temperatures even during frozen periods where
there is limited recharge.
3.5 Nutrients and chlorophyll-a
Phosphate is usually considered the limiting nutrients in freshwaters, but nitrate is increasingly
recognised as being important in certain systems (James
et al
. 2003; May
et al
. 2010). Due to the
uncertainty regarding turlough ecology, we present data for both N and P species.
Thirteen measurements of Total Phosphorus (TP) and Orthophosphorus (= soluble reactive
phosphorus: SRP) have been collected. Unfortunately the detection limit used to analyse these is
20
µ
g/l which represents the upper end of mesotrophic for most standing waters (JNCC, 2005).
Consequently it is not possible to calculate mean values. However, 12 of 14 samples had SRP
values <20
µ
g/l, and 8 of 13 samples had TP values <20
µ
g/l. High TP values that give cause for
significant concern (117 and 68
µ
g/l) were found on 2 and 27 July 2008. Lower but still elevated
TP values occurred again on 28 November 2008 and 20 January 2009 (40 and 38
µ
g/l
respectively). SRP values were all lower than TP indicating no analytical anomalies, but due to
the high LOD for SRP most values were below the LOD and so trends could not be established.
Total phosphorus averages 27µg/l in groundwater. Dyllgoed Isaf borehole is highest on average
at 35µg/l, compared to an average of 16µg/l at Gorswen.
Chlorophyll-a concentration is a measure of the concentration of microscopic algae
(phytoplankton) in the water column and is a good measure of an ecological response to
nutrients. Ten measurements of chlorophyll-a were analysed between February 2008 and March
2009. Chlorophyll concentrations were generally low, apart from marked spikes in July 2008 and
winter 2008-9. There is a striking correspondence between chlorophyll-a spikes and spikes in
both TP and TN (Figure 19) as well as turbidity, suspended solids, conductivity, biological
CCW Staff Science Report 12/8/1
40
oxygen demand and dissolved organic carbon (data not shown), suggesting that Pant-y-Llyn may
be highly sensitive to increases in nutrient loading.
Although the recorded chlorophyll spike is a cause for concern, the results should be interpreted
with some caution. The increase occurred during July when water levels would be expected to be
at or close to their minimum value (Section 2.2), so the recorded spike may represent a natural
phenomenon. Weather data for Wales over this period indicates a dry May followed by slightly
above average rainfall in June and almost double the long-term average rainfall for July
(MetOffice 2011). Additionally, much of the June rainfall fell in a few heavy rain events which
could have resulted in a rapid influx of nutrients following low water levels. However, smaller
increases in chlorophyll in winter 2008-9 cannot be explained in this way. Further investigation
of this is required, including modelling of water levels in July 2008 and collection of
simultaneous water level and water chemistry data.
The Redfield ratio (here calculated as TN:TP) was calculated for all sample points where both
determinands were available. Redfield ratios indicate whether a water body is likely to be N or P
limited: at ratios of 13:1 or greater water bodies are P limited, whereas at ratios of less than 13:1
water bodies are N limited. Redfield ratios ranged between 23.7 on 2 July 2008 and 81.6 on 26
February 2008, with a mean of 48.7, indicating that the turlough is P limited at all times.
Chlorophyll values have not been measured in groundwater, because algae cannot survive
without light and chlorophyll concentrations are therefore negligible.
0
20
40
60
80
100
120
140
Nov-07 Jun-08 Dec-08 Jul-09 Jan-10 Aug-10 Feb-11 Sep-11
Date
TP or chl-a ug/l
TP
Chl-a
Figure 19 TP concentrations (solid line) and chlorophyll concentrations (dotted line) in Pant-y-Llyn. The blue
shaded area indicates the lower limit of detection for the TP analytical method used. All chl-a values were above the
detection limit.
LOD 20ug/l
CCW Staff Science Report 12/8/1
41
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Nov-07 Jun-08 Dec-08 Jul-09 Jan-10 Aug-10 Feb-11 Sep-11
N (mg/l)
NO3-N
TN
Figure 20 Nitrate (dashed line) and Total nitrogen (solid line) concentrations in Pant-y-Llyn.
Nitrate is low, both in groundwater (<0.196 -2.4mg/l) and Pant-y-Llyn (<0.2 - 0.585mg/l). The
highest nitrate readings are from Dyllgoed Isaf borehole which averages at 1.5mg/l. This may
reflect localised land use and farming practices. Nitrate is much lower in Gorswen borehole
(where land use is less intensive) averaging 0.8mg/l. In phosphate enriched freshwater lakes a
limit of 2mg/l Total N should be used (Gonzalez Sagrario
et al
. 2005; James
et al
.2005; Jeppesen
et al
. 2005, 2007). Nitrate within Pant-y-Llyn is well below this suggested 2mg/l threshold and is
generally low in the groundwater body as a whole (EAW, 2008).
Nitrite is low in both groundwater (0.016- 0.728 mg/l) and Pant-y-Llyn which is consistently
below detection of 0.004mg/l. The highest nitrite readings are from Dyllgoed Isaf (0.016-
0.728mg/l) and averages 0.09mg/l. Nitrite is lower in the other groundwater borehole at Gorswen
averaging 0.004mg/l which is very close to the lower detection limit. Higher nitrite at Dyllgoed
Isaf is again most likely related to the local more intensive land use.
3.6 Ammonia
Ammonia as N averages 0.029mg/l in groundwater with the highest readings in the far west of
the area from Dyllgoed Isaf borehole 0.036mg/l. Other boreholes have lower levels of ammonia,
such as Gorswen 0.020mg/l. Ammonia in Pant-y-Llyn is more variable than in the groundwater
ranging from 0.003mg/l (lowest detection limit) to 0.287mg/l in June 2008 and averaging
0.065mg/l.
3.7 Other determinands
Potassium as K averages 0.706mg/l in Pant-y-Llyn. More elevated concentrations are seen in
groundwater averaging 1.46mg/l at Gorswen and 5.3mg/l at Dyllgoed Isaf. A maximum of
18.1mg/l is recorded at Dyllgoed Isaf borehole and reflects the immediate land use where
improved grassland, cattle and the use of fertilisers is the most likely source.
Dissolved Organic Carbon (DOC) has only been recorded in Pant-y-Llyn and averages 4.63 ±
2.19 mg/l, varying from 2.6mg/l – 9.57 mg/l.
CCW Staff Science Report 12/8/1
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The major ions for groundwater and Pant-y-Llyn have been plotted on a Piper diagram (Figure
21). Only data from 2010 and 2011 was plotted as previous datasets did not include a full set of
major ions. The groundwater is dominated by calcium and bicarbonate ions, as was also reported
in EAW, 2008 and would be expected for groundwater from Carboniferous Limestone aquifers.
Two samples from Dyllgoed Isaf show a different, less mineralised chemistry (15/10/2007 and
9/04/2008). No explanation can be offered at this stage. The other major ions are fairly consistent
within the groundwater samples.
Calcium is the dominant ion in the Pant-y-Llyn samples. It would be expected for bicarbonate to
be dominant also. The calcium levels are generally lower than observed in groundwater.
Magnesium and potassium are also present in lower levels. The water types are very similar at
Gorswen and Pant-y-Llyn, whereas samples from Dyllgoed Isaf and Glangwenlais Quarry
boreholes did not closely resemble Pant-y-Llyn.
Figure 21 Piper diagram showing water quality in Pant y Llyn Turlough and Environment Agency monitoring
boreholes in the Cernydd Carmel SAC.
Organic chemical analysis has been undertaken on groundwater but not at Pant-y-Llyn. Of the
organic chemicals tested none were present or they were below the detection limit. This included
pesticides such as atrazine, cypermethrin and simazine. Due to cost it is not proposed to
undertake further organic analysis on groundwaters at this stage. It would be advisable to
undertake an organic analysis at Pant-y-Llyn to quantify what if any organic compounds are
present.
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43
4 SCRUB ENCROACHMENT
Aerial photography was used to examine the extent of scrub encroachment on the turlough.
Aerial photographs were available from 20-22/7/2000, 5/4/2006, and 11/9/2009. The extent of
the turlough in 2009 (Figure 22) was digitized and compared with the other two years.
The turlough was full in 2006 and 2009 but only about half full in 2000. There were also
differences in quality between the different images, with the most recent 2009 images being
much better quality than previously collected images. The seasonal differences between the
images has also affected interpretation, because the trees were leafless in April 2006 but not in
2000 and 2009.
In spite of these differences, it was possible to compare the aerial images (Figure 23, Figure 24).
This showed that the turlough was significantly affected by scrub encroachment in 2000, but was
open in 2006 and 2009. There is however some evidence of scrub regrowth in 2009, although
this may partially reflect the improved resolution of the photograph and that trees were in leaf.
Figure 22 Pant-y-Llyn from the air, photographed on 11/9/2009. Aerial photograph © Infoterra Ltd, reproduced
under CCW Licence.
CCW Staff Science Report 12/8/1
44
Figure 23 Pant-y-Llyn from the air, photographed on 20-22/7/2000. The blue line indicates the outline of the
turlough in September 2009. Aerial photograph © Getmapping Ltd, reproduced under CCW Licence.
Figure 24 Pant-y-Llyn from the air, photographed on 5/4/2006. The blue line indicates the outline of the turlough in
September 2009. Aerial photograph © COWI-Vexcel Ltd, reproduced under CCW Licence.
Scrub control work was carried out in 2002 and again in 2011. Based on the above, scrub
encroachment is not considered to be a serious problem at present, but periodic clearance work is
likely to be required.
CCW Staff Science Report 12/8/1
45
5 RISKS TO THE ECOLOGICAL FUNCTIONING OF PANT-Y-LLYN
5.1 Hydrology
As an ecosystem that depends on a variable ecological process, Pant-y-Llyn is highly vulnerable
to hydrological changes. Potential risks have been assessed in the past and included the
numerous quarrying operations, especially those at Glangwenlais Quarry directly to the west and
also in Cilyrychen Quarry to the east. Historic maps (not shown) show land use in 1850 and
1954. The most obvious change is Glangwenlais quarry, which is now part of the Cernydd
Carmel SAC and quarrying here has ceased. Tracers have proved a connection between Pant-y-
Llyn and Glangwenlais Quarry.
However, quarrying operations still pose a potential risk. Many of the existing quarries are
currently not worked, but mineral rights still exist. Further work is required to prove a link
between Pant-y-Llyn and the currently inactive Cilyrychen Quarry (LRG, 2007). Any deepening
of Allt Y Garn Quarry, SW of Pant-y-Llyn, and Pwllymarch Quarry would need further
assessment to assess what if any risk this would pose to the hydrological functioning of Pant-y-
Llyn.
Land use change or land drainage could also impact Pant-y-Llyn. Any changes of this type
within the small surface water catchment should be carefully considered.
Changing climate and rainfall patterns could also affect the hydrology of the site and the periods
of emptying and filling. A better model of the hydrological behaviour of the turlough would
greatly improve our ability to predict and manage changes to the ecosystem as a result of
changing rainfall patterns.
Other factors that could affect the hydrology and water quality include private water
abstractions. There are no registered abstractions in the area and current data do not suggest that
lack of water is an issue.
5.2 Nutrients and other Pollution
Nutrients derived from local land use (within the surface water catchment for Pant-y-Llyn)
would be the primary risk to water quality. Additional water quality risks could come from septic
tanks and other private sewage treatment systems that discharge to ground. The Cernydd Carmel
SAC area is not served by foul mains drainage and it should be assumed that all properties within
the SAC area have private sewage treatment, most probably discharging to ground. This is not
such a large concern as there are only about 8 properties on private treatment systems within the
entire SAC area. Many of these systems will not drain towards Pant-y-Llyn. There is only one
private sewage treatment system near Pant-y-Llyn and this is outside the surface catchment. The
mains sewers do serve all of the small villages to the north and south of Pant-y-Llyn, including
Carmel, Pentre Glanwenlais, and Pant-y-Llyn (Figure 25).
At present there is no intensive farming within the immediate surface water catchment area and
the majority of the surface water catchment comprises managed natural woodland (Appendix 5).
However, the road running adjacent to Pant-y-Llyn drains into the turlough, and drains running
under the road channel water from the immediate surface water catchment. Any spills on this
stretch of road would migrate rapidly to Pant-y-Llyn. In winter any salt or grit added to this
section of road would also find its way into Pant-y-Llyn, and there is a general risk of silt, slurry,
oil and grease and other chemicals associated with traffic entering the lake. As a matter of good
practice we recommend that action is taken by the relevant highways authority to divert road
drainage away from the turlough.
CCW Staff Science Report 12/8/1
46
The land use in the wider groundwater catchment is dominated by managed grassland and semi-
natural deciduous woodland, much of which is within the CCW-managed Carmel National
Nature Reserve. Groundwater quality data shows elevated nutrients, mainly at Dyllgoed Isaf
borehole (situated to the extreme west of the Cernydd Carmel SAC, see Figure ). The elevated
nutrients at Dyllgoed Isaf borehole are no doubt a direct result of the land use in the immediate
area (improved grassland and grazing by cattle). Nutrients at Gorswen borehole are lower and
again this is a reflection of the immediate land use: the fields are semi-improved, and are mainly
grazed by horses.
It is not clear whether the higher nutrient levels recorded at Dyllgoed Isaf borehole, in the west,
are a threat to Pant-y-Llyn. This cannot be assessed in the absence of a definitive groundwater
catchment. Despite these uncertainties, due to the distance from Pant-y-Llyn, dilution effects and
our present understanding of the hydrology of the system, we suspect that water from the
Dyllgoed Isaf area is at most a weak and intermittent influence on the water quality of Pant-y-
Llyn.
Figure 25 Location of mains sewers (red) and areas served by them (yellow) in the Cernydd Carmel area. The
surface water catchment of the turlough is shown by a heavy blue line. The green line is the SAC boundary. OS base
maps reproduced with permission of HMSO. Crown copyright reserved. CCW licence No. 100018813
The risk of groundwater eutrophication from private domestic sewage treatment plants
(including septic tanks and cesspits) is a possibility, especially in the area closer to Pant-y-Llyn.
The lack of a clear groundwater catchment map and inadequate information on the location of
domestic sewage treatment prevents a detailed assessment of this risk. However, the area is
connected to mains sewerage and is sparsely populated. In particular, there is only a single
property within the immediate environs of Pant-y-Llyn. The water chemistry results suggest that
no source of nutrients can be entirely ruled out at this stage – for example, it is possible that
leakage from one or more properties is an issue. Other than properties within the surface water
and immediate groundwater catchment of Pant-y-Llyn, we consider the risk of eutrophication
from domestic sources to be negligible.
CCW Staff Science Report 12/8/1
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6 SUGGESTED WATER QUALITY LIMITS FOR PANT-Y-LLYN
The available evidence allows us to provide actual limits for nutrient input to Pant-y-Llyn.
Conservation objectives are required by the 1992 ‘Habitats’ Directive (92/43/EEC) and the data
presented within this report should allow numeric limits to be applied to the water quality at
Pant-y-Llyn. These ‘performance limits’ are part of the conservation objectives of the Core Site
Management Plan.
It is CCWs intention to include more detailed water quality parameters, which is made clear in
the core management plan (see Factor F1-2 Water Quality Factors). Having now obtained a
more detailed data set it is possible to define “good quality” and we can provide performance
indicators for both physical parameters and nutrient levels within Pant-y-Llyn.
6.1 pH and Conductivity
Current water quality performance limits in the management plan are based on Blackstock
et al
(1993). Their data was based on only two field readings of water chemistry, on 29 May (pH 6.7,
conductivity 275
µ
S/cm) and 28 July 1992 (pH 7.1, conductivity 634
µ
S/cm). Due to a lack of
data there are no limits for nutrients such as nitrate or phosphate. The pH results in Blackstock
et
al
. (1993) are unusually low both in the context of calcareous waters and the current dataset.
Additionally, one of the conductivity readings is unusually high and may indicate instrument
error. In any event, the small size of their dataset prevents calculation of means or identification
of outliers, and we therefore propose to abandon use of these measurements.
Recent data (EPA, 2007) shows that Irish Turloughs have a high pH, and this is reflected by
sampling at Pant-y-Llyn. In view of the current recorded pH range (Mean = 7.92: Range = 7.51-
8.32), we propose the following performance indicator for pH:
pH: Annual mean upper limit 8.1 and lower 7.7. Excludes measurements during very low
water levels (0.5m or less above the estavelle).
Based on the above values, Pant-y-Llyn would pass this attribute.
The conductivity data produced by Blackstock
et al
(1993) include a high reading of 634 µS/cm.
This level has not been approached in any subsequent sampling. The only conductivity value
exceeding 300 µS/cm was associated with the chl-a and nutrient spike in July 2008. We propose
the following performance indicator for conductivity:
Conductivity at 20
˚
C: Annual mean upper limit 235 µS/cm, lower limit 200 µS/cm.
Excludes measurements during very low water levels (0.5m or less above the estavelle).
Based on the above values, Pant-y-Llyn would pass this attribute.
6.2 Chlorophyll and Nutrients
Water clarity in turloughs is typically very high. Low nutrient levels should result in a low
standing crop of phytoplankton, whilst very little silt normally enters the system. With this in
mind, annual mean chlorophyll levels should be very low.
The only available nutrient and chlorophyll data for Pant-y-Llyn are those collected during the
EA/CCW sampling programme (2008). We have used these as a baseline to inform targets. In
view of the lack of clear impacts on Pant-y-Llyn we have assumed that median values generally
represent favourable condition. However, mean values are affected by the nutrient and
CCW Staff Science Report 12/8/1
48
chlorophyll spikes identified in section 2.5. Moreover, the strong relationship between
chlorophyll and nutrients indicates that the turlough is sensitive to increased nutrient levels and
responds rapidly to them, but also that (unlike in lakes) these effects are short-lived, possibly
because the repeated cycles of filling and emptying help to flush out nutrients, preventing a
build-up of nutrients in the sediments. It is also possible that samples taken when water has
receded and has ponded around the estavelle are elevated in chlorophyll and phosphate, and any
water quality would only affect a small area of the turlough.
Chlorophyll levels in Pant-y-Llyn were always below 10µg/l, except when TP concentrations
exceeded 30µg/l, in which case chlorophyll concentrations increased rapidly. When chlorophyll
levels associated with high TP were removed, the mean chlorophyll concentration was 4.57±2.6
µg/l. When the high values were included, the mean value was 17.20±22.48µg/l.
When assessing chlorophyll concentrations, use of a geometric mean (geomean) is often
recommended. Geomeans are calculated by taking the mean of the logarithm of each datapoint,
and then antilogging the resulting value. The resulting value has the advantage that undue
weighting is not given to very high values, such as might occur during a short-lived algal bloom.
Unless there is no variation in the dataset, geomeans are usually somewhat lower for the same
dataset than arithmetic means. This needs to be taken into account when setting targets.
Chlorophyll geomeans for Pant-y-Llyn are 3.46 with high nutrient datapoints removed, and 8.28
with high nutrient datapoints included.
In our view, the high chlorophyll levels seen in July and November 2008 and January 2009
require more investigation. We need to understand if these spikes are an annual feature of Pant-
y-Llyn or if the results are simply a one off, perhaps representing small areas of receded water
that gathers around the estavelle. Further sampling will be required to answer this question. We
propose the following chlorophyll target for Pant-y-Llyn, which reflects the geometric mean
value when the high nutrient levels are removed:
Chlorophyll-a: Annual geometric mean upper limit 5 µg/l. Excludes measurements during
very low water levels (0.5m or less above the estavelle).
In support of the chlorophyll-a target, we have proposed nutrient standards (Table 2). Various
determinands have been analysed at Pant-y-Llyn, and the evidence from chlorophyll
concentrations and Redfield ratios suggests that phosphorus is overwhelmingly the most
important nutrient type at this site. Unfortunately the phosphorus data has not been analysed to a
sufficient limit of detection, and a key requirement for future monitoring of the turlough is that
further P monitoring be carried out to the low detection limit (currently around 2µg/l).
Our understanding of turlough ecology is limited at present. In particular, information on which
P species to monitor is limited. Typically, TP is monitored in lakes (which tend to have
significant standing plankton crops) and SRP in rivers (which do not). As a standing water body
TP seems the most relevant determinand, but SRP should continue to be measured for the time
being, especially in the light of the potential nutrient pollution.
In the light of the subordinate role played by nitrogen, we recommend that nitrogen species
monitoring be scaled back to cover only nitrate-N, and that only an indicative target be set for
nitrogen at this site.
For ammonia, which can be both a nutrient and a toxic pollutant we have set a target that reflects
the Water Framework Directive target for high ecological status in high alkalinity rivers.
Ammonia levels are very low most of the time in Pant-y-Llyn, but increased ammonia would
reflect impacts of intensive farming.
CCW Staff Science Report 12/8/1
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Nutrient Determinand Unit Statistic Target
Phosphorus Total Phosphorus (TP as
P) µg/l Annual mean:
Upper limit 20
Phosphorus Soluble Reactive
Phosphorus (SRP as P) µg/l Annual mean: Upper
limit 10
Nitrogen Nitrate-N as N mg/l Annual mean: Upper
limit 0.4
Nitrogen Ammoniacal N mg/l 90%ile: upper limit 0.3
Table 6. Suggested nutrient limits for Pant-y-Llyn. Determinands in bold are required; determinands in plain text are
recommended.
Based on the current water quality at Pant-y-Llyn these nutrient targets should be manageable.
The results should be included in CCWs Core Site Management plan as performance indicators.
CCW Staff Science Report 12/8/1
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7 REVISED PERFORMANCE INDICATORS AND CONDITION ASSESSMENT FOR PANT-Y-LLYN
A revised set of performance indicators is summarised in Table 7 below. This is based on the investigation and review in sections X to Y. We have also
made minor changes to the wording of some other performance indicators. Most of the limits remain unchanged, except where we have identified
issues as above.
Table 7. Revised Performance Indicators for the Turlough feature of Cernydd Carmel SAC.
Performance indicators for feature condition
Attribute Attribute rationale and other comments Rationale for Change Specified limits
A1. Extent The lower limit is based on extent during the wet phase. In
winter the water level will reach the upper limits of
inundation, approx. 3 m above the swallow hole to the
marginal Salix woodland zone. No upper limit has been set,
as the extent is naturally limited by the size of the turlough
basin.
This attribute can be monitored via simple visual checks of
winter water levels.
Unchanged. Upper limit: Not required
Lower limit: Turlough basin will fill with water
during wet phase
A2. Habitat Structure The lower limit is based mainly on the continued presence of
a number of vegetation zones in the turlough basin during
the dry phase, originally identified by Blackstock, et al.
(1993). No upper limit is required.
Monitoring of this attribute should be carried out during the
dry phase, ideally in July. Full monitoring should be
undertaken on a six-year cycle, although brief checks for
non-native arrivals etc can be carried out more regularly.
Minor modifications
to format only (not
considered in detail as
part of this report).
Upper limit: Not required
Lower limit:
Each of the following vegetation zones including
their associated species (listed) should be present:
1. Hydrophytic bryophyte zone (Fontinalis
antipyretica, Drepanocladus aduncus) –
currently occurs in the immediate vicinity of the
swallow hole.
2. Equisetum fluviatile zone (E. fluviatile, Galium
palustre, Mentha aquatica, Veronica scutellata,
Persicaria hydropiper, P. maculosa, Fontinalis
antipyretica, D. aduncus, Calliergon
cordifolium. – currently occurs c. 0.6 m above
and to the south of the swallow hole.
3. Carex vesicaria zone (C. vesicaria, M. aquatica,
CCW Staff Science Report 12/8/1
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Phalaris arundinacea, Solanum dulcamara, F.
antipyretica, D. aduncus, C. cordifolium. –
currently dominates most of the turlough basin,
c. 1.2-2.2 m above the swallow hole.
4. Phalaris arundinacea zone (P. arundinacea, S.
dulcamara, G. palustre, F. antipyretica, D.
aduncus) – currently occupies the northern end
of the basin, c. 2.2 m above the swallow hole.
5. Salix cinerea-Galium palustre (S. cinerea, G.
palustre, M. aquatica, S. dulcamara, Agrostis
stolonifera) woodland zone – extends as a
narrow zone around the edge of the turlough
basin, up to c. 3.5 m above the swallow hole.
And alien plant species are absent. Potentially
invasive non-native species include Impatiens
glandulifera, Crassula helmsii, Hydrocotyle
ranunculoides, Myriophyllum aquaticum and
Azolla filiculoides.
A3. Phytoplankton
(as chlorophyll a) Turloughs characteristically have clear water with low
phytoplankton biomass, partly due to low available nutrient
levels and partly due to grazing by zooplankton. Low
chlorophyll levels in the water column are a prerequisite for
the characteristic ecology of the turlough.
Chlorophyll readings should be based on at least 6
measurements taken over the course of a single year when
water levels are more than 0.5m above the estavelle
(swallow hole)
Site-specific: linked to
TP levels Upper limit: Annual geometric mean 5µg/l
Lower limit: None
A4. Water quality Good water quality is essential to the ecological integrity of
the turlough. Increased nutrient levels in particular could be
detrimental to the characteristic flora and fauna of the
turlough.
In line with CSM
guidance, water
quality determinands
and limits are site-
specific and have been
CCW Staff Science Report 12/8/1
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Water quality values should be based on at least 6
measurements taken over the course of a single year when
water levels are more than 0.5m above the estavelle
(swallow hole)
updated based on
recent data.
A4a. pH
Limits are based on EA data sampled during 2007-9. See above Upper limit: Annual mean pH 8.1
Lower limit: Annual mean pH 7.7
A4b. Conductivity
Limits are based on EA data sampled during 2007-9. See above Upper limit: Annual mean 235µS/cm
Lower limit: Annual mean 200µS/cm
A4c Total
Phosphorus
Limits are based on EA data sampled during 2007-9. See above Upper limit: Annual mean 20µg/l
Lower limit: None
A4e Ammoniacal
Nitrogen
Limits are based on EA data sampled during 2007-9. See above Upper limit: 90%ile 0.3mg/l
Lower limit: None
A4f Turbidity
The turlough is fed entirely by groundwater and its waters
are normally very clear. Turbidity is most likely to arise
from pollution, either via the groundwater or from surface
water runoff. Monitoring should take place during the wet
phase, and may be undertaken throughout the monitoring
cycle.
Minor changes to
wording to improve
clarity – no change to
meaning or target.
Upper limit: Not required
Lower limit: Water should be sufficiently clear that
the bed of the turlough can be clearly seen from the
surface at all times during the wet phase. .
A5. Characteristic
Fauna Turloughs should be dominated by species with life history
characteristics that enable them to survive the dry phase.
This includes specific cladocerans, ostracods, gastropods and
amphibians.
A performance
indicator is needed to
define the fauna, but
will not be used in this
monitoring cycle.
No limits set at present pending further
investigation of the invertebrate community.
Performance indicators for factors affecting the feature
Factor Factor rationale and other comments Operational Limits
F1a Nutrient Pollution:
Soluble Reactive Phosphorus
In natural systems, phosphorus is a limiting nutrient and available levels of
SRP are typically very low. Elevated SRP likely indicates surface or
groundwater pollution. Increased SRP will increase the TP and lead to
increased chlorophyll (see attributes, above).
Limits are based on EA data sampled during 2007-9. Values ill be reviewed
following further data collection using a better limit of detection.
Upper limit: Annual mean 10µg/l
Lower limit: None
F1b Nutrient Pollution:
Nitrate-N In natural systems, nitrate levels are typically very low. Elevated nitrate likely
indicates surface or groundwater pollution. Increased nitrate may have various
negative effects including increased filamentous algal and plant growth, and
Upper limit: Annual mean 0.4mg/l
Lower limit: None
CCW Staff Science Report 12/8/1
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increased chlorophyll.
Limits are based on EA data sampled during 2007-9. Will be reviewed
following further data collection.
F3 Siltation Increased siltation, such as runoff from farmland, will smother the plants
growing in the turlough and transport nutrients, especially phosphate.
F4. Water levels The hydrology of the turlough is determined by seasonal fluctuations in the
groundwater table of the underlying aquifer. Any alterations to the cyclical
fluctuation of water levels could have a detrimental impact on the ecological
and hydrological integrity of the turlough.
Limits relating to water levels in the turlough are
addressed in Attributes A1 and A2 above. Any
concerns highlighted through monitoring of
Attributes A1 and A2 should trigger investigations
into Factor F4. Simple visual checks of water
levels can also be carried out at various stages of
the annual fill-drain cycle.
F5. Scrub encroachment Development of willow and alder scrub in the turlough basin is a potential
threat to the characteristic flora and fauna of the turlough. Scrub encroachment
in the turlough basin is unacceptable and a upper limit of 5% scrub cover has
been set.
Upper limit: 5% scrub cover
Lower limit: Not required
CCW Staff Science Report 12/8/1
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7.1 Condition Assessment
Table 8.Condition assessment for the Turlough feature of Cernydd Carmel SAC, based on the revised performance indicators in Table 7.
Attribute Specified limits Status Comments
A1. Extent Upper limit: Not required
Lower limit: Turlough basin will fill with water during
wet phase ✓✓ Hydrological monitoring shows that the turlough
underwent full empty / fill cycle in 2010.
A2. Habitat Structure Upper limit: Not required
Lower limit:
Each of the following vegetation zones including their
associated species (listed) should be present:
1. Hydrophytic bryophyte zone (Fontinalis
antipyretica, Drepanocladus aduncus) – currently
occurs in the immediate vicinity of the estavelle
2. Equisetum fluviatile zone (E. fluviatile, Galium
palustre, Mentha aquatica, Veronica scutellata,
Persicaria hydropiper, P. maculosa, Fontinalis
antipyretica, D. aduncus, Calliergon cordifolium. –
currently occurs c. 0.6 m above and to the south of
the estavelle.
3. Carex vesicaria zone (C. vesicaria, M. aquatica,
Phalaris arundinacea, Solanum dulcamara, F.
antipyretica, D. aduncus, C. cordifolium. –
currently dominates most of the turlough basin, c.
1.2-2.2 m above the swallow hole.
4. Phalaris arundinacea zone (P. arundinacea, S.
dulcamara, G. palustre, F. antipyretica, D.
aduncus) – currently occupies the northern end of
the basin, c. 2.2 m above the estavelle.
5. Salix cinerea-Galium palustre (S. cinerea, G.
palustre, M. aquatica, S. dulcamara, Agrostis
stolonifera) woodland zone – extends as a narrow
zone around the edge of the turlough basin, up to c.
Not Assessed No formal assessment but thought to be favourable –
no evidence of significant change [region to confirm]
CCW Staff Science Report 12/8/1
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3.5 m above the estavelle.
And alien plant species are absent. Potentially invasive
non-native species include Impatiens glandulifera,
Crassula helmsii, Hydrocotyle ranunculoides,
Myriophyllum aquaticum and Azolla filiculoides.
A3. Phytoplankton (as
chlorophyll a) Upper limit: Annual geometric mean 5µg/l
Lower limit: None
%
Annual geometric mean = 8.28µg/l, based on 10
measurements between February 2008 and January
2009.
However, since water level data was not collected at
the time of the chlorophyll measurements, no firm
interpretation can be placed on this data.
A4. Water quality
A4a. pH
Upper limit: Annual mean pH 8.1
Lower limit: Annual mean pH 7.7 ✓✓ Annual mean = 7.92
A4b. Conductivity
Upper limit: Annual mean 235µS/cm
Lower limit: Annual mean 200µS/cm ✓ Annual mean = 231. One high value (327) is a cause
for concern and is linked to elevated nutrient levels.
A4c Total Phosphorus
Upper limit: Annual mean 20µg/l
Lower limit: None %
13 measurements of TP were available between
February 2008 and March 2011. Exact calculation of
mean TP is not possible due to problems with the
detection limit (20µg/l). However, if all values <20
are assumed to be 1µg/l, the annual mean is 25µg/l; if
all values <20 are assumed to be 19µg/l, the mean is
33.8µg/l. At an intermediate value (10µg/l), the mean
is 28.2µg/l, Thus, despite the detection problems,
there is high confidence that this attribute is failed in
absolute terms.
The above notwithstanding, this impact appears to
have been short-lived and it is unclear whether it is
linked to
A4e Ammoniacal Nitrogen
Upper limit: 90%ile 0.3mg/l
Lower limit: None ✓ 14 measurements of ammoniacal nitrogen were
available, for which the 90%ile was 0.133mg/l. The
highest value was 0.287 on 2
nd
July 2008.
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A4f Turbidity
Upper limit: Not required
Lower limit: Water should be sufficiently clear that the
bed of the turlough can be clearly seen from the surface
at all times during the wet phase. .
Not assessed Turbidity readings are available from some of the
water samples and we will relate these to water depth
in the turlough.
A5. Characteristic Fauna No limits set at present pending further investigation of
the invertebrate community. Not assessed Not applicable at present.
Factor Factor limits Operational Limits
F1a Nutrient Pollution:
Soluble Reactive Phosphorus Upper limit: Annual mean 10µg/l
Lower limit: None Not assessed Present limit of detection is inadequate to assess this
factor.
F1b Nutrient Pollution:
Nitrate-N Upper limit: Annual mean 0.4mg/l
Lower limit: None ✓ Annual mean = 0.313. However, the standard
deviation overlaps the target value.
F3 Siltation Increased siltation, such as runoff from farmland, will
smother the plants growing in the turlough and transport
nutrients, especially phosphate. Not assessed No evidence of elevated siltation. However, road
drainage should be investigated in this context.
F4. Water levels Limits relating to water levels in the turlough are
addressed in Attributes A1 and A2 above. Any
concerns highlighted through monitoring of Attributes
A1 and A2 should trigger investigations into Factor F4.
Simple visual checks of water levels can also be carried
out at various stages of the annual fill-drain cycle.
✓ No issues with attributes A1 or A2
F5. Scrub encroachment Upper limit: 5% scrub cover
Lower limit: Not required ✓ Current scrub encroachment is within operational
limits, but management in the next 5-10 years may be
required.
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Based on the above, we consider the Turlough feature at Cernydd Carmel to be in Favourable
condition. This is because the hydrology at the site was favourable, there was no evidence of
significant scrub invasion, and water quality was generally good.
This assessment should be considered as low confidence and is subject to the following caveats
and concerns:
•
Inadequate biological data was available.
•
There are intermittent chlorophyll and nutrient spikes which require further investigation.
8 DISCUSSION AND RECOMMENDATIONS
8.1 Hydrology
8.1.1 General Hydrological Findings
There is a clear relationship between rainfall, groundwater levels and water level fluctuations
within Pant-y-Llyn.
The maximum fill level is just over 3m depth above the estavelle.
Water level peaks lagged by between 1 and 8 days behind heavy rainfall events, depending on
storage of the aquifer and SMD.
Recharge is more rapid during wetter months where the water
table is higher (e.g winter) and slower when the water table is depressed (e.g late summer).
Hydrologically Pant-y-Llyn is considered to be meeting existing performance indicators for
emptying and filling. The empty and filling cycle of Pant-y-Llyn is also reflected in the
distribution and type of vegetation, which again is probably in good condition and a proxy
indicator that the hydrology of the site is functioning well.
•
Water level performance indicators should be added to CCWs Core Site
Management Plan.
8.1.2 Defining the Groundwater Catchment
At present the area of the Gwenlais impounded karst is considered to be the potential
groundwater catchment. This position was adopted on the recommendation of LRG as a
precautionary approach and reflects uncertainty regarding the real extent of the groundwater
catchment. LRGs tracer tests show the main discharge point is the spring that feeds the Nant
Gwenlais brook. We have produced a 3D ‘catchment visualisation diagram’ (a large scale
conceptual model ) (Figure 26) to help visualise the spatial distribution of the catchment, and the
positions of monitoring boreholes and tracer tests within it.
Surface water enters the main karstic system through five main sink holes, from west to east at
Dyllgoed Isaf, Pwll Edrychiad, Garn Farm, Sinc Llinos and Bwlchau Sink. These discharge
predominantly at the Nant Gwenlais spring southwest of Glanwenlais Quarry. Based on the
tracer data, there is no clear evidence that Pant-y-Llyn is predominantly fed by the main karst
system. While groundwater hydrographs within the Gwenlais karst area do show very similar
hydrographs and responses to water levels at Pant-y-Llyn, this is not necessarily indicative of a
direct connection. Instead, as all karst systems in the area receive the same precipitation this may
just be similar karst systems responding to the same input. Water level and water quality data
(which indicate shallow, rapid flow paths within the local vicinity of Pant-y-Llyn) may provide
further consideration for a smaller, possibly fault-bounded karstic limestone catchment within
the immediate vicinity of Pant-y-Llyn.
•
Further detailed work is recommended to define a better groundwater catchment for
Pant-y-Llyn, so that future management decisions can be taken from the best possible
knowledge base.
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Figure 26 ‘Catchment Visualisation Diagram’ showing Pant-y-Llyn within the Cernydd Carmel SAC. LRG/CCW tracer tests and other key features such as quarries, and Environment
Agency Wales boreholes are annotated.
CCW Staff Science Report 12/8/1
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8.2 Water Quality
8.2.1 General Water Chemistry
Water chemistry at Pant-y-Llyn is typical of a high alkalinity system, with high pH and calcium
concentrations. The water in Pant-y-Llyn is less mineralised than deeper groundwater monitored
in the area, reflecting surface water inputs.
The importance of the surface water catchment should be considered. There are no significant
water quality risks within the surface water catchment.
•
Appropriate land management within the surface water catchment may provide the
best water quality protection to Pant-y-Llyn.
•
It would be advisable to undertake an organic analysis at Pant-y-Llyn to identify if any
organic compounds are present, including compounds likely to result from road
drainage.
8.2.2 Nutrients
Nutrient levels at Pant-y-Llyn are generally low, but there are some unexplained spikes in both
nutrients and chlorophyll that require further investigation, especially in the absence of any
obvious sources of nutrients. We do not know if these results are caused by evaporation and
concentration of existing nutrients towards the end of the dry phase, or whether they represent
pollution. Our understanding of the dynamics of Pant-y-Llyn would be much improved if we
could combine water quality and water level monitoring for a period of at least 12 months, and
this data could also be used for condition assessment for the 2012-2018 monitoring cycle.
•
Water Quality monitoring should continue at Pant-y-Llyn (for a period of at least 12
months) to address why large phosphate and chlorophyll spikes are observed in late
summer.
•
The following determinands should be continued to be measured either for
monitoring or interpretation purposes: pH; conductivity; alkalinity; calcium;
chlorophyll-a; total phosphorus; soluble reactive phosphorus; nitrate-N; total
nitrogen; ammoniacal nitrogen.
•
The protocol for measuring phosphate species (both TP and SRP) needs to be
amended so that in future, the limit of detection is 2 µg/l.
8.2.3 Water Quality Targets
We propose the following water quality targets for Pant-y-Llyn. All values exclude
measurements taking during periods of very low water level (0.5m above the estavelle or less):
- pH: Annual mean upper limit 8.1 and lower 7.7.
- Conductivity at 20
˚
C: Annual mean upper limit 235 µS/cm, lower limit 200 µS/cm.
- Chlorophyll-a: Annual geometric mean upper limit 5 µg/l.
- Total Phosphorus: Annual mean upper limit 20 µg/l.
- Soluble Reactive Phosphorus (guideline): Annual mean upper limit 10 µg/l.
- Nitrate-N as N (guideline): Annual mean upper limit 0.4mg/l
- Ammoniacal nitrogen as N: 90%ile upper limit 0.3mg/l
CCW Staff Science Report 12/8/1
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•
CCW and EAW should adopt the above targets for future monitoring of Pant-y-
Llyn.
8.3 Biological Monitoring
Recent biological data for the turlough is inadequate. An improved understanding of the flora
and fauna of this unique environment is necessary so that we can measure ecological changes
and / or impacts.
•
EAW should carry out a detailed invertebrate survey of the turlough, including
both benthic species and zooplankton. A survey is planned for 2012.
•
CCW should survey the vegetation in Pant-y-Llyn. This would be best carried out
by wetland specialists. A repeat survey is planned for 2012.
8.4 Site Management
The risk assessment did not identify any immediate and serious causes for concern. However,
there are three longer-term issues that could not be quantified and any of which could cause
serious impacts:
•
A renewal of quarrying has the potential to cause serious hydrological problems.
•
Within the surface water catchment, the most likely risk is drainage from the road
which has the potential to act as a conduit for nutrients and other pollutants. Action
should be taken to divert road drainage away from the turlough.
•
Scrub clearance may be needed periodically.
•
There is some potential for nutrient pollution of the groundwater catchment but in
the absence of a clear groundwater catchment and an inventory of nutrient sources
this is difficult to quantify. Water quality results in the turlough do not suggest
groundwater contamination is a serious issue at present.
9 ACKNOWLEDGEMENTS
We thank Simon Neale, Environment Agency Wales, and Professor John Gunn for providing
critical comments on an earlier draft.
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CCW Staff Science Report 12/8/1
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APPENDIX 1: ORIGINAL PERFORMANCE INDICATORS FOR THE
TURLOUGH FEATURE (CCW, 2008)
Performance indicators for feature condition
Attribute Attribute rationale and other comments Specified limits
A1. Extent This attribute was developed by CCW’s SAC
monitoring team (Lovering, 2006). The
lower limit is based on extent during the wet
phase. It assumes that in winter the water
level will reach the upper limits of
inundation, approx. 3.5 m above the swallow
hole to the marginal Salix woodland zone.
No upper limit has been set, as the extent is
naturally limited by the size of the turlough
basin.
This attribute can be monitored via simple
visual checks of winter water levels.
Upper limit: Not required
Lower limit: Turlough basin will fill with
water during wet phase
A2. Quality This attribute was developed by CCW’s SAC
monitoring team (Lovering, 2006). The
lower limit is based mainly on the continued
presence of a number of vegetation zones in
the turlough basin during the dry phase. The
various zones were originally identified by
Blackstock, et al. (1993). No upper limit is
required in this case.
Monitoring of this attribute should be carried
out in during the dry phase, ideally in July.
Full monitoring should be undertaken on a
six-year cycle, although brief checks for non-
native arrivals etc can be carried out more
regularly.
Upper limit: Not required
Lower limit:
Each of the following vegetation zones
should be present:
6. Hydrophytic bryophyte zone – currently
occurs in the immediate vicinity of the
swallow hole.
7. Equisetum fluviatile zone – currently
occurs c. 0.6 m above and to the south of
the swallow hole.
8. Carex vesicaria zone – currently
dominates most of the turlough basin, c.
1.2-2.2 m above the swallow hole.
9. Phalaris arundinacea zone – currently
occupies the northern end of the basin, c.
2.2 m above the swallow hole.
10. Salix cinerea-Galium palustre woodland
zone – extends as a narrow zone around
the edge of the turlough basin, up to c.
3.5 m above the swallow hole.
And associated species for each vegetation
zone are present. Associated species for each
zone include:
A2. Quality
(cont.d) 1. Hydrophytic bryophyte zone – Fontinalis
antipyretica, Drepanocladus aduncus.
2. Equisetum fluviatile zone – Galium
palustre, Mentha aquatica, Veronica
scutellata, Persicaria hydropiper, P.
maculosa, Fontinalis antipyretica,
Drepanocladus aduncus, Calliergon
cordifolium.
3. Carex vesicaria zone – Mentha aquatica,
Phalaris arundinacea, Solanum
dulcamara, Fontinalis antipyretica,
Drepanocladus aduncus, Calliergon
cordifolium.
4. Phalaris arundinacea zone – Solanum
dulcamara, Galium palustre, Fontinalis
antipyretica, Drepanocladus aduncus.
5. Salix cinerea-Galium palustre woodland
zone – Mentha aquatica, Solanum
dulcamara, Agrostis stolonifera.
CCW Staff Science Report 12/8/1
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And alien plant species are absent.
Potentially invasive non-native species
include Crassula helmsii, Hydrocotyle
ranunculoides, Myriophyllum aquaticum and
Azolla filiculoides.
Performance indicators for factors affecting the feature
Factor Factor rationale and other comments Operational Limits
F1-2. Water
quality factors Good water quality is essential to the
ecological integrity of the turlough. Increased
nutrient levels in particular could be
detrimental to the characteristic flora and
fauna of the turlough. Common Standards
Monitoring guidance states that, for
turloughs, water quality determinands and
limits should be defined on a site-by-site
basis, ideally following collection of a data-
set of readings over time – refer to Lovering
(2006) for a fuller review of potentially
relevant water quality parameters. Initial
limits for pH and conductivity were proposed
by Lovering (2006), based on readings taken
in 1992 by Blackstock et al. (1993).
Performance indicators for additional
parameters (including nutrient determinands)
will be developed in future following further
water quality sampling.
See below
F1. Water quality:
pH Limits are based on pH measurements taken
in 1992 (see above). Water sampling should
take place during the wet phase, ideally in
March to enable monitoring of both dry and
wet phases in the same reporting year.
Upper limit: pH 7.1
Lower limit: pH 6.7
F2. Water quality:
conductivity Limits are based on conductivity
measurements taken in 1992 (see above).
Water sampling should take place during the
wet phase, ideally in March to enable
monitoring of both dry and wet phases in the
same reporting year.
Upper limit: 634 µs cm
Lower limit: 275 µs cm
F3. Turbidity The turlough is fed entirely by groundwater
and its waters are normally very clear.
Turbidity is most likely to arise from
pollution, either via the groundwater or from
surface water runoff. Monitoring should take
place during the wet phase, and may be
undertaken throughout the monitoring cycle.
Upper limit: Not required
Lower limit: Entire bed of turlough should be
visible during wet phase.
F4. Water levels The hydrology of the turlough is determined
by seasonal fluctuations in the groundwater
table of the underlying aquifer. Any
alterations to the cyclical fluctuation of water
levels could have a detrimental impact on the
ecological and hydrological integrity of the
turlough.
Limits relating to water levels in the turlough
are addressed in Attributes A1 and A2 above.
Any concerns highlighted through
monitoring of Attributes A1 and A2 should
trigger investigations into Factor F4. Simple
visual checks of water levels can also be
carried out at various stages of the annual
fill-drain cycle.
F5. Scrub
encroachment Development of willow and alder scrub in
the turlough basin is a potential threat to the
characteristic flora and fauna of the turlough.
Scrub encroachment in the turlough basin is
unacceptable and a upper limit of 5% scrub
cover has been set.
Upper limit: 5% scrub cover
Lower limit: Not required
APPENDIX 2: BOREHOLE LOGS
CCW Staff Science Report 12/8/1
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APPENDIX 3: LRG (2006) TOPOLOGICAL SURVEY MAP
CCW Staff Science Report 12/8/1
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APPENDIX 4: WATER CHEMISTRY DATA
All values are presented in mg/l. Asterisks indicate samples that are not consistent with the remainder of the dataset.
Site Date Ca
2+
Mg
2+
Na
+
K
+
HCO
3-
Cl
-
SO
42-
NH
3
-N NO
x
NO
3-
NO
2-
SRP TP
07/02/2001 83.1 4.88
257 14.7 0.05 1.242 1.23 0.016 0.005 0.007
10/04/2002 89 4.11
260 14.1 0.01 1.751 1.75 0.005 0.012 0.016
15/11/2002 89.6 5.07
264 13.8 0.01 1.638 1.64 0.002 0.004 0.009
20/03/2003 87.4 4.24
254 13.5 0.01 1.925 1.92 0.007 0.004 0.136
08/09/2003 84.5 3.88
6.46 0.57
13.2 12.4 0.02 2.159 2.16 0.003 0.051
20/02/2004 83.3 4.72
6.72 0.46
11.9 12.9 0.01 2.02 2.02 0.002 0.004
23/07/2004 63.2 3.45
7.38 2.23
23.8 19.8 0.06 2.569 2.4 0.17 0.012
11/03/2005 75.1 3.74
6.5 1.2 17.9 17.1 0.01 0.39 0.388 0.002 0.007
03/08/2005 90.9 3.66
6.53 0.721
13.4 12.4 0.03 1.92 1.92 <0.004 <0.02
01/03/2006 77 3.81
6.13 0.57
23.9 14.3 0.075 1.77 1.77 <0.004 <0.02
15/10/2007* 5.5 0.3 10.9 18.1
45.1 11.7 12 0.119 1.21 0.482 0.728 <0.02 <0.02
09/04/2008* 4.9 0.3 9.3 15.7
37.8 11 13 0.03 0.8 0.649 0.151 <0.02 <0.02
Dyllcoed Isaf
EAW Borehole
14/04/2010* 18.7 0.328
6.34 7.74
41.5 10.1 18.1 <0.03 1.25 1.25 <0.004 <0.02
Glanwenlais Quarry
EAW Borehole 15/04/2010 11.5 5.52
8.3 0.457
70.8 11.9 <10 0.054 <0.2 <.196
<0.004 <0.02
07/02/2001 91.1 4.11
267 110 0.01 1.022 1.02 0.002 0.023 0.023
10/04/2002 91.1 4.18
262 16.5 0.01 1.099 1.09 0.007 0.008 0.013
15/11/2002 91.7 4.58
264 15.5 0.01 1.116 1.11 0.002 0.004 0.014
20/03/2003 92.8 4.36
265 15.6 0.01 1.136 1.13 0.004 0.004 0.002
11/03/2005 98.5 5.45
8.22 1.4 16.1 26 0.01 1.54 1.54 0.003 0.009
03/08/2005 96 5.5 8.26 1.31
18.5 20.1 0.03 1.05 1.05 <0.004 <0.02
04/04/2006 103 4.79
9.51 1.44
16.86 21.5 0.04 0.39 0.386 <0.004 <0.02
15/10/2007 91.9 5.42
8 1.54
290 14 16 0.034 0.27 0.266 <0.004 <0.02 <0.02
09/04/2008 106 5.27
8.4 1.59
301 17.5 18 0.03 <0.2 0.196 <0.004 <0.02 0.0242
Gorswen
EAW Borehole
55m
14/04/2010 95.8 4.66
8.17 1.52
303 14.6 14 <0.03 0.24 0.236 <0.004 <0.02
CCW Staff Science Report 12/8/1
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Site Date Ca
2+
Mg
2+
Na
+
K
+
HCO
3-
Cl
-
SO
42-
NH3-N NO
x
NO
3-
NO
2-
TN SRP TP
26/02/2008 39.5 2.17 7.76 0.769
14.2 0.03 0.43 <0.004 0.7 <0.02 <0.02
06/03/2008 40 2.22 8.19 0.783
15.7 0.0929 0.317 <0.004 0.6 <0.02 <0.02
28/03/2008 38 2.15 7.81 0.809
14.1 0.03 0.25 <0.004 0.31 <0.02 <0.02
29/04/2008 38 2.1 7.78 0.6 13.7 0.03 0.2 <0.004 0.28 <0.02 <0.02
21/05/2008 38.5 2.09 7.1 0.92
15.5 0.03 0.2 <0.004 0.47 <0.02 0.0239
02/06/2008 69.3 2.51 11 0.987
16.1 0.287 0.26 <0.004 1.25 0.0206 0.117
28/07/2008 41 2.68 8.87 0.283
15.2 0.114 0.2 <0.004 1.39 0.0328 0.068
18/09/2008 40.9 2.28 6.63 0.867
11 0.0336 0.33 <0.004 0.42 <0.02 <0.02
28/11/2008 41.1 2.4 7.29 1.06
11.6 0.03 0.422 <0.004 0.79 <0.02 0.04
25/08/2010 54.8 2.4 7.6 0.328
160 10.7 16.8 0.03 0.226 <0.004 <0.02
18/01/2011 39.8 2.05 6.9 0.8 118 12.9 10 0.041 0.446 <0.004 <0.02 <0.02
Pant Y Llyn
Turlough
19/04/2011 40.9 2.06 8.17 0.263
126 15 10 0.03 0.196 <0.004 <0.02 <0.02
CCW Staff Science Report 12/8/1
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APPENDIX 5: 3D VIEW OF SURFACE WATER CATCHMENT
Surface water catchment (blue) and geological faults (dashed line). The location of the turlough is indicated by the
white arrow. Aerial Photography © Infoterra Ltd, 2009.
APPENDIX 5: DATA ARCHIVE APPENDIX
No data outputs were produced as part of this project. All data referred to are stored on
Environment Agency systems.
