I
I
I
I
1
HXDROGEOTHERMAL CONDXTXONS IN EIRE
by
C.R. Aldwell
and D.J. Burdon
HYDROGEOTHERMAL CONDITIONS IN EIRE
by
C.R. Aldwell * and D.J. Burdon **
An original paper prepared for section 14.2
Fossil fuels,
of the XXVI International
Geological Congress, Paris, 1980.
Ref. No. 14.0068.
SUMMARY
Hydrogeothermal conditions in Eire have not yet been substantially
investigated, but a project entitled "Geothermal Energy Potential of Ireland"
is planned to commence in mid-1980, with.EEC assistance.
The data presented
here, mainly on warm springs, forms part of the preparation for this project.
The 17 known warm springs are listed in Table 1, and shown on Figs. 1
and 2.
Temperatures of the Leinster Group of warm springs have been
collected since 1971, and most were chemically analysed at end-1979, Tables 3
and 4.
From a study of the location, geological and geophysical setting and
hydrological, thermal and hydrochemical characteristics, a number of
inferences have been reached.
These may be summarised here:
(i) Several
warm springs occur on higher ground than might be expected;
(ii) Some have
been made functional only by man's activities;
(iii) The Leinster warm
springs appear to be distributed at 8 krn distances apart;
(iv) All the warm
springs issue from Dinantian limestones, mainly from the Visean;
(v) Some
small Tertiary intrusives have been reported from their vicinity;
(vi) The occurrences can often be related to fold structures, as along a
syncline;
(vii) Certain gravity lows may have some connection with these
warm springs;
(viii) Discharges increase in spring and fall in autumn,
indicating groundwater flow controlled by the normal hydrological cycle;
(ix) Temperatures peak in spring and decline in autumn, but are irregular;
(x) Most of the warm springs show no abnormal hydrochemical characteristics;·
(xi) High Cl and TDS are found in three springs occurring along a synclinal
structure, but two other springs located there have as yet not been analysed;
(xii) There are faint indications that Si and I increase with increasing
temperature.
Of the Munster warm springs, those in Cos. Clare and Limerick appear
to be similar to those of Leinster.
However, the Mallow Spa spring, the
best-known in Ireland, issues where the Dinantian limestones are highly
compressed, downfolded and thrust.
Boreholes in the Mallow area adjacent
to the spring have located warm water.
While the mineralization of the Lower Carboniferous in Ireland has been
attributed to hydrothermal concentration and precipitation of base metals,
present-day geothermal circulation bears no relationship to such ancient
activity.
However, the groundwater circulation is considered as due to the
effects of heat, on cirCUlation cells of the order of 50 km 2 , with conduction-
*Geological Survey of Ireland, 14 Hume Street, Dublin 2.
** Consultant, Geological Survey of Ireland;
and Director, Minerex Ltd.
convection uprising of warm water in the centre of each cell, and recharge
and downflow at the edges, as in Fig. 7.
Depths of circulation may be
of the order of 700 metres.
As yet, the source of the heat has not been
identified, while the problem of heat conservation by impermeable strata or
cover is under consideration.
In brief, the problem has been but stated;
intensive investigation under the "Geothermal Energy Potential of Ireland"
project will be required to solve the mechanisms and lead to the proper
utilisation and management of this natural energy resource of the country.
HYDROGEOTHERMAL CONDITIONS IN EIRE
CONTENTS
I
INTRODUCTION
I-I. Scope and Objective
I-2. Historic warm Springs
1-3. Terrestial Heat Flow in Ireland
II
WARM SPRINGS OF IRELAND
II-I.
1I-2.
1I-3.
1I-4.
1I-5.
1I-6.
1I-7.
III
ORIGIN AND FUNCTIONING
III-I.
III-2.
1II-3.
1II-4.
IV
Location
Geological Setting
Geophysical Setting
Hydrological Characteristics
Thermal Characteristics
Hydrochemical Characteristics
Main Inferences
Palaeozoic Thermal Springs and Mineralization
Present-Day Circulation of Warm Groundwater
Heat Sources and Heat Conservation
Conclusion
ACKNOWLEDGEMENTS AND REFERENCES
Table 1.
The seventeen Warm Springs located in Leinster and Munster, to
which reference is made in the text.
Table 2.
Swnmary of Lithological Data from Trim No.1 Well (sunk, 1962)
as affecting the main group of Leinster Warm springs (from
Sheridan, 1972).
Table 3.
Temperatures and major chemical constituents of the waters of the
Irish warm springs, based on samples taken on 15 Nov., and on
4 & 7 Dec., 1979.
Analyses made at State Laboratory, College
of Science, Dublin.
Table 4.
Trace elements found in the waters of Irish warm springs, based
on samples taken on 15 Nov., and on 4 & 7 Dec., 1979.
Analyses
made at the state Laboratory, College of Science, Dublin.
Fig. 1.
Location of the recorded Warm Springs of Ireland
Fig. 2.
Location characteristics and geological setting of the main group
of Leinster Warm Springs
Fig. 3.
Annual Temperature Variations for Enfield (No. 1005) and for
St. Gormans (No. 1007) for the years 1976 and 1978.
Fig. 4.
Triangular Diagram plotting of
of Irish Warm Springs
Fig. 5.
Trace elements Silicon and Iodine plotted against Water Temperatures
Fig. 6.
Convection Circulation of groundwater in Lower Carboniferous times
(after Russell, 1978)
Fig. 7.
Possible present-day groundwater circulation feeding the Leinster
Group of Warm Springs.
Chemi~al
Composition of the waters
Ballinalack
No .1011
Hurler's
No.2004
.Cro~5
Newca5tle
No.2003
~~~~i.~S'€~rl;allOW
We~t
Spa
&. Borehole
r~
L::::J
....'.4l:n1t tMdt-d tjrn.r:stonr\ ~*'
DoLin .nd Dolo:n.;lLS
Culm
o
FIGURE 1 - LOCATION OF THE RECORDED WARM SPRINGS
or
IRELAND
(Carboniferous lithology after Charlesworth)
HYDROGEOTHERMAL CONDITIONS IN EIRE
C.R. Aldwell and D.J. Burdon
I
INTRODUCTION
The paper attempts to bring together and present existing data on
hydrogeological conditions in Ireland, with emphasis on the known warm springs
of the country.
It does not deal with the granites and other potentially
hot dry rocks.
The paper marks a preliminary stage in initiating the study
of the geothermal energy potential of the island.
I-I.
Scope and Objective
On the "Atlas of Subsurface Temperatures in the European Community"
(Haenel, editor, 1980), the Republic of Ireland is shown as a blank.
Though
some temperature data has been obtained, it has not been published, except
in very abbreviated form as in "Terrestial Heat Flow in Europe" (Cermak and
Rybach, editors, 1979).
However, some data has been collected over the past 9 years on the
warm springs of Ireland, and there are also scattered records from the past.
This paper endeavours to bring together such data, plus that obtained by the
authors, relating to the 17 known warm springs (including a very few
warm boreholes) recorded in Ireland.
The warm springs occur in rather
restricted areas, as shown on Fig. 1; and no attempt has been made to study
or record hydrogeothermal conditions outside the areas of the known warm
springs.
The field data collection was intensified towards the end of 1979,
when almost all the known warm springs were sampled and their waters chemically
analysed under uniform conditions to produce comparable results.
The establishment of a projectto make a preliminary investigation
and assessment of the "Geothermal Energy Potential of Ireland" has given
added impetus to the compilation of this present paper on hydrogeothermal
conditions in Ireland.
The 'Geothermal Project' is expected to receive
the support of the Commission of the European Communities (R. & D. Energy
Programme), and to commence formally on I July, 1980.
It is hoped that
the collection and analysis of existing hydrogeothermal data made here will
contribute a little to the successful launching of this Project.
As part
of the Project, it is hoped to obtain a better understanding of the
functions of known and still-to-be-discovered warm springs and boreholes,
together with their potential for energy development and use.
Low enthalpy
sources, such as the Irish warm springs, have potential in many fields,
including horticulture, carp ponds, swL~ing pools, feed waters to industry
and possibly space heating.
1-2.
Historic Warm Springs
There are innumerable springs and spring wells in Ireland; they
are enshrined in place names, in myths and in legends.
There are many holy
wells and springs, christianized sites of druidic worship and healing; these
usually bear the name of a saint.
However, warm springs or wells are rare;
the holy wells are not marked by any physical difference from ordinary wells.
In the eighteenth and nineteenth centuries, there were many spas, of varying
reputation and importance; except for Mallow, their groundwaters were at
normal temperatures.
At the still important spa of Lisdoorivarna, the waters
are warmed prior to drinking
- 2 -
0
0
Normal groundwater in Ireland varies from 9.5 to 10.5 C.
It
tends to lie in the cooler range in the north, and possibly at higher
elevations.
Warm springs are considered as having appreciably higher
temperatures than "normal", at least for a part of the year.
Thus a spring
0
or well whose temperature was not reliably recorded as more than 12 C at
0
some time could not be called a warm spring or well; and plus 13 C would be
a more normal lower temperature limit.
Such a low-temperature "warm spring"
would feel cool to cold in summer, and would only feel very slightly warm
even in winter.
Hence, nmny slightly warm springs may well have escaped
notice.
It will be difficult to identify all Irish warm springs in the lowest
temperature range.
The Mallow Spa spring is the oldest recorded warm spring in Ireland.
Rutty (1957) states "Mallow-water was first discovered and introduced into
practice by Dr. Rogers of Cork about the year J724." Rutty also reports on
the "two tepid springs in the county of Dublin", at St. Margarets (No. 1009
here) and Ballydowd (No. 1013 here, but not relocated on the ground).
Two
of the main warm springs of Leinster were brought into operation by man's
activities.
Louisa Bridge spring (No. 1001) came into operation when the
Royal Canal was being constructed in 1794; Enfield spring became known
in the 1890s, when gravel was being excavated for a railway line.
Until
quite recently, the number of warm springs recognised in Leinster were limited
to two or three.
Grainger and Davies in 1966 state "Now, however, a third
warm spring has come to light in the region", referring to st. Gorman's
(No. 1007), visited by Du Noyer on 21 July, 1859, but thereafter forgotten
by investigators.
In the 1970s however, attention began to be given to the warm
springs, in particular those of Leinster.
Numerous temperature readings
have been collected by one of the authors since 1971.
Enquiries were made
regarding hitherto unidentified warm springs.
So, when Fahy, (1974, 1975)
studied the biology of the Enfield spring (No. 1005) he was able to present
a map showing the location of 10 warm springs.
Horne (1977) drew attention
to two warm springs in Munster, at Newcastle West (No. 2003) in Co. Limerick,
and Hurler's Cross (No. 2004) in Co. Clare.
Even as this paper was nearing
completion news was received of a 'new' warm spring at Clonee, Co. Meath;
this has been included in Table 1.
So, by Spring 1980, it can be said that there are 17 warm springs
(including boreholes) known in Ireland.
They are listed in Table 1, with
positions indicated on Figs. 1 and 2.
Dunlavin spring (No. 1012) is
rejected; it could not be found on the ground when its reputed locality
was examined by one of the authors.
Of the 17 accepted, 11 occur in
Cos. Dublin, Kildare and Meath, and are shown in some detail in Fig. 2; if
Ballydowd (No. 1013) were relocated, i t would fall within this area too.
Further west in Leinster, there is Ballinalack (NO. 1011) spring or bore,
reported by Prof. P. Br~ck, but not as yet examined by the authors.
In
Munster, there is the famous Spa at Mallow, issuing from a pool and in the
well constructed within the Spa House.
Warm water was also encountered in
the borehole in the nearby Cattle Mart, and possibly in another bore (now
shut) close to the Spa itself.
The warm springs at Newcastle West and at
Hurler's Cross have already been mentioned.
/
...
FIGURE 2
-
LOCATION CHARACTERISTICS AND GEOLOGICAL SETTING OF THE MAIN GROUP OF LEINSTER WARM SPRINGS
2.1°.".---,
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STRUCTURE
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SPRINGS
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GEOLOGY
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3 -
Name or Names
Ref. No.
County
1001
Louisa Bridge, Leixlip
Kildare
1002
Bride's River, or Breda's Well, Kilcock
Kildare
1003
Kewnins Mills Spring
Meath
1004
st. Patrick's Well, Celbridge
Kildare
1005
Enfield, Kilbrook or Cappagh Spring
Kildare
1006
Dysart Spring
Kildare
1007
St. Gorman's or Hotwell House Spring
Meath
1008
Ardenew Spring, Longwood
Meath
1009
St. Margaret's Well, Tobar Mairaide
Dublin
1010
Spring on Cash's Farm, Clane
Kildare
1011
Ballinalack Spring or Bore
Westmeath
1013
Ballydowd or Hermitage Spring (Not located)
Dublin
1014
Clonee Spring (reported in March, 1980)
Meath
2001
Spa Well, Mallow
Cork
2002
Mart Borehole, Mallow
Cork
2003
st. Bridget's Well, Newcastle West
Limerick
2004
Tobar Mackann, Hurler's Cross
Clare
TABLE 1.
Seventeen warm springs located in Leinster and Munster,
to which reference is made in the text.
- 4 -
1-3.
Terrestial Heatflow in Ireland
The heat produced in the crust of the earth by decay of radioactive elements was studied by Joly (1857-1933) and considered by him to be
a prime factor in causing the major orogenies; he summarised his findings
in the 1924 Halley Lecture at Oxford.
Joly's interest had been stimulated
by the radio-activity of the Leinster granite, but the subject of
terrestial heat (from depth or crust-generated) was not pursued in Ireland.
At the end of 1979, Prof. A. Brock prepared a comprehensive report
on "Geothermal Energy in Ireland" for the National Board for Science and
Technology.
In Chapter 6 he deals with the "Geothermal Data in Ireland". While
this comprehensive review tends to emphasise data existing or inferrable on
the "hot dry rocks", it covers the whole position, including the low-enthalpy
wet rocks.
With Dr. Brock's permission, the data relevant to wet rocks is
summarised here.
From 18 heat flow values from deep boreholes in the
Carboniferous Limestone determined by ~tr. Wheildon (unpublished and
-2
uncorrected for climatical factors) heat flow under Ireland averages 60 mWm ,
with rather higher values in the north.
From data on 7 boreholes analysed
by Cermak & Ryback (1979), the average heatflow value was calculated at
64.6 mwm- 2 , with a value as high as 81 mWm- 2 for the Portmore borehole in
the extreme north.
Dr. Brock then examines the geophysical evidence, in
particular gravity lows which might indicate buried granites, including
possibilities at Navan and Drogheda(Leinster warm springs) of small granitic
bodies, and a much larger buried granite under Kerry (as Howard, 1975).
Dr. Brock briefly reviews possible conditions favourable for useable heat in
wet rocks, identifying two possible areas - the Limerick/Shannon area with
thick Namurian shale cover over Dinantian limestones, and the Munster Basin
of Old Red Sandstone cover over a possible buried granite.
The terrestial heat flow in Ireland is therefore very close to the
world average, which is reported at 65 mwm- 2 ; the average thermal gradient
is 25 0 C/kilometer depth.
Useful energy is extracted in areas of above
normal heat flow and temperature gradient.
However, warm springs do exist
in Ireland, and seem to indicate that there are at least limited zones of
increased heat flow, with convection flow of warm groundwater assisting the
primary conduction heat transfer through 'the rocks.
So, the investigation
of the warm springs of Ireland is now proceeding, and the position reached
by Spring, 1980 is reported here.
II
- WARM SPRINGS OF IRELAND
Based on old records, and following field data collection from 1971,
a first report on Irish warm springs formed part of a paper "Groundwater
Investigations in Ireland" read at the "Hydrology in Ireland" meeting of May,
1979 (Aldwell and Burdon, 1979).
In this present paper, all available data
has been collected and presented under the headings of location, geological
and geophysical settings, and hydrological, thermal and hydrochemical
characteristics.
The twelve main inferrences drawn from the data conclude
this item.
/
...
- 5 -
II-l. Location
The general location of the 17 warm springs listed in Table 1 are
shown on Fig. 1, while the main Leinster group (Nos. 1001 to 1010) are
located in more detail on Fig. 2.
There are rumours of other warm springs
which need to be investigated; it is anticipated that the planned
geothermal energy investigation will reveal more such springs.
The Leinster springs tend to occur along the Meath/Kildare border,
as in Fig. 2.
The country is open, slightly rolling, with varying depths
of persistant Quaternary cover.
A noticable feature is the fact that
several of the springs (as Nos. 1002, 1007, 1008) occur on high ground,
small rises or mounds.
This is particularly remarkable in the case of
Bride's River Kilcock (No. 1002) from which the stream splits and flows in
two directions.
At least two of the Leinster springs were brought into
existance by man's activities.
Louisa Bridge spring, Leixlip (No. 1001)
started to flow or to be identified when the Royal Canal was being constructed
in 1794; it was quickly developed as a spa (Maxwell, 1974).
Enfield
spring became known in the 1890s, when gravel was being excavated for
railway line construction (Davies & Hill, ]965)..
The other springs appear
to be long established; that at St. Germans (1007) was examined by geologist
Du Noyer on 21 July, 1859, and his comments are written on the back of the
6" sheet.
Signi!icance is attach.ed to the fact that some of th.e warm springs
issue from high ground, and to the fact that two of the larger were brought
into existance by man's activities.
Another point of possible significance
for the Leinster warm springs shown on Fig.2 is their distance apart.
When plotted, their rather uniform distance apart was noticed, and when
scaled appeared to be of the order of 8 kilometres.
Circles of 8 km
diameter have accordingly been drawn ~round the 11 warm springs on Fig. 2.
Could the enclosed areas (about 50 km ) be those contributing to each
individual warm spring?
Are additional warm springs more likely to be
found in such gaps as those between Nos. 1009 and 1001, and between Nos.
1010 and 1006?
Of the Munster group of springs, those at Mallow are by far the bestknown and unique in their occurrence.
Rutty (1757) gives much information.
Dalton (1889) gives several references, including Jephson (1834) who states
0
0
that in autumn/winter 1833, temperatures ranged from 67 F (19.33 C) to
.
o
o
71.3/16 F (2l.77 C) at the main Spa, while a spring 100 yards to the north
~ow Lady's Well) was lOF warmer (max. 22.33 0 C).
Mallow warm springs occur
in a deep glen, on the edge of the town, and close to the Blackwater river.
There is a Spa House.
In the past decade, two boreholes drilled nearby
encountered warm water;
the one at the Mart (No. 2002) is in use, the other
is closed and the warmth of its water has been queried.
The two other
known Munster springs, at Newcastle West (No. 2003) and at Hurler's Cross
(No. 1004) occur in open rolling country, with no special topographic
expression, though No. 2003 is near an old rath and on slightly higher ground.
/
...
- 6 -
II-2.
Geological Setting
The known warm springs all issue from Dinantian limestones. For
the main Leinster group of 11, Fig. 2 shows that they emerge mainly from
Visean limestones, except the small spring (No. 1010) at Cash's farm
issuing from the underlying Tournaisian.
This applies to the warm springs
in Co. Limerick and in Co. Clare.
The Mallow springs and boreholes are
also from the limestone, but are associated with a major thrust zone.
Fig. 2 brings together such geological data as is available from
the main area of warm springs in Leinster.
Most of the area is covered by
Quaternary deposits, of varying thickness and nature; rock exposures are
poor.
The main data comes from MacDermot & Sevastopulo (1972), and
Sheridan (1972), while Turner (1952), Nevill (1957), Bruck (1971) and
Williams & McArdle (1978) have also been studied and relevant data extracted.
Data on ~rtiary sills/dykes comes from Sheridan (1972), Selwyn Turner et al
(1972) and Byrne et al (1971).
The region of the warm springs lies between the granite, metamorphics
and Silurian of the Leinster Caledonian granite massif on its south-east, and
the Ordovician-Silurian Balbriggan and Louth blocks on its north.
Here,
conglomerates, sandstones and siltstones of desertic and fluvio-esturine
environments were laid down in Devonian and into Lower Carboniferous times,
as the Tournaisianseas gradually transgressed from the south and south-west,
as Clayton & Higgs (1979).
It could have been well into Lower Tournaisian
time before marine limestones began to interdigitate with coastal sandstones
under the area of Fig. 2.
By the Upper Tournaisian, shelf limestones with
isolated Waulsortian reef mounds were forming under the main portion of
Fig. 2. The Waulsortian mounds were more frequent and much thicker to the
west and away to the southwest, to the areas of warm springs Nos. 2003 and
2004; this is shown clearly in Figs. 1 and 2 of MacDermot & Sevastopulo
(1972) .
There was a break at the end of the Tournaisian, with some uplift
and faulting, but marked also by increased marine transgression, as onto the
edges of the Leinster granite massif.
In the region of the Leinster warm
springs, there was a sharp change to deposition of argillaceous limestones
of basinal facies;
the 800 or so metres of the Rathmolyon Shale Formation
(with pyrite) cut in the Trim NO. 1 boreholes is dated as "Cl or younger,
ranging up to C2Sl" (Sheridan, 1972, p. 330), and so of Lower Visean age.
These shales were followed by limestones, in part with reefs of Visean
type and towards the top of the Visean with bedded limestones, argillaceous,
with some chert and silica.
These limestones are succeeded by Namurian
strata, often found occurring as perched synclines, as at Summerhill (Nevill
1957) .
The total thickness of the Tournaisian + Visean is of the order of
2,220 metres (Sheridan, 1972) plus more than 760 m of overlying Namurian
(Nevill, 1957).
The thickness of the underlying Devonian is unknown for the
area of Fig. 2.
The detailed information obtained from the Trim No.1 borehole is
very relevant to the study of the rocks from which the warm springs issue.
The lithology/thicknesses are summarised in Table 2.
The fact that the
boreholes intersected some 4.5 metres of an olivine dolerite dyke or sill
at from 740.7 to 745.2 metres below surface is also of importance when
considering local sources of heat within the sedimentary and underlying
sequences.
/
...
- 7 -
Lithology
Thickness (m)
+ 335 m
Agher Formation, (P l - 2 ).
Bedded limestone, dark grey,
with shales; agrillaceous, pyritic, with some chert
and silica.
+ 183 m
Bray Hill Formation, exposed above the collar of the
borehole; mainly reef micrites
Collar of Borehole at 79,03 (259.3');
+ 4.3 m.
l.8m
Superficial deposits;
measurements from Rotary Table @
Quaternary
Bray Hill Formation.
A reef complex, with two major
reef successions, separated by thin transition zones.
401 m
37 m
Rathmolyon Limestone Formation.
Argillaceous
limestones, grading upwards into the Bray Hill Formation,
but with a sharp basal junction.
Mainly oosparites
193 m
Rathmolyon Oolite Formation.
806 m to 770 m
Rathmolyon Shale Formation (Cl to C2Sl) .
Shales, dark
grey, platy, indurated, calcareous; crinoid debris
common, pyritic and siliceous; with some limestone
zones, especially in the middle of the succession.
(4.5 m)
Olivine-Dolerite Dyke/Sill.
Intruded into upper part
of the Rathmolyon Shale Formation.
Potassium-argon
age dating gave 65 (+ 11) million years, or PalaeoceneEocene on the New Holmes Scale.
330 m to 300 m
TABLE 2.
~thmolyon
Basal Clastic Formation, (Z2-Cl).
The
formation commences at the first sandstone bed
encountered, though beneath it there are limestones
and shales.
It ends in sandstones and siltstones. It
is noted that "By analogy with other boreholes in
Ireland, it is suspected that a continental facies of
Upper Old Red Sandstone type is present at no great
depth belmv the bottom of Trim No. 1 Well"
Summary of lithological data from Trim No. 1 Well (sunk, 1962)
as affecting the main group of Leinster Warm Springs (from
Sheridan, 1972).
-
8 -
The presence of sills/dykes of intrusive basalt has been reported
from at least three places within or bordering the region of these warm
springs.
The earliest report is by Cruise (1871), an 18" wide dyke in a
road-cutting about l~ miles WNW of Duleek, Co. Meath, as noted by Selwyn
Turner et al (1972), when reporting on Tertiary igneous activity at Navan,
Co. Meath.-- They found that a sill at the Tara mine was some 51 million
years old by K-Ar dating, and so "not later than Lower Eocene, not earlier
than Middle Eocene".
In describing the occurrences, Byrne et al (1971)
note "The basalt sills transgress the Carboniferous series, independent of
structures, and are considered to be Tertiary ..• at Tara's Navan mine, there
are generally two such sills, varying in thickness from one to ten feet".
The third recorded occurrence is that in the Trim No. 1 Well, where the
4.5 metre thick sill or inclined dyke of olivine basalt was dated as 65
(± 11) million years.
It is accepted that these sills/dykes are related to the Antrim
Basalts,whose K-Ar determinations indicate a Palaeocene-Eocene age some 65
million years BP.
According to Miller & Brown (1963) the Upper Lava Series
of the Antrim basalts have an K-Ar age of 74 million years;
later they
indicated that the presence of zeolites in basalt tend to give K-Ar ages
which are too great (Miller & Fitch, 1964).
However, on palynological
grounds, the Antrim basalts are considered to be of Miocene or even
Pliocene age, and so much younger (Sabine & Watson, 1965, p. 506).
The
matter is of importance here, as late Tertiary intrusions (say 15 to 20
million years ago) might have initiated some thermal groundwater
circulations which could have persisted, after the initiating heat source
had diminished or ceased to act.
The rocks underlying the w~rm springs of Leinster have been gently
folded and deformed during the Hercynian orogeny.
The folds tend north-east
to south-west, rather than the prime east-west direction seen in the south
of Ireland.
They appear to have been reoriented by the resistant blocks in
their neighbourhood, or to follow deeper Caledonian structural trends.
On Fig. 2, three anticlinal and two synclinal axial traces are indicated;
the two of the north-west are based on Fig. 1 of Sheridan (1972); the
other three are based on the form of the Tournaisian/Visean contact.
While
detailed and more precise mapping of the springs and some of the structures
is required, i t may be noted here that:
(i) Springs 1009, 1001, 1004, 1010
and 1014 lie along the synclinal structure inferred for the two Tournaisian
outcrops: and (ii) The warmest springs are not far from the axial plane of
the Summerhill syncline (No. 1005) and from the the Rathmolyon anticline
(No. 1007).
The rocks were thus folded and deformed long before the intrusion
of the Tertiary dyke/sill intersected 740.7 to 745.2 metres below surface
in the Trim No.1 Well.
Thus, the intrusion will cross-cut the bedding
whether it be a sill or a dyke (vertical or inclined) .
Such an intrusion
may be expected to swell, and form a larger body of magma, where the plane
of intrusion intersects a shattered (as anticlinal crests) or open (as
palaeokarstic limestone) volume in the intruded rocks.
Such large volumes
of intrused lava could have provided areas favourable for heat storage and
the initiation of thermally-controlled groundwater circulation in the
enclosing rocks; how long such a mechanism could continue would depend on
the size, initial temperature and rate of cooling of the intrusive body.
/
...
- 9 -
Boreholes drilled for water, particularly in the western portion
of Fig. 2, often encountered a black shale or mud, which is very fine and
can be separated from the water obtained only by long sedimentation. The
material could be weathered shale, or residual from limestone weathered
under peri-glacial conditions.
In the south of Ireland, the geological setting for the two warm
springs (No. 2003 at Newcastle West and No. 2004 at Hurler's Cross) is not
dissimilar to that of the main group of Leinster warm springs.
As yet,
however no start has been made on relating information on the geology of
these areas to the occurrence of these two warm springs.
The Mallow warm spa, and the boreholes near it, appear to be in
an unique geological setting for Ireland.
The waters issue from the
Lower Carboniferous limestone where i t was strongly compressed and deeply
downfolded in the Hercynian orogeny.
The thickness of the Dinantian
limestones is about 1,700 metres thick tas determined by Hudson & Philcox
(1965) some distance to the north.
The limestone has been eroded from
the anticlinal hills to the south, but is preserved in the synclines.
At
Mallow, the intensity of the folding was such that i t was relieved by
thrusting; a major thrust can be traced westwards from Mallow to the sea
at the head of Dingle bay.
There is also much faulting at Mallow.
The
depths to which the limestones have been down folded and downthrust at Mallow
are uncertain.
But the depth of burial and the possibilities of deep
groundwater circulation in such limestones must be major factors in
bringing this warm spring into existance.
11-3.
Geophysical Setting
As yet, the existing geophysical data has not been fully examined
and used to assist in understanding the factors controlling the location
and functioning of the warm springs of Ireland.
Under the Leinster warm springs area, i t is considered that the
suture which is postulated as marking the closing of the Iapetus ocean
extends from Salterstown through Navan and to the Slieve Bloom mountains,
(as Fig. 8, Phillips et aI, 1976).
Thus the region was in Caledonian
times and into the Devonian/Lower Carboniferous, a place where the edges
of two plates were in collision and subduction.
As such, it was a
probable place for thermal circulation of oceanic brines producing ore
deposits in the accumulating Carboniferous sediments, (Russell, )978) as in
Fig. 6 here.
However, there could be little in such conditions to
affect present day thermal activity, though some relationship cannot
entirely be discarded.
Gravity investigations (as Murphy, 1962, 1974) have revealed a
number of gravity lows of medium size (as under Navan) as well as groups
of lows of very small extent.
It seems probably that the medium sized
lows are not due to buried granites, but to a greater thickness of sediments
than originally estimated.
The very small gravity anomalies (Murphy, 1962)
occur rather to the west of the main area of Leinster warm springs, but
some could occur in the actual spring area.
These lows show a marked NE-SW
elongation, and could be solution caves in the Carboniferous limestone
located or initiated by faulting which has opened up the limestone to
circulating groundwater.
But as such, the water-filled (or gravel-filled)
shallow cavities would not necessarily have any connection with the
circulation of warm groundwater.
It may be noted that the main Iapetus
suture is supported by a fault postulated on lithostratigraphical grounds
/
- 10 -
Cas Harper & Brenchley, 1972).
'I'his has also been associated with the
lines of small gravity anomalies, and there could be some reactivation of
such a major fault in later times.
It could prove to be a zone where
abnormally deep groundwater circulation occurs.
With regard to Mallow, and the other warm springs of the south,
a major gravity low under Kerry was first considered as due to a great
thickness of Devonian-Lower Carboniferous sediments.
Later, it was
indicated that a major buried granite (Armorican ?) was more likely to
have produced the large gravity low, (Howard, 1975).
Such a deep granite,
with a thick cover of low-conductivity rocks, might possibly have some
genetic relationship to the warm springs of Munster.
Brock (1979) has
drawn attention to the thickness of sediments, and the low conductivity
of the Namurian cover, in the Shannon basin; this may have some influence
on the small warm springs of Cos. Limerick (No. 2003) and Clare (No. 2004).
The limited work done on geothermal gradients and heat flow in
Ireland was noted under Item 1-3 of this paper.
In general, geophysical
investigations of very many types have been used in Europe over the past
decade.
They have proved very useful in elucidating the characteristics
of already-located geotherw~l fields and assisting in their development
and management, as reported in the many papers presented to the First
(December, 1977) and Second (March, 1980) Seminars on Geothermal Energy of
the commission of the European Communities.
Indirectly, they can assist
in prospecting for geothermal anomalies, but such investigations have to
be specifically oriented to obtain such information.
It does not appear
that they have been very successful in finding new geothermal fields.
11-4.
Hydrological Characteristics
No systematic gaugings have been made of the yields of the Irish
warm springs.
However, it is certain that most, probably all, show strong
seasonal variations in discharge, with maximum flows in spring and into
summer, while many of them go dry, or have negligible flows, in the autumn and
into winter.
Minimum flow, or middle point of dryness, can be SeptemberOctober.
None of the springs have definitely large flows; maxima for
the bigger springs would be in the range of 5 to 10 litres per second;
if the springs were developed, this might be increased slightly under
gravity discharge.
Usually, spring discharge is in the 1 to 4 lit/sec.
range, and some are wells with little obvious discharge.
The spring regime is thus the normal one for groundwater
recharged during the winter months, and discharging this water some four
to eight months later.
No doubt, careful gaugings will permit closer
correlation wie1 precipitation-infiltration, while recession curves can be
constructed and analysed, permitting some deductions as to the hydraulic
characteristics of the aquifer.
11-5.
Thermal Characteristics
In general, temperature observations on the waters of the warm
springs have not been made in a systematic manner, or extended over a long
range of time.
However, over the past nine years, temperatures of the
Leinster group of springs have been measured throughout the years by one of
the authors.
In November-December, 1979 temperatures were measured with the
same standardized instrument and in the same manner for almost all of the
Irish warm springs; these temperatures form part of Tables 3 and 4.
I
I
I
I
J
.J
J.
S
0
--
o
.N
--~--f -22C:
1
. - 0
Is=.
o
·21-
·0
.19_
l.
.
0
-18-
.:
f - - - i -l
0
4=
--
13
FIG.
3.
0
Annual Temperature Variations for Enfield (No.1005)
and for St. Gorman1s (No.I007) Springs for years 1976 and 1978.
-
11 -
It might be possible to go back over old records and the limited
publications, to see if there have been significant changes in temperature
over tQe past 200 years or so.
But the only old reliable data appears to
lie is that given by Rutty for the period about 1757.
He notes Mallow spa
0
0
0
water as issuing at 68 F (20 C), as compared with 20.7 e measured on 15 Nov.
1979 (Table 36.
Rutty also notes that st. Margarets (NO. 1009)
0
o
registered 51 F (lO.SoC) or 5S F C12.8 C) on 20 July, 1752; this would
0
compare with 17.2 C on the morning of 7 December 1979.
In a study of the
biology of the Enfield spring, Fahy (1975) gave some temperature figures,
based apparently on a recording thermometer placed in the spring; he
0
0
reports in his Table I a range from 6 to 22.5 C.
He also states that the
spring has a day-degree quota of 6,606, as against 3,835 for an Irish
isothermic spring.
Fahy (1975, p. 112) mentions but three thermal springs
recorded in Ireland, and introduces the supposedly warm spring at Dunlavin
in Co. Wicklow, with the footnote "The area was visited by me; the spring
was not located".
From the periodic temperature measurements made on the main
Leinster group of springs (Fig. 2), those for the Enfield (NO. 1005) and
st. Gormans (No. 1007) have been selected for presentation here, Fig. 3.
During the eight years to end-1978, Enfield water temperature was recorded
62 times and that of St. Gormans 64 times.
No self-registering or min-max
thermometers were used.
Periods when the springs had dried-up or become
stagnant were noted; such low/nil flows tended to occur from August to
November.
Figure 3 shows water temperature for Enfield and for st.
Gormans over the calander years 1976 and 1978.
These were years for which
good records were available, and which come after the 1974-76 drought.
The plottings show how temperatures reach a peak in the spring,
and then fall quite sharply from May'to October, to rise again during the
winter.
The discharges also increase in spring and fall in autumn.
So,
as flow increases, so does temperature; as flow decreases, so do temperatures
fall.
It is considered that this relationship is of considerable
significance.
The source of the heat which warms the water must be much
greater than the amount of heat taken away by the water.
It suggests that
the larger the volume of water moving underground, the more it is brought
into contact with zones of more heat or higher temperatures.
This may
indicate a deeper circulation in the ground when winter infiltration has
recharged the aquifers and the water is moving strongly towards its
discharge point.
The degree-days for the two springs have also been calculated,
due to irregularity of measurement and that fact that st. Gormans was dry
for part of the year, degree-days are only approximate.
Enfield showed
o
0
7820 degree-days (21.3S C average) in 1976 and 7380 degree-days (20.22 C
0
average) in 1978. St. Gormans showed_6230 degree-days in 1976 (17.06 e
0
average) and 6790 degree-days (18.61 C average) in 1978.
The differences
in average and from year to year may have significance, but certainly
need to be related to volumes of water discharged if the data is to be
used as an analytical tool.
/
...
- 12 -
i
Spring No.
Sample NO.
Date, 1979
°c
2001
I
15/11
11.3°
2001
II
15/11
20.7°
2002
III
15/11
17.6°
2003
IV
4/12
13.7°
2003
V
4/12
13 .8°
1009
VI
7/12
17.2°
1001
VII
7/12
16.2°
1002
VIII
7/12
13 .2°
1003
IX
7/12
14.2°
1005
X
7/12
23.3°
1005
XI
7/12
23.3°
1007
XII
7/12
20.2°
1008
XIII
7/12
13.0°
1006
XIV
7/12
13.8°
1004
Ca mgE/lit
Mg
K
4.72
2.64
0.31
0.10
3.60
1.28
0.50
0.03
4.20
1.04
0.47
0.05
3.44
1.44
0.60
0.03
3.40
1.32
0.59
0.03
6.84
0.88
0.56
0.06
5.92
2.72
9.09
0.23
4.60
2.04
0.43
0.03
5.60
0.76
0.39
0.03
3.92
2.16
1.13
0.05
3.84
2.08
1.13
0.05
4.80
1.20
0.41
0.03
6.56
0.64
0.31
0.03
5.76
1.40
0.48
0.07
4.64
2.40
2.61
0.08
Sum Cations
HC03 mgE/ li t
Cl
"
N03
"
S04
"
7.77
4.96
0.68
0.21
1. 92
5.41
4.16
0.62
0.17
0.46
5.76
4.56
0.62
0.31
0.27
5.51
4.72
0.59
0.36
5.34
4.64
0.59
0.24
8.34
5.88
1.10
0.26
l.10
17.96
5.40
12.57
0.10
7.10
6.44
0.48
0.24
6.78
6.24
0.48
0.24
7.26
5.84
1.27
0.19
7.10
5.92
1.33
0.20
-
-
-
7.54
6.64
0.61
0.04
0.25
7.71
6.48
1.25
0.04
-
6.44
5.76
0.49
0.06
0.15
-
9.73
5.60
4.00
0.03
0.12
Sum Anions
7.77
5.41
5.76
5.67
5.47
8.34
18.07
7.16
6.96
7.30
7.45
6.46
7.54
7.77
9.75
236
132
7
4
302
24
13
92
180
64
12
1
214
22
10
22
210
52
11
2
278
22
19
13
172
72
14
1
282
21
22
170
66
14
1
282
21
15
296
136
209
9
329
446
6
230
102
10
1
393
17
15
280
38
9
1
381
17
15
196
108
26
2
316
45
12
192
104
26
2
341
47
12
-
-
342
44
13
3
359
39
16
53
232
120
60
3
342
142
2
-
-
-
328
32
7
1
405
22
2
12
288
70
11
3
395
45
2
-
240
60
9
1
354
18
4
7
-
-
810
3.0
0.02
0.29
370
248
368
7.40
565
2.4
0.02
0.02
286
2208
244
7.52
607
4.4
0.02
0.02
334
228
262
7.41
589
5.0
0.02
0.02
296
236
244
7.60
869
3.6
0.01
0.02
528
296
386
7.23
431
1.4
0.23
0.01
1160
296
432
7.28
768
3.4
0.02
0.07
370
322
332
7.24
741
3.4
0.03
0.06
380
312
318
7.26
745
2.6
0.04
0.01
386
292
304
7.31
744
2.8
0.01
0.01
404
296
296
7.35
690
0.8
0.01
0.01
412
288
300
7.26
809
0.6
0.03
0.02
440
332
360
7.11
814
0.6
0.01
0.01
480
324
358
7.19
907
0.45
0.15
0.01
636
280
352
7.32
Ma
Ca mg/lit
Mg
"
Na
"
K
"
HC0 mg/lit
3
Cl
"
11
N03
11
S04
Sum of above
Nitrate (as N)
Free/Sol.Am. (N)
Albuminoid (N)
TDS (ppm)
Total Alkali
Total Hardness
pH
TABLE 3.
-
-
569
3.4
0.01
0.01
292
232
236
7.84
-
-
Temperatures and main chemical constituents of the waters of the Irish warm springs, based on samples taken
on 15 November and 4 and 7 December, 1979.
Analyses made at State Laboratory, College of Science, Dublin.
XV
7/12
12.2(
-----
Dol.
t;"
2001
2002
2003
p.p.m.
~H)l22-
15.11.79
15.11. 79
4.12.79
7.12.79
7.12.79
7.12.79
7.12.79
7.12.79
7.12.79
7.12.79
7.12.79
7.12.79
Noh:
' ..
1001
1002
1003
1004
1005
1JJ_O~
10Ql
J..QQB
565
607
569
1431
768
7~1
907
745
814
680
~---------~
Rr mod.•
u
20.7 C;
u
17.6 C;
13.:guC;a
16·.2 C:
u
2 •
Mallow 5Q~ )
l'lill.o.ML.BtL- )
Newcastle \,/ )
-t.. OU1 "o!l' A d rl
i;:.~Ef~~
~. ~ _.~
~o
Three from Munster (1'1)
i/p.r
1%1
12.2 Cj Ce1bridg~
23. 3:[; EofieJ...d~
1 3 • 8 C.L____12Y 5 aLi
20.2: C..;--S.t.cGOIJUan!....:
_JL~_ J..:t;J)oW~.d.eJl£W~
A6q
.lL2-=-.c.;--..SL_"J'1ar.-gaI.£J
..
-
tlumbrlS 01. in 0«01(10'1(. with wolrr - point
numbtl~ ;n th. study 0'.0.
12
0
20°C ..••••••••••
_
o
+ 20 C ...•.•. _ ••..•••
0
Temperature range in
December,
FIG.4.
1979."
Triangular Diagram Plotting of Chemical Composition of the
waters of Iris~ Warm Springs. I~os. 1001, 1004 and 1009 form a group
apart.
- FIGURE 5
-
TRACE ELEMENTS SILICON AND IODINE PLOTTED AGAINST WATER TEMPERATURES
25°
25°
24°
....5c:r
23° ~
I .
0
220
a:
L..
210
w
- a:
200
....c:r
::J
.
w
....
L9
a
L1
cI. M2
I
.I
15°
J
°
14
I
o'
15°
pLS
140
.
130 mg!li tre
2
3
.I
PM3
.
I
4
/
/
OL3
/
~30~6
\.
......
./
/ 16°
0L1
/
OM3
L4
. . . . . . . . . -0- -- -- -:-
o I'll
1
/
/
/
/
I I
ISO
/
/
5
19°
/
o?s '",
.
64/
M1
/
9'L9
L2
;'
20°
-/
\
\
\
."
OLS
~1°
/
I
I
I
0
22°
/
/
10L?
0/
./
9'
/
oj
/
l'12
°t
i
.0
i
I
'
0
/
/
/
L3.1
-/
L6
(/
.OQ2
M3
I
Silicon in /
1
./
.I
.I
./
.I
/
/
./
I-
/
/-
Ml
L5
o
/
./
.I
./
/
./
.I
.J
./
O.1rt1
.I .'.1
19° ~
a
./
'OL?
a:
w
-ieo
./
./
./
/
0
/
.
./
.I
t.:l
OLS'
OL5
./
.
::J
/
I
./
:3
"
/"
./
/
/
/
.
/
/
-)
/
15°
-.
.. :-13°
00
.
I
3
I
4
I
T
6
I
7
I
l
1
I
12°
1
I
Iodine in~g/litr8
2
I
14°
o
8
I
9
I
_:1
11
12
I
(
,I
i
I
j
I
I,
! I
I !
,
r
- 13 -
11-6.
Hydrochemical Characteristics
Investigators of one or other of the warm springs have in the past
taken samples and had chemical analyses made.
In this respect, the report
of Plunkett & Studdert (1883) on the Mallow Spa spring is of interest;
Fahy (1974, 1975) also had some chemical analyses made on the Enfield (1005)
spring.
But it was only towards the end of 1979 that there was a
systematic sampling of almost all of the known springs, with the analyses
carried out under uniform conditions at the State Laboratory in the College
of Science.
The results of these analyses are presented here in Table 3
and 4.
The results are strictly comparable, though the absolute values
of some of the trace elements (not normally determined in water in this
laboratory) are less certain.
The percentage amounts of the major ions are
plotted on the triangular diagram of Fig. 4, while the trace elements si
and I are plotted against temperatures on Fig. 5.
(i)
Major Anions and Cations.
In general, the major ions
indicate rather normal groundwater issuing from these springs.
TDS tend
to lie in the range of 280 to 480 ppm, which is normal for Irish groundwaters
from limestone aquifers.
Much of the chlorine is in the 15 to 30 mg/lit
range, normal for such waters and reflecting the rather high Cl content of
Irish groundwaters (as Burdon & Cullen, 1980).
However, it is clear that
there are three springs in Leinster which have definitely abnormal chemical
characteristics and this is also well-marked on Fig. 4.
They are:
NO. 1001 (Louisa Bridge) with Cl = 446 and TDS = 1160; No. 1004 (st. Patrick's
Well) with Cl = 142 and TDS = 636; and No. 1009 (St. Margarets) with Cl = 39
and TDS = 582.
From Fig. 2, i t can be seen that these three springs have
already been grouped together as lying "along the synclinal structure
inferred from two Tournaisian outcrops" (Item 11-2. (i».
It is unfortunate
that No. 1010 was not sampled in December, 1979, and that the existence
of No. 1014 (Clonee) was reported only when this study had almost been
completed.
Repeated chemical analyses from these five springs (1009, 1014, JOO]
1004 and 1010) will be awaited with interest.
It will be noted that the other high Cl values (45 and 47 mg/lit)
come from springs Nos. 1005 and 1006, which are close together, but which
differ much in temperature.
In fact, all efforts to find a direct relationship between the
major anions and cations in solution and the temperatures of the springs
proved unavailing.
It may be said that the main chemical composition of
the Irish warm springs is not related to the temperature of these springs.
The most mineralized (No. 1001, Louisa Bridge) had a temperature of l6.2 o C
on 7.12.79; the hottest (No. 1005, Enfield) has but some 400 ppm of TDS,
0
but a temperature of 23.3 C on 7.12.79.
The coolest is also rather highly
mineralized.
(ii)
Trace Elements.
The trace elements determined were Si, I,
F, B, Se and Ba, as in Table 4.
In addition total Fe and dissolved Fe were
determined, though it seemed improbable that they would yield diagnostic
information.
The accuracy of some of the determinations is doubtful, since
such elements are not normally determined in water analyses.
It can be seen
that they are erratic.
Efforts were made to relate them to temperature.
In the cases of Si and I, there was some limited success, as shown on Fig. 5.
- 14 -
Spring No.
Sample No.
Date, 1979
2001
2001
2002
2003
I
II
III
IV
15/11
17.6
4/12
13.7
15/11
11.3
°c
pH
TDS ppm
EC ~mhos)
Total Fe (mg/li t)
Dissolved
mgsi/lit
If/g/lit
F flg/li t
B /f.g/lit
SeJtg/lit
Ba
Ca: Mg Ratio
TABLE 4.
7.40
370
570
2.90
15/11
20.7
7.52
286
7.60
296
42
7.41
334
525
0.25
0.05
3.3
1.0
100
28
Nil
Neg
Nil
Neg
Nil
Neg
Nil
1. 79
2.81
4.04
2.40
Nil
3.2
0.9
100
73
0.75
0.20
0.10
5.2
1.5
-
0.30
0.03
3.8
3.0
63
2003
V
4/12
13 .8
7.84
292
510
0.35
0.10
4.2
6.0
160
23
0.75
1009
1001
1002
1003
VI
VII
VIII
IX
7/12
17.2
7/12
16.2
7/12
13 .2
7/12
14.2
7.23
528
830
0.25
7.28
1160
1970
1.90
Nil
Nil
0.9
6.0
140
50
0.9
9.8
440
60
Nil
Neg
Nil
2.57
7.77
Neg'
Nil
2.18
1005
X
7/12
23.3
1005
1007
1008
1006
XI
XII
XIII
XIV
7/12
23.0
7/12
20.2
7/12
13 .0
7/12
13 .8
7.24
370
665
1.00
0.05
3.4
1.3
140
53
7.26
380
659
1.00
0.05
3.2
2.5
120
30
7.31
386
715
0.80
0.05
5.4
10.9
280
60
7.35
404
708
0.35
0.05
5.7
12.0
290
92
7.26
412
645
0.20
0.15
3.9
3.0
220
78
7.11
440
751
0.20
0.15
2.6
1.1
100
153
7.19
480
765
0.20
0.20
3.6
3.9
200
182
Neg
Nil
Neg
Nil
Neg
Nil
Neg
Nil
Neg
Nil
Neg
Nil
Neg
Nil
2.25
7.37
1.81
1.85
4.00
10.25
4.11
I
Trace elements found in the waters of Irish warm springs, based on samples taken on 15 November
and on 4 and 7 December, 1979.
Analyses made at the State Laboratory, College of Science, Dublin.
1004
XV
7/12
12.2
.- 7.32
636
1070
0.35
0.15
3.7
4.3
270
52
Neg
Nil
1. 93
•
- 15 -
They merit only very brief discussion.
For Silicon, it will be noted that
1009 and 1001 lie well away from the main envelope showing Si increasing
as temperature increases.; these are springs noticably high in Cl and TDS.
The third spring of this little group, No. 1004, lies well within the
envelope.
It is generally reported that the si of thermal waters is
affected by the TDS of such waters.
For Iodine, there is only a very slight
tendancy for I to increase as temperature increases.
The points most off
the line of Fig. 5, are 2001 and 2002 (Mallow Spa), 1007 (st. Germans) and
1001 (Louisa Bridge) .
While it could be said that the I of the first three
of these was lower than might be expected, and that the I of No. 1001 is
higher than might be expected considering their temperatures, the basis for
such an attribution is slight and weak.
Further work on trace elements
is clearly required.
(iii)
Gases.
In some of the springs, as Enfield and Mallow, an
appreciable amount of gas is given off, bubling up through the waters.
Its composition has not been determined, though the earlier work of Plunkett
& Studdert (1883) is of interest.
Generally, the gases are odourless,
though a faint whiff of sulphur or hydrogenated sulphur is sometimes
detected; i t may arise from decomposition of organic matter in the mud of
the spring.
(iv)
11-7.
Isotopes.
As yet, there have been no isotope determinations.
Main Inferences
It is considered of use to present here the 12 main inferences
which have been drawn with regard to these warm springs.
Location
(i)
(ii)
(iii)
Geology
Hydrology
Hydrothermal
Hydrochemical
Some have been made functional by man's activities.
The Leinster springs appear to be 8 km apart.
(iv)
All issue from Dinantian limestones, some 1,500 to
2,500 m thick.
(v)
Some small Tertiary intrusives have been reported
from their vicinity.
(vi)
~eophysics
Several occur on higher ground than might be expected.
(vii)
(viii)
(ix)
(x)
(xi)
(xii)
Their occurrences can often be located with respect
to fold structures.
Certain gravity lows may have some connection with
these warm springs.
Discharges increase in spring, as expected for
meteoric waters under the hydrological cycle.
Temperatures peak in spring, decline from May to Oct;
they are irregular.
Most of the warm springs show no abnormal hydrochemical
characteristics.
High CI and TDS are restricted to three springs occurring
along a synclinal structure.
There are faint indications that Si and I increase with
increasing temperature.
M'ddl~ Slo9~
CrySlol\'n~
bos.m.nt
or ductilp' lower crust
FIGURE 6
10 om
CONVECTION CIRCULATION OF GROUNDWATER IN LOWER CARBONIFEROUS TIMES (after Russell, 1978)
- 16 -
III
ORIGIN AND FUNCTIONING
As yet, the data presented and analysed in the foregoing item
of this paper is too slight to allow of any firm hypothesis to be
advanced as to the origin and functioning of the warm springs of Ireland.
However, some ideas have been formulated and are put forward for
discussion here.
III-I.
Palaeozoic Thermal Springs and Mineralization
Over the past two decades, the ability of thermal springs, often
of brines, to leach and deposit metalliferous elements and form ore
deposits has received renewed attention and confirmation, as in the Red Sea
investigations.
Such an origin has been postulated for the Irish base
metal deposits found and exploited in the Lower Carboniferous of Ireland,
as Williams & McArdle (1978) and Russell (J978).
The latter postulated infiltration of sea water of the
transgressive Tournaisian seas into the great thickness of underlying Lower
Palaeozoic sediments. In these the waters circulated under thermal forces,
leaching out lead, zinc, copper and sulphur, and deposited them in the
limestone sediments forming on the bed of the sea.
Russell's diagram
representing these events is reproduced here as Fig. 6.
Since then, much heat has flowed out of the crust, and there can
be no possibility of such past thermal circulation of groundwater
influencing the supply of warm water to certain springs in Ireland.
But
it seemed desirable to include this hypothesis of past circulation of warm/
hot brines through the rocks which underlie the Dinantian limestones of
Ireland, and from which the present day warm springs issue.
III~2.
Present-Day Circulation of Warm Groundwater
It is thought that the present day groundwater circulation feeding
warm water to certain springs is due to the effects of a heat source or
sources which has modified the normal groundwater circulation of the region
on the main warm springs of Leinster, and possibly those of Cos. Limerick
and Clare.
Mallow would be an exception, as usual.
Fig. 7 sets out the manner in which hydrotherrr01 cells are
considered to function.
The idea is often applied to low-temperature
circulation of groundwater, as in Facca's (1973) diagram of the basic model
of a low temperature hot water field.
The cells are some 8 kilometres in
diameter.
Main infiltration occurs in lows (from streams ?) while
discharge is on highs.
These are based on location features.
The springs
form part of the hydrological cycle, with winter infiltration resulting in
increased yields (or higher groundwater levels) in spring.
Some of the
springs dry-up in summer/autumn.
This concept should become more definite
as actual discharges are gauged and related to the precipitation and
infiltration.
/
...
FIGURE 7
-
PRESENT~DAY
POSSIBLE
~
"I:,
"
'.
oJ.
~~,
~ It
..
~
"
It
~~
~
,tV"
V'
~ ~
~..
"v
'-'_1,
.AI
I
~
. i- --.L
Downward Flow ofl
Cold Water
•
~
I
~
-:}
~
A
'f
1
permitting free
Groundwater
~
~
~
CircUlati~ ~ ."
~
"-.
.::Jr
¥t '" t
'b -¥ ~
,.,\
.
.
.-)I tv .,
~
4:'"
1
~r
';-:a
.,., .
1\I \"'
+
PorOU5 Formation5
~
V,'.
~~.>;r
;:r
.;:;,
~~
.
~
-'7
""
u
200 m
+J
Gl
co
3
-0
300 m
C
::J
'0
~
1
.
,
400 m
t-'l
~
o
t+-
500 m
1
600 m
700 m
CD
U
til
t;..
H
:J
U1
3
~
Gl
Form of Geother.. ........
mal Gradient
.<
'.
.0
800 m
III
..c
+J
0-
900 m
•
less PoroU5 Formation-at Depth
100 m
1
1
...
::J
U
~
If
)1-,
/f
'\
.
~ ~~ c ~~ ~~;t
~
of Warm Water
+J
.-j
~
,'-1
IJ,
Upward F1 ow~ - _ .......
l'
11
1
"')\
»-ooV
l' \
""co·
~-f\;----""'"
I
\J-i'JIJ"' It
c
o
_"~I
fA'
-/~-~
;f/\
I
I
Warm Spring
I
~f
t.
1-1 ~. if
~.)"::J
~~'_'*1~/. ~
6f
·1-.... ~
,.. ~ ~ Jt
-(,
l' .\A\'\.
1. ..
~
It:
1\
M
.1-
I
I~~
J'
HALF-CELL>,
I
Infilt-!
Warm Spring
ration Zone ~ I'~
~
.........t
~
CELL
I Main
,
'~+-.
~
8 KILOMETER DIAMETER
I
Spring
I
I
.,.......
HALF - CELL
I Warm
I .t
~.
GROUNDWATE8 CIRCULATION FEEDING THE LEINSTER GROUP OF WARM SPRINGS
g
- 17 -
The effects of heat on the circulation of groundwater can be very
striking, in particular when considered from the hydrogeological viewpoint.
By raising temperatures of groundwater, its viscosity will be decreased, so
that its transmissibility will be increased.
Thus, raising temperatures
from lOoe to 20 0 e will increase flow by over 30%, all other conditions
remaining the same.
Heating also decreases the density of the warmed
groundwater, and such density differences reinforce gravity-controlled flows.
As such thermal flow continues, much more heat will be brought up in the
water by convection than by the earlier conduction in the rock and/or
static water.
Rising temperatures can induce thermal shattering of the
rock, and increase permeability.
While such flow mechanisms are
considered here for basins of say 8 kilometres diameter, they have been
studied on a continental scale by one of the authors in the great artesian
basins of North Africa and Arabia (Burdon, 1978).
The depths to which groundwater circulation might extend can be
inferred according to the assumed geothermal gradient and the amount of
0
mixing.
Accepting a geothermal gradient of 25 e/kilometer, and an increase
o
0
0
of water temperature of 15 e (from 10 e to 25 e), then the depth of
circulation below surface would be of the order of 600 metres with no
mixing.
If there is much admixture of cooler (and shallower) groundwater
to the rising warm water, then the depth of circulation could be much greater.
The fact that spring water temperatures rise as flow increases canbe
explained in a number of ways.
strong thermal upflows may push aside other
waters and result in less mixing.
The greater the volume of upflow, the
more heat is brought up, so that water temperatures tend to rise.
The hydrochemistry does not shed too much light on the problem.
The geology indicates that evaporite deposits occur and are to be expected
in the shelf limestones, as in the Upper Tournaisian.
Sulphate nodules
and lenses have been reported, but i t would seem that halite was not
precipitated.
High chlorine and total dissolved solids characterise
three springs (No. 1001, Lousia Bridge;
No. 1004, st. Patrick's well;
and No. 1009, St. Margarets) lying on a NE to SW line and possibly along the
axis of a syncline in the Lower Visean.
There would be reason for
believing that these
springs obtain their
more-then-average mineralised
groundwaters from deeper beds with some evaporites.
The other springs
have chlorine which can in part be due to cyclic chlorine in the
precipitation, though as already noted NOS. 1005 and 1006 also are
comparatively high in chlorine.
While there are no apparent direct
correlationships between temperatures and hydrochemistry, the latter supports
the idea that the groundwater circulation is sufficiently deep in at least
one group of warm springs to have gone down to the evaporite beds thought
to exist in the Upper Tournaisian.
There is little information as to the formation conditions which
allow groundwater to circulate freely down to depths of the order of
700 metres.
The Hercynian folding of the rocks must have opened up flow
paths on anticlines and synclines.
The geological location of several
of the warm springs would suggest connections with anticlinal or synclinal
axes.
The extent and depth of Quaternary karstification in Ireland is
still very uncertain; but karstification below say 100 metres from surface
is not very likely, controlled as i t should be by low sea levels during
and soon after the glacials.
However, if evaporite beds, as gypsum,
anhydrite and halite, form appreciable sequences in the general carbonate
succession, then their solution in circulating groundwater (not controlled
by the availability of free carbon dioxide in the water), could result in
deep karstification.
~uuld
- 18 -
The effects of the varying thicknesses of Quaternary cover also
call for some comment, since at least two of the warm springs became
active only as the result of man's activities.
It would seem that the
boulder clay is effective in holding down such other rising warm
groundwaters as may occur, and dissipating their heat (which is not large)
into the overburden and so to the atmosphere.
Under natural conditions,
springs can break through where there are fluvio-glacial patches of sand
and gravel in the main cover of boulder clay. In such areas, hot
springs could have come into existence without man's intervention.
111-3.
Heat Sources and Heat Conservation
The present review and analysis of existing data have shown that
Tertiary basic intrusives occur in the carbonate formations through which
the warm springs rise and from which they issue.
Dating back probably
to the Palaeocene-Eocene on K-Ar evidence (though possibly younger on
palyneological indications), these basic intrusives are no longer sources
of geothermal heat.
But i t is just possible that they initiated the
present cellar circulation of groundwater in this area of Leinster, and
that once initiated it has continued to function with lesser amounts of
o
heat supplied by the normal geothermal gradient (say 2S C/km) of the area.
Gravity measurements showed lows around Navan and Drogheda.
It was inferred that these indicated the presence of small masses of
buried granite.
Now it seems more probably that they are due to the much
greater thicknesses of Balaeozoic sediments underlying the region.
Buried granites are thus an unlikely source of the heat for the circulation
of these warm springs.
No study has been made of the possible amounts of heat generated
by radio-active decay within the sediments, still less of the basement.
Such would seem to be a very unlikely heat source, and may be put aside for
the present.
Finally, there are possible effects of heat conservation by a
cover of low-conductivity formations.
The 800 or so metres of Rathmolyon
Shale Formation (Lower Visean age) could indeed act as a cap rock to hold
down heat arriving in the underlying Tournaisean limestones.
Limited
opening (as on antclines and synclines, or along faults) might permit
concentrated upflow of heat (water-bo'rne) and provide localised heat
upflows for the waters in the overlying limestones, (1150 metres thick
according to Table 2) .
The effects of the cover of mainly impermeable Quaternary deposits
has already been noted;
it will not affect the deep circulation of the
groundwater.
111-4.
Conclusion
This compilation and analysis of the Jata on the warm springs of
Ireland has shed a little light on hydrogeothermal conditions in some of the
wet rock areas of Ireland.
In particular twelve main inferences have been
drawn regarding the warm springs of Leinster, as in Item 11-7. In turn,
this data has been used to formulate a possible picture of thermally
controlled circulation of groundwater in cells some B kID in diameter and
extending down to say 700 metres below surface, as in Fig. 7.
Almost
- 19 -
nothing can be inferred as to the source of the heat driving this thermal
circulation of warmed groundwater.
IV
ACKNOWLEDGEMENTS AND REFERENCES
The thanks of the authors are due to Dr. C.E. Williams who has
encouraged and facilitated this investigation, and to colleagues in the
Geological Survey and outside who have supplied information on many
aspects of the wor~ not least to the staff of the State Laboratory.
In
the course of preparing the "Geothermal Energy Potential of Ireland" project
the authors have had very fruitful discussions with their colleagues
Professors Peter Br~ck of Cork, Professor Andrew Brock of Galway, as well
as Messers Eamon Kinsella and Keith Robinson of the National Board for
Science and Technology.
And as the paper was being finalized, there was
a useful visit to several of the springs in the company of Drs. K. Louwrier,
R. Haenel and W.M. Edmunds.
The practical field discussions will facilitate
the more efficient collection of further data and the application of
findings to the possible development of a valuable energy resource of the
country.
The debt of the authors to other workers is acknowledged by the
following list of references.
REFERENCES
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BROCK, A.
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"
BRUCK,
P.M. (1971) "The Lower Carboniferous Rocks of the Naas District,
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BURDON, D.J. (1978) "Infiltration Conditions of Major Sandstone Aquifer
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and CULLEN, K. (1980) "Hydrochemistry of Caradocian Volcanics
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(1980)
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- 20 -
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- 21 -
MURPHY, T.(1962) "Some Unusual Low Bouguer Anomalies of Small Extent in
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