Hydrogeothermal Conditions in Éire - Geological Survey of Ireland

IIII1HXDROGEOTHERMAL CONDXTXONSIN EIREbyC.R. Aldwelland D.J. Burdon

HYDROGEOTHERMAL CONDITIONSIN EIREbyC.R. Aldwell* and D.J. Burdon**An original paper prepared for section 14.2Fossil fuels, of the XXVI InternationalGeological Congress, Paris, 1980.Ref. No. 14.0068.SUMMARYHydrogeothermal conditions in Eire have not yet been substantiallyinvestigated, but a project entitled "Geothermal Energy Potential of Ireland"is planned to commence in mid-1980, with.EEC assistance. The data presentedhere, 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. 1and 2. Temperatures of the Leinster Group of warm springs have beencollected since 1971, and most were chemically analysed at end-1979, Tables 3and 4. From a study of the location, geological and geophysical setting andhydrological, thermal and hydrochemical characteristics, a number ofinferences have been reached. These may be summarised here: (i) Severalwarm springs occur on higher ground than might be expected; (ii) Some havebeen made functional only by man's activities; (iii) The Leinster warmsprings appear to be distributed at 8 krn distances apart; (iv) All the warmsprings issue from Dinantian limestones, mainly from the Visean; (v) Somesmall Tertiary intrusives have been reported from their vicinity;(vi) The occurrences can often be related to fold structures, as along asyncline; (vii) Certain gravity lows may have some connection with thesewarm 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 synclinalstructure, but two other springs located there have as yet not been analysed;(xii) There are faint indications that Si and I increase with increasingtemperature.Of the Munster warm springs, those in Cos. Clare and Limerick appearto be similar to those of Leinster. However, the Mallow Spa spring, thebest-known in Ireland, issues where the Dinantian limestones are highlycompressed, downfolded and thrust. Boreholes in the Mallow area adjacentto the spring have located warm water.While the mineralization of the Lower Carboniferous in Ireland has beenattributed to hydrothermal concentration and precipitation of base metals,present-day geothermal circulation bears no relationship to such ancientactivity. However, the groundwater circulation is considered as due to theeffects 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 rechargeand downflow at the edges, as in Fig. 7. Depths of circulation may beof the order of 700 metres. As yet, the source of the heat has not beenidentified, while the problem of heat conservation by impermeable strata orcover 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 properutilisation and management of this natural energy resource of the country.

HYDROGEOTHERMAL CONDITIONS IN EIRECONTENTSIINTRODUCTIONI-I. Scope and ObjectiveI-2. Historic warm Springs1-3. Terrestial Heat Flow in IrelandII WARM SPRINGS OF IRELANDII-I. Location1I-2. Geological Setting1I-3. Geophysical Setting1I-4. Hydrological Characteristics1I-5. Thermal Characteristics1I-6. Hydrochemical Characteristics1I-7. Main InferencesIIIIVORIGIN AND FUNCTIONINGIII-I. Palaeozoic Thermal Springs and MineralizationIII-2. Present-Day Circulation of Warm Groundwater1II-3. Heat Sources and Heat Conservation1II-4. ConclusionACKNOWLEDGEMENTS AND REFERENCESTable 1. The seventeen Warm Springs located in Leinster and Munster, towhich 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 (fromSheridan, 1972).Table 3.Table 4.Temperatures and major chemical constituents of the waters of theIrish warm springs, based on samples taken on 15 Nov., and on4 & 7 Dec., 1979. Analyses made at State Laboratory, Collegeof Science, Dublin.Trace elements found in the waters of Irish warm springs, basedon samples taken on 15 Nov., and on 4 & 7 Dec., 1979. Analysesmade at the state Laboratory, College of Science, Dublin.Fig. 1.Fig. 2.Fig. 3.Fig. 4.Fig. 5.Fig. 6.Fig. 7.Location of the recorded Warm Springs of IrelandLocation characteristics and geological setting of the main groupof Leinster Warm SpringsAnnual Temperature Variations for Enfield (No. 1005) and forSt. Gormans (No. 1007) for the years 1976 and 1978.Triangular Diagram plotting of Chemi~al Composition of the watersof Irish Warm SpringsTrace elements Silicon and Iodine plotted against Water TemperaturesConvection Circulation of groundwater in Lower Carboniferous times(after Russell, 1978)Possible present-day groundwater circulation feeding the LeinsterGroup of Warm Springs.

BallinalackNo .1011Hurler's .Cro~5No.2004Newca5tle We~tNo.2003~~~~i.~S'€~rl;allOWSpa&. Boreholer~L::::Jo....'.4l:n1t tMdt-d tjrn.r:stonr\ ~*'DoLin .nd Dolo:n.;lLSCulmFIGURE 1 - LOCATION OF THE RECORDED WARM SPRINGS or IRELAND(Carboniferous lithology after Charlesworth)

HYDROGEOTHERMAL CONDITIONSIN EIREC.R. Aldwell and D.J. BurdonIINTRODUCTIONThe paper attempts to bring together and present existing data onhydrogeological conditions in Ireland, with emphasis on the known warm springsof the country. It does not deal with the granites and other potentiallyhot dry rocks. The paper marks a preliminary stage in initiating the studyof the geothermal energy potential of the island.I-I.Scope and ObjectiveOn the "Atlas of Subsurface Temperatures in the European Community"(Haenel, editor, 1980), the Republic of Ireland is shown as a blank. Thoughsome temperature data has been obtained, it has not been published, exceptin very abbreviated form as in "Terrestial Heat Flow in Europe" (Cermak andRybach, editors, 1979).However, some data has been collected over the past 9 years on thewarm springs of Ireland, and there are also scattered records from the past.This paper endeavours to bring together such data, plus that obtained by theauthors, relating to the 17 known warm springs (including a very fewwarm boreholes) recorded in Ireland. The warm springs occur in ratherrestricted areas, as shown on Fig. 1; and no attempt has been made to studyor record hydrogeothermal conditions outside the areas of the known warmsprings. The field data collection was intensified towards the end of 1979,when almost all the known warm springs were sampled and their waters chemicallyanalysed under uniform conditions to produce comparable results.The establishment of a projectto make a preliminary investigationand assessment of the "Geothermal Energy Potential of Ireland" has givenadded impetus to the compilation of this present paper on hydrogeothermalconditions in Ireland. The 'Geothermal Project' is expected to receivethe support of the Commission of the European Communities (R. & D. EnergyProgramme), and to commence formally on I July, 1980. It is hoped thatthe collection and analysis of existing hydrogeothermal data made here willcontribute a little to the successful launching of this Project. As partof the Project, it is hoped to obtain a better understanding of thefunctions of known and still-to-be-discovered warm springs and boreholes,together with their potential for energy development and use. Low enthalpysources, such as the Irish warm springs, have potential in many fields,including horticulture, carp ponds, swL~ing pools, feed waters to industryand possibly space heating.1-2. Historic Warm SpringsThere are innumerable springs and spring wells in Ireland; theyare enshrined in place names, in myths and in legends. There are many holywells and springs, christianized sites of druidic worship and healing; theseusually 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 varyingreputation and importance; except for Mallow, their groundwaters were atnormal temperatures. At the still important spa of Lisdoorivarna, the watersare warmed prior to drinking

- 2 -Normal groundwater in Ireland varies from 9.5 0 to 10.5 0 C. Ittends to lie in the cooler range in the north, and possibly at higherelevations. Warm springs are considered as having appreciably highertemperatures than "normal", at least for a part of the year. Thus a springor well whose temperature was not reliably recorded as more than 12 0 C atsome time could not be called a warm spring or well; and plus 13 0 C would bea more normal lower temperature limit. Such a low-temperature "warm spring"would feel cool to cold in summer, and would only feel very slightly warmeven in winter. Hence, nmny slightly warm springs may well have escapednotice. It will be difficult to identify all Irish warm springs in the lowesttemperature range.The Mallow Spa spring is the oldest recorded warm spring in Ireland.Rutty (1957) states "Mallow-water was first discovered and introduced intopractice by Dr. Rogers of Cork about the year J724." Rutty also reports onthe "two tepid springs in the county of Dublin", at St. Margarets (No. 1009here) and Ballydowd (No. 1013 here, but not relocated on the ground). Twoof the main warm springs of Leinster were brought into operation by man'sactivities. Louisa Bridge spring (No. 1001) came into operation when theRoyal Canal was being constructed in 1794; Enfield spring became knownin the 1890s, when gravel was being excavated for a railway line. Untilquite recently, the number of warm springs recognised in Leinster were limitedto two or three. Grainger and Davies in 1966 state "Now, however, a thirdwarm 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 forgottenby investigators.In the 1970s however, attention began to be given to the warmsprings, in particular those of Leinster. Numerous temperature readingshave been collected by one of the authors since 1971. Enquiries were maderegarding hitherto unidentified warm springs. So, when Fahy, (1974, 1975)studied the biology of the Enfield spring (No. 1005) he was able to presenta map showing the location of 10 warm springs. Horne (1977) drew attentionto 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 nearingcompletion 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, withpositions indicated on Figs. 1 and 2. Dunlavin spring (No. 1012) isrejected; it could not be found on the ground when its reputed localitywas examined by one of the authors. Of the 17 accepted, 11 occur inCos. Dublin, Kildare and Meath, and are shown in some detail in Fig. 2; ifBallydowd (No. 1013) were relocated, it 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. InMunster, there is the famous Spa at Mallow, issuing from a pool and in thewell constructed within the Spa House. Warm water was also encountered inthe borehole in the nearby Cattle Mart, and possibly in another bore (nowshut) close to the Spa itself. The warm springs at Newcastle West and atHurler's Cross have already been mentioned./ ...

FIGURE 2 - LOCATION CHARACTERISTICS AND GEOLOGICAL SETTING OF THE MAIN GROUP OF LEINSTER WARM SPRINGS..zso-21-0IJ. J: fJ.GEOLOGYNamurian•••••Vi5aan•••••••Tourna16ean••Old Red Sand.'-Silurian•••••2.1° .".---," ,/ ,I \~WARf'l;/;80xSPRINGS• ?I 8 \ 1'h .\ • J / w -,rIm\ /"",-- --'f.. • A{,.1\ / /'" / .., \~-~- I\.- _-......I 7. Y cy..,(-r..",\ /}, I I\ '.;-- /', A' 5'.+ 20°C ••••••• •15° C - 20°C.... ()a12 C -015 C •••• •STRUCTURE. Anticline•• / • % .-,Z:JO"\ - '/.... " ". \,'---r\ " \~." ~I \ . '.... // "6 \ ~ . '.... ..... -- -",r,. ;. .. " - -",.I ~ .\ .... --_ ..., ,./"..-- ......./ "I "I \"..- L __/1.t., \/"~ ........ -.\ Jt" \ Io ...... I /- .....-I.~ ,," I\--.././/'X' ,/IJ310I.---"" .......Matamorphic~.Granite=•••••+Syncline•••• __ • ~{.Deep Borehole... -$-

- 3 -Ref. No.Name or NamesCounty1001100210031004100510061007100810091010101110131014200120022003Louisa Bridge, LeixlipBride's River, or Breda's Well, KilcockKewnins Mills Springst. Patrick's Well, CelbridgeEnfield, Kilbrook or Cappagh SpringDysart SpringSt. Gorman's or Hotwell House SpringArdenew Spring, LongwoodSt. Margaret's Well, Tobar MairaideSpring on Cash's Farm, ClaneBallinalack Spring or BoreBallydowd or Hermitage Spring (Not located)Clonee Spring (reported in March, 1980)Spa Well, MallowMart Borehole, Mallowst. Bridget's Well, Newcastle WestKildareKildareMeathKildareKildareKildareMeathMeathDublinKildareWestmeathDublinMeathCorkCorkLimerick2004Tobar Mackann,Hurler's CrossClareTABLE 1.Seventeen warm springs located in Leinster and Munster,to which reference is made in the text.

- 4 -1-3. Terrestial Heatflow in IrelandThe heat produced in the crust of the earth by decay of radioactiveelements was studied by Joly (1857-1933) and considered by him to bea prime factor in causing the major orogenies; he summarised his findingsin the 1924 Halley Lecture at Oxford. Joly's interest had been stimulatedby the radio-activity of the Leinster granite, but the subject ofterrestial heat (from depth or crust-generated) was not pursued in Ireland.At the end of 1979, Prof. A. Brock prepared a comprehensive reporton "Geothermal Energy in Ireland" for the National Board for Science andTechnology. In Chapter 6 he deals with the "Geothermal Data in Ireland". Whilethis comprehensive review tends to emphasise data existing or inferrable onthe "hot dry rocks", it covers the whole position, including the low-enthalpywet rocks. With Dr. Brock's permission, the data relevant to wet rocks issummarised here. From 18 heat flow values from deep boreholes in theCarboniferous Limestone determined by ~tr. Wheildon (unpublished and -2uncorrected for climatical factors) heat flow under Ireland averages 60 mWm ,with rather higher values in the north. From data on 7 boreholes analysedby Cermak & Ryback (1979), the average heatflow value was calculated at64.6 mwm- 2 , with a value as high as 81 mWm- 2 for the Portmore borehole inthe extreme north. Dr. Brock then examines the geophysical evidence, inparticular gravity lows which might indicate buried granites, includingpossibilities at Navan and Drogheda(Leinster warm springs) of small graniticbodies, and a much larger buried granite under Kerry (as Howard, 1975).Dr. Brock briefly reviews possible conditions favourable for useable heat inwet rocks, identifying two possible areas - the Limerick/Shannon area withthick Namurian shale cover over Dinantian limestones, and the Munster Basinof Old Red Sandstone cover over a possible buried granite.The terrestial heat flow in Ireland is therefore very close to theworld average, which is reported at 65 mwm- 2 ; the average thermal gradientis 25 0 C/kilometer depth. Useful energy is extracted in areas of abovenormal heat flow and temperature gradient. However, warm springs do existin Ireland, and seem to indicate that there are at least limited zones ofincreased heat flow, with convection flow of warm groundwater assisting theprimary conduction heat transfer through 'the rocks. So, the investigationof the warm springs of Ireland is now proceeding, and the position reachedby Spring, 1980 is reported here.II - WARM SPRINGS OF IRELANDBased on old records, and following field data collection from 1971,a first report on Irish warm springs formed part of a paper "GroundwaterInvestigations in Ireland" read at the "Hydrology in Ireland" meeting of May,1979 (Aldwell and Burdon, 1979). In this present paper, all available datahas been collected and presented under the headings of location, geologicaland geophysical settings, and hydrological, thermal and hydrochemicalcharacteristics. The twelve main inferrences drawn from the data concludethis item./ ...

- 5 -II-l. LocationThe general location of the 17 warm springs listed in Table 1 areshown on Fig. 1, while the main Leinster group (Nos. 1001 to 1010) arelocated in more detail on Fig. 2. There are rumours of other warm springswhich need to be investigated; it is anticipated that the plannedgeothermal 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 depthsof persistant Quaternary cover. A noticable feature is the fact thatseveral of the springs (as Nos. 1002, 1007, 1008) occur on high ground,small rises or mounds. This is particularly remarkable in the case ofBride's River Kilcock (No. 1002) from which the stream splits and flows intwo directions. At least two of the Leinster springs were brought intoexistance by man's activities. Louisa Bridge spring, Leixlip (No. 1001)started to flow or to be identified when the Royal Canal was being constructedin 1794; it was quickly developed as a spa (Maxwell, 1974). Enfieldspring became known in the 1890s, when gravel was being excavated forrailway line construction (Davies & Hill, ]965).. The other springs appearto be long established; that at St. Germans (1007) was examined by geologistDu Noyer on 21 July, 1859, and his comments are written on the back of the6" sheet.Signi!icance is attach.ed to the fact that some of th.e warm springsissue from high ground, and to the fact that two of the larger were broughtinto existance by man's activities. Another point of possible significancefor the Leinster warm springs shown on Fig.2 is their distance apart.When plotted, their rather uniform distance apart was noticed, and whenscaled appeared to be of the order of 8 kilometres. Circles of 8 kmdiameter have accordingly been drawn ~round the 11 warm springs on Fig. 2.Could the enclosed areas (about 50 km ) be those contributing to eachindividual warm spring? Are additional warm springs more likely to befound 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 bestknownand unique in their occurrence. Rutty (1757) gives much information.Dalton (1889) gives several references, including Jephson (1834) who statesthat in autumn/winter 1833, temperatures ranged from 67 0 F (19.33 0 C) to .71.3/16 o F (2l.77 o 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 occurin 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 nearbyencountered warm water; the one at the Mart (No. 2002) is in use, the otheris closed and the warmth of its water has been queried. The two otherknown Munster springs, at Newcastle West (No. 2003) and at Hurler's Cross(No. 1004) occur in open rolling country, with no special topographicexpression, though No. 2003 is near an old rath and on slightly higher ground./ ...

- 6 -II-2.Geological SettingThe known warm springs all issue from Dinantian limestones. Forthe main Leinster group of 11, Fig. 2 shows that they emerge mainly fromVisean limestones, except the small spring (No. 1010) at Cash's farmissuing from the underlying Tournaisian. This applies to the warm springsin Co. Limerick and in Co. Clare. The Mallow springs and boreholes arealso from the limestone, but are associated with a major thrust zone.Fig. 2 brings together such geological data as is available fromthe main area of warm springs in Leinster. Most of the area is covered byQuaternary deposits, of varying thickness and nature; rock exposures arepoor. The main data comes from MacDermot & Sevastopulo (1972), andSheridan (1972), while Turner (1952), Nevill (1957), Bruck (1971) andWilliams & 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, metamorphicsand Silurian of the Leinster Caledonian granite massif on its south-east, andthe Ordovician-Silurian Balbriggan and Louth blocks on its north. Here,conglomerates, sandstones and siltstones of desertic and fluvio-esturineenvironments 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 Tournaisiantime before marine limestones began to interdigitate with coastal sandstonesunder the area of Fig. 2. By the Upper Tournaisian, shelf limestones withisolated Waulsortian reef mounds were forming under the main portion ofFig. 2. The Waulsortian mounds were more frequent and much thicker to thewest and away to the southwest, to the areas of warm springs Nos. 2003 and2004; 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 upliftand faulting, but marked also by increased marine transgression, as onto theedges of the Leinster granite massif. In the region of the Leinster warmsprings, there was a sharp change to deposition of argillaceous limestonesof 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 Viseantype and towards the top of the Visean with bedded limestones, argillaceous,with some chert and silica. These limestones are succeeded by Namurianstrata, often found occurring as perched synclines, as at Summerhill (Nevill1957) . The total thickness of the Tournaisian + Visean is of the order of2,220 metres (Sheridan, 1972) plus more than 760 m of overlying Namurian(Nevill, 1957). The thickness of the underlying Devonian is unknown for thearea of Fig. 2.The detailed information obtained from the Trim No.1 borehole isvery relevant to the study of the rocks from which the warm springs issue.The lithology/thicknesses are summarised in Table 2. The fact that theboreholes intersected some 4.5 metres of an olivine dolerite dyke or sillat from 740.7 to 745.2 metres below surface is also of importance whenconsidering local sources of heat within the sedimentary and underlyingsequences./ ...

- 7 -Thickness(m)Lithology+ 335 m+ 183 mAgher Formation, (P l- 2). Bedded limestone, dark grey,with shales; agrillaceous, pyritic, with some chertand silica.Bray Hill Formation, exposed above the collar of theborehole; mainly reef micritesCollar of Borehole at 79,03 (259.3'); measurements from Rotary Table @+ 4.3 m.l.8m Superficial deposits; Quaternary401 mBray Hill Formation. A reef complex, with two majorreef successions, separated by thin transition zones.37 m Rathmolyon Limestone Formation.limestones, grading upwards intobut with a sharp basal junction.Argillaceousthe Bray Hill Formation,193 m Rathmolyon Oolite Formation. Mainly oosparites806 m to 770 m Rathmolyon Shale Formation (Cl to C2Sl) . Shales, darkgrey, platy, indurated, calcareous; crinoid debriscommon, pyritic and siliceous; with some limestonezones, especially in the middle of the succession.(4.5 m) Olivine-Dolerite Dyke/Sill. Intruded into upper partof the Rathmolyon Shale Formation. Potassium-argonage dating gave 65 (+ 11) million years, or Palaeocene­Eocene on the New Holmes Scale.330 m to 300 m ~thmolyon Basal Clastic Formation, (Z2-Cl). Theformation commences at the first sandstone bedencountered, though beneath it there are limestonesand shales. It ends in sandstones and siltstones. Itis noted that "By analogy with other boreholes inIreland, it is suspected that a continental facies ofUpper Old Red Sandstone type is present at no greatdepth belmv the bottom of Trim No. 1 Well"TABLE 2. Summary of lithological data from Trim No. 1 Well (sunk, 1962)as affecting the main group of Leinster Warm Springs (fromSheridan, 1972).

- 8 -The presence of sills/dykes of intrusive basalt has been reportedfrom at least three places within or bordering the region of these warmsprings. The earliest report is by Cruise (1871), an 18" wide dyke in aroad-cutting about l~ miles WNW of Duleek, Co. Meath, as noted by SelwynTurner 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 millionyears old by K-Ar dating, and so "not later than Lower Eocene, not earlierthan Middle Eocene". In describing the occurrences, Byrne et al (1971)note "The basalt sills transgress the Carboniferous series, independent ofstructures, and are considered to be Tertiary..• at Tara's Navan mine, thereare 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 the4.5 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 AntrimBasalts,whose K-Ar determinations indicate a Palaeocene-Eocene age some 65million years BP. According to Miller & Brown (1963) the Upper Lava Seriesof the Antrim basalts have an K-Ar age of 74 million years; later theyindicated that the presence of zeolites in basalt tend to give K-Ar ageswhich are too great (Miller & Fitch, 1964). However, on palynologicalgrounds, the Antrim basalts are considered to be of Miocene or evenPliocene age, and so much younger (Sabine & Watson, 1965, p. 506). Thematter is of importance here, as late Tertiary intrusions (say 15 to 20million years ago) might have initiated some thermal groundwatercirculations which could have persisted, after the initiating heat sourcehad diminished or ceased to act.The rocks underlying the w~rm springs of Leinster have been gentlyfolded and deformed during the Hercynian orogeny. The folds tend north-eastto south-west, rather than the prime east-west direction seen in the southof Ireland. They appear to have been reoriented by the resistant blocks intheir 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); theother three are based on the form of the Tournaisian/Visean contact. Whiledetailed and more precise mapping of the springs and some of the structuresis required, it may be noted here that: (i) Springs 1009, 1001, 1004, 1010and 1014 lie along the synclinal structure inferred for the two Tournaisianoutcrops: and (ii) The warmest springs are not far from the axial plane ofthe Summerhill syncline (No. 1005) and from the the Rathmolyon anticline(No. 1007).The rocks were thus folded and deformed long before the intrusionof the Tertiary dyke/sill intersected 740.7 to 745.2 metres below surfacein the Trim No.1 Well. Thus, the intrusion will cross-cut the beddingwhether it be a sill or a dyke (vertical or inclined) . Such an intrusionmay be expected to swell, and form a larger body of magma, where the planeof intrusion intersects a shattered (as anticlinal crests) or open (aspalaeokarstic limestone) volume in the intruded rocks. Such large volumesof intrused lava could have provided areas favourable for heat storage andthe initiation of thermally-controlled groundwater circulation in theenclosing rocks; how long such a mechanism could continue would depend onthe size, initial temperature and rate of cooling of the intrusive body./ ...

- 9 -Boreholes drilled for water, particularly in the western portionof Fig. 2, often encountered a black shale or mud, which is very fine andcan be separated from the water obtained only by long sedimentation. Thematerial could be weathered shale, or residual from limestone weatheredunder peri-glacial conditions.In the south of Ireland, the geological setting for the two warmsprings (No. 2003 at Newcastle West and No. 2004 at Hurler's Cross) is notdissimilar to that of the main group of Leinster warm springs. As yet,however no start has been made on relating information on the geology ofthese areas to the occurrence of these two warm springs.The Mallow warm spa, and the boreholes near it, appear to be inan unique geological setting for Ireland. The waters issue from theLower Carboniferous limestone where it was strongly compressed and deeplydownfolded in the Hercynian orogeny. The thickness of the Dinantianlimestones is about 1,700 metres thick tas determined by Hudson & Philcox(1965) some distance to the north. The limestone has been eroded fromthe anticlinal hills to the south, but is preserved in the synclines. AtMallow, the intensity of the folding was such that it was relieved bythrusting; a major thrust can be traced westwards from Mallow to the seaat the head of Dingle bay. There is also much faulting at Mallow. Thedepths to which the limestones have been downfolded and downthrust at Malloware uncertain. But the depth of burial and the possibilities of deepgroundwater circulation in such limestones must be major factors inbringing this warm spring into existance.11-3. Geophysical SettingAs yet, the existing geophysical data has not been fully examinedand used to assist in understanding the factors controlling the locationand functioning of the warm springs of Ireland.Under the Leinster warm springs area, it is considered that thesuture which is postulated as marking the closing of the Iapetus oceanextends from Salterstown through Navan and to the Slieve Bloom mountains,(as Fig. 8, Phillips et aI, 1976). Thus the region was in Caledoniantimes and into the Devonian/Lower Carboniferous, a place where the edgesof two plates were in collision and subduction. As such, it was aprobable place for thermal circulation of oceanic brines producing oredeposits in the accumulating Carboniferous sediments, (Russell, )978) as inFig. 6 here. However, there could be little in such conditions toaffect present day thermal activity, though some relationship cannotentirely be discarded.Gravity investigations (as Murphy, 1962, 1974) have revealed anumber of gravity lows of medium size (as under Navan) as well as groupsof lows of very small extent. It seems probably that the medium sizedlows are not due to buried granites, but to a greater thickness of sedimentsthan originally estimated. The very small gravity anomalies (Murphy, 1962)occur rather to the west of the main area of Leinster warm springs, butsome could occur in the actual spring area. These lows show a marked NE-SWelongation, and could be solution caves in the Carboniferous limestonelocated or initiated by faulting which has opened up the limestone tocirculating groundwater. But as such, the water-filled (or gravel-filled)shallow cavities would not necessarily have any connection with thecirculation of warm groundwater. It may be noted that the main Iapetussuture is supported by a fault postulated on lithostratigraphical grounds/

- 10 -Cas Harper & Brenchley, 1972). 'I'his has also been associated with thelines of small gravity anomalies, and there could be some reactivation ofsuch a major fault in later times. It could prove to be a zone whereabnormally 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 greatthickness of Devonian-Lower Carboniferous sediments. Later, it wasindicated that a major buried granite (Armorican ?) was more likely tohave produced the large gravity low, (Howard, 1975). Such a deep granite,with a thick cover of low-conductivity rocks, might possibly have somegenetic relationship to the warm springs of Munster. Brock (1979) hasdrawn attention to the thickness of sediments, and the low conductivityof the Namurian cover, in the Shannon basin; this may have some influenceon the small warm springs of Cos. Limerick (No. 2003) and Clare (No. 2004).The limited work done on geothermal gradients and heat flow inIreland was noted under Item 1-3 of this paper. In general, geophysicalinvestigations of very many types have been used in Europe over the pastdecade. They have proved very useful in elucidating the characteristicsof already-located geotherw~l fields and assisting in their developmentand management, as reported in the many papers presented to the First(December, 1977) and Second (March, 1980) Seminars on Geothermal Energy ofthe commission of the European Communities. Indirectly, they can assistin prospecting for geothermal anomalies, but such investigations have tobe specifically oriented to obtain such information. It does not appearthat they have been very successful in finding new geothermal fields.11-4. Hydrological CharacteristicsNo systematic gaugings have been made of the yields of the Irishwarm springs. However, it is certain that most, probably all, show strongseasonal variations in discharge, with maximum flows in spring and intosummer, while many of them go dry, or have negligible flows, in the autumn andinto winter. Minimum flow, or middle point of dryness, can be September­October. None of the springs have definitely large flows; maxima forthe bigger springs would be in the range of 5 to 10 litres per second;if the springs were developed, this might be increased slightly undergravity 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 groundwaterrecharged during the winter months, and discharging this water some fourto eight months later. No doubt, careful gaugings will permit closercorrelation wie1 precipitation-infiltration, while recession curves can beconstructed and analysed, permitting some deductions as to the hydrauliccharacteristics of the aquifer.11-5. Thermal CharacteristicsIn general, temperature observations on the waters of the warmsprings have not been made in a systematic manner, or extended over a longrange of time. However, over the past nine years, temperatures of theLeinster group of springs have been measured throughout the years by one ofthe authors. In November-December, 1979 temperatures were measured with thesame standardized instrument and in the same manner for almost all of theIrish warm springs; these temperatures form part of Tables 3 and 4.

IIII.JJJ.S 0 .No-- --~--f-22C:1. - 0Is=.o·21-·0.19_l.. 0-18-.: 0f---i-l 4=-- 013FIG. 3. 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 limitedpublications, to see if there have been significant changes in temperatureover tQe past 200 years or so. But the only old reliable data appears tolie is that given by Rutty for the period about 1757. He notes Mallow spawater as issuing at 68 0 F (20 0 C), as compared with 20.7 0 e measured on 15 Nov.1979 (Table 36. Rutty also notes that st. Margarets (NO. 1009)registered 51 F (lO.SoC) or 5S o F C12.8 0 C) on 20 July, 1752; this wouldcompare with 17.2 0 C on the morning of 7 December 1979. In a study of thebiology of the Enfield spring, Fahy (1975) gave some temperature figures,based apparently on a recording thermometer placed in the spring; hereports in his Table I a range from 6 0 to 22.5 0 C. He also states that thespring has a day-degree quota of 6,606, as against 3,835 for an Irishisothermic spring. Fahy (1975, p. 112) mentions but three thermal springsrecorded in Ireland, and introduces the supposedly warm spring at Dunlavinin Co. Wicklow, with the footnote "The area was visited by me; the springwas not located".From the periodic temperature measurements made on the mainLeinster group of springs (Fig. 2), those for the Enfield (NO. 1005) andst. Gormans (No. 1007) have been selected for presentation here, Fig. 3.During the eight years to end-1978, Enfield water temperature was recorded62 times and that of St. Gormans 64 times. No self-registering or min-maxthermometers were used. Periods when the springs had dried-up or becomestagnant were noted; such low/nil flows tended to occur from August toNovember. Figure 3 shows water temperature for Enfield and for st.Gormans over the calander years 1976 and 1978. These were years for whichgood 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 thewinter. The discharges also increase in spring and fall in autumn. So,as flow increases, so does temperature; as flow decreases, so do temperaturesfall. It is considered that this relationship is of considerablesignificance. The source of the heat which warms the water must be muchgreater than the amount of heat taken away by the water. It suggests thatthe larger the volume of water moving underground, the more it is broughtinto contact with zones of more heat or higher temperatures. This mayindicate a deeper circulation in the ground when winter infiltration hasrecharged the aquifers and the water is moving strongly towards itsdischarge point.The degree-days for the two springs have also been calculated,due to irregularity of measurement and that fact that st. Gormans was dryfor part of the year, degree-days are only approximate. Enfield showed7820 degree-days (21.3S o C average) in 1976 and 7380 degree-days (20.22 0 Caverage) in 1978. St. Gormans showed_6230 degree-days in 1976 (17.06 0 eaverage) and 6790 degree-days (18.61 0 C average) in 1978. The differencesin average and from year to year may have significance, but certainlyneed to be related to volumes of water discharged if the data is to beused as an analytical tool./ ...

- 12 -iSpring No. 2001 2001 2002 2003 2003 1009 1001 1002 1003 1005 1005 1007 1008 1006 1004Sample NO. I II III IV V VI VII VIII IX X XI XII XIII XIV XVDate, 1979 15/11 15/11 15/11 4/12 4/12 7/12 7/12 7/12 7/12 7/12 7/12 7/12 7/12 7/12 7/12°c 11.3° 20.7° 17.6° 13.7° 13 .8° 17.2° 16.2° 13 .2° 14.2° 23.3° 23.3° 20.2° 13.0° 13.8° 12.2(Ca mgE/lit 4.72 3.60 4.20 3.44 3.40 6.84 5.92 4.60 5.60 3.92 3.84 4.80 6.56 5.76 4.64Mg 2.64 1.28 1.04 1.44 1.32 0.88 2.72 2.04 0.76 2.16 2.08 1.20 0.64 1.40 2.40Ma 0.31 0.50 0.47 0.60 0.59 0.56 9.09 0.43 0.39 1.13 1.13 0.41 0.31 0.48 2.61K 0.10 0.03 0.05 0.03 0.03 0.06 0.23 0.03 0.03 0.05 0.05 0.03 0.03 0.07 0.08Sum Cations 7.77 5.41 5.76 5.51 5.34 8.34 17.96 7.10 6.78 7.26 7.10 6.44 7.54 7.71 9.73HC03 mgE/lit 4.96 4.16 4.56 4.72 4.64 5.88 5.40 6.44 6.24 5.84 5.92 5.76 6.64 6.48 5.60Cl " 0.68 0.62 0.62 0.59 0.59 1.10 12.57 0.48 0.48 1.27 1.33 0.49 0.61 1.25 4.00N03 " 0.21 0.17 0.31 0.36 0.24 0.26 0.10 0.24 0.24 0.19 0.20 0.06 0.04 0.04 0.03S04 " 1. 92 0.46 0.27 - - l.10 - - - - - 0.15 0.25 - 0.12Sum 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 9.75Ca mg/lit 236 180 210 172 170 342 296 230 280 196 192 240 328 288 232Mg " 132 64 52 72 66 44 136 102 38 108 104 60 32 70 120Na " 7 12 11 14 14 13 209 10 9 26 26 9 7 11 60K " 4 1 2 1 1 3 9 1 1 2 2 1 1 3 3HC0 mg/lit 3302 214 278 282 282 359 329 393 381 316 341 354 405 395 342Cl " 24 22 22 21 21 39 446 17 17 45 47 18 22 45 14211N0313 10 19 22 15 16 6 15 15 12 12 4 2 2 211S0492 22 13 - - 53 - - - - - 7 12 - -Sum of above 810 565 607 589 569 869 431 768 741 745 744 690 809 814 907Nitrate (as N) 3.0 2.4 4.4 5.0 3.4 3.6 1.4 3.4 3.4 2.6 2.8 0.8 0.6 0.6 0.45Free/Sol.Am. (N) 0.02 0.02 0.02 0.02 0.01 0.01 0.23 0.02 0.03 0.04 0.01 0.01 0.03 0.01 0.15Albuminoid (N) 0.29 0.02 0.02 0.02 0.01 0.02 0.01 0.07 0.06 0.01 0.01 0.01 0.02 0.01 0.01TDS (ppm) 370 286 334 296 292 528 1160 370 380 386 404 412 440 480 636Total Alkali 248 2208 228 236 232 296 296 322 312 292 296 288 332 324 280Total Hardness 368 244 262 244 236 386 432 332 318 304 296 300 360 358 352pH 7.40 7.52 7.41 7.60 7.84 7.23 7.28 7.24 7.26 7.31 7.35 7.26 7.11 7.19 7.32TABLE 3. Temperatures and main chemical constituents of the waters of the Irish warm springs, based on samples takenon 15 November and 4 and 7 December, 1979. Analyses made at State Laboratory, College of Science, Dublin.

----- ~---------~t;" Dol. p.p.m. Rr mod.•2001 15.11.79 565 20.7 u C; Mallow 5Q~2002 15.11. 79 607 17.6 u C; l'lill.o.ML.BtL-2003 4.12.79 569 13.:guC;- Newcastle \,/a1001 7.12.79 1431 16·.2 C:-t.. OU1 "o!l' Ad rl1002 7.12.79 768 i; 2 u • ~. ~ _.~ i/p.r1003 7.12.79 7~1 :.~Ef~~ ~o 1%11004 7.12.79 907 12.2 Cj Ce1bridg~1005 7.12.79 745 23. 3:[; EofieJ...d~1JJ_O~ 7.12.79 814 1 3 •8 C.L____12Y 5 aLi10Ql 7.12.79 680 20.2:C..;--S.t.cGOIJUan!....:J..QQB~H)l22- 7.12.79 A6q .lL2-=-.c.;--..SL_"J'1ar.-gaI.£J7.12.79 _JL~_ J..:t;J)oW~.d.eJl£W~..Noh: ' ..tlumbrlS 01. in 0«01(10'1(. with wolrr - pointnumbtl~ ;n th. study 0'.0.-)) Three from Munster (1'1))12 0 _ 20°C ..••••••••••o+ 20 C ...•.•. _••..••• 0Temperature range inDecember, 1979."FIG.4. Triangular Diagram Plotting of Chemical Composition of thewaters of Iris~ Warm Springs. I~os. 1001, 1004 and 1009 form a groupapart.

- FIGURE 5 - TRACE ELEMENTS SILICON AND IODINE PLOTTED AGAINST WATER TEMPERATURES25° 25°24°.... 5c:r:323° ~/ I./OLS' o//L5 0I . OL5::J. /9'0 ..a:220 / / //t.:l22°. I . ./L../ / /210 0./ . .Ml/- / ~1°w- a:O.1rt1/ / 0 / /200 ::J.....'OL?10L?//20°c:ra: I .'.1 ./w ./19° ~ . J I / /19°w . ./.... .oj-ieo / . I ./-/.l'12 ISO/cI I- I /M2 . 0//a L9 .9'L9I / I/I . ../a/L1 I / / I 0L1 / 16°.. .I / /. .'//15° I I I \ /° J L3.1 - /\/t L6 (/ \OL314 o' ° OM3I .OQ2 M3 L2 \. ~30~6 / .15° pLS PM3 ;' o?s '",140 . I 6 4 // ...... L4 ." "/"./00//-./OLS15°/ 14°-)..:--.. .................-0- -- -- -:-.12°Silicon in / i . / 1 I130 mg!litre i .0 M1 I o I'll Iodine in~g/litr8 I1 2 3 4 5II1213°I l1o (j3 4 6 7 8 9 11 12 II I I T I I I I _:1 , iII! I,II !,r

- 13 -11-6. Hydrochemical CharacteristicsInvestigators of one or other of the warm springs have in the pasttaken samples and had chemical analyses made. In this respect, the reportof 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 asystematic sampling of almost all of the known springs, with the analysescarried out under uniform conditions at the State Laboratory in the Collegeof Science. The results of these analyses are presented here in Table 3and 4. The results are strictly comparable, though the absolute valuesof some of the trace elements (not normally determined in water in thislaboratory) are less certain. The percentage amounts of the major ions areplotted on the triangular diagram of Fig. 4, while the trace elements siand I are plotted against temperatures on Fig. 5.(i) Major Anions and Cations. In general, the major ionsindicate rather normal groundwater issuing from these springs. TDS tendto lie in the range of 280 to 480 ppm, which is normal for Irish groundwatersfrom limestone aquifers. Much of the chlorine is in the 15 to 30 mg/litrange, normal for such waters and reflecting the rather high Cl content ofIrish groundwaters (as Burdon & Cullen, 1980). However, it is clear thatthere are three springs in Leinster which have definitely abnormal chemicalcharacteristics 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'sWell) with Cl = 142 and TDS = 636; and No. 1009 (St. Margarets) with Cl = 39and TDS = 582. From Fig. 2, it can be seen that these three springs havealready been grouped together as lying "along the synclinal structureinferred from two Tournaisian outcrops" (Item 11-2. (i». It is unfortunatethat No. 1010 was not sampled in December, 1979, and that the existenceof No. 1014 (Clonee) was reported only when this study had almost beencompleted. 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 whichdiffer much in temperature.In fact, all efforts to find a direct relationship between themajor anions and cations in solution and the temperatures of the springsproved unavailing. It may be said that the main chemical composition ofthe 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 Con 7.12.79; the hottest (No. 1005, Enfield) has but some 400 ppm of TDS,but a temperature of 23.3 0 C on 7.12.79. The coolest is also rather highlymineralized.(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 weredetermined, though it seemed improbable that they would yield diagnosticinformation. The accuracy of some of the determinations is doubtful, sincesuch elements are not normally determined in water analyses. It can be seenthat 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. 2001 2001 2002 2003 2003 1009 1001 1002 1003 1005 1005 1007 1008 1006 1004Sample No. I II III IV V VI VII VIII IX X XI XII XIII XIV XVDate, 1979 15/11 15/11 15/11 4/12 4/12 7/12 7/12 7/12 7/12 7/12 7/12 7/12 7/12 7/12 7/12°c 11.3 20.7 17.6 13.7 13 .8 17.2 16.2 13 .2 14.2 23.3 23.0 20.2 13 .0 13 .8 12.2.--pH 7.40 7.52 7.41 7.60 7.84 7.23 7.28 7.24 7.26 7.31 7.35 7.26 7.11 7.19 7.32TDS ppm 370 286 334 296 292 528 1160 370 380 386 404 412 440 480 636EC ~mhos) 570 - 525 - 510 830 1970 665 659 715 708 645 751 765 1070Total Fe (mg/lit) 2.90 0.20 0.25 0.30 0.35 0.25 1.90 1.00 1.00 0.80 0.35 0.20 0.20 0.20 0.35Dissolved Nil 0.10 0.05 0.03 0.10 Nil Nil 0.05 0.05 0.05 0.05 0.15 0.15 0.20 0.15mgsi/lit 3.2 5.2 3.3 3.8 4.2 0.9 0.9 3.4 3.2 5.4 5.7 3.9 2.6 3.6 3.7If/g/lit 0.9 1.5 1.0 3.0 6.0 6.0 9.8 1.3 2.5 10.9 12.0 3.0 1.1 3.9 4.3F flg/lit 100 - 100 - 160 140 440 140 120 280 290 220 100 200 270B/f.g/lit 73 42 28 63 23 50 60 53 30 60 92 78 153 182 52SeJtg/lit 0.75 Neg Neg Neg 0.75 Neg Neg' Neg Neg Neg Neg Neg Neg Neg NegBa Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil NilCa: Mg Ratio 1. 79 2.81 4.04 2.40 2.57 7.77 2.18 2.25 7.37 1.81 1.85 4.00 10.25 4.11 1. 93ITABLE 4.Trace elements found in the waters of Irish warm springs, based on samples taken on 15 Novemberand on 4 and 7 December, 1979. Analyses made at the State Laboratory, College of Science, Dublin.

- 15 -•They merit only very brief discussion. For Silicon, it will be noted that1009 and 1001 lie well away from the main envelope showing Si increasingas temperature increases.; these are springs noticably high in Cl and TDS.The third spring of this little group, No. 1004, lies well within theenvelope. It is generally reported that the si of thermal waters isaffected by the TDS of such waters. For Iodine, there is only a very slighttendancy for I to increase as temperature increases. The points most offthe line of Fig. 5, are 2001 and 2002 (Mallow Spa), 1007 (st. Germans) and1001 (Louisa Bridge) . While it could be said that the I of the first threeof these was lower than might be expected, and that the I of No. 1001 ishigher than might be expected considering their temperatures, the basis forsuch an attribution is slight and weak. Further work on trace elementsis clearly required.(iii) Gases. In some of the springs, as Enfield and Mallow, anappreciable 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 sometimesdetected; it may arise from decomposition of organic matter in the mud ofthe spring.(iv)Isotopes.As yet, there have been no isotope determinations.11-7. Main InferencesIt is considered of use to present here the 12 main inferenceswhich have been drawn with regard to these warm springs.LocationGeology~eophysicsHydrologyHydrothermalHydrochemical(i)(ii)(iii)(iv)(v)(vi)(vii)(viii)(ix)(x)(xi)Several occur on higher ground than might be expected.Some have been made functional by man's activities.The Leinster springs appear to be 8 km apart.All issue from Dinantian limestones, some 1,500 to2,500 m thick.Some small Tertiary intrusives have been reportedfrom their vicinity.Their occurrences can often be located with respectto fold structures.Certain gravity lows may have some connection withthese warm springs.Discharges increase in spring, as expected formeteoric 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 hydrochemicalcharacteristics.High CI and TDS are restricted to three springs occurringalong a synclinal structure.(xii) There are faint indications that Si and I increase withincreasing temperature.

M'ddl~Slo9~CrySlol\'n~ bos.m.ntor ductilp' lower crust10 omFIGURE 6 CONVECTION CIRCULATION OF GROUNDWATER IN LOWER CARBONIF­EROUS TIMES (after Russell, 1978)

- 16 -III ORIGIN AND FUNCTIONINGAs yet, the data presented and analysed in the foregoing itemof this paper is too slight to allow of any firm hypothesis to beadvanced as to the origin and functioning of the warm springs of Ireland.However, some ideas have been formulated and are put forward fordiscussion here.III-I.Palaeozoic Thermal Springs and MineralizationOver the past two decades, the ability of thermal springs, oftenof brines, to leach and deposit metalliferous elements and form oredeposits has received renewed attention and confirmation, as in the Red Seainvestigations. Such an origin has been postulated for the Irish basemetal 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 thetransgressive Tournaisian seas into the great thickness of underlying LowerPalaeozoic sediments. In these the waters circulated under thermal forces,leaching out lead, zinc, copper and sulphur, and deposited them in thelimestone sediments forming on the bed of the sea. Russell's diagramrepresenting these events is reproduced here as Fig. 6.Since then, much heat has flowed out of the crust, and there canbe no possibility of such past thermal circulation of groundwaterinfluencing the supply of warm water to certain springs in Ireland. Butit seemed desirable to include this hypothesis of past circulation of warm/hot brines through the rocks which underlie the Dinantian limestones ofIreland, and from which the present day warm springs issue.III~2.Present-Day Circulation of Warm GroundwaterIt is thought that the present day groundwater circulation feedingwarm water to certain springs is due to the effects of a heat source orsources which has modified the normal groundwater circulation of the regionon the main warm springs of Leinster, and possibly those of Cos. Limerickand Clare. Mallow would be an exception, as usual.Fig. 7 sets out the manner in which hydrotherrr01 cells areconsidered to function. The idea is often applied to low-temperaturecirculation of groundwater, as in Facca's (1973) diagram of the basic modelof a low temperature hot water field. The cells are some 8 kilometres indiameter. Main infiltration occurs in lows (from streams ?) whiledischarge is on highs. These are based on location features. The springsform part of the hydrological cycle, with winter infiltration resulting inincreased yields (or higher groundwater levels) in spring. Some of thesprings dry-up in summer/autumn. This concept should become more definiteas actual discharges are gauged and related to the precipitation andinfiltration./ ...

FIGURE 7 - POSSIBLE PRESENT~DAY GROUNDWATE8 CIRCULATION FEEDING THE LEINSTER GROUP OF WARM SPRINGS~ HALF - CELLII.,.......II8 KILOMETER DIAMETER CELL ~ HALF-CELL>,Warm Spring I IMain Infilt-! Warm Spring I Warm Spring I.t , ration Zone ~ I'~ I _"~I~. '. I~~ ~f I ~-f\;----""'""I:, .........t ~ J' t. .AI fA' l'" ~~, oJ. ,tV" -/~-~ "')\ 11'~+-. V' \J-i'JIJ"' It »-ooV .::Jr 1~ It " .1- ~ ~ ~.. l' \ ¥t '" t~ It M ~ '-'_1, I ;f/\ ~ 'b -¥ ~ 4:'" ~ '\ 1.~ ~~ "v ~ I ~ ,.,\ . 1 ~. . i- --.L • ~ 1-1 ~. if . .-)I tv ., ~r ,'-11\ ~.)"::J ~~'_'*- 1- ';-:a .It: ~/. ~ -:} 6f ·1-.... ~ .,., . ~ ~ -'71,.. Jt -(, l' A\'\ V,'. . ... 11\ ~\~ ~ .\ .1- . .. IJ,I Downward Flow ofl ~ A+ 1PorOU5 Formation5 ~ ~permitting free ~ ~~ "' ~~ ~;t1Cold Water 'f ~~ c ~ ~Groundwater CircUlati~ ~ ."Upward F1 ow~ - _ ....... Ifof Warm Water )1-, Form of Geother-/f.. ........ mal Gradient.;:;,;:r.

- 17 -The effects of heat on the circulation of groundwater can be verystriking, in particular when considered from the hydrogeological viewpoint.By raising temperatures of groundwater, its viscosity will be decreased, sothat its transmissibility will be increased. Thus, raising temperaturesfrom lOoe to 20 0 e will increase flow by over 30%, all other conditionsremaining the same. Heating also decreases the density of the warmedgroundwater, and such density differences reinforce gravity-controlled flows.As such thermal flow continues, much more heat will be brought up in thewater by convection than by the earlier conduction in the rock and/orstatic water. Rising temperatures can induce thermal shattering of therock, and increase permeability. While such flow mechanisms areconsidered here for basins of say 8 kilometres diameter, they have beenstudied on a continental scale by one of the authors in the great artesianbasins of North Africa and Arabia (Burdon, 1978).The depths to which groundwater circulation might extend can beinferred according to the assumed geothermal gradient and the amount ofmixing. Accepting a geothermal gradient of 25 0 e/kilometer, and an increaseo 0 0of water temperature of 15 e (from 10 e to 25 e), then the depth ofcirculation below surface would be of the order of 600 metres with nomixing. If there is much admixture of cooler (and shallower) groundwaterto the rising warm water, then the depth of circulation could be much greater.The fact that spring water temperatures rise as flow increases canbeexplained in a number of ways. strong thermal upflows may push aside otherwaters and result in less mixing. The greater the volume of upflow, themore 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 expectedin the shelf limestones, as in the Upper Tournaisian. Sulphate nodulesand lenses have been reported, but it would seem that halite was notprecipitated. High chlorine and total dissolved solids characterisethree 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 theaxis of a syncline in the Lower Visean. There would be reason forbelieving that these springs obtain their more-then-average mineralisedgroundwaters from deeper beds with some evaporites. The other springshave chlorine which can in part be due to cyclic chlorine in theprecipitation, though as already noted NOS. 1005 and 1006 also arecomparatively high in chlorine. While there are no apparent directcorrelationships between temperatures and hydrochemistry, the latter supportsthe idea that the groundwater circulation is sufficiently deep in at leastone group of warm springs to have gone down to the evaporite beds thoughtto exist in the Upper Tournaisian.There is little information as to the formation conditions which~uuld allow groundwater to circulate freely down to depths of the order of700 metres. The Hercynian folding of the rocks must have opened up flowpaths on anticlines and synclines. The geological location of severalof the warm springs would suggest connections with anticlinal or synclinalaxes. The extent and depth of Quaternary karstification in Ireland isstill very uncertain; but karstification below say 100 metres from surfaceis not very likely, controlled as it should be by low sea levels duringand soon after the glacials. However, if evaporite beds, as gypsum,anhydrite and halite, form appreciable sequences in the general carbonatesuccession, then their solution in circulating groundwater (not controlledby the availability of free carbon dioxide in the water), could result indeep karstification.

- 18 -The effects of the varying thicknesses of Quaternary cover alsocall for some comment, since at least two of the warm springs becameactive only as the result of man's activities. It would seem that theboulder clay is effective in holding down such other rising warmgroundwaters 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 sandand gravel in the main cover of boulder clay. In such areas, hotsprings could have come into existence without man's intervention.111-3. Heat Sources and Heat ConservationThe present review and analysis of existing data have shown thatTertiary basic intrusives occur in the carbonate formations through whichthe warm springs rise and from which they issue. Dating back probablyto the Palaeocene-Eocene on K-Ar evidence (though possibly younger onpalyneological indications), these basic intrusives are no longer sourcesof geothermal heat. But it is just possible that they initiated thepresent cellar circulation of groundwater in this area of Leinster, andthat once initiated it has continued to function with lesser amounts ofheat supplied by the normal geothermal gradient (say 2S o C/km) of the area.Gravity measurements showed lows around Navan and Drogheda.It was inferred that these indicated the presence of small masses ofburied granite. Now it seems more probably that they are due to the muchgreater thicknesses of Balaeozoic sediments underlying the region.Buried granites are thus an unlikely source of the heat for the circulationof these warm springs.No study has been made of the possible amounts of heat generatedby 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 forthe present.Finally, there are possible effects of heat conservation by acover of low-conductivity formations. The 800 or so metres of RathmolyonShale Formation (Lower Visean age) could indeed act as a cap rock to holddown heat arriving in the underlying Tournaisean limestones. Limitedopening (as on antclines and synclines, or along faults) might permitconcentrated upflow of heat (water-bo'rne) and provide localised heatupflows for the waters in the overlying limestones, (1150 metres thickaccording to Table 2) .The effects of the cover of mainly impermeable Quaternary depositshas already been noted; it will not affect the deep circulation of thegroundwater.111-4. ConclusionThis compilation and analysis of the Jata on the warm springs ofIreland has shed a little light on hydrogeothermal conditions in some of thewet rock areas of Ireland. In particular twelve main inferences have beendrawn regarding the warm springs of Leinster, as in Item 11-7. In turn,this data has been used to formulate a possible picture of thermallycontrolled circulation of groundwater in cells some B kID in diameter andextending 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 thermalcirculation of warmed groundwater.IVACKNOWLEDGEMENTS AND REFERENCESThe thanks of the authors are due to Dr. C.E. Williams who hasencouraged and facilitated this investigation, and to colleagues in theGeological Survey and outside who have supplied information on manyaspects of the wor~ not least to the staff of the State Laboratory. Inthe course of preparing the "Geothermal Energy Potential of Ireland" projectthe authors have had very fruitful discussions with their colleaguesProfessors Peter Br~ck of Cork, Professor Andrew Brock of Galway, as wellas Messers Eamon Kinsella and Keith Robinson of the National Board forScience and Technology. And as the paper was being finalized, there wasa 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 facilitatethe more efficient collection of further data and the application offindings to the possible development of a valuable energy resource of thecountry.The debt of the authors to other workers is acknowledged by thefollowing list of references.REFERENCESALDWELL, C.R. and BURDON, D.J., (1979) "Groundwater Investigations in Eire"Proc. 'Hydrology in Ireland' meeting May 1979; IHPNat. Committee.BROCK, A. (1979) "Geothermal Energy in Ireland" Internal Report, Nat. BoardSci. and Tech., Dublin, 68p." BRUCK, P.M. (1971) "The Lower Carboniferous Rocks of the Naas District,Co. Kildare" Bull. Geol. Survey Ireland, Vol. I, pp. 211-221.BURDON, D.J. (1978) "Infiltration Conditions of Major Sandstone Aquiferaround Ghat, Libya" Sec. Sym. on Geology of Libya, Tripoli;(in press) •and CULLEN, K. (1980) "Hydrochemistry of Caradocian Volcanicsin S.E. Ireland and the Effects of Preclpitation Composition onthe Groundwater Chemistry". Fourth Internat. Symp. on Groundwater,Catania, Feb. 1980.BYRNE, B., DOWNING, D. and ROMER, D. (1971) "Some Aspects of the Genesisof the Lead-Zinc Orebody at Navan" Irish Geological Assoc.,Galway meeting on "Genesis of Base Metal Deposits in Ireland".C.E.C. (1977) "First Seminar on Geothermal Energy" Brussels, 2 Vols., pp. 732C.E.C. (1980) "Second Seminar on Geothermal Energy", Strasbourg.CERMAK, V. and RYBACH, L. (1979) "Terrestial Heat Flow in Europe"Springer-Verlag.

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- 21 -MURPHY, T.(1962) "Some Unusual Low Bouguer Anomalies of Small Extent inCentral Ireland, -and their Connection with Geological structure"Geophy. Prosp., Vol. X, pp. 258-269.MURPHY, T. (1974) "Gravity Anomaly Map of Ireland" Conun. Dublin Inst.Advanced Studies, Geophys. Bull No. 32D.NEVILL, W.E. (1957) "The Geology of the Summerhill Basin, Co. Meath"Proc. Roy. Irish Acad., Vol. 58B, pp. 293-302.NORTON, D. (1978) "Source-lines, Source-regions and Path-lines for Fluidsin Hydrothermal Systems related to Cooling Plutons" Econ. Geol.,Vol. 73, pp. 21-28.PHILLIPS, W.E.A., STILLMAN, C.J. and MURPHY, T. (1976) "A Caledonian PlateTectonic Model" Jour. Geol. Soc., Vol. 132, pp. 579-609.PLUNKETT, W. and STUDDERT, L. (1883) "Report on the Solid and GaseousConstitutents of the Mallow Spa, in the County of Cork" Proc.Roy. Irish Acad., Vol. 3, pp. 75-78.RUSSELL, M.J. (1978) "Downward-excavating Hydrothermal Cells and Irish-typeOre Deposits: Importance of an Underlying thick Caledonian Prism"Trans. I.M.M., Section B, Vol. 87, pp. 168-171.RUTTY, (1757) "Mineral Waters of Ireland" - "Book V, of the Warm Waters,and particularly those of Mallow" Nat. Library cat. No. J. 61338.SABINE, P.A. and WATSON, J.V. (1965-67) "Isotopic Age-Determinations ofRocks and Minerals from the British Isles" Q.J.G.S., Vol. 120,121, 122.SELWYN TURNER, J., HORNUNG, G. and REX, D.C. (1972) "Tertiary IgneousActivity at Navan, Co. Meath" Irish Naturalist Jour., Vol. 17,pp. 262-264.SHERIDAN, D. (1972) "The Stratigraphy of the Trim No.1 Well, Co. Meath;and its Relationships to the Lower Carboniferous Outcrops inEast-Central Ireland" Bull. Geol. Survey Ireland, Vol. 1,pp. 311-344.STROGEN, P. (1974) "The sub-Palaeozoic Basement in Central Ireland" Nature,Vol. 250, pp. 562-3.TURNER, J.S. (1952) "The Lower Carboniferous Rocks of Ireland" L'pool andManchester Geol. Jour., Vol. I, pp. 113-147.~HLLIAMS, C.E. and McARDLE, P. (1978) "Mineral Deposits of Europe - Vol. I:Northeast Europe - Ireland" Inst. Min. and Met., London, pp. 319-345.