Hydrogeothermal Conditions in Ãire - Geological Survey of Ireland

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- 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
Page 1 and 2: IIII1HXDROGEOTHERMAL CONDXTXONSIN E
Page 3 and 4: convection uprising of warm water i
Page 5 and 6: BallinalackNo .1011Hurler's .Cro~5N
Page 7 and 8: - 2 -Normal groundwater in Ireland
Page 9 and 10: - 3 -Ref. No.Name or NamesCounty100
Page 11 and 12: - 5 -II-l. LocationThe general loca
Page 13 and 14: - 7 -Thickness(m)Lithology+ 335 m+
Page 15 and 16: - 9 -Boreholes drilled for water, p
Page 17 and 18: IIII.JJJ.S 0 .No-- --~--f-22C:1. -
Page 19 and 20: - 12 -iSpring No. 2001 2001 2002 20
Page 21 and 22: - FIGURE 5 - TRACE ELEMENTS SILICON
Page 23 and 24: - 14 -Spring No. 2001 2001 2002 200
Page 25 and 26: M'ddl~Slo9~CrySlol\'n~ bos.m.ntor d
Page 27: FIGURE 7 - POSSIBLE PRESENT~DAY GRO
Page 31 and 32: - 20 -CLAYTON, G. and HIGGS, K. (19

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Hydrogeothermal Conditions in Ãire - Geological Survey of Ireland