Hydrogeothermal Conditions in Éire - Geological Survey of Ireland

- 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.