Hydrogeothermal Conditions in Éire - Geological Survey of Ireland

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.