Hydrogeothermal Conditions in Éire - Geological Survey of Ireland

- 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.