| Abstract |
Far from plate boundaries, with no recent volcanism or tectonism, and with geothermal gradients of <25C/km, Ireland has few geothermal resources apart from 42 warm springs ranging in temperature from 13-24.7C. These are concentrated in two groups, in the south-west and east central parts of the country. Recently groundwater at 26C was encountered at a depth of 40m during routine well drilling operations near the town of Mitchelstown in the south-west, the warmest groundwater encountered to date in the shallow Irish subsurface. It is interpreted to have migrated upwards from greater depth via a steep fault structure. A research project is in progress with the aim of identifying the source aquifer and fault conduit, controlling upwards movement of the warm water, and also to assess the potential of the warm water for district heating purposes. A major NE–SW lineament, identified on landsat images, of probable Caledonian (425-395 Ma) age, possibly subsequently reactivated during the Variscan Orogeny (295-315 Ma), passes close to the Mitchelstown well. It extends 30 km SW to the town of Mallow, where a 22C warm spring, which formed the basis for a spa resort in the 19th century, is today being utilised with a heat pump to heat the municipal swimming pool. Geophysical surveys are being conducted to accurately delineate this structure on the ground. A temperature survey of all water wells and springs in the Mallow-Mitchelstown area and further to the NE has been conducted. Although average groundwater temperatures in Ireland are of the order of 9-11.5C, a number of the wells surveyed record anomalous temperatures in excess of 12C, interpreted to represent mixing of warm deep groundwater with cooler near surface groundwater. To test this hypothesis, a programme of hydrochemical analyses has been undertaken for normal and trace components and the stable isotopes 18O/16O and 2D/1H. Lower nitrate and chloride/bromide ratios and possibly higher lithium in the anomalous wells appears to differentiate the warmer water from depth from cooler near surface water with which it has mixed. This it is hoped will also fingerprint the source of the warm waters. The ultimate objective is to locate the fault conduit sufficiently accurately to make it possible to drill to intersect the fault at moderate depth in order to tap the migrating warm water where it can be utilized, and to this end it is hoped to develop a methodology which can be applied in other similar situations. |