A multi-disciplinary investigation of the provenance, pathways and geothermal potential of Irish thermal springs

Abstract

The geothermal energy of thermal groundwater is currently being exploited for district-scale heating in many locations world-wide. The Carboniferous bedrock in the south and east of Ireland hosts a number of thermal springs with temperatures ranging from 12 – 25 °C. These temperatures are elevated with respect to average Irish groundwater temperatures (9.5 – 10.5 °C), and represent a geothermal energy potential, which is currently under evaluation. This thesis furthers our understanding of the sources, circulation pathways and temporal variations of the Irish thermal springs, by using a multi-disciplinary methodology (including audio-magnetotelluric (AMT) geophysical surveying, time-lapse temperature and chemistry measurements, and hydrochemical analysis) to develop hydrogeological conceptual models for several of these springs. A sub-set of six springs in the Carboniferous limestones of the Dublin Basin were examined. Seasonal hydrochemical data were explored using multivariate statistical analysis to investigate the source aquifers of the thermal groundwaters. The analysis indicates that the thermal waters flow within the limestones of the Dublin Basin, and there is evidence that some springs receive a contribution from deep-basinal, saline fluids. Three-dimensional electrical resistivity models of the subsurface were constructed from AMT data collected at Kilbrook spring (maximum of 25.0 °C) and St. Gorman’s Well (maximum of 21.8 °C). These models revealed two types of geological structure beneath the springs; (1) Carboniferous normal faults, and (2) Cenozoic strike-slip faults. The karstification of these vertically-persistent structures, particularly where they intersect, has provided conduits that facilitate the operation of a relatively deep hydrothermal circulation pattern (likely estimated depths between 240 and 1,000 m) within the Dublin Basin. The thermal maximum and simultaneous increased discharge observed at several of the springs each winter must be the result of rapid infiltration, heating and re-circulation of meteoric waters within a structurally- and recharge-controlled hydrothermal circulation system.