The hydrogeology and hydrochemistry of the thermal waters at Taffs Well, South Wales, UK

Taffs Well is the only thermal spring in Wales, with an average temperature of 21.6°C ± 0.5°C. The River Taff is adjacent to the spring and removal of a weir and work on flood defences has reduced mixing with flood water from the river. This has enabled data to be gathered that more closely represent the thermal water end-member than previously possible. Limited interaction with modern waters is confirmed by tritium, nitrate, CFC and SF6 concentrations below or close to lower detection limits, showing at most 6% mixing with modern waters. 14C dating suggests a conservative age estimate of at least 5000 years. Values for dissolved noble gases suggest that the waters originate as rainfall at an altitude several hundred metres higher than the spring. The northern Carboniferous Limestone outcrop is proposed, which would then require recharged waters to flow to a depth of 400m and distance of 25km, following the synclinal structure of the South Wales Coalfield, to discharge at the spring. Sr isotope data suggest interaction with the Marros Group (formerly known as the Millstone Grit), the waters flowing within or close to the contact between the Carboniferous Limestone and Marros Group before rising via the Tongwynlais Fault.