Taff's Well thermal spring is located in Taff's Well or Taffs Well (Welsh: Ffynnon Taf) north of Cardiff, South Wales, UK (National Grid Reference ST 11925 83639) and an elevation of about 30m AOD. It is the only thermal spring in Wales. The spring emerges on the eastern bank of the River Taff and has been contained within a brick well structure and building. Access is via Taff's Well park, a public park, owned and maintained by "Rhondda Cynnon Taff Council".
The spring is enclosed inside a well and stone building constructed in the 19th century. The well built to contain the spring waters is brick lined and approximately 4m deep. The building has an internal dimension of 5.3m x 3.9m and water fills this area. A brick built spiral staircase is incorporated in the inside of the well. There is one visible overflow pipe which emerges several meters to the west of the spring, on the bank of the River Taff.
Thermal springs are also located in Bath, Somerset Bristol Hotwells,Buxton in England and Mallow, County Cork in Ireland. There are no reported thermal springs in Scotland.The thermal springs are all located in Carboniferous Limestone synclines and basins. These structural settings allow water to descend to sufficient depth for it to be heated by the geothermal gradient, and to return to the surface without a significant fall in temperature Taffs Well spring is also located in a similar setting, the southern limb of the south Wales Coalfield syncline.he site is not protected by any statutory legislation; it is however a registered 'Regionally Important Geological Site' or RIGS site and a Grade II listed building site.
Taff's Well spring emerges from the southern limb of the south Wales Coalfield ,Carboniferous Millstone Grit occurs in the immediate area of the Taff's Well thermal spring. The underlying Carboniferous Limestone can be seen outcropping just south of Taff's Well.The underlying Devonian Old Red Sandstone outcrops over 2 km southeast, just north of the M4 motorway.The superficial geology in the area of the Taff's Well spring comprises an unknown thickness of river alluvium and river terrace deposits.
The Taffs Well sub-unit is bound to the south west by the Tongwynlais or Taff's Well Fault, which runs north-south, crossing the River Taff, and passing very close to the Taff's Well Thermal Spring. The Taffs Well Fault is a continuation of the Daren-ddu fault which is a major NW-SE trending fault in the Coal Measure rocks of the South Wales Coalfield.The Tongwynlais Fault is a normal fault and affects both the Carboniferous Limestone and underlying Devonian Upper Old Red Sandstone. The fault has a downthrow of 85m to the west.
Environment Agency Wales have monitored the temperature of the well since 2008, using a Solinst LTC logger. During this period the maximum recorded temperature was 22.12 °C and the average temperature 21.6 °C. The temperature varies +/- 0.5 °C about the average temperature of 21.6 °C. The temperature can, on the whole be described as constant.The average temperature at Taff's Well spring, 21.6 °C is over 10 °C warmer than the average groundwater temperature of 11.3 °C in Wales
There are no historical water level readings within the Taff's Well spring, this is due to the overflow pipe which helps to maintain the level of the well. Iron and manganese staining on the inside of the well house show that there is fluctuation of water levels. Flooding in front of the well is also observed, forming a warm water temporary ‘thermal pond’.
Measurements of flow from the overflow pipe are rare. Slaters Dictionary mentions the well is close to the river into which the waters are continually flowing .In 1877 the well was described as ‘very powerful, and appears from rough calculation to afford about 800 gallons per hour.’ (or about 0.001m3/s).Modern readings show the volume of water from the overflow pipe can vary between 0.001m3/s - 0.005m3/s. The spring has never run dry.