 The temperature varies +/- 0.5 °C about the average temperature of 21.6 °C.The temperature can, on the whole be described as constant, with lower temperatures on occasion being attributed to mixing with Taff water or groundwater after heavy rain.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

"Environment Agency Groundwater Quality Review / Baseline Reports".