Llantrisant Water Hardness & Quality Report (2026)

Dŵr Cymru Welsh Water supplies Llantrisant, the historic hilltop town and home of the Royal Mint in Rhondda Cynon Taf at the Vale of Glamorgan fringe, from Llwyn-On Reservoir in the upper Taf Fawr valley beneath the Brecon Beacons, supplemented by the wider south Wales upland reservoir supply chain, treated at regional Rhondda Cynon Taf works. At 131.5 mg/L (9.2°Clark), Llantrisant's water is slightly hard, somewhat more mineralised than the softest South Wales valley communities — reflecting the geological transition between the upland reservoir catchment and the Carboniferous Limestone and Old Red Sandstone country of the Glamorgan Vale fringe.

Llwyn-On Reservoir is impounded in the upper Taf Fawr above Merthyr Tydfil, draining the southern scarp of the Brecon Beacons — an upland of Old Red Sandstone (Devonian) and Carboniferous Limestone at the Beacons escarpment. As this water is distributed southward to the Vale of Glamorgan fringe at Llantrisant, it traverses the Carboniferous Limestone of the south Wales coalfield northern fringe and the Lower Coal Measures, contributing modest calcium to the supply blend. The resulting 131.5 mg/L with TDS 305.3 mg/L is slightly more mineralised than the pure upland reservoir baseline.

At 131.5 mg/L, limescale is a moderate and manageable domestic concern in Llantrisant. Descaling the kettle every six to eight weeks is typically adequate. The combi-boiler benefits from a standard scale inhibitor as a sensible precaution. Washing-up liquid lathers adequately at everyday quantities. Taps and shower heads develop only light deposits over several weeks; a monthly wipe with white vinegar keeps fixtures clean. The slightly hard supply is characteristic of the Vale of Glamorgan fringe communities where the soft Brecon Beacons reservoir supply picks up modest mineralisation from the southward traverse of the limestone and Coal Measures geology.