Treharris Water Hardness & Quality Report (2026)

Dŵr Cymru Welsh Water supplies Treharris, a former colliery village in the Taff Valley in Merthyr Tydfil County Borough — once the home of the Deep Navigation Colliery that produced high-quality steam coal and employed generations of the valley community — from Llwyn-on Reservoir in the Brecon Beacons, a reservoir on the upper Taff Fechan catchment in the Brecon Beacons National Park, treated at Llwyn-on Water Treatment Works. At 109 mg/L (7.6°Clark), Treharris' water is slightly hard — somewhat more mineralised than many south Wales valley communities — reflecting modest calcium contributions from the calcareous Carboniferous Limestone (Dinantian) outcrops at the northern rim of the Brecon Beacons catchment.

Treharris lies in the Taff Valley coal measures below the Brecon Beacons plateau. Welsh Water supplies from Llwyn-on Reservoir on the upper Taff Fechan — a catchment bordered to the north by the Carboniferous Limestone escarpment of the Beacons, which occasionally contributes modest calcium to the otherwise soft upland reservoir water. The resulting 109 mg/L with TDS 230.3 mg/L is slightly hard — harder than the fully upland shale and grit catchments further west — reflecting the limestone fringe influence in the upper Taff Fechan Brecon Beacons supply.

At 109 mg/L, limescale is a moderate and manageable domestic concern in Treharris. Descaling the kettle every six to eight weeks is typically adequate. The combi-boiler benefits from a standard scale inhibitor as a precaution. Washing-up liquid lathers adequately at everyday quantities. Taps and shower heads develop light deposits over several weeks; a monthly wipe with white vinegar keeps fixtures in good condition. The slightly hard Brecon Beacons reservoir supply at Treharris is characteristic of the upper Taff valley communities where the limestone-fringed Beacons catchment adds a modest calcium increment above the softest south Wales upland baseline.