Gelligaer Water Hardness & Quality Report (2026)

Dŵr Cymru Welsh Water supplies Gelligaer, a former ironworks community in the Rhymney Valley in the Caerphilly County Borough of south Wales. Supply is drawn from Llwyn-on Reservoir and Pontsticill Reservoir in the Brecon Beacons, impounded on tributaries of the River Taf Fechan, treated at Llwyn-on Water Treatment Works before distribution down the Rhymney Valley. At 119 mg/L (8.3°Clark), Gelligaer's water is moderately soft, reflecting the upland Brecon Beacons catchment geology mixed with contributions from the Carboniferous Limestone of the South Wales coalfield rim.

The Brecon Beacons reservoir catchments drain Old Red Sandstone (Devonian) and ancient volcanic rocks — formations low in soluble calcium — producing naturally soft upland water. However, as the supply is distributed down the Rhymney Valley, it passes through and is drawn from zones where the Lower Carboniferous Limestone outcrops at the valley heads, adding moderate calcium hardness. The resulting moderately soft character at 119 mg/L reflects this geological transition between the soft sandstone Beacons and the limestone ridges that frame the South Wales valleys.

At 119 mg/L, Gelligaer's water is moderately soft with manageable limescale demands. Descaling the kettle every six to eight weeks is typically adequate. The combi-boiler benefits from a fitted scale inhibitor as a precaution. Washing-up liquid lathers satisfactorily at normal quantities. Taps and shower heads develop only light limescale deposits over several weeks; a monthly clean with white vinegar keeps fixtures in good condition. The soft Brecon Beacons water is characteristic of the South Wales valleys supply zone, and household appliances generally enjoy a longer service life than in harder water areas of England.