Caerphilly Water Hardness & Quality Report (2026)

Caerphilly, the famous castle town in the Rhymney valley in Caerphilly County Borough, renowned as the original home of Caerphilly cheese, is supplied by Dŵr Cymru Welsh Water. Supply draws on upland impoundments in the upper Rhymney and Sirhowy valleys, principally from the Llyn Aber and Bargoed Rhymney reservoir systems and catchment sources within the Caerphilly and Merthyr Tydfil uplands. These reservoir watersheds drain across Carboniferous Namurian shale and Pennant Sandstone — the characteristic hard silicic rocks of the south Wales coalfield — that contribute minimal dissolved calcium to surface runoff. Water is treated at Llanishen Water Treatment Works north of Cardiff and distributed northward through the Caerphilly valley network.

The Carboniferous Pennant Sandstone and lower Coal Measures that dominate the Rhymney and Sirhowy valley geology are calcium-poor, producing soft to moderately soft reservoir water across the south Wales coalfield. Where minor thin limestone horizons within the Carboniferous sequence crop out in the lower valleys, small amounts of calcium bicarbonate are dissolved into river flows. This accounts for Caerphilly's hardness being slightly above the very soft level seen in the upper Rhondda or Cynon valleys — the lower altitude and more mixed lithology near Caerphilly town introduce modest mineral content. The TDS of 285.9 mg/L remains low, confirming a predominantly rainfall-and-reservoir character.

At 126 mg/L Caerphilly's water is moderately soft and limescale is a relatively minor concern for residents. Kettles accumulate a light film of scale after two to three months of regular use — a brief descaling with a citric acid tablet every couple of months keeps elements in good condition. Shower screens remain clean for longer than in hard-water areas. Washing-up liquid lathers well. Combi-boilers and washing machines face low scaling risk. As with all soft Welsh valley water, older metallic plumbing may be at slight risk of corrosion — properties with lead or early copper pipework should have water quality checked. The castle and its deep moat may be Caerphilly's showpiece, but its soft water supply is a daily practical asset.