St Austell Water Hardness & Quality Report (2026)

St Austell, the china clay capital of mid-Cornwall in the St Austell Bay hinterland, is supplied by South West Water. The supply for mid-Cornwall draws on Stithians Reservoir near Redruth and the inter-linked Colliford and Crowdy Reservoirs on Bodmin Moor, supplemented by abstraction from the River Par and River Fal catchments. The striking hardness of 246 mg/L — exceptional for a town surrounded by the soft St Austell Granite moorland and Devonian slate country — reflects the blend of supply sources in South West Water's mid-Cornwall distribution grid. The local granite and Devonian slate catchments produce inherently very soft water (20–40 mg/L), but South West Water's regional distribution network incorporates supply transfers from areas underlain by Devonian limestone and calcareous mudstone outcrops in the Liskeard and east Cornwall fringe, which contribute harder groundwater to the supply blend. The TDS of 724.5 mg/L, disproportionately high for the hardness level, suggests additional mineral contributions from evaporite or calcareous geological horizons in the wider supply network.

The St Austell Granite (Hensbarrow Granite) and surrounding Devonian Killas (slate, phyllite and siliceous mudstone) of the St Austell area are calcium-depleted rocks that in pure local drainage produce the soft water typical of Cornish moorland. The anomalous 246 mg/L hardness in St Austell's supply is thus not a product of local geology but of water transfer within South West Water's integrated distribution system, which routes harder supply from calcareous formations in the wider western peninsula into the mid-Cornwall network to balance demand and resource availability.

At 246 mg/L St Austell's water is hard — a significant surprise for a Cornish town. Kettles should be descaled monthly with a citric acid tablet. Shower screens develop a calcium film requiring regular cleaning with white vinegar. Washing-up liquid must be used generously. Combi-boilers and white goods benefit from inline scale inhibitor cartridges and regular servicing. The landscape of gleaming white china clay spoil tips above St Austell hints at the unusual geological setting — and its unexpectedly hard water supply is an equally distinctive feature of everyday domestic life in this corner of Cornwall.