Penzance Water Hardness & Quality Report (2026)

South West Water supplies Penzance, the most westerly major town in mainland England, primarily from Drift Reservoir on the River Drift near Sancreed and a network of surface water and groundwater sources across the far west of Cornwall, treated at Drift Water Treatment Works before distribution to the Penzance and Newlyn area. At 187.5 mg/L (13.2°Clark), Penzance's water is moderately hard — a level considerably higher than would be expected for a Cornish town underlain predominantly by granite, reflecting blending with mineralised sources from the broader supply network.

West Cornwall is underlain by the Land's End Granite — a hard, chemically resistant igneous rock with no calcium carbonate that typically yields very soft, acidic groundwater. However, the granite is intruded into Devonian sedimentary country rocks that include thin limestones and calcareous beds, and the thermal metamorphism at the granite margins produces calcareous skarn zones. Groundwater circulating through these contact zones and through any Devonian limestone inclusions acquires dissolved calcium hardness far above granite levels. Blending sources from across the west Cornwall network introduces this mineralised component to Penzance's supply.

At 187.5 mg/L, limescale is a noticeable domestic concern in Penzance. Kettles benefit from descaling every four to six weeks to maintain element efficiency and avoid calcium deposits in drinks. The combi-boiler should be fitted with a scale inhibitor and serviced annually to check for calcium build-up in the heat exchanger. Washing-up liquid performs satisfactorily at normal quantities, though some reduction in lather is noticeable. Taps and shower heads accumulate moderate limescale deposits over several weeks; a monthly wipe with white vinegar or a proprietary descaling solution keeps surfaces clean and prevents hard-water staining from becoming established on fixture surfaces.