Truro Water Hardness & Quality Report (2026)

Truro, the cathedral city and county town of Cornwall at the head of the Fal estuary, is supplied by South West Water from the Colliford Reservoir (Bodmin Moor) and the River Fal catchment, supplemented by transfers from South West Water's mid-Cornwall regional supply network. Like St Austell (246 mg/L) to the east, Truro's hardness of 198.5 mg/L is anomalously high for a Cornish city surrounded by the Cornish Devonian Killas (slate and phyllite) and the Carmenellis Granite — rock types that normally produce very soft water at 20–50 mg/L. The Colliford and Bodmin Moor reservoirs drain purely granitic and Devonian metamorphic catchments that should yield near-distilled-quality soft water. The elevated 198.5 mg/L hardness with TDS 531.7 mg/L reflects South West Water's integrated distribution network, which incorporates harder supply from calcareous Devonian mudstone and siltstone horizons in the east Cornwall and Tamar valley fringe, as well as supply transferred from the regional infrastructure to balance demand across the Cornish peninsula.

The Devonian Killas and granites of mid-Cornwall produce inherently soft water. The deviation to 198.5 mg/L in Truro's supply indicates South West Water's network routing of harder water from east Cornwall (Tamar valley Devonian limestone and calcareous formations) into the Truro distribution zone. The River Fal and its tributaries drain areas of Devonian calcareous sedimentary rock east of the Carnmenellis granite, and surface-water abstraction from the Fal catchment contributes harder water than pure granite-granite-drainage alone. The TDS of 531.7 mg/L further suggests sulphate and mineral contributions beyond calcium bicarbonate alone.

At 198.5 mg/L Truro's water is moderately hard — a surprising characteristic for the remotest cathedral city in England. Kettles benefit from monthly descaling with a citric acid tablet. Shower screens and tap surfaces accumulate a calcium film requiring regular white vinegar treatment. Washing-up liquid must be used in moderate amounts. Combi-boilers benefit from inline scale inhibitor protection. Truro's elegant Georgian and Victorian city character, its cathedral, its ancient quays on the Fal and its Cornish identity are unmistakable — but its moderately hard domestic water supply is geographically anomalous for a city at the granite heart of Cornwall.