Sutton Water Hardness & Quality Report (2026)

Sutton, a south London borough in the London Borough of Sutton, is supplied by Thames Water from the capital's combined surface and groundwater infrastructure. The borough is served by a blend of River Thames surface water stored in the Thames Valley Reservoir Group — including Queen Mother Reservoir near Datchet — and contributions from the North Downs Chalk Aquifer, which underlies Sutton directly. The North Downs chalk dips northwards beneath south London, and licensed boreholes in the Sutton and Epsom area extract groundwater from this chalk. Water is treated at Kempton Park Water Treatment Works and Carshalton Water Treatment Works before distribution through the south London network serving the borough.

Sutton's hardness of 193 mg/L (13.5°Clark) reflects its position above the North Downs Chalk. The North Downs are formed from Cretaceous Upper Chalk — the same highly porous, calcium-rich formation that produces hard water across the Home Counties. Sutton sits on the dip slope of the Downs where the chalk dips northward beneath the London Clay, and the chalk groundwater accessed by local boreholes carries high calcium carbonate concentrations from decades of percolation through this rock. Thames surface water blended into the supply adds a further mineral contribution from its chalk and limestone upstream catchments, resulting in water classified as moderately hard by the Drinking Water Inspectorate (DWI).

Limescale is a regular feature of Sutton households. At 193 mg/L, limescale develops in kettles within three to four weeks and monthly descaling is advisable. Combi-boiler heat exchangers accumulate limescale deposits steadily at this hardness — annual boiler servicing with limescale inspection is important, and an in-line scale inhibitor is a sensible addition to any boiler cold feed. Showerheads, taps, and glass shower screens develop limescale deposits at a steady rate. Washing-up liquid lathers less well than in soft-water areas. Using Calgon monthly in the washing machine and keeping descaling products to hand is standard household practice for Sutton residents.