Croydon Water Hardness & Quality Report (2026)

Croydon, one of Greater London's most populous boroughs, is supplied by Thames Water drawing from its combined South London infrastructure. The borough receives a blend of River Thames surface water stored in Thames Valley reservoirs and groundwater abstracted from the North Downs Chalk Aquifer, which extends beneath Croydon from the Surrey chalk escarpment northward under London. Licensed boreholes in the Croydon area — including abstraction sites at Beddington and the surrounding south London area — tap into chalk groundwater that has percolated through the North Downs over decades. Water is treated at Beddington Water Treatment Works and Kempton Park Water Treatment Works before distribution through the south-east London network to the Croydon borough.

Croydon's very high hardness of 328 mg/L (23.0°Clark) results from its proximity to the North Downs Chalk. The North Downs are formed from Cretaceous Upper and Middle Chalk — the same highly porous, calcium-rich limestone that produces very hard water across the South East. The chalk dips northward beneath Croydon, and chalk borehole water here carries extremely high calcium carbonate concentrations — among the highest in Greater London — from its slow passage through the chalk bedrock. Thames surface water blended into the supply adds further mineral content from chalk and Jurassic limestone upstream catchments, placing Croydon's water at the very hard end of the Drinking Water Inspectorate (DWI) scale.

Limescale is one of the most pressing household maintenance challenges in Croydon. At 328 mg/L, limescale forms extremely rapidly in kettles — a visible crust appears within one to two weeks, requiring weekly or fortnightly descaling with a proprietary descaler. Combi-boilers are at significant risk from limescale accumulation in the heat exchanger, which can cause early failure and dramatically reduce efficiency; annual servicing is essential, and fitting an in-line polyphosphate scale inhibitor is strongly recommended. Showerheads, taps, and shower screens require aggressive, regular descaling. Washing-up liquid lathers very poorly at 328 mg/L. Croydon homeowners should seriously consider an ion-exchange water softener for comprehensive long-term limescale protection.