Mitcham Water Hardness & Quality Report (2026)

Mitcham, in the London Borough of Merton in inner south London, is supplied by Thames Water from the River Thames storage reservoir system. Thames Water abstracts from the Thames at Hampton and Walton-on-Thames, treating at Walton Water Treatment Works, and distributes through the inner south London network serving Merton and Mitcham. The Thames supply carries dissolved calcium from the southern England chalk and Jurassic limestone catchments — the Chalk Aquifer drainage of the Kennet, Wey, and Mole tributaries contributing the dominant calcium load to the Thames at west London intakes. The Merton distribution network distributes treated Thames reservoir water to Mitcham, Morden, and the surrounding south London area.

Mitcham's hardness of 234 mg/L (16.4°Clark) is characteristic of the inner south London Thames supply zone — consistently hard from chalk-stream tributary chemistry accumulated in the Thames. This is slightly lower than the most extreme Lee Valley chalk supply zones (above 300 mg/L) in north-east London, reflecting the south London network's greater reliance on Thames reservoir storage dilution. The Drinking Water Inspectorate (DWI) classifies this supply as hard.

Limescale is a persistent household challenge in Mitcham. At 234 mg/L, limescale forms in kettles within two to three weeks and monthly descaling is advisable. Combi-boiler heat exchangers accumulate deposits steadily — annual boiler servicing and fitting an in-line scale inhibitor are recommended. Showerheads, taps, and shower screens develop regular deposits. Washing-up liquid lathers poorly. Using Calgon monthly in the washing machine and maintaining a regular descaling routine is important limescale management for Mitcham and south London Merton households.