Chiswick Water Hardness & Quality Report (2026)

Chiswick, in the London Borough of Hounslow on the north bank of the Thames at the Chiswick reach, is supplied by Thames Water from the River Thames storage reservoirs at Hampton and Walton-on-Thames and from the Chiltern Chalk Aquifer and Thames Valley Chalk groundwater. Thames Water operates the west London reservoir ring — Queen Mary, Knight, and Wraysbury — storing Thames water rich in chalk calcium from the Wye, Colne, and other chalk-stream tributaries. The chalk borehole network beneath the Thames valley west of London adds further dissolved calcium from the Cretaceous Chalk of the Chilterns. Chiswick's position on the inner Thames reach in the west London supply zone is within one of Thames Water's hardest supply areas, with the chalk groundwater proportion high.

Chiswick's very hard water — 249.5 mg/L (17.5°Clark) — reflects the Chiltern Chalk Aquifer and Thames reservoir chalk chemistry in Thames Water's inner west London supply zone. The TDS of 640.9 mg/L confirms high mineral content. The Drinking Water Inspectorate (DWI) classifies this supply as very hard.

Limescale is a very serious daily challenge in Chiswick. At 249.5 mg/L, limescale forms in kettles within one to two weeks requiring fortnightly descaling. Combi-boiler heat exchangers face serious limescale risk — annual boiler servicing and a polyphosphate scale inhibitor are essential. Showerheads, taps, and shower screens develop very heavy deposits. Washing-up liquid lathers very poorly. Chiswick homeowners should strongly consider a whole-house water softener for effective protection against the very hard inner west London Thames supply.