Mortlake Water Hardness & Quality Report (2026)

Thames Water supplies Mortlake, the south-west London riverside district in the London Borough of Richmond upon Thames — a historic Thames-side community famous as the finish line of the annual Oxford-Cambridge Boat Race, home of the Stag Brewery (whose riverside buildings are being redeveloped), and the burial place of the explorer Sir Richard Burton in the extraordinary Arabic tent-tomb at St Mary Magdalen Church — from the River Thames and Lee Valley chalk-tributary blend in the Thames Water south-west London Richmond distribution zone, treated at Lee Valley Water Treatment Works. At 309.5 mg/L (21.7°Clark) and a TDS of 898.3 mg/L, Mortlake's water is extremely hard — reflecting a dominant Lee Valley chalk supply fraction in the Thames Water Richmond distribution zone, producing water considerably harder than the Hampton-zone west London communities to the west.

Mortlake lies in the Thames Water Richmond distribution zone where the supply draws on a blend with a substantial Lee Valley chalk-tributary fraction, rather than the predominantly Thames Hampton (moderate hardness) supply of outer west London. The Lee Valley chalk contribution produces 309.5 mg/L with TDS 898.3 mg/L — extremely hard water with an elevated TDS/hardness ratio (2.90) approaching the 1,000 mg/L TDS level, among the highest values in the Thames Water London distribution network at this south-west London location.

At 309.5 mg/L, limescale is a severe and relentless household problem in Mortlake. Kettles should be descaled every week to ten days. The combi-boiler requires a fitted scale inhibitor and annual professional servicing as an absolute minimum. Washing-up liquid requires very generous quantities. Taps and shower heads develop heavy white chalk deposits within days; frequent descaling with white vinegar or a proprietary product is necessary. A whole-house water softener is strongly recommended for Mortlake households — at 309.5 mg/L, the chalk supply will significantly reduce appliance lifespan without active protection, whatever the Boat Race and the Thames scenery may suggest about soft south-west London living.