Thames Water Tap Water Quality

Thames Water supplies London & Thames Valley with drinking water drawn from blended sources including the River Thames, chalk and limestone aquifers, and Lea Valley reservoirs.

Thames Water supplies the largest population of any UK water company. Water hardness across the supply area is among the highest in the country — typically 250–320 mg/L CaCO₃ — due to the chalk and Oolitic limestone geology beneath the Thames Valley. Chlorination and coagulation treatment is standard. THM disinfection byproducts are monitored at all zones; most are well within the 100 µg/L legal limit though some south-west London zones have historically recorded mean values above 50 µg/L.

Typical water profile

With typical hardness of approximately 280 mg/L CaCO₃, water in the Thames Water supply area is classified as very hard. This means significant limescale buildup on kettles, showerheads, and heating elements over time. A water softener or quality filter jug is a worthwhile investment for most households in this area.

Notable: Very hard water — significant limescale risk for kettles, boilers, washing machines. Some zones have elevated THM mean values. 29 active PFAS improvement programmes under DWI enforcement.

Is Thames Water water safe to drink?

Yes. All treated water supplied by Thames Water meets the UK Drinking Water Regulations. The Drinking Water Inspectorate (DWI) audits every water company annually — Thames Water serves 15 million customers across 169 supply zones. Each zone is tested thousands of times per year and results are published in the annual compliance report.

However, water quality varies significantly between zones within the Thames Water supply area. Your specific zone may have different hardness, nitrate, or THM levels from your neighbour in a different zone. That's why it's worth checking your exact postcode above.

How to check your Thames Water zone

Enter your postcode in the box above to see zone-specific data for your address — including hardness, nitrates, lead, chlorine, fluoride, and THMs — drawn directly from Thames Water's most recent annual compliance report.