Osterley Water Hardness & Quality Report (2026)

Thames Water Utilities Limited supplies Osterley, located in the London Borough of Hounslow, Greater London, UK. This mixed water supply draws from the Chalk aquifer within the London Basin, supplemented by surface water from the River Thames and its tributaries. Water is abstracted from boreholes in the Lee Valley and processed at treatment facilities including Hampton, Isleworth, and Mogden. These plants purify raw water through essential steps like coagulation, filtration, chlorination, and fluoridation, before it's distributed to millions across London and the Thames Valley. The journey begins in the Thames River Basin watershed, which includes the chalky London Basin.

The primary source is the Cretaceous Chalk Group aquifer, a porous limestone that naturally dissolves minerals into the water. While overlying clays like the London Clay affect recharge, the underlying calcareous bedrock is the main contributor to the supply's very hard character. This geological makeup consistently results in mineral-rich water, a common trait for supplies across southeast England. The karstic nature of the limestone also promotes significant mineral dissolution.

This very hard water leads to noticeable limescale buildup in appliances like kettles, boilers, and showerheads, diminishing their efficiency and shortening their lifespan. You'll find deposits affecting dishwashers and washing machines, potentially increasing energy consumption by up to 20%. Homeowners often combat this by regularly descaling with vinegar, fitting scale collectors in kettles, or lowering hot water temperatures to 60°C. Many residents opt for whole-house water softeners to protect plumbing and appliances. Thames Water maintains the water's pH around 7.5-8.0. The supply meets all UK tap water standards, including limits for lead and copper, following extensive lead pipe replacement efforts. Trace levels of PFAS are monitored and treated if necessary, and past issues like cryptosporidium have been addressed with improved filtration processes.