Cobham Water Hardness & Quality Report (2026)

South East Water supplies Cobham and surrounding areas in Surrey, serving over 2 million customers across Kent and Sussex. The water primarily originates from groundwater abstracted from boreholes tapping the Chalk aquifer in the Surrey Hills and Weald region. Key sources include boreholes in the Leatherhead and Dorking areas, with treatment at works like the Effingham or Leatherhead plants. Here, the groundwater undergoes aeration, filtration, and disinfection before distribution through mains. While Thames Water also contributes in parts of the region via river abstractions from the Thames and treatment at sites like Island Water Works, Cobham falls under South East Water's primary groundwater-dominated supply.

The dominant geology is the Cretaceous Chalk Group, a thick sequence of marine limestone deposited 100 million years ago, forming a major confined aquifer beneath London Clay. This karstic aquifer imparts a hard character to the water through natural dissolution of calcite, with minor contributions from magnesium-bearing minerals in associated Greensand layers. Rainwater percolates through the fissured and karstic chalk, dissolving high levels of calcium and magnesium ions from the carbonate bedrock. The slightly acidic nature of rainfall enhances mineral dissolution, resulting in a characteristically hard supply. The enclosed nature of the aquifer limits dilution, preserving the mineralised profile from recharge areas on the North Downs.

Severe limescale buildup affects boilers, kettles, showers, and washing machines the most in hard water areas like Cobham, reducing their efficiency and lifespan. Hot water systems can suffer pipe narrowing, while visible deposits form on taps and fixtures. To combat this, regular descaling of appliances every 2-3 months is recommended, along with using scale collectors in kettles and lowering hot water temperatures to 60°C. Whole-house water softeners are strongly recommended for protection, though a bypass tap for drinking water is advised as softened water alters taste and isn't ideal for consumption. South East Water maintains compliance with UK drinking water standards, with annual reports showing neutral pH, low lead and copper levels, and no notable PFAS exceedances.