Witney Water Hardness & Quality Report (2026)

Witney, the west Oxfordshire market town in the Windrush valley, historically famous for blanket manufacturing, is served by Thames Water. Supply draws on two complementary geological sources: boreholes into the Jurassic Great Oolite and Inferior Oolite limestone of the Cotswold uplands west of Witney, and the Upper Thames Valley Chalk Aquifer further east. Water is also supplemented by Farmoor Reservoir near Oxford, which stores Thames water abstracted from the upper river. Treatment is at Farmoor Water Treatment Works and at Swinford Water Treatment Works, with distribution west to the Witney and Burford supply zone. The very high TDS of 802.7 mg/L reflects a supply dominated by limestone and chalk groundwater with very long residence times and high dissolved mineral concentrations.

The Jurassic Great Oolite limestone — the Cotswold building stone responsible for the honey-coloured villages of west Oxfordshire — is a highly permeable oolitic limestone that dissolves readily into percolating groundwater, producing calcium bicarbonate concentrations of 250–280 mg/L in deep boreholes. The Inferior Oolite below similarly contributes hard groundwater. Where this Jurassic limestone-sourced water is blended with chalk groundwater from the upper Thames chalk further east, a very hard composite supply results. The TDS greatly exceeding 800 mg/L suggests additional dissolved sulphate from Oxford Clay and deeper Jurassic formations interbedded with the limestone.

At 271.5 mg/L Witney's water is very hard and limescale demands regular household attention. Kettle elements develop a substantial white crust within a week or two and need fortnightly descaling with a concentrated citric acid solution. Shower screens and bathroom glass should be treated weekly with a limescale remover. Washing-up liquid is consumed heavily to maintain lather. Combi-boilers are at significant scaling risk and scale inhibitors plus annual power-flushing are strongly recommended. Witney's Cotswold limestone buildings and hard limestone water are products of the same geological force — calcium carbonate that built the Cotswolds is dissolved into the water supply every time it rains.