Sevenoaks Water Hardness & Quality Report (2026)

Sevenoaks, the Kent market town on the North Downs greensand and chalk ridge between the Holmesdale vale and the Weald, is served by South East Water. Supply to the Sevenoaks district draws on deep boreholes into the North Downs Chalk Aquifer of west Kent, where the chalk dip slope and the confined chalk beneath the Weald clay provides a substantial groundwater resource. Treatment is carried out at Horton Kirby Water Treatment Works in the Darent valley near Dartford, with supply to Sevenoaks via trunk mains from the north Kent chalk distribution zone. South East Water also operates boreholes at Wrotham and the Maidstone chalk supply area eastward. The very high TDS of 821.6 mg/L reflects long-residence confined chalk groundwater highly saturated in dissolved calcium carbonate and associated minerals.

The North Downs Chalk of west Kent is thickly developed, and where it dips beneath the Gault Clay and Lower Greensand below the Sevenoaks Weald dip slope, groundwater residence times in the confined aquifer extend over many decades. Over those timescales calcium bicarbonate dissolves to saturation from the chalk matrix, producing groundwater hardness values of 290–310 mg/L in boreholes. The Lower Greensand and Gault Clay confining layers above the deep chalk prevent dilution by younger, softer water, allowing the extreme mineral concentration to persist. The TDS exceeding 800 mg/L additionally reflects sulphate from Upper Greensand and residual evaporite minerals in the deeper confined zone.

At 300 mg/L Sevenoaks has very hard water — at the threshold between very hard and extremely hard — and limescale is a substantial daily maintenance challenge. Kettle elements fur up rapidly and need descaling every week or two with concentrated citric acid. Shower screens require weekly limescale treatment to prevent thick opaque deposits from building up. Combi-boilers in Sevenoaks properties face high scaling risk and need fitted scale inhibitors and annual servicing. Washing-up liquid must be used liberally. A whole-house water softener is strongly recommended in Sevenoaks; many local households have already invested in softening systems given the area's reputation for punishing limescale.