Mendip Water Hardness & Quality Report (2026)

Mendip District in Somerset is supplied primarily by Bristol Water, drawing from the two major reservoir impoundments in the Mendip foothills — Blagdon Lake (completed 1905) and Chew Valley Lake (completed 1956), both fed by rivers draining the northern slopes of the Mendip Hills. The River Chew, the River Blagdon, and smaller streams flowing off the Mendips drain both the Carboniferous Limestone plateau at the core of the Mendip Hills and the surrounding Triassic and Jurassic country. The Mendip Hills' limestone karst — famous for Cheddar Gorge, Wookey Hole Caves, and the cave systems carved by millennia of limestone dissolution — contributes calcium-rich spring and river water. Bristol Water treats this supply and distributes it across north Somerset and south Gloucestershire, including the Mendip district.

Mendip's hardness of 240 mg/L (16.8°Clark) directly reflects the Carboniferous Limestone geology of the Mendip Hills. The Mendips are formed from Carboniferous Dinantian limestone — the same Carboniferous limestone age as the Peak District and the Yorkshire Dales — but here tilted into a series of east–west anticlines that expose the limestone at the surface across the central Mendip plateau. Groundwater in the Mendip limestone acquires high dissolved calcium through karst dissolution. The spring water discharging at Cheddar and Wookey Hole is famously hard from this limestone contact. This limestone-derived calcium is carried into the feeder rivers of Blagdon and Chew Valley, reflecting in the hard supply. The Drinking Water Inspectorate (DWI) classifies this supply as hard.

Limescale is a consistent and significant challenge in Mendip homes. At 240 mg/L, limescale forms in kettles within two to three weeks and fortnightly descaling is advisable. Combi-boiler heat exchangers accumulate limescale deposits steadily — annual servicing with a limescale check is important, and fitting an in-line scale inhibitor is recommended. Showerheads, taps, and shower screens develop regular deposits requiring monthly attention. Washing-up liquid lathers noticeably poorly. Fitting a scale inhibitor or water softener is a worthwhile investment for Mendip homeowners seeking to manage the district's hard limestone water.