Darwen Water Hardness & Quality Report (2026)

Darwen, in the Borough of Blackburn with Darwen south of Blackburn in the Darwen valley, is supplied by United Utilities from Entwistle Reservoir and the broader east Lancashire supply network including contributions from Carboniferous Limestone groundwater from the Ribble/Pendle limestone country north of Blackburn. The Entwistle Reservoir on the Wayoh valley drains Carboniferous Millstone Grit moorland, providing a soft upland base supply, but Darwen's supply zone receives additional blending from the east Lancashire United Utilities network, which can incorporate contributions from the Great Scar Limestone and Carboniferous limestone of the Ribble valley and Forest of Bowland country to the north. This limestone-influenced blending raises the Darwen supply hardness well above the typical soft Pennine reservoir level. The TDS of 482.7 mg/L — high relative to hardness — confirms additional dissolved minerals beyond calcium alone.

Darwen's moderately hard water — 168.5 mg/L (11.8°Clark) — is notably harder than nearby east Lancashire Pennine towns, reflecting Carboniferous limestone groundwater contributions in the Blackburn with Darwen United Utilities supply zone. The Drinking Water Inspectorate (DWI) classifies this supply as moderately hard.

Limescale is a regular household concern in Darwen. At 168.5 mg/L, limescale forms in kettles within three to four weeks and monthly descaling is sensible. Combi-boiler heat exchangers accumulate deposits at a moderate rate; annual servicing is recommended. Showerheads and taps develop consistent deposits. Washing-up liquid lathers moderately. Using Calgon monthly in the washing machine and a regular descaling routine is appropriate limescale management for Darwen households.