Londonderry County Borough Water Hardness & Quality Report (2026)

Londonderry (also known as Derry), the second-largest city in Northern Ireland on the River Foyle, is supplied by NI Water drawing from upland reservoir sources in the Sperrin Mountains and the broader Foyle catchment. Primary sources include Lough Fea reservoir in the Sperrin uplands of County Tyrone and river and reservoir abstractions from the Mourne Mountains and Sperrins system, distributed through NI Water's north-west network. The Foyle catchment drains a varied geology — including Carboniferous Sandstone, Carboniferous Limestone, Tertiary Basalt in the Antrim plateau, and ancient Precambrian metamorphic rocks of the Sperrins — contributing moderate mineral content to the supply. Water is treated at NI Water's Dunore Point Water Treatment Works on Lough Neagh (supplemented for north-west supply) before distribution to Londonderry.

Londonderry's hardness of 136 mg/L (9.5°Clark) is moderately high for a Northern Ireland city — higher than Belfast's typically softer Mourne Mountain supply but well below the hardness of most English midlands or south-east cities. The catchment blend from the Sperrin Mountains and Foyle watershed traverses Carboniferous Sandstone and some Carboniferous Limestone beds, dissolving moderate calcium content in the river and reservoir system. The Tertiary Basalt and Precambrian metamorphic rocks in the wider catchment contribute softer water that dilutes the limestone component. The Drinking Water Quality Regulator for Northern Ireland classifies this supply as moderately soft to moderately hard.

Limescale is a moderate household concern in Londonderry. At 136 mg/L, limescale builds gradually in kettles and descaling every one to two months is typically sufficient. Combi-boiler heat exchangers accumulate deposits at a moderate rate and annual servicing is sensible. Showerheads and taps develop modest limescale deposits over several months. Washing-up liquid lathers reasonably well. The moderate hardness is manageable — Calgon monthly in the washing machine and a regular kettle descale with white vinegar or a proprietary descaler is adequate limescale management for most Londonderry households.