Derry Water Hardness & Quality Report (2026)

Derry (also known as Londonderry), Northern Ireland's second city on the River Foyle, is supplied by NI Water from upland reservoir sources in the Sperrin Mountains and the Foyle catchment, managed through NI Water's north-west Northern Ireland distribution network. The primary supply is drawn from Lough Fea reservoir in the Sperrins and from supplementary sources within the Mourne and Bann inter-basin transfer. The Sperrin Mountains catchment drains over a varied geology — including Precambrian and Dalradian metamorphic rocks, Tertiary Basalt flows at the Antrim plateau margins, and Carboniferous Sandstone and Limestone in the lowland catchment — contributing moderate dissolved mineral content from the more soluble formations. Water is treated at NI Water's facilities before distribution to Derry and the north-west.

Derry's hardness of 136 mg/L (9.5°Clark) is relatively high for a Northern Irish city, reflecting the mixed geology of the Foyle watershed. The upper Sperrin catchment drains over ancient Precambrian and Dalradian rocks — largely insoluble, producing very soft water — while the lower Foyle plain and tributary streams drain through Carboniferous Sandstone and Limestone that contribute dissolved calcium and magnesium. The Tertiary Basalt of the Antrim margins, where present in the eastern catchment, contributes additional mineral content. The blended result is classified as moderately soft to moderately hard by the Drinking Water Quality Regulator for Northern Ireland.

Limescale is a moderate household concern in Derry. At 136 mg/L, limescale accumulates 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 with a limescale check is sensible. Showerheads and taps develop modest deposits. Washing-up liquid lathers reasonably well at this hardness. Adding Calgon monthly to the washing machine and keeping a kettle descaler to hand provides adequate limescale management for most Derry households.