Coleraine Water Hardness & Quality Report (2026)

Coleraine, the university and market town on the lower River Bann in County Londonderry at the Causeway Coast, is served by NI Water. Supply for the Coleraine area draws on two principal sources: Altnahinch Reservoir — impounded in the Tardree uplands of the north Antrim plateau west of Larne — and abstraction from the River Bann treated at Ballinrees Water Treatment Works south of Coleraine. The Altnahinch catchment drains the Antrim Plateau Basalt (the same flood basalt landscape that forms the Giant's Causeway coast) — a geologically young volcanic rock with minimal calcium carbonate content, yielding inherently very soft, low-mineral runoff. The River Bann at Ballinrees carries water drained from the Sperrin Mountains (metamorphic schist and gneiss) to the south-west, reinforcing the soft-water character. The very low TDS of 106.4 mg/L confirms minimal dissolved mineral content throughout the Coleraine supply zone.

The Antrim Plateau Basalt (Palaeogene) — a thick sequence of Cenozoic flood basalts — forms one of Northern Ireland's most distinctive geological landscapes. Basalt is a calcium-depleted mafic volcanic rock that weathers slowly and contributes very little dissolved calcium to surface runoff. Combined with the metamorphic Dalradian and Sperrin schists draining into the Bann headwaters, the result is a supply of 59 mg/L hardness — soft water typical of upland volcanic and metamorphic catchments in Northern Ireland and Scotland.

At 59 mg/L Coleraine's water is soft and limescale is rarely a significant domestic concern. Kettles develop only a thin mineral film and need descaling every two to three months with a brief white vinegar rinse. Shower screens remain clear for extended periods. Washing-up liquid lathers freely with modest amounts. Combi-boilers and white goods face very low scaling risk and enjoy extended lifespans. Coleraine's position as the gateway to the Causeway Coast and Antrim Glens is reflected in its water chemistry — the same Antrim basalt landscape that draws tourists also provides the town with a pleasantly soft domestic water supply.