Cosham Water Hardness & Quality Report (2026)

Portsmouth Water supplies Cosham, the northern suburb of Portsmouth at the foot of Portsdown Hill — the chalk ridge that forms the northern boundary of the Portsmouth urban area, providing panoramic views across the Portsmouth Harbour and the Solent, and an area dominated by the chalk downland of Portsdown above the Victorian terraces of Cosham and the Portchester Castle Roman and medieval fortification to the west — from the South Downs Chalk (Cretaceous) aquifer boreholes directly beneath Portsdown Hill, treated at Farlington Water Treatment Works east of Cosham. At 193.5 mg/L (13.6°Clark), Cosham's water is hard — consistent with the Cretaceous Chalk aquifer of the South Downs and Portsdown Hill that supplies the Portsmouth area, with a moderate TDS/hardness ratio (2.31) confirming predominantly chalk carbonate hardness.

Cosham sits directly beneath Portsdown Hill, the chalk ridge underlain by the South Downs Chalk aquifer. Portsmouth Water draws from deep chalk boreholes at the base of Portsdown Hill at Farlington, with the chalk delivering calcium carbonate groundwater at 193.5 mg/L with TDS 447.5 mg/L — hard water consistent with the Portsmouth Water chalk distribution zone at Havant (190 mg/L), Waterlooville (192 mg/L), and Fareham in the same South Downs chalk supply corridor.

At 193.5 mg/L, limescale is a consistent household challenge in Cosham. Kettles should be descaled monthly. The combi-boiler benefits from a fitted scale inhibitor and annual professional servicing. Washing-up liquid requires more product per wash. Taps and shower heads develop visible white limescale deposits within one to two weeks; a fortnightly wipe with white vinegar or a proprietary descaling product keeps fittings clean. Cosham's hard chalk supply is a product of the Portsdown chalk ridge visible above the town — the same chalk escarpment that gives panoramic views across the harbour also delivering mineralised chalk groundwater to every household in this Portsmouth suburb.