Monmouth Water Hardness & Quality Report (2026)

Welsh Water supplies Monmouth (Trefynwy), the Monmouthshire market town at the confluence of the Wye and Monnow rivers — a handsome border town of Georgian buildings with a Norman castle (birthplace of King Henry V in 1386), a magnificent medieval fortified bridge gate (Monnow Bridge) that is the only complete remaining example in Britain, and a setting among the hills of the lower Wye valley in south-east Wales — from Wentwood Reservoir in the Gwent upland south-west of Monmouth, treated at Llandegfedd Water Treatment Works near Pontypool. At 77.5 mg/L (5.4°Clark), Monmouth's water is soft — consistent with the impermeable Devonian Old Red Sandstone and Silurian geology of the Gwent upland and Monnow Valley catchment that contributes very little calcium to the Wentwood reservoir supply.

Monmouth lies in the lower Wye valley at the Monnow confluence where Welsh Water delivers supply from Wentwood Reservoir — an upland reservoir in the Devonian Old Red Sandstone and Silurian upland of south-east Gwent. The impermeable Old Red Sandstone upland yields very little calcium, producing 77.5 mg/L with TDS 140.9 mg/L — soft water with a very low TDS/hardness ratio (1.82) characteristic of the Welsh Water Gwent and Wye valley distribution zone.

At 77.5 mg/L, limescale is a moderate and manageable domestic concern in Monmouth. Descaling the kettle every two to three months is typically adequate. The combi-boiler has low scaling risk. Washing-up liquid lathers readily with modest quantities. Taps and shower heads develop light deposits; a quarterly wipe with white vinegar keeps fixtures clean. Monmouth's soft supply is as characteristic of the Wye valley borderland as the ancient Monnow Bridge gatehouse — the impermeable Old Red Sandstone hills of Gwent contributing minimal calcium to both the Wye scenery and the tap water of Wales's oldest continuously inhabited town.