Stockton-on-Tees Water Hardness & Quality Report (2026)

Stockton-on-Tees, the town in the Tees Valley on the north bank of the River Tees, is supplied by Northumbrian Water from the River Tees catchment and the Kielder Transfer Scheme. Northumbrian Water's Tees supply draws from Cow Green Reservoir in upper Teesdale, supplemented by the Kielder Water transfer into the Tees, and is treated at Lartington Water Treatment Works and Broken Scar Water Treatment Works near Darlington before distribution through the Teesside network. The River Tees drains the North Pennines — traversing Carboniferous limestone country in upper Teesdale, including the limestone gorge at High Force and Cauldron Snout — contributing a moderate calcium increment to the supply from limestone dissolution.

Stockton's hardness of 100.5 mg/L (7.0°Clark) reflects the Tees catchment blend of very soft Pennine moorland source water and moderate limestone contribution from upper Teesdale. Cow Green Reservoir sits on the North Pennine moorland above the limestone gorge country, collecting predominantly soft peat moorland runoff. The Kielder transfer adds further very soft Northumberland moorland water. However, the River Tees in its limestone gorge section acquires moderate dissolved calcium from the Carboniferous Great Limestone formation, contributing the moderate hardness seen at Stockton. The Drinking Water Inspectorate (DWI) classifies this supply as soft to moderately soft.

Limescale is a moderate concern in Stockton-on-Tees. At 100.5 mg/L, limescale builds up gradually — kettles need descaling every two to three months and deposits on taps and showerheads are modest. Combi-boiler heat exchangers accumulate limescale at a moderate rate; annual servicing is sensible. Washing-up liquid lathers reasonably well. The Tees supply gives Stockton easier limescale management than most of southern England — a monthly Calgon tablet in the washing machine and occasional kettle descaling provides adequate maintenance for most households.