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

Thornaby-on-Tees, the Borough of Stockton-on-Tees town on the south bank of the Tees between Stockton and Middlesbrough — historically part of the North Riding of Yorkshire — is supplied by Northumbrian Water from the River Tees catchment and Kielder Water transfer, supplemented by groundwater from the Permian Magnesian Limestone of the Cleveland and south Durham coastal hinterland. Northumbrian Water treats Tees water at Linthorpe and Tees Road Water Treatment Works and distributes through the Teesside network. The River Tees flows from the Pennine limestone (Great Scar Limestone) of the North Pennines through Upper Teesdale and the limestone dales of north Yorkshire, acquiring moderate carbonate hardness. At 195 mg/L with TDS 558.1 mg/L (ratio 2.86), the Thornaby supply shows the elevated-TDS signature of Magnesian Limestone (Zechstein dolomite) groundwater blending in the Teesside distribution zone — calcium sulphate from the gypsum-bearing Zechstein evaporite interbeds raising TDS well above carbonate hardness alone. Similar supply chemistry to Newton Aycliffe (208 mg/L) and Seaham (178 mg/L) in the broader Northumbrian Water south Durham network.

The Permian Magnesian Limestone of Cleveland and south Durham runs east from the Tees valley toward the North Sea coast, contributing gypsum-enriched groundwater to the Teesside supply blend. The Tees itself carries moderate Pennine limestone hardness at 120–150 mg/L, while Kielder Water adds soft Northumberland upland supply. The final Thornaby blend at 195 mg/L reflects Magnesian Limestone groundwater moderating the blend away from the softer Kielder-dominant zones of the Tyne valley to the north.

At 195 mg/L Thornaby's water is moderately hard and limescale management is a regular domestic concern. Kettles benefit from monthly descaling with a citric acid tablet. Shower screens develop a calcium film requiring regular white vinegar treatment. Washing-up liquid lathers adequately. Combi-boilers benefit from inline scale inhibitor protection. Thornaby's South Tees industrial heritage — aircraft engineering, chemicals, the ICI complex and the original Teesside steel tradition — sits above the same Magnesian Limestone belt whose mineral-enriched groundwater defines domestic tap water across the south Durham and Teesside coast.