Darlington Water Hardness & Quality Report (2026)

Darlington, the market town in County Durham in the Tees Valley, is supplied by Northumbrian Water from the River Tees catchment and associated infrastructure. Primary sources include the Broken Scar Water Treatment Works on the Tees immediately west of Darlington, which treats water from the River Tees directly, supplemented by storage from Cow Green Reservoir in upper Teesdale and transfers from the Kielder Transfer Scheme. However, Darlington's supply is notably harder than Middlesbrough (100 mg/L) or Sunderland (80 mg/L), reflecting a significant contribution from Permian Magnesian Limestone groundwater in the Darlington area. Northumbrian Water operates boreholes in the Magnesian Limestone belt that runs through Durham south of Darlington, blending this harder groundwater with Tees river supply for the town.

Darlington's hardness of 189 mg/L (13.3°Clark) — considerably above the typical Northumbrian Water softness — reflects the Permian Magnesian Limestone groundwater component distinctive to the Darlington supply zone. The Magnesian Limestone escarpment runs from south of Darlington northward through County Durham to the Tyne, and groundwater in this Permian dolomitic limestone carries elevated calcium and magnesium. The Tees river supply is blended with this Magnesian Limestone groundwater in the Northumbrian Water network serving south and central Durham. The resulting supply is classified as moderately hard by the Drinking Water Inspectorate (DWI) — unusual for north-east England.

Limescale requires regular attention in Darlington — more so than most North East England towns. At 189 mg/L, limescale forms in kettles within three to four weeks and monthly descaling is advisable. Combi-boiler heat exchangers accumulate limescale deposits and annual servicing with a limescale check is important. Showerheads and taps develop regular deposits. Washing-up liquid lathers moderately. Using Calgon monthly in the washing machine and maintaining a regular descaling routine is appropriate for Darlington's harder than typical north-eastern supply.