Land drainage has caused a significant increase in surface water salinity in the River Thurne catchment of northeast Norfolk which is related to saline intrusion in the underlying Crag aquifer. A survey of surface water C1 concentration revealed values ranging from <100 mg 1-1 to >12 000 mg 1-1, with C1 exceeding 10 000 mg 1-1 at the centre of the coastal marshes. In the sub-surface, an electromagnetic induction survey demonstrated elevated apparent conductivities of 100–200 mS m-1 at the greatest exploration depth, except for areas having higher topography where fresh, or slightly brackish water persists. An electrical resistivity survey identified a low true resistivity layer (typically <1 Ωm), assumed representative of saline water, at a depth of typically <10 m below sea level under the coastal marshes. Higher land associated with dunes and isolated blocks of elevated land within the marshes gave a saline interface at 10–15 m below sea level, and beneath the catchment watersheds at >20 m. Solute transport modelling of an artificial vertical section through the aquifer supports the field observation that the distribution of surface water salinity is primarily determined by the spatial distribution of water levels in the main drains, all of which are maintained below sea level by pumping.