LAND DRAINAGE AND SALINE INTRUSION IN THE COASTAL MARSHES OF NORTHEAST NORFOLK

Land drainage has caused a significant increase in surface water salin ity in the River Thurne catchment of northeast Norfolk which is relate d to saline intrusion in the underlying Crag aquifer. A survey of surf ace water Cl concentration revealed values ranging from <100 mg l(-1) to >12 000 mg l(-1), with Cl exceeding 10 000 mg l(-1) at the centre o f the coastal marshes. In the sub-surface, an electromagnetic inductio n 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 elect rical resistivity survey identified a low true resistivity layer (typi cally <1 Ohm m), assumed representative of saline water, at a depth of typically <10 m below sea level under the coastal marshes. Higher lan d associated with dunes and isolated blocks of elevated land within th e marshes gave a saline interface at 10-15 m below sea level, and bene ath the catchment watersheds at >20 m. Solute transport modelling of a n artificial vertical section through the aquifer supports the field o bservation that the distribution of surface water salinity is primaril y determined by the spatial distribution of water levels in the main d rains, all of which are maintained below sea level by pumping.