A study of the hydrogeological controls on the Pleistocene Crag aquifer in northeast Norfolk is reported. Geophysical methods, in particular multichannel seismic reflection profiling, have provided useful information on the internal structure of the aquifer. The Crag represents a layered aquifer, with horizons of clay and silt strata able to limit the overall vertical permeability of the aquifer. A laterally continuous clay horizon is identified at around -25 mOD and is mapped as far as the coast. Shallow wells and borehotes in the Crag are prone to contamination. Nitrate is widely recorded, with concentrations of typically up to 150 mg l-1 in shallow wells, derived from mainly agricultural sources, although deeper boreholes are less affected given the restricted downward movement of ground-water. The land drainage systems within this lowland area dominate the hydrology and hydrogeology of the catchment. Surface flows into the River Thurne are controlled by the operation of drainage pumps and a catchment water balance demonstrates that groundwater principally discharges to the drainage ditches within the extensive arable and grazing marshes in the catchment.