Hydrogeological investigations were conducted around the Gorleben salt dome, in the northeastern part of Lower Saxony in Germany, to study the aquifer system in the cover deposits consisting of unconsolidated Quaternary and Tertiary sediments up to 250 m thick. An upper freshwater body is underlain here by saline water. The salinity of the water generally increases with depth. Saturated brines are found in the lower aquifer in a subglacial erosion channel above the salt dome. Hydrochemical analyses and geophysical and isotope studies provide information on development history and behaviour of the flow system. Highly saline groundwater formed by subrosion of salt collects in the lower aquifer above the salt dome. It generally flows north through the erosion channel in the direction of the regional flow field and collects in a basin-like aquifer structure north of the dome. The post-Pleistocene groundwater circulation is progressively replacing the Pleistocene brines with Holocene groundwater. Only a small amount of salt water is being transported into the near-surface aquifer.