Hydrochemical investigations form an important part of the Nirex Site Characterization Programme at Sellafield. They support the development of a conceptual hydrogeological model of the area as it is now, are the main basis for reconstructing the palaeohydrogeological evolution of the area over the recent geological past (which will assist predictions of future evolution) and allow characterization of the baseline hydrochemical conditions in the potential repository rock volume. The deep hydrochemistry of the area of interest is dominated by its location on the margin of the East Irish Sea Basin. To the west of the potential repository zone, the influence of basinal brines has been a feature of the deep sedimentary rocks and the Borrowdale Volcanic Group (BVG) basement for a considerable period of geological time. The brines currently present are inferred to be the result of partial dissolution of Permo-Triassic halite by ancient (probably pre-Pleistocene) meteoric recharge. Within the BVG of the potential repository zone itself, stable isotopic data and estimates of recharge temperature (based on noble gas data) indicate a predominance of old meteoric recharge (probably Pleistocene). These groundwaters are moderately saline and of NaC1 type. Br/CI ratios suggest that a significant component of this salinity is derived from a putative saline groundwater within the Lake District basement further to the east. The BVG within the potential repository zone is overlain by a Permo-Triassic sedimentary sequence containing fresh groundwaters of Ca-HCO3 type, which are separated from the underlying saline groundwaters by a fairly sharp salinity transition near the base of the sedimentary formations and locally within the topmost part of the BVG.