Geochemical indicators of deep groundwater movements at Sellafield, UK

Boreholes were drilled in an area of 52 km2 at Sellafield, on the coast of northwest England, to investigate groundwater conditions in Permo-Triassic sedimentary rocks and underlying Ordovician metavolcanic basement. The basement rocks are fractured with low permeability and were being investigated as a potential host for a nuclear waste repository until the project was stopped in 1997.Water samples were collected by pumping from discrete intervals in 19 deep boreholes to a maximum depth of 1950 m. Chemical analyses of the water samples showed a range of salinities from fresh/brackish to brine and, in addition, various isotopic analyses were carried out. Data for Na, K, Ca, Mg, Sr, Cl, SO4, HCO3, Br, 18O/16O, 2H/1H, 3H, 14C, 13C/12C, 36Cl/Cl, 87Sr/86Sr, 34S/32S, 4He and other inert gas contents are reported here for 16 of the boreholes. The reliabilities of most of these data as representative of in situ conditions are considered to be high, although pH, alkalinity and tritium are less reliable because of contamination with drilling fluid, and carbon isotopic data are definitely erroneous.The groundwater system in the sedimentary cover rocks and basement formation comprises water masses with different origins, ages and salinities. Fresh water in the sedimentary rocks, typically to 350 m depth in the centre of the area, was mostly recharged during the Holocene. Fresh and brackish waters, which occur locally at greater depth (about 700 m), have stable isotopic compositions that indicate cold climate recharge, probably in the late Pleistocene. Saline water in the basement formation contains meteoric water that recharged during the Pleistocene, i.e. between 10 ka and 2 Ma ago. Brine in offshore basinal sedimentary rocks, west of the investigated area, is more than 2 Ma old and probably originated as meteoric recharge during the Tertiary period, becoming saline by dissolution of halite.The most likely origin of salinity in groundwaters in basement rocks here is that it is primarily evolved from basinal brine that migrated from overlying sedimentary rocks before Tertiary erosion. Saline groundwater in the basement is now chemically distinct from the basinal brine, for example having a higher Br/Cl ratio. Mixtures between the groundwater types occur in salinity transition zones between the major water masses. The transition zone between fresh/brackish and saline water in the centre of the area is a few tens of metres thick with a steep salinity gradient and is located at the boundary between sedimentary cover rocks and basement. The transition between saline basement water and basinal brine near to the coast is 500 m wide. 36Cl data suggest that groundwater movements and mixing of solutes at this transition have occurred within the last 1.5 Ma.Interpretations of the sources of water and salinity and of ages and mixing of groundwater masses are integrated with geological, structural and climatic information in a synthesis of palaeohydrogeology. This explains how the groundwater system in the Sellafield area has evolved over time from the Tertiary period to the present. The evolution and stability of these groundwaters are particularly related to the location of the Sellafield area at a structural and hydrogeological boundary between a basin margin and an upland massif.