Hydrogeochemical evidence supporting models for groundwater flow around Sellafield, UK

Recently, United Kingdom Nirex Limited has investigated a site near Sellafield, north-west England, to assess its suitability as the potential location for a deep underground repository for the disposal of intermediate- level, and some low-level, solid radioactive waste. Groundwater flow at the site was simulated using a variety of computer codes, based upon conceptual models of the hydrogeological system. Chemical data for groundwaters aided the development of these conceptual models, and also provided a check upon the computer models' validity. Mineralogical information can be accommodated within the conceptual and theoretical framework. Sellafield lies near the coast, with the East Irish Sea Basin (EISB) to the west, and the upland Lake District Massif to the east. The EISB is filled mainly with Permo-Triassic sedimentary rocks, including halite-bearing, Permian and Triassic evaporites. In contrast, the Lake District Massif consists of Lower Palaeozoic rocks, including metasediments, granites and the Borrowdale Volcanic Group (BVG). The potential locality for the repository lies at the edge of the EISB, where Permo-Triassic sedimentary rocks form a cover to BVG basement rocks. The hydrochemical data reveal several chemically distinct groundwaters, but in general terms, these can be classified into two groups: Group I. deep, Na-Cl dominated saline waters and brines, occurring in the cover and basement in the west, but entirely in the basement in the east; and Group 2. shallow, fresh Ca-HCO3 and Na-Ca-HCO3 dominated waters, occurring mostly in the cover. The transition between these two groups is termed the Saline Transition Zone (STZ). Studies of fracture infilling minerals have identified several 'Mineralisation Episodes' (MEs). The most recent MEs are termed ME8 and ME9, and are post- early Tertiary in age. The ME8 comprises dominantly manganese and iron oxyhydroxide minerals, which increase in abundance towards the land surface. ME9 is calcite-dominated, and lines the walls of open fractures. At depth, in the east of the site, this calcite occurs with finely disseminated, fresh pyrite. In the west, where brine occurs at depth, calcite and anhydrite together represent ME9. The base of the ME8-bearing zone coincides approximately with the STZ. ME9 sulphides occur only beneath the STZ. Systematic, depth-related changes in ME9 calcite morphology, from c-axis flattened forms to c-axis elongate forms, also occur near to the STZ. The depths of these mineralogical features and the STZ vary by c. 350 m across the site. This implies that calcite morphologies, and distributions of ME8 oxides and ME9 sulphides, are related to variations in present groundwater chemistry. Computer simulations have been developed which reproduce the spatial arrangement of different groundwaters. The ME8 and ME9 mineral distributions are consistent with predictions based upon the groundwater distributions. ME8 occurs where fresh waters are predicted to have been recharged relatively recently; ME9 sulphides occur where groundwaters are saline and predicted to be older; ME9 anhydrite occurs only where brine is located. Potentially, the mineralogical data may be used to constrain the evolution of groundwater flow over time. The approximate coincidence of the morphological change in ME9 calcite, and the base of ME8-bearing zone, with the STZ, implies that the STZ was approximately at its present location throughout ME8 and ME9.