Estimating hydrogeological conditions over the last 120 ka: an example from the Sellafield area, UK

Abstract

For a number of years, United Kingdom Nirex Limited investigated the suitability of the area around Sellafield in NW England to assess its suitability to host a deep repository for intermediate-level and some low-level radioactive wastes resulting from Britain9s nuclear programme. The investigation programme collected a large amount of lithological, hydrogeological and hydrochemical data from 18 boreholes up to 2000 m deep. A potential route by which radionuclides may leave the repository and return to surface is the groundwater pathway. It was recognized that the groundwater regime could be disturbed significantly by major climate changes, particularly glacial–interglacial cycles, over the very long time scales considered in the assessment of the radiological safety of a repository. Available geological and hydrogeological data for the Sellafield area are reviewed in the context of current thinking on glacial hydrogeology, showing that disturbance of the hydrogeological regime is possible during glaciation, and that there are significant uncertainties in determining the timing and magnitude of the effects caused by glaciation. However, the evidence available, both from Quaternary fracture-filling minerals and from the chemistry of groundwater now present, suggests that any disturbance to the deeper hydrogeological regime was relatively minor and that any subglacial recharge has been removed by the present flow regime. There is a possibility that high hydraulic heads at depth in the low-permeability basement rock considered as a host for the repository, the Borrowdale Volcanic Group of Ordovician age beneath a cover of Triassic sandstones, may be residual.