Chemical Characterisation of Core Pore-Waters for Deep Borehole Investigations at Sellafield, Cumbria

Abstract

A site near Sellafield, north-west England, is currently being assessed by UK Nirex Ltd for its suitability to host a radioactive waste repository. At this site, a sequence of relatively permeable sedimentary cover rocks, consisting mainly of Permo-Triassic red beds, overlies a sequence of low permeability Ordovician volcanic basement rocks, the Borrowdale Volcanic Group (BVG). A major part of the investigation has been the chemical characterisation of the groundwaters in these varied lithologies, in order to aid the interpretation of groundwater flow paths and water-rock interaction. This has involved sampling groundwaters from deep (up to ca. 2km) boreholes. However, the coverage of such sampling is restricted by the relatively low frequency of fractures from which groundwater flow can be induced and from which waters can be collected. This means that often only relatively large-scale hydrochemical variations down a borehole may be resolved using samples obtained from these zones. It is therefore desirable to construct continuous geochemical profiles down the boreholes using alternative sources of data as an additional interpretation tool. One source of such data are geophysical logs. However, these usually allow only an indication of gross salinity variations down a borehole. Chemical data may also be extracted from core samples collected from down the borehole. The purpose of the present paper is to describe the evaluation of such data using (a) porewater extraction from relatively low porosity sandstones by inert, heavy liquid displacement centrifugation and (b) aqueous leaching of residual solutes.