Hydrogeological characterization of the onshore Quaternary sediments at Sellafield using the concept of domains

Abstract

As part of an assessment of the performance of a potential deep repository for intermediate- and some low-level radioactive wastes at Sellafield, numerical models of the regional scale groundwater flow were built. Quaternary sediments play an important role in the flow because they control recharge to the deeper groundwater system. This paper outlines research carried out under the Nirex Quaternary Characterization programme at Sellafield and shows how lithological and structural characterization of the sediments has enabled the development of Quaternary Hydrogeological Domains which have been used in the regional groundwater models. The Quaternary sediments in the Sellafield area are very varied and result from more than one ice advance. Sediments include glaciogenic diamictons, coarse glaciofluvial gravels probably deposited as sandar and silts of probable glaciolacustrine origin. These sediments are locally glacitectonically deformed. Recent alluvial, marine and lacustrine sediments are also present. The lateral extent of many of the sediment bodies is expected to be small, and therefore their complete characterization by the traditional approach of interpolating between measurements in boreholes and at outcrop is likely to be difficult, without an impracticably large number of boreholes. Thus the hydrogeology was characterized on the basis of the geology, supported by a relatively small number of boreholes and hydraulic measurements. Quaternary Geological Domains defined by distinctive landform-sediment associations and structural characteristics were mapped. Distinctive vertical profiles of sediments were identified from subsurface data (e.g. borehole data). This enabled the Geological Domains to be refined to Hydrogeological Domains. The overall performance of the system can be confirmed by reference to detailed field scale measurements in specific areas. The behaviour varies between providing little impediment to recharge to, or discharge from, the underlying strata of the Sherwood Sandstone Group over most of the area, but with local areas where recharge or discharge is considerably impeded. Locally, the Quaternary deposits contribute a significant part of the transmissivity of the regional aquifer. The hydraulic characteristics of individual sediment types are known from laboratory data and limited field data. These data have been integrated and upscaled using analytic calculations based on the lithological profiles recognized in the Hydrogeological Domains. This has enabled the derivation of effective properties, which were used in the regional-scale flow models. The resulting parameter values are consistent with those independently derived from regional aquifer modelling.