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

As part of an assessment of the performance of a potential deep repository fro intermediate- and some low-level radioactive wastes a Sellafield, numer ical models of the regional scale groundwater flow were built. Quaternary s ediments play an important role in the flow because they control recharge t o the deeper groundwater system. This paper outlines research carried out u nder the Nirex Quaternary Characterization programme at Sellafield and show s how lithological and structural characterization of the sediments has ena bled 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 on ice advance. Sediments include glacigenic diamictons, coa rse glaciofluvial gravels probably deposits as sandar and silts of probable glaciolacustrine origin. these sediments are locally glacitectonically def ormed. Recent alluvial, marine and lacustrine sediments are also present. T he lateral extent of many of the sediment bodies is expected to be small, a nd therefore their complete characterization by traditional approach of int erpolating between measurements in boreholes and at outcrop is likely to be difficult, without and impracticably large number of boreholes. This the hydrogeology was characterized on the basis of the geology, suppor ted by a relatively small number of boreholed and hydraulic measurements. Q uaternary Geological Domains defined by distinctive landform-sediment assoc iations and structural characteristics were mapped. Distinctive vertical pr ofiles of sediments were identified from subsurface data (e.g. borehole dat a). This enabled the Geological Domains to be refined to Hydrogeological Do mains. The overall performance of the system can be confirmed by reference to deta iled field scale measurements in specific areas. The behaviour varies betwe en providing little impediment to recharge to, or discharge from, the under lying Strata: of the Sherwood Sandstone Group over most of the area, but wi th local areas where recharge or discharge is considerably impeded. Locally , the Quaternary deposits contribute a significant part of the transmissivi ty of the regional aquifer. The hydraulic characteristics of individual 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 i n the Hydro-geoglogical Domains. This has enabled the derivation of effecti ve properties, which were used in the regional-scale, flow models, The resu lting parameter values are consistent with those independently derived from regional aquifer modelling.