Developing 3D geological and hydrogeological models for the Low Level Waste Repository site, west Cumbria, UK

Abstract

This case study presents the recent development of new 3D geological models for the Low Level Waste Repository in Cumbria through the review of historical and new datasets. The site is located in an area of complex Late Pleistocene glacial geology and geomorphology, which control near-surface groundwater movement, and benefits from extensive historical site characterization data. This work emphasizes the value of a quality review of existing borehole records, drilling techniques and supplementary information to build a predictive geological model. The integration of different datasets has proved crucial to our understanding of the ground conditions, which are characterized by complex Quaternary sequences, and the depositional processes responsible for their formation. The updated geological interpretation suggests that the Low Level Waste Repository is located on a former ice-dammed lake plain. This is contrary to the previous interpretation of multiple and extensive till units in the area. Although the permeabilities may be similar in both interpretations, the extents may be different. Understanding the depositional processes and extent and the hydrogeological characteristics of the deposits is key to developing a hydrogeological model that is appropriate for the assessment of the environmental safety of the site, both now and into the future.Thematic collection: This article is part of the Ground models in engineering geology and hydrogeology collection available at: https://www.lyellcollection.org/cc/Ground-models-in-engineering-geology-and-hydrogeology

High-resolution 3D geological modelling of heterogeneity in poorly exposed glacial deposits using sedimentary and glaciotectonic architectural element analysis: a case example from Sellafield in west Cumbria, UK

“…If correct, the new 'glaciolacustrine' interpretation presented by Coleman et al (2020) has profound implications for future 3D modelling and site investigations, both in the district and in other areas of comparable glacial history. For example, by adopting their glaciolacustrine model, these authors have…”

“…The evidence that Coleman et al, (2020) cite against subglacial deposition of pebbly clay units in the Drigg Beach area includes sedimentological, glaciotectonic and accumulation style. We examine each facet in turn: Sedimentology: Coleman et al, (2020) interpret particle size distribution curves of the controversial units to indicate gap-graded clays of glaciolacustrine origin, especially as they assume that the well dispersed lonestone pebbles are dropstones.…”

“…The idea of mud drapes, however, has been resurrected recently by Coleman, et al, (2020) who conclude that that the massive, clay-dominated 'upper tills' exposed in the cliff at Drigg Beach and elsewhere in the district are glaciolacustrine clay, rather than till. Together with the intervening glaciofluvial units, they identify three lake fill / lake drainage cycles, rather than the generally accepted glacial advance and retreat scenario.…”

“…The upper sequence (Whinneyhill Clay Member, Figure 3) includes similar graded couplets (Merritt and Auton, 2000), but other occurrences may simply represent episodic input of slightly coarser material during fluvial or slump activity into otherwise still water. Deposition mostly occurred within ice-dammed lakes that formed at several localities and times (Trotter, et al, 1937;Huddart, 1991;Merritt & Auton, 2000;Coleman, et al, 2020).…”

“…It is important to critically examine the genesis of this element and the underlying one described in the next section, as together, they are subject of some contradictory interpretations (Coleman, et al, 2020). The clay-rich, pebbly lithology, with an overall absence of sedimentary structures (for example grading or lamination) suggests that subaqueous reworking or deposition is unlikely, and that deposition directly from glacier ice is more plausible.…”