Interpretation of pumping tests in the Sherwood Sandstone Group, Sellafield, Cumbria, UK

Abstract

In 1995 as part of the process to assess the suitability of a site near Sellafield, Cumbria, for a deep geological repository for solid intermediate level and some solid low level radioactive waste, United Kingdom Nirex Limited (Nirex) commissioned a series of long duration pumping tests in the various stratigraphic horizons at the site. Two of the pumping tests were conducted in vertically adjacent sections of the lower Sherwood Sandstone Group (SSG) whilst pressures were monitored at discrete intervals in surrounding monitoring boreholes. Responses were observed in over twenty monitoring zones at various levels around the pumped sections during both tests. In order to arrive at an understanding of the hydraulic system in the SSG, a systematic methodology was adopted to analyse the data sets from the pumping tests. Preliminary analysis of the data from each responding zone used the standard 2D radial flow analytical approach. This indicated that, whilst responses conformed broadly to this model, there were important deviations. Three different analytical modifications were adopted to assess the significance of these deviations; a partial penetration model, a bounded aquifer model and a generalized radial flow model. The use of the different models allowed a conceptual model of flow processes occurring in the SSG to be developed.

Characterization of a fluvial aquifer at a range of depths and scales: the Triassic St Bees Sandstone Formation, Cumbria, UK

“…Optical logs in the shallow St Bees Sandstone aquifer (< 150 m BGL) also show development of karst pathways in correspondence of intersections between bedding planes and stratabound fractures (Medici et al 2016). This suggests how~2.5 Ma of exposure to the processes of enhancement of fracture permeability, typical of shallow aquifers, might have played a key role in influencing transmissivity distribution in this fractured fluvial aquifer, which is markedly decreased where tested at depths > 150 m below the ground surface (Chadwick et al 1994;Streetly et al 2000;Worthington et al 2016). Hence, a second groundwater alteration zone in the aquifer (see Fig.…”

“…2; Nirex 1992bNirex , 1993a. Results from these tests were then compared to multiborehole packer tests undertaken by Streetly et al (2000), which represent a larger investigation scale (~50-1,000 m; Fig. 2).…”

“…Transmissivity values were converted into mean well-test scale hydraulic conductivity by dividing by the screen length; this assumes sub-horizontal flow. This assumption is reasonable given the layered nature of the aquifer (K h > K v at the well-test scale, Streetly et al 2000).…”

“…The Brockram Formation, which consists of low-permeability deposits of Permian age (conglomerate with a silty matrix, evaporites and shale) underlies the St Bees Sandstone aquifer forming an aquitard unit (Allen et al 1997;Streetly et al 2000).…”

“…1) in both wells penetrating the fluvial (n = 9; T = 30-700 m 2 /day) and aeolian (n = 3; T = 100-220 m 2 /day) deposits (Allen et al 1997;Medici et al 2016). Pumping tests (n = 33; T = 0.46-8.82 m 2 /day) were also undertaken at greater depth (200-400 m BGL; Streetly et al 2000) in the fluvial St Bees Sandstone aquifer. In this work hydro-geophysical data from the St Bees Sandstone fluvial aquifer at up to 1,100 m BGL will also be considered, in order to achieve a better understanding of the mechanisms that control hydraulic conductivity with depth.…”