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.