A hydrogeological characterisation of the superficial deposits of the Severn valley upstream of the Shrewsbury

This report summarises the results of a collaborative study jointly funded by the Environment Agency and the British Geological Survey (BGS). BGS was commissioned to carry out a geological and hydrogeological characterisation of the superficial drift deposits of the Severn valley, upstream of Shrewsbury. The principal aims of the project were identified as follows: • to assess the heterogeneity of the superficial deposits within the buried channel systems that underlie this part of the River Severn • to identify hydrogeologically significant drift lithologies with respect to ground water storage and aquifer recharge, and • to assess their spatial distribution. A model of the sub-drift topography was constructed, based on information from over 700 boreholes. These revealed an irregular rockhead surface, deeply dissected locally by steep-sided, buried channels and broader, over-deepened depressions. The deepest of these is the Severn Trench, which probably links eastwards with a separate south-easttrending system of buried channels that passes beneath Shrewsbury. The drift varies considerably in thickness but on the lower ground commonly exceeds 40 m and in places reaches 120 m. In the Severn Trench, the sediment infill is dominantly one of clays and silts, but in the more easterly channels there are substantial sand bodies, which provide hydraulic connection between the River Severn and the underlying Permo-Triassic aquifer. A detailed assessment of the drift variation across the district was carried out from a study of borehole records. Despite the marked lateral and vertical variability encountered, it proved possible, by constructing a series of stratigraphic cross-sections, to rationalise and classify the drift into a relatively small number of hydrogeological domains and sub-domains. The resulting domain map distinguishes zones in which differing hydrogeological behaviour can be expected. For example, it shows where potential inflow from the River Severn via the drift to the Permo-Triassic aquifer is likely and where it is unlikely. It also indicates where precipitation recharge is likely to be restricted by the surface clays. The results of the study will be used by the Environment Agency to build an investigative groundwater model of the Alberbury groundwater unit. An electronic version of this report and digital outputs are to be found on the accompanying CD.