Bromsgrove Aquifer Groundwater Modelling Study : results from Task 1.1 3D visualisation and geological framework of the Bromsgrove Aquifer

The Bromsgrove Sandstone aquifer is over-abstracted. This has resulted in a long-term fall in groundwater levels, the reduction or loss of baseflow and the derogation of surface water features. To support flows in Battlefield Brook (a BAP site and notable amenity feature in Bromsgrove), Bow Brook (BAP site) and water levels in Hewell Grange Lake (SSSI), four alleviation of low-flow (ALF) boreholes are operated (one by the Environment Agency and three by Severn Trent Water). These ALF boreholes were installed as short term measures, prior to the implementation of a long-term solution to reduce the impacts of groundwater abstraction on surface water features which is under discussion between the water company and the Environment Agency. In 2001 an existing groundwater model of the Bromsgrove Sandstone aquifer (developed by Birmingham University in 1990) was adopted and updated as part of the Environment Agency Midlands Region Groundwater Modelling Strategy. However, monitoring data collected since 2002 has shown that this groundwater model does not accurately simulate groundwater flows and levels in critical areas. The Bromsgrove aquifer groundwater modelling project aims to develop a new groundwater model that will be used to determine a more optimal groundwater abstraction regime which benefits the surface water environment, with the minimum of overall groundwater abstraction reduction and affordability. The British Geological Survey (BGS) was contracted to undertake Environment Agency Task 1.1 of the Bromsgrove aquifer groundwater modelling study, namely the production of a three dimensional geological model of the investigation area. The model was specified to cover the outcrop of the Bromsgrove Sandstone Formation, the outcrop of the Clent Formation to the north and the confined Sherwood Sandstone Group to the west. The geographical limits of the area are approximately Droitwich Spa and Astwood Bank in the south (Northing 261550) and Rubery in the north (Northing 279560), Elmley Lovett (Easting 387134) in the west and Redditch (Easting 405456) in the east. The outline of the project area is given in Figure 1. The 3D geological model will be used in a concurrent Task (Task 1.2) to develop the conceptual model of groundwater flow between the principal formations of the Bromsgrove Sandstone aquifer system, as well as providing the geometrical information for building the groundwater model (Task 2). This report outlines the methods used in the BGS 3D geological visualisation work and provides a brief summary of the stratigraphy, facies relationships and structure of the bedrock geology. Much of the information in the report has not been published before, and results from an extensive reinterpretation of existing borehole lithological descriptions and geophysical logs. The model integrates information from BGS 1:50000 geological sheets (E167 Dudley, E168 Birmingham, E182 Droitwich and E183 Redditch), borehole descriptions derived from core or cuttings, geophysical logs and NEXTMap digital terrain data. Published information on the regional geological framework was also incorporated into the model (e.g. Old et al., 1991, Old et al. 1987, Powell et al. 2000)