Data review leading to a conceptual model of the hydrogeology of the Combe Down area, Bath

Bath and North East Somerset Council (B&NES) has obtained funds from the Department of Environment, Transport and the Regions to carry-out stabilisation of stone mines located in the Combe Down area to the south-east of the city of Bath. There are potentially significant engineering geology and hydrogeology implications associated with the project. The British Geological Survey (BGS) has been appointed by B&NES to provide independent geological advice. In this role, BGS has undertaken a review of available data to allow a conceptual model of the hydrogeology of the Combe Down area to be developed. Combe Down is located on a spur, formed by the incised valley of the River Avon. The aquifer system underlying Combe Down can be simplified to a three layer system, the Great Oolite limestones and the Inferior Oolite limestones separated by the clays of the Fuller's Earth. Springs issue from the base of the two main aquifer units and provide both public and private water supplies. The hillslopes have undergone a great deal of land slippage and those on the southern side of the plateau are still oversteepened. The review has collated the following hydrogeology-related information on the Combe Down area: • existing reports on the geology, geomorphology and hydrogeology • geological borehole logs held within the National Geosciences Data Centre, BGS, Keyworth • well, borehole and spring information held within the National Groundwater Archive, BGS, Wallingford • rainfall data for the locality and Meteorological Office estimates of actual evapotranspiration for the region • available groundwater-level, springflow and stream data collected by Halcrow and Wessex Water • groundwater and spring water quality analyses undertaken by Halcrow and Wessex Water • licenced abstractions, consented discharges and an estimate of mains leakage • groundwater source protection zones, obtained from the Environment Agency The development of the conceptual model of the hydrogeology was aided by undertaking a preliminary water balance on the shallow aquifer, as part of the review, in which known inputs to the aquifer are compared with known outputs. Total groundwater recharge and spring flow was calculated for a hydrological year from mid-August 1994 to mid-August 1995. It is anticipated that any impacts on the hydrogeology of Combe Down due to stabilisation are likely to be the result of mine voids becoming less permeable due to their total or partial infill and/or leachate from mine infill affecting the quality of spring outflows. If infill of some or all of the existing mined areas does not allow water to infiltrate to the shallow aquifer following the present pattern, then the groundwater flow directions may alter. This could affect the location of existing springs and cause changes to the overall volumes or temporal variations in flow, possibly to the detriment of the environment and the present users. The review has produced improved estimates of the size of catchment areas for the springs discharging from the shallow aquifer. It has shown that the delineation of the catchments of individual springs, in particular those of Prior Park and the Whittaker and Tucking Mill Springs, is very approximate. due in particular to the possible importance of structural controls and the difficulty in assessing groundwater recharge in urban areas. The review presents evidence that the deep aquifer. formed by the Inferior Oolite limestones and the Midford Sands, is not isolated from the shallow aquifer in the Combe Down area, and therefore that springs discharging from this aquifer may potentially be affected by changes to groundwater flow and quality associated with stabilisation of the mines.