ARC (Southern) intend to extend and deepen working from two quarries in the Folkestone Formation, near Reigate in Surrey, UK. During the planning process, credible forecasts of the impact of future dewatering strategies are required to reassure the concerns of the Environment Agency, environmentalists and local water users. A hydrogeological and modelling study was carried out to investigate the present groundwater flow in the Lower Greensand aquifer, and to forecast the future pumping rates required to maintain dry excavations, the impact of further dewatering on groundwater levels and quality, and water levels after the end of quarrying. At the site, the Folkestone Formation consists of two discrete sand horizons, separated hydraulically by a clay layer, which supports head differences of 10 m near the quarries. Apart from ARC's recent monitoring of flows and water levels, very limited information on the aquifer was available, but the problem was minimized by careful calibration of the model for the three years with the most data. The results were used to modify working plans to minimize the impact on the surrounding aquifer and to support ARC's application for planning permission to extend the quarries.