Application of numerical modelling to investigate recharge to the Chalk Aquifer beneath thick till deposits in East Anglia

The importance of the Chalk aquifer as a groundwater source and the uncertainty associated with the occurrence of recharge to the Chalk when it is overlain by thick till deposits has led to the development of a project to investigate the Chalk-till groundwater system. An area in East Anglia, where the Chalk is mainly covered by the till-deposits but still exposed in some places especially at the river valleys, has been selected for investigation. This study involves drilling cored boreholes, monitoring groundwater levels, sampling Chalk and till groundwaters and porewaters and the development of a conceptual model of Chalk-till groundwater hydrogeology (Marks et al., 2004). This report discusses the application of numerical modelling to validate the conceptual model built by Marks et al. (2004). This is achieved by determining the likely recharge rates and transmissivity distribution required to reproduce the groundwater heads observed in the study area. A one-dimensional groundwater model has been constructed and both steady state and time-variant modelling are undertaken. A one-dimensional model that extends 11 km starting from the river Stour to the south is considered. This model includes a 1 km of exposed Chalk next to the river and 10 km of tilldeposits covered Chalk. A fixed head is considered at the end where the river is located while an impermeable boundary is imposed at the other end. The steady state modelling showed that recharge rates of 5 mm/a through the till and 300 mm/a over the exposed Chalk were required to produce the observed groundwater heads. A transmissivity value of between 10 and 50 m2/d was used under the till and a transmissivity value of greater than 400 m2/d was used in the valley. Under time variant conditions, the numerical results are compared to the groundwater head fluctuations recorded at two observation wells. The first is approximately 1000 m away from the river bank and drilled in the exposed part of the Chalk, and the second is approximately 9000 m away from the river bank and drilled in the covered part of the Chalk. The use of constant recharge under the till and seasonally varying the recharge over exposed part of the Chalk produced reasonable match between the numerical and observed results at the first observation well when the parameter distribution for the steady state model is used. However, this parameter distribution could not reproduce the observed fluctuations in the groundwater heads at the second observation well drilled in the covered part of the Chalk. A small, approximately 1%, seasonal component of recharge under the till was required to produce the observed groundwater head fluctuations. This is justified by calculating seasonal changes in the vertical hydraulic gradients in the system. While simple calculations showed that there should be a lag time between the groundwater hydrographs at the two observation wells, the field hydrographs at these two observation wells do not show any time lag between the occurrence of the peaks or the troughs. This is further evidence for some degree of seasonality in the recharge under the till.