Pumping Test Analysis Using a Layered Cylindrical Grid Numerical Model in a Complex, Heterogeneous Chalk Aquifer

50 A groundwater investigation including several pumping tests has been carried out by 51 Thames Water Utilities Limited (TWUL) to improve the understanding of the 52 distribution of hydraulic properties of the Chalk in the Swanscombe area of Kent in 53 south-eastern England. The pumping test behaviour is complicated by: the fractured 54 condition of the Chalk, simultaneous pumping from adjacent boreholes, and variable 55 pumping rates during the test. In addition, the groundwater flow system is 56 complicated by quarrying of the Chalk. Analytical solutions for pumping test analysis 57 fail to represent these complex flow processes and cannot reproduce the observed 58 time drawdown curves. A layered cylindrical grid numerical model has been applied 59 to the results of the Swanscombe pumping test. This model can represent the 60 heterogeneity of the aquifer and the detailed flow processes close to the abstraction 61 borehole such as well storage, seepage face and well losses. It also includes a 62 numerical representation of the moving water table using a grid that deforms to 63 eliminate numerical instabilities. The analyses of the test results demonstrate that 64 they are significantly influenced by fracture flow, which needs to be included to 65 improve the simulation of the groundwater system; not withstanding this, the layered 66 cylindrical grid numerical model reproduced many of the features in observed time67 drawdown, which allowed an assessment of the hydraulic characteristics of the 68 aquifer as well as the investigation of the impact of quarries on the test results. This 69 has demonstrated that the numerical model is a powerful tool that can be used to 70