Analytical and numerical analysis of constant-rate pumping test data considering aquifer boundary effect

The constant-rate pumping test (CRT) is commonly used to determine the aquifer properties. In this method, measured drawdown in the pumping well and the monitoring wells are usually matched with type curves developed by various analytical methods. However, numerous assumptions used to develop the analytical solution are not always compatible with the actual site conditions. In this study, drawdown records collected in a pumping well and two monitoring wells located near a river, were analyzed to investigate the boundary effects on the estimated aquifer properties. This site condition violates the assumption that the aquifer is of infinite areal extent. Moreover, time varying water heads during the pumping and recovery tests were simulated numerically to determine the effect of hydraulic gradient, which does not satisfy the assumption of horizontal potentiometric surface used in the analytical solution. Calibrated aquifer properties without the boundary effect showed clear differences, where the transmissivity, anisotropy ratio and specific yield varied by 12%, 34% and 53%, respectively, as compared to the results obtained by including the boundary effect. Numerical simulation conducted by considering a horizontal potentiometric surface as in the analytical model produced an averaged root-mean-squared-error (RMSE) of 0.055 m, which was approximately 57% higher than the RMSE value estimated with the effect of the hydraulic gradient. Therefore, it was concluded that the simplified analytical solutions may lead to misleading estimations of aquifer properties when the pumping test was conducted in an area with complex site conditions.