Simulation of groundwater flow within observation boreholes for confined aquifers

Summary Observation wells around a pumping well are the primary means of monitoring the change of hydraulic head in an aquifer. Pipe flow in open observation boreholes will affect the measurement of hydraulic head in the observation wells and alter the flow patterns around the wells, but the influences of non-Darcian pipe flow in observation wells on the accuracy of hydraulic head measurements are not fully recognized. This paper presents a coupled seepage-pipe flow theory which integrates the aquifer and well as a continuous medium for assessing the influences. The objective of this study is to examine the pipe-flow effects in observation wells of varied diameters under several pumping scenarios including different penetration lengths of the pumping well. A numerical model based on the Galerkin finite element method and the finite difference method was developed for the study. Numerical simulations were conducted for a homogeneous and isotropic confined aquifer in which a pumping well penetrated a different thickness of the aquifer and a number of full penetrating observation wells were placed to obtain drawdowns. The simulation results indicated that vertical pipe flow within the observation wells was significant and the flow was non-linear. The pipe flow resulted in a conduit effect that disturbed the distribution of the hydraulic head within the observation wells. After comparing with the results of the Hantush (1961) integral by using the case study, which neglected the wellbore effect on the groundwater flow system, the authors found that the Hantush integral may be only conditionally applicable for estimating the drawdown within the observation wells when well diameter is greater than 0.20 m and radial distance is greater than the thickness of the aquifer. Our study suggests that use of hydraulic heads from observation wells for the evaluation of groundwater flow systems must be cautious before the pipe-flow effect can be assumed to be negligible.