A STUDY ON SALTWATER TRANSPORT TOWARDS A PUMPING WELL

Abstract

The available analytical solutions for upconing of saltwater interface are based upon many restrictive assumptions which may not always be satisfied. In the present study, an attempt has been made to develop a numerical model for simulation of saltwater transport occurring in consequence of pumping water from a partially penetrating well. The well is assumed to tap only the freshwater zone and saltwater is assumed to occur below the well screen. The model, accounting for both convective and diffusive components of saltwater transport, is based upon a numerical solution of the differential equations governing the pressure distribution and the mass transport in a two-dimensional axi-symmetric flow domain. While calculating the pressure distribution, the variation of the specific weight and dynamic viscosity of fluid due to time and space variation of saltwater concentration is accounted for. The pressure distribution is computed by the finite difference employing iterative alternating direction implicit explicit (IADJ.E) scheme. To avoid numerical dispersion, computation of the total saltwater transport is accomplished in three stages. First, the convective transport is computed by the method of characteristics. The necessary velocities are calculated from the precomputed pressure distribution. Subsequently, the diffusive transport is computed by the finite difference employing iterative alternating direction implicit explicit (IADIE) scheme. Finally, the two transports are integrated to get the total transport. Thus, the proposed model basically simulates the vertical and radial movement of the saltwater during and subsequent to the closure of IV pumping. Such a simulation leads to estimates of spatially and temporally distributed saltwater concentration in the aquifer and temporally distributed saltwater concentration in the pumped water. Further, the fractional saltwater settlement [i.e., the fraction of saltwater lifted (during the pumping), settling down to the initial position of the interface] at different discrete times since the closure of the pumpage may also be estimated. The model has been implicitly validated by comparing its response with Bear and