Understanding saline water dynamics in coastal aquifers using sand tank experiment and numerical modeling

A better understanding of seawater intrusion (SWI) problem in coastal aquifers is important for a perspicacious management of groundwater resources. SWI is affected by various hydrogeological and hydrological parameters such as: hydraulic conductivity (K_sat) of the aquifer, abstraction rate, recharge rate, density of seawater, etc. The objective of this paper is to explore saline water dynamics in an unconfined aquifer under different hydraulic gradients and under managed aquifer recharge (MAR) by using sand tank experiments and numerical simulations using SEAWAT code. Also, the efficiency of MAR in countering SWI malady was explored under different values of K_sat by using SEAWAT code. Numerical modeling is an effective tool to investigate the effect of K_sat on seawater dynamics. Modeling is cheaper and required less time as compared to the sand tank experiment. The sand tank experiment showed that the retreat rate of the saline water interface is always higher than the intrusion rate. As the hydraulic gradient across the sand tank increases, the saline water interface recedes further in the seaward direction. Injection of 1,060 cm³ freshwater into a well located at the toe of a saline water interface caused its retreat seaward by 40%. The calibrated model was used to simulate the effect of aquifer’s hydraulic conductivity on the dynamics of saline water under MAR. The results show that MAR practiced in highly conductive aquifers was less effective in combatting SWI because the injected water discharges rapidly from the aquifer. A small water table mound develops when MAR is practiced in a highly conductive porous medium and hence there is only a small effect in controlling SWI. In contrast, a low aquifer’s hydraulic conductivity slows down water flow, develops a higher water table mound and thus induces a significant effect on controlling SWI. Therefore, optimizing MAR requires close consideration of geological settings and hydrological conditions to ensure high efficiency of MAR in mitigation of salinized aquifer.