Experimental and numerical investigations on the fresh-saline water interface dynamics in a coastal aquifer due to saline groundwater pumping

In coastal aquifers, pumping from the dispersion zone extracts intruded saltwater into saline groundwater (SGW). There has been limited research on visualizing and evaluating situations when well screens capture saltwater and the extent of dilution occurring, when it is combined with freshwater within the well. The present study attempts to quantify mixing through the dissipation of salinity variance and analyze the most significant parameters contributing to various pumping situations. To examine the influence of freshwater velocity distribution on the distribution of salinity, we conducted experiments with (i) changing hydraulic gradient, (ii) with and without groundwater extraction well, and (iii) three varying pumping rates. Pumping induced changes in vertical fluxes that resulted in disparities in the length of the toe (L_toe) of the wedge, salinity of pumped water, distribution of velocities, and overall mass balance. The characteristic behaviour of the system was further evaluated using a dimensional analysis which differentiated the response of the system for buoyancy dominated and balanced flow conditions. The salinity of pumped water was dependent on the freshwater flux in the vicinity. The buoyant flow regime increased salinity because seawater compensated for the pressure deficit due to proximity to the saltwater barrier, while the balanced regime decreased salinity by diluting salt flux with freshwater. The well location ratio and conductivity anisotropy reduce saltwater extent more in buoyancy-dominated flow scenarios than in balanced flow. Despite its simplified approach, the results provide insights for practitioners aiming to implement a technique of extracting SGW to restore a salinized unconfined coastal aquifer system.