Three-Dimensional Numerical Simulation of Density-Dependent Groundwater Flow and Salt Transport Due to Groundwater Pumping in an Unsaturated Fractured Porous Coastal Aquifer System

A series of three-dimensional numerical simulations using a generalized multidimensional hydrodynamic dispersion numerical model is performed to analyze density-dependent groundwater flow and salt transport before and during groundwater pumping in an unsaturated fractured porous coastal aquifer system, which is heterogeneous and anisotropic. The coastal aquifer system is located on the western coast of Korea and is composed of the Quaternary alluvium (i.e., alluvial aquifer) underlain by a series of the Precambrian and Cretaceous geologic formations (i.e., bedrock aquifer), which contain numerous planar geologic structures such as bedding planes and joints (joint sets) of various orientations as well as porous rock matrices, with a major fault. The upper part of the geologic formations forms a weathered zone, which is covered by a marine sediment below the mean sea level. A steady-state numerical simulation with model calibration is performed first to obtain initial steady-state spatial distributions of density-dependent groundwater flow and salt transport in the coastal aquifer system before groundwater pumping, and its results are illustrated and analyzed and are also validated reasonably well with respect to the measured groundwater levels and the inferred seawater intrusion front lines (i.e., transition zones between fresh water and salt water). A series of transient-state numerical simulations is then performed using the above-mentioned initial steady-state spatial distributions as initial conditions to obtain spatial and temporal distributions of density-dependent groundwater flow and salt transport in the coastal aquifer system during groundwater pumping with the six different pumping rates at each of the six different potential locations for a new pumping well in the bedrock aquifer, and its results are illustrated and analyzed to suggest optimal location and pumping rate for the new pumping well using the well response function and secondary drinking water quality limit of the seawater-normalized salt concentration. The results of both steady-state and transient-state numerical simulations show that heterogeneity and anisotropy of the geologic media, especially the major fault, as well as the topography and unsaturated zone have significant effects on the spatial and temporal distributions of density-dependent groundwater flow and salt transport in the coastal aquifer system before and during groundwater pumping.