Mechanisms of Non-Fresh Groundwater Presence at Water Tables in Highly Permeable Coastal Aquifers

Coastal aquifers with high hydraulic conductivities on the order of 10^-2 m s^-1 have unconventional salinity distributions with the presence of non-fresh groundwater at the water table over a wide swath near the coast. This study aims to unravel the mechanisms underlying the phenomenon via numerical simulations for variably saturated, density-driven flow and solute transport in porous media. The simulation results indicate that the existence of non-fresh groundwater at the water table is attributed to the upward mass flux in the saturated zone near the coast, which transports solute from deeper groundwater toward the water table. With high hydraulic conductivity, the upward mass flux becomes prominent at shallower elevations because of the high Darcy flux and the shallow saline groundwater. The upward mass flux has two main drivers, upward advection by the upward flow component and transverse dispersion by the seaward flow component. The advective mass flux dominates over the transverse dispersion in the deep part of the saturated zone where only groundwater with sea water salinity exists. In contrast, the transverse dispersion becomes more pronounced than the upward advection in the shallow saturated zone just beneath the water table and in the unsaturated zone immediately above the water table. Our findings help interpret the unconventional salinity distributions observed and elucidate the unique dynamics of groundwater flow and solute transport in highly permeable coastal aquifers.