Effects of inland water level oscillation on groundwater dynamics and land-sourced solute transport in a coastal aquifer

Inland groundwater level variation widely exists but is usually neglected when conducting research on flow and solute transport in coastal aquifers. A variable-saturation and variable density flow and transport model (FEFLOW) is applied to investigate groundwater dynamics and land-sourced solute transport in a coastal aquifer under the influence of groundwater level oscillation at inland boundary and tidal fluctuation at seaward boundary over a period of two years. The wave number shows that the damping rate of the seasonal oscillation at the inland boundary when propagating in the aquifer is much smaller than that of the tidal fluctuation, thus the seasonal oscillation can propagate a longer distance. The damping forms of the amplitude of both signals are exponential when propagating in the aquifer even though the amplitudes and the periods are different. For the base conditions simulated, the size of the upper saline plume (USP), i.e., recirculated seawater driven by tidal fluctuation, fluctuates periodically in response to the groundwater level oscillation at inland boundary. The fluctuation of the USP controls the movement of freshwater discharge tube (FDT) and the upper part of seawater wedge (SW). Therefore, the SW-freshwater interface shows a rotating instead of parallel movement. As groundwater system in a coastal aquifer is a mixed convection system, a modified Rayleigh number is adopted to investigate the density-induced instabilities associated with the transport of dense solute under the effect of groundwater level oscillation at inland boundary and tidal fluctuation. The more obvious gravity-induced instabilities (i.e., stronger solute downwelling and freshwater upwelling) are observed at lower groundwater level. The finger plume and the upwelled freshwater mix well when approaching the discharge zone where strongly affected by tidal fluctuation. Consequently, the high concentration of the plumes is diluted when migrating toward the sea. Sensitive analysis indicates that the hydraulic conductivity and the density of the solute are the dominant impact factors for the morphology, horizontal and vertical movements of the land-sourced solute plume. The amplitude and period of the fluctuating signal at the inland boundary have minor influence on the morphology and instabilities of the finger plume.