Effects of inland water level oscillation on groundwater dynamics and land-sourced solute transport in subterranean estuary

Abstract:

Inland water level variation widely exists but is usually neglected when conducting research on flow and solute transport in coastal aquifers. A numerical model considering variable-saturation and variable-density based on FEFLOW was used to investigate the influence of inland groundwater level oscillation on groundwater dynamics and land-sourced solute migration in a subterranean estuary over a period of two years. Results show that groundwater level fluctuation induced by water level change at the inland boundary decays linearly seaward but that induced by tidal fluctuation decays exponentially landward. The size of the upper saline plume fluctuates with the inland groundwater level oscillation and the size of the largest USP is more than two times that of the smallest. The largest and the smallest USPs are observed about 25 and 45 days after the lowest and highest inland groundwater level respectively for the parameters and aquifer configuration used in this study. The seawater wedge-freshwater interface shows a rotated movement in response to groundwater level oscillation at inland boundary because its upper part is controlled by the USP movement. Considering that the relatively large horizontal velocity exists in coastal aquifers, a modified Rayleigh number is used to investigate the density-induced unstable phenomenon involving in solute transport in a subterranean estuary. A high Rayleigh number indicates the onset of instability. Fingers and freshwater protuberance are observed in both cases with and without inland groundwater level oscillation. Due to small seaward hydraulic gradient, a high Rayleigh number is reached at the low inland groundwater level, resulting in a long finger plume and high freshwater protuberance morphology. Because of mechanical dispersion induced by the large horizontal velocity when groundwater level is high at the inland boundary, the finger plume and freshwater protuberance well mixes. Consequently, the high concentration plumes will be diluted when migrating toward the sea. Sensitive analysis indicates that the USP and land-sourced solute plume are sensitive to hydraulic conductivity of the aquifer, amplitude of the groundwater level oscillation at the inland boundary and density of the land-sourced solute.