An Analytical Study Of Tide-induced Seawater-groundwater Exchange Rate Through A Horizontal Seabed

The interaction between seawater and groundwater has a significant effect on the groundwater flow in many coastal areas. It may cause many problems in hydrogeology and engineering geology at the same time, e.g., shortage of fresh groundwater resources, disappearance of springs, land subsidence, ground surface(karst) collapse, land desertification, water quality deterioration caused by groundwater pollution, and the degradation of the ecological environment, etc. The study of tide-induced groundwater level fluctuations and the groundwater-seawater exchange rate can help us to understand the vital significance of coastal hydro-environment evolution and groundwater exploitation and utilization.In this paper, a submarine aquifer with impermeable bottom is investigated. The aquifer is assumed to be directly connected with the seawater and to extend horizontally infinitely. A mathematical model is used to describe this system and the analytical solution of this model is derived. Some assumptions used in the process of model conceptualization. It is further assumed that the density difference between the groundwater and the seawater is negligible due to its slight impact on groundwater level fluctuation. A vertically one-dimensional mathematical model is used to describe the problem.We obtained the influence of the hydrogeological parameters on the groundwater head and seawater-groundwater exchange rate. These hydrogeological parameters include the vertical hydraulic conductivity, the aquifer’s thickness, specific storativity and the tidal efficiency.On the basis of the derivation of the analytical solution to the model, we discussed how the amplitudes and phase shifts of the groundwater head at different depth change with the model parameters. It is found that the decrease of the tidal loading coefficient Te and hydraulic conductivity, the increase of the depth, specific storativity and the aquifer’s thickness tend to reduce the amplitudes and enhance the phase shifts.Based on the system model and the analytical solution, we analyzed the observation data of the tide and groundwater head fluctuations observed at Laizhou Bay. The amplitude and phase shift of the main tidal component was determined by the Least-squares method. We obtained the multi-component analytical solution of groundwater head and the seawater-groundwater exchange rate. Calibration of the simulation is conducted by comparing the multi-component analytical solution with the observation data. The seawater-groundwater exchange rate increases with the vertical hydraulic conductivity, the aquifer’s thickness and specific storativity and decreases with the tidal efficiency.Finally, the observation data and the analytical fitting results were used separately to estimate seawater-groundwater exchange rate. Corresponding comparison and analysis were made.