Numerical modeling of tidal effects on groundwater dynamics in a multi-layered estuary aquifer system using equivalent tidal loading boundary condition: case study in Zhanjiang, China

This paper presents a numerical modeling study of tidal effects on groundwater dynamics of a multi-layered estuary aquifer system. In this study, to generalize the tidal loading effects on the confined aquifer, we used equivalent tidal loading boundary condition based on existing analytical solutions for the confined aquifer in a three-dimensional numerical model and then good simulated result was obtained. Using the numerical model, the tide-induced groundwater level and confined groundwater discharge dynamics were discussed and the following conclusions were obtained. The combined action of sea tide and river tide can enhance the tidal response of groundwater level in the aquifer system, especially for the confined groundwater. The river tidal effect is more significant in the areas away from coastline. Although the confined aquifer is separated by a layer of clay from the ocean, because of the tidal loading effects, the confined groundwater discharge also changes periodically displaying a negative correlation with the tide level fluctuations like the unconfined groundwater discharge. Using linear functions that express the relationship between the mean confined groundwater flux and sea tide level, the confined groundwater flux through the tidal loading boundaries can be approximately estimated. When the sea tide level is higher than 4.4 m, there would be a risk of seawater intrusion into the confined aquifer in the study area. This case study is useful for numerical studying of tidal effects on groundwater resource dynamics in a coastal multi-layered estuary aquifer system.