Modeling seawater intrusion upon water withdrawal from coastal aquifers

Seawater intrusions into coastal aquifers have been the object of study by hydrogeologists since long ago. A wide range of methods are used to predict the development of intrusions: analytical, numerical analytical and numerical. The purpose of this paper is to identify and evaluate seawater intrusion using a coastal groundwater intake as an example. The paper considers a coastal groundwater intake, where mineralization, total hardness and chlorides have increased due to intensive exploitation. Seawater intrusion is assumed to be the main reason for the groundwater quality deterioration. To prove the seawater intrusion motion, chemical sampling of water from the intake wells was carried out, which resulted in chlorine-bromine coefficient values indicating the presence of seawater admixture in fresh groundwater. The seawater intrusion was evaluated using numerical-analytical and numerical modelling. The estimation obtained by the analytical element method (GFLOW code) indicates the presence of seawater intrusion at the studied water intake. However, the analytical element method calculations provide the ultimate steady-state estimate of the intrusion development. Also, the method uses the assumption of a sharp freshwater-saltwater boundary. Numerical modelling provides a more realistic assessment. In building the three-dimensional numerical model, special attention was paid to the nature of the connection between the exploited aquifer and the sea. In the onshore part, the Quaternary aquifer is separated from the exploited aquifer by a clay layer with interbedded gravel and sand, but there is no data on the composition of sediments beneath the sea. To clarify their composition, profile numerical models were built in the FEFLOW program with different degrees of connection between the sea and the exploited aquifer. One of the models suggests a direct connection between the sea and groundwater through sand, while the other one is complicated by clay. According to the results of the profile models, the direct connection between the exploited aquifer and the sea was proved, which was taken into account in the construction of the three-dimensional numerical model. On the numerical model in the SEAWAT program, the change in the seawater intrusion position in different periods of water intake operation with different water withdrawal values was obtained, after which the calculation results according to numerical-analytical and numerical models were compared.