Prediction of saltwater intrusion’ dynamics in coastal aquifer using modeling techniques: a case study in Northeastern Tunisia

The groundwater, which present 30.1% of the global freshwater, is at risk of being contaminated by saltwater intrusion. The saltwater intrusion is the induced flow of seawater into freshwater aquifers. Saltwater intrusion can occur due to natural processes as well as over-abstraction of groundwater from coastal aquifers. Numerical modelling is a useful tool in helping hydrologists to understand and predict how saltwater intrusion occurs in coastal aquifers. These numerical models are based on the governing equations of groundwater flow and contaminant transport.

 In this paper, the extension of saltwater intrusion into the coastal aquifers, of the Bouficha region, has been investigated by modelling using SEAWAT in MODFLOW. The abstraction from the Bouficha groundwater had increased more than fourfold between 1993 and 2021. Numerical groundwater modelling is a powerful tool for evaluation, development and management of groundwater resources of this basin. A numerical groundwater model for Bouficha groundwater was developed using MODFLOW software (pm8) to simulate regional groundwater changes in the Bouficha groundwater under steady and transient state. The flow model was calibrated based on 29-years historical period.

For controlling the quality of Bouficha groundwater, and as the Bouficha groundwater is a costal aquifer, a transport model related to salinity was developed using SEAWAT package, in MODFLOW, based on historical salinity data of 28-years. The transport model was successfully calibrated in the steady and transient state.

The transport model was applied to examine how far the seawater transition zone will moved based on five future scenarios (pumping and climate change). The five flow scenarios were used to predict the salinity distribution in the Bouficha groundwater, using SEAWAT package, by extended the transport model until 2050.

The scenarios results indicate the total deterioration of the Bouficha groundwater’s quality. The predicted salinity shows that the Bouficha groundwater will be in critical status.

The sources of the Bouficha groundwater quality degradation are multiple: the seawater intrusion from the sea and from the Sebkha and a chloride input from agricultural activities (as transport boundary conditions).