With 80% of the world's population living in coastal areas, aquifers in direct contact with the sea are widely exploited and are experiencing rising temperatures, sea levels and droughts, making them vulnerable to saltwater intrusion. The Roussillon plain is a region in the south of France, bordering the Mediterranean Sea, which has suffered severe drought in recent years, from an average rainfall of 600 mm per year to a total of 250 mm for 2023, affecting both surface and groundwater. The main activities in the region are tourism and agriculture, which require water especially in spring and summer for crop irrigation and drinking water. In summer, when surface water levels are at their lowest, the region's water supply depends heavily on groundwater. The plain is covered by sediments deposited during the Pliocene and Quaternary, which form aquifers with favorable transmissivities for groundwater exploitation, extending several kilometers under the sea shore. Intensive pumping in recent years has lowered the groundwater level, making it more vulnerable to saltwater intrusion. We show that the seasonality of pumping has a strong effect on groundwater levels, and that the seasonal oscillations of the groundwater level increase the risk of possible seawater intrusion. Our work therefore focuses on building a transient land-sea Modflow model that matches the observed oscillations and helps to manage the Roussillon aquifer by anticipating possible saltwater intrusions. The heterogeneity of the aquifer, with alternating sand and clay layers, is an important factor in this aquifer and must be carefully considered when attempting to model saltwater intrusion. We show that high-permeability layers are the main pathway for intrusion during low-water periods, and that low-permeability layers that become salt-contaminated by dispersion are salt reservoirs, making the intrusion more difficult to reverse.