This master thesis aims at studying the effect that the presence of the caves has on saline intrusion in the aquifer. Thus, the final objective is to elaborate a numerical modeling of the groundwater flow and salt transport in the karstic aquifer. Despite the fact that many researchers have invested their efforts in studying the behavior of continental karsts, the modeling examples of coastal karstic aquifers are very rare. Due to the complexity of the problem, it is necessary to apply drastic simplifications. In this case, the karstic system has been characterized as a porous matrix crossed by one conduit. In the matrix it is considered that the water flow is driven by the variable density. The flow in the conduit does not take into account the mass transport processes and, thus, it is supposed to be governed by the Saint Venant equation. This approach has not been applied to coastal aquifers previously. The main advantage of this methodology is that it considers the exchange between the conduit and the surrounding matrix. In order to reach the objective, different models have been implemented in COMSOL Multiphysics, a Finite Element Method code. Initially, the groundwater flow in the karstic system has been modeled without considering the variable density of the water. Next, the variable density problem in two dimensions with and without the conduit is introduced. Finally, the whole problem is extended to three dimensions.