Simulation of intense salinization phenomena in coastal aquifers—the case of the coastal aquifers of Thrace

Seawater intrusion in porous media and accompanying transport processes were investigated. The present study concerns the coastal area of the Thrace region, Greece that is a typical Mediterranean coastal aquifer system. This aquifer system consists of several highly permeable zones separated by less-permeable layers. The aquifer system is hydraulically interconnected in variable ways. The intensive pumping together with the absence of any integrated water resources management plan, resulted in the permanent presence of a reverse regional cone of depression and the degradation of the groundwater quality. The decline in water level reached −40 m in the centre of the study area. Hydrochemical diagrams and ion balance demonstrated that cation exchange phenomena take place due to salinization processes in the coastal zone and refreshening processes in recharge areas. In these areas, groundwater is locally characterized by increased salinity, which may be partly connate in origin. The code PHREEQC, which is suitable for transport modeling, successfully simulated the extent of the saline front from Vistonis Lagoon to approximately 6 km inland. The application of the code proved that the aquifer system has properties of a dual porosity medium, where exchanges of chemical species take place between flowing and stagnant water through the process of diffusion. The current chemical characteristics of the groundwater under the simulated conditions were achieved when the amount of groundwater equals 1.7 times the pore water volume of the aquifer system.