Seawater Intrusion Characterization in a Coastal Aquifer Using Geophysical and Geochemical Approach (Northeastern Tunisia)

The shallow aquifers in the Mediterranean coastal areas are potential groundwater resources for many purposes. Groundwater from sands and sandstone deposits in the Medjerda lower valley aquifer (Northeastern Tunisia) represents a source which is characterized by high saline variations with Total Dissolved Solids (TDS) levels up to (19,000 mg/l) and with high Cl⁻ (>4000 mg/l) and Na⁺ (>3000 mg/l) contents near the coastline, indicating the saline nature of the groundwater. This study consisting in an integrated geophysical approach (using Vertical Electrical Sounding (VES)) and hydrochemical analyses (60 groundwater samples) to circumscribe seawater intrusion phenomena. Several VES results were utilized using Schlumberger configurations with AB/2 « ρa = ʄ (AB/2) » varying from 1 to 200 m. The low resistivity (<5 Ω m), as saline water intruded up, extended over several km inland within the eastern parts of the study area. Mapping of TDS, Cl⁻, Na⁺ and iso-resistivity using GIS tools and electrical pseudo-sections revealed the extent of the seawater intrusion inland and the presence of direct cation exchange linked to seawater intrusion and dissolution processes associated with cation exchange. Finally, we demonstrate that these results may be used to enhance the management of the groundwater resources in the Medjerda lower valley aquifer in a complex terrain.