Evolution of the fresh to salt water interface from electrical sounding near Oued Nador, Tipaza (North Algeria)

With growing urbanization and associated aquifer overexploitation added to ongoing climatic changes effects (with related sea level rise), coastal aquifers are increasingly threatened by marine transgression. This results in an increase of salt content in underground waters leading to expensive salt removal treatments. The aim of this study is to perform a spatio-temporal monitoring of the fresh to salt water interface using vertical electrical sounding in the Nador plain (Tipaza region, North Algeria), using a Schlumberger electrode array. The fresh to salt water interface is marked by a resistivity contrast reaching 50 Ω m in fresh water formations and less than 10 Ω m for formations with saline water. Three campaigns were conducted a year apart in May 2015, 2016 and 2017. The results illustrate a recurrent interface oscillation over time in response to seasonal changes and overexploitation. Furthermore, the fresh to salt water interface is particularly observed to rise over time away from the sea due to overexploitation and related upconing. Clay layers present natural barriers against an otherwise very large salt water intrusion at depth, portioning at depth the intrusion process. This study highlights the effectiveness of electrical sounding, repeated over time and automated. It demonstrates to be as a valuable tool for environmental assessment and water quality management, including the monitoring of salt water intrusion processes in clay bearing alluvial formations.