4D Electrical Resistivity Tomography Monitoring of Talesh Mahaleh-Rasht Coastal Aquifer Polluted by Caspian Seawater

Abstract

Sea saltwater influx into coastal aquifers, a phenomenon brought by the flow of seawater into freshwater aquifers, has long been recognised around the world. In this study, Geophysical surveys are carried out in the coastal area of Talesh-Mahaleh (North-eastern Rasht-Gilan Province of Iran) to study the time evolution of seawater contamination of the coastal aquifer, probably increased due to human impact. The study is based on a processing and integrated analysis of hydrogeological and geophysical data. In particular, in the September to March 2012, seasonal time-lapse electrical resistivity tomography (on 15 Dipole-Dipole profiles) has been carried out. The resistivity data were acquired with electrode spacing (a) of 25 m, and expansion factor (n) of 40. The interpretation of geophysical data made it possible to reconstruct a 4D model of the electrical resistivity of the aquifer, aimed at defining the extent and the boundaries of seawater intrusions. Furthermore, the execution of a series of high-resolution time-lapse electrical tomography and a correlation analysis between geophysical measures and, geological and hydrogeological data allowed specifying the effects of the salt concentration in the groundwater and the porosity and saturation degree of the geology formations on the time variations of the measured electrical resistivity.