Large Scale Electrical Resistivity Tomography Survey Correlated to Hydrogeological Data for Mapping Groundwater Salinization: A Case Study from a Multilayered Coastal Aquifer in Rhodope, Northeastern Greece

Abstract

A large scale two dimensional electrical resistivity survey was carried out to investigate groundwater salinization phenomena of a multilayered coastal aquifer in south Rhodope area, northeastern Greece. The electrical resistivity tomography (ERT) profiles, calibrated with lithological, physicochemical and geophysical log data, were used to demarcate the geometry of the aquifer system and characterize the extent of the interface between saline and freshwater in a coastal groundwater system, which has been subject to overexploitation for the last 35 years. In total, 12 deep high resolution ERT profile lines, covering a distance of approximately 15 km were carried out at key locations across the downgraded part of the aquifer system. The resistivity sections were calibrated using a reference dataset of 45 information wells incorporating 28 lithological logs, 31 groundwater quality samples from a variety of screen placements and five gamma ray logs. Results revealed the presence of four major resistivity zones ranging in values between 0.1 and 150 Ωm at various depths and locations. A very low resistivity zone (0.1–5 Ωm) correlated with saline water saturated layers, a medium resistivity zone (5–15 Ωm) attributed to clays and silts, and two high resistivity zones generally representing freshwater saturated sediments of coarse grain size (15–40 Ωm) or alterations of thin marl and fine sand layers (40–150 Ωm). It is demonstrated that extensive use of high resolution deep geoelectrical sections, calibrated with a large number of hydrogeological data can successfully delineate aquifer geometry dimensions, identify hydraulic boundaries and clarify ambiguous field measurements, thus allowing the development of a thorough hydrogeological conceptual model.