Use of salinity and resistivity measurements to study the coastal aquifer salinization in a semi-arid region: a case study in northeast Nile Delta, Egypt

To understand the impact of sea level rise (SLR) and aquifer intensive use would cause to the groundwater level and saltwater intrusion, an integrated relationship between salinity and electrical resistivity of a Quaternary aquifer is established in the northern part of East Nile Delta (El Sharkia area), Egypt. Historical data of groundwater salinity are mapped and compared with the current water samples to better understand the salinity spatial variability. The salinity maps show that salinization has increased sustainably. Additionally, the transition/dynamic zone related to SLR and/or excessive pumping can be addressed. In particular, the surface DC resistivity soundings were carried out to demonstrate the vertical and horizontal salinity distributions in the area. In the course of this study, the 1D model generation using a hybrid genetic algorithm (GA) was applied and tested using borehole information. The constructed geoeletrical cross-sections emphasize and delineate the extension of saltwater intrusion. Cleary, it is found that due to excessive pumping from shallow wells over the last decades, the subsurface resistivity and TDS vertical distributions can change rapidly within a short distance. Additionally, the results show that despite the dominance of brackish and saltwater at the northern part of the area, perched low conductive lenses are observed reflecting a low level of groundwater salinization. To obtain a link between water salinity and aquifer resistivity, an empirical relationship was derived to predict the salinity variations at different depths. In comparison with the measured total dissolved solids (TDS), the predicted salinity map appears realistic. These results demonstrate the important role of the integration between resistivity and salinity measurements for mapping the groundwater salinization with depth, and call for further research to plan and manage the area's water resources.