Geophysical and geochemical studies to delineate seawater intrusion in Bagoush area, Northwestern coast, Egypt

•Seawater intrusion in the o-olitic limestone, Northwestern coast, Egypt.•Use ERT and isotopes to profile the brackish water thickness in coastal aquifers.•Evaluate Recharge and salinization origins, Northwestern coast, Egypt.•Electrical resistivity, geochemistry and isotopes for SWI characterization.

Coastal aquifers are the main source for sustainable freshwater in many arid and semi-arid regions around the earth. In such regions, groundwater extraction far exceeds the natural replenishment rates due to additional demands on groundwater resources especially in the last few decades. The characterization of the seawater intrusion in the Baghoush area along the northwestern coast of Egypt assesses the risk of seawater intrusion for the purpose of managing the groundwater resources in coastal areas. The (SI) in the oolitic Pleistocene aquifer is affected by several natural factors, including the drainage patterns, geological structures, distance from the sea and the manipulation of groundwater. Electrical Resistivity Tomography (ERT) has been implemented to identify the geometry of the brackish/saline water interface and to map the distribution of brackish water zone floating over the denser saline water. Seven 2-D resistivity imaging profiles were conducted using a Wenner array with different electrode distance spacings. The inverse resistivity models of these profiles indicate that these profiles are composed of three zones: the upper dry zone, the middle brackish water zone, and the lower saline water zone. The thickness of the brackish groundwater zone decreases toward the sea and the resistivity decreases with depth due to increase in water salinity. Water table along these profiles decreases from south to north, which indicates that groundwater flow is from south (inland) to north (sea). Groundwater chemistry and stable isotopes were used to determine the fresh groundwater recharge source(s), to identify mixing of different groundwaters, to evaluate seawater intrusion zone along the coast, and to investigate the upwelling of deep saline groundwater underneath the brackish zone. The recharge of fresh groundwater originates from the mountain watershed located upstream as well as the annual rainfall; however, seawater is the main source of groundwater salinization. The estimated mixing ratios between the fresh groundwater recharge to seawater ranges between 32% and 75% groundwater to 68% and 25% seawater. The outcomes of this study emphasize the importance of closely monitoring groundwater management to limit upwelling of the underlying salt-water into the overlying groundwater along the coast to limit further seawater intrusion.