ELECTRICAL RESISTIVITY TOMOGRAPHY, GEOCHEMISTRY AND ISOTOPE TRACERS FOR SALT WATER INTRUSION CHARACTERIZATION ALONG THE NORTHWESTERN COAST, EGYPT

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 in the last decades. The characterization of the salt-water intrusion (SWI) in Baghoush area along the northwestern coast of Egypt assesses the risk of (SWI) for the purpose of managing the groundwater resources in coastal areas. The (SWI) in the O-Olitic Pleistocene aquifer is linked to natural factors including the drainage patterns, geological structures, distance from the sea and the manipulation of groundwater. This approach based on Electrical Resistivity Tomography (ERT) that 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 Wenner array with different electrode distance spacing. 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 resistivity of the brackish water zone decreases with depth due to increase in water salinity. Water table along these profiles decrease from south to north which indicate that the groundwater flow is from south (inland) to north (sea). Depth to the brackish/saline water interface increased from north to south due to salt water intrusion. The results obtained from the (ERT) were interpolated and used as to build a 3-D groundwater flow model (MODFLOW) coupled with a variable-density flow and transport model (SEAWAT). The model were developed and calibrated to simulate the upwelling of saline water beneath existing wells, and (SWI) along the coast under different pumping stresses. In addition, Groundwater chemistry and stable isotope tracers were used to determine the groundwater recharge source(s), mixing of different groundwater from the mountain recharge area, as well as to confirm the (ERT) results. Future groundwater pumping should be closely monitored to limit upwelling of the underlying salt-water into the overlying groundwater along the coast to limit further (SWI).