CONTRIBUTION OF GEOELECTRICAL SURVEY METHODS IN STUDYING THE CONDITION OF GROUNDWATER OCCURRENCE IN EL ALAMEIN AREA – NORTHWESTERN COAST - EGYPT

The present work deals with the use of the geoelectrical methods to study the condition of groundwater occurrence in El Alamein area. The stratigraphic section of this area extends from Early Miocene to Holocene. It includes several water - bearing formations, the most important of them is the Post Miocene aquifers.
A total of 35 Vertical Electrical Soundings distributed in the form of a grid were carried out in the study area to determine the aquifer parameters (depth, thickness, resistivity & extension). Ten 2-D electrical profiles were also carried out to accurately detect the brackish/saline water interface. In addition, an attempt was made to determine the porosity of the oolitic limestone aquifer using 2-D measurements. In this respect, the fluid and bulk resistivities of the part of the aquifer saturated with saline water were determined and used in conjunction with Archie’s formula.
The results revealed the presence of a shallow oolitic limestone aquifer. The depth to top of that aquifer varies from 3 m under the depression to 19 m under the ridge. The water quality ranges from brackish to saline. The aquifer is regarded to be consisting of three parts; an upper part saturated with brackish water, a middle part saturated with saline water and a lower part, partially saturated with saline water. The brackish/saline water interface was found to be at a depth varying from 6 m to 37.7 m. Calculated porosity of the saline water saturated portion of this aquifer was found to vary from 33% to 39% that exhibits a good agreement between the measured porosities and that calculated from 2-D measurements.
This shallow aquifer is invaded by sea water from the north direction (Mediterranean Sea). As a result, the denser saline water penetrated inland below the brackish water. Consequently, the brackish/saline water interface becomes deeper towards the south under the present ridge which, in turn, increases the thickness of the brackish water layer in that direction.
Hand dug wells are recommended on the ridge with total depth less than that of the brackish/saline water interface. The discharge of such wells should be under control to avoid depletion and rising of that interface and hence,
contamination with salt water.