Mapping the saline/fresh groundwater interface beneath the Judean Desert using deep TDEM

A deep TDEM (Time Domain Electro-Magnetic) survey was carried out at the Judean Desert to delineate the geometry of the interface between fresh, brackish and saline groundwater bodies up to the depth of approximately 1.5 km. The survey was conducted at 20 locations on the desert plateau, between Nebi Musa in the north and Nahal Hever in the south. At this area, fresh groundwater flows from the replenishment area at the eastern slopes of the Judea Mountains toward the Feshha, Kaneh, Samar and Ein-Gedi springs, whose total mean annual discharge is estimated to be 85 million m3/y. At deeper aquifers, saline groundwater is found, which is mixed at specific locations with the shallow fresh groundwater; thus the salinity of some of these springs varies from fresh to saline. Results of all TDEM soundings show that a low resistivity layer of 5-15 ohm-m exists below a relatively higher resistivity layer of 50-500 ohm-m. The low resistivity layer is found at a depth range between 500 and 1000 m below land surface. Comparison of these geoelectric results with stratigrafic data testifies to the existence of an interface most likely separating fresh and brackish groundwaters within the above mentioned depth range. Theoretical estimations using Archie's law as well as comparison of TDEM resistivities with groundwater salinities measured in oil and deep water wells in the southern Judea desert and central Israel verify this hypothesis. The expected resistivity of the Dead-Sea brine is about 1 ohm-m or less. Although not detected in most locations of this survey, yet at three sites located at the eastern edge of the plateau (adjacent to the rift faults), an additional interface probably separating the brackish groundwater (5-15 ohm-m) and the Dead-Sea brine (3-5 ohm-m) is detected at greater depth. These observations show that the upper Judea Group aquifer is usually saturated with fresh groundwater, while the lower Judea Group aquifer, as well as the underlying Kurnub Group aquifer, are saturated with brackish groundwater, the salinity of which does not exceed the normal seawater salinity. The underlying Jurassic formations are normally saturated with the Dead-Sea brine.