THE EFFECT OF BASE LEVEL CHANGES AND GEOLOGICAL STRUCTURES ON THE LOCATION OF THE GROUNDWATER DIVIDE, AS EXHIBITED IN THE HYDROLOGICAL SYSTEM BETWEEN THE DEAD SEA AND THE MEDITERRANEAN SEA

The effect of base (sea and lake) level changes on the location and elevation of the groundwater divide were examined in the hydrogeological system between the Mediterranean Sea and the Dead Sea. Steady-state simulations were conducted with a 1-D analytical model and transient conditions were simulated using FEFLOW groundwater modeling software. Two hydrological scenarios were simulated: a) a transition to a new steady-state, following the expected drop of 150 m of the Dead Sea level; and b) the time of the precursor of the Dead Sea (Lisan Lake), some 20,000 years ago, when the lake level was about 250 m above the present-day Dead Sea level and the Mediterranean Sea level was 120 m below its present one. The results of the simulations show that the Dead Sea level drop has led to a progressive decline in the groundwater level up to several kiometers inland from the shoreline. the hydraulic gradient increases, and thus the discharge to the lake also increases at the expense of the storage, and also due to a small enlargement of the recharge zone by a ~600 m shift of the divide.

Broadly, the subsurface hydrologic system is compartmentalized into several sections, separated by low permeability fold structures bounding the mountainous uplands. Most of the groundwater recharge occurs in the uplands. Given this geometry and the available water level measurements, analytical and numerical modeling results show that only changes in the Dead Sea level and recharge rate result in dramatic changes for the section examined. limited by the low permeability folds, the effect of the Dead Sea level on groundwater level are most pronounced in the vicinity of the Judea Desert lowlands. The folds also greatly limit migration of the groundwater divide associated with changes in the Dead Sea level. Transient simulations show a peak increase of ~30% in the Dead Sea groundwater flux in ~75 years, resulting from drainage of lowlands aquifers. After more than 500 years the groundwater flux is expected to return to values no more than 4% higher than at the beginning of the simulation. This small net increase is consistent with the small shift in the groundwater divide.