Israel Water Context

7. The Coastal Aquifer

The coastal aquifer is a groundwater reservoir extending in the sand and kurkar rocks beneath the coastal plain of Israel. The aquifer extends over an area of about 1800 km² from the Karmel (Carmel) in the north to the Gaza district in the south and to the width of the Gaza strip that continues 7-20 km eastwards from the coastline in the west. The natural recharge to this aquifer is estimated at ~250 million m³/yr (around 15% of the total consumption in Israel), and is exploited fully through hundreds of boreholes spread throughout its area.

The coastal aquifer in vertical section is triangular in shape, expanding from several meters in the east to around 150-200 m in the west. Its upper flank is parallel to the surface. Its lower flank, which is the base of the aquifer, overlies thick layers of clay and is inclined westward. These layers seal the aquifer at its base and sever it from the deep aquifers. The aquifer itself is built mainly of calcareous sandstone (locally named kurkar) with lenses and thin layers of clay. The sandstone has high hydraulic conductivity and high porosity. The hydraulic conductivity of the clay lenses is lower by several orders. The clay layers thicken in the west and constitute partitioning layers that divide the aquifer into several sub-aquifers.

Schematic E-W cross section of the coastal aquifer.

The third side of the aquifer connects the coastline (surface) with the sealed base. This flank is outlined on the interface (the imaginary surface that constitutes a boundary between the freshwater body in the east and the saline water body, the Mediterranean seawater, in the west). The exact location of the interface depends on the volume of the water flowing to the sea and on the hydraulic gradient (the slope of the groundwater level) of the aquifer. In a natural state, wherein the gradient of the groundwater is 1-2 ‰, the edge of the interface penetrates a few hundred meters westwards. To the extent that the gradient of the water level increases, the interface retreats towards the west, and vice versa, to the extent that the gradient decreases, the interface advances eastwards. This matter is of utmost importance in planning the exploitation of the aquifer – overpumping is liable to bring about an eastward penetration of the interface and the intrusion of saline water into the pumping wells.

On the one hand, the shallow depth of the coastal aquifer makes it available and very convenient to exploit, but on the other hand, highly sensitive to contamination. The unsaturated zone naturally constitutes a natural "sieve" for the water infiltrating from the surface. The water infiltrating through it undergoes various chemical processes and reaches the aquifer "cleansed" of different contaminants. In several areas, particularly in the west of the aquifer, the unsaturated zone is quite thin and the passage time through it is relatively fast. Thus, precisely under the most settled and industrialized area in the country the aquifer is the most sensitive. This sensitivity is manifested in the penetration of contaminants from different sources – beginning with seepage from garbage dumps and ending with fuel leaks. In addition, there is infiltration from surplus irrigation of fields situated above the aquifer. Nevertheless, the quality of the water in half the wells in the aquifer is good – with less than 250 mg/Cl/l (maximum concentration desirable according the Ministry of Health) and less than 45 mg/NO₃^-/l. In the rest of the wells the chloride concentration is higher and reaches around 600 mg/l or the nitrate concentration is higher and reaches around 70 mg/l (the maximum concentrations allowed according to the Ministry of Health) and in the remaining minority [of wells], the water is of bad quality, mainly as a result of the nitrate concentrations that exceed 70 mg/l.

In average years the contribution of water from the coastal aquifer to the Israeli water economy has been ~240-300 million m³/yr, putting it in third place after the mountain aquifer and the Kinneret. However, this aquifer constitutes an important operational reservoir because it is the only one that can store a large volume of water perennially. And therefore, in very rainy years water that is collected in other reservoirs in the country and floodwaters are penetrated into the aquifer at various points.