In Israel seven major groundwater basins are distinguished:
- The Coastal basin
- The Yarqon-Taninim basin
- The Western Galilee basin
- The Mount Carmel basin
- The Kinneret basin
- The Eastern Mountain basin
- The Negev and Arava basin
The coastal basin, also called the coastal aquifer, to which a Pleistocene age is attributed, is connected to the country-wide water system (the National Carrier) and constitutes one of the main sources of groundwater in Israel.
This aquifer extends from the slopes of Mount Carmel in the north to Sinai in the south, and from the foot of the mountains in the east to the sea in the west.
Vertically, the section of the aquifer is wedge-shaped, broadening from a few meters in the east to 150 m close to the western shore.
The aquifer is built of layers of sand, calcareous sandstone and conglomerates, which are conductive rocks, and clayey sand and loam, which are partially conductive. Confining layers of clay and marl found in the western part of the aquifer (up to 5 km from the coast) separate the aquifer into secondary units termed sub-aquifers. Some of these layers continue westward into the sea.
At the base of the aquifer are clays of the Neogene Saqiye Group, which constitute a confining layer for the passage of water, except at a few sites in which fractures or faults enable the passage of saline water from depth to the aquifer.
At its eastern boundary, in the Emeq Hefer area and between Rehovot and southward to Nir Am, the aquifer is in contact with partially confined layers of Eocene age. This contact enables the passage of significant amounts of mainly saline water to the eastern cells of the aquifer. Likewise, in the central part of the eastern boundary, in the area between the southern Sharon plain and Rishon LeZiyyon, there is direct contact between the coastal aquifer and the Yarqon-Taninim basin that enables the passage of water between them.
In the west the aquifer is in direct contact with the sea, which enables the flow of fresh groundwater to the sea and penetration of sea water to the aquifer.
The upper boundary of the aquifer is the phreatic zone that absorbs water from the surface from both natural (precipitation) and artificially controlled (infiltration) and uncontrolled (return flow of water and treated sewage from consumption systems and surface drainage) sources.
The sea constitutes the natural drainage outlet of the aquifer. Today the aquifer is drained mainly by pumping wells.
The Yarqon-Taninim basin extends between the Judea and Samaria mountains in the east and the Mediterranean Sea shore in the west. In the north, the basin borders the margins of the southern slopes of Mount Carmel and the Taninim stream, and in the south, the Beer Sheva Val ley.
The basin is divided into three longitudinal strips which have hydrological significance:
- the mountain region
- the strip at the foot of the mountains
- the coastal plain
The mountain region comprises the western slopes of the Hebron, Judea and Samaria mountains, which are the natural recharge areas.
The strip at the foot of the mountains is covered by impermeable rocks (the Shephela Group) that confine the basin.
In the coastal plain, the basin is confined beneath younger rocks (the Saqiye Group of Plio-Pleistocene age and the underlying Shephela Group).
The main stratigraphic units building the geologic section are (in ascending order):
The Kurnub Group (Lower Cretaceous) at the base of the basin. It is composed of shales, limestone, chalk and sandstone.
The Judea Group (Late Cretaceous), which constitutes the basin aquifer, includes three lithologic units:
- Lower conductive unit (Yagur and Yakhini fms.-Early Cenomanian-
- Albian); dolomite and limestone totaling 150-600 m.
- Partially conductive intermediate unit (Deir Hanna Fm. – Early
Cenomanian); chalky limestone, chalk and marl with marked
and facies changes, 100-150 m thick.
- Upper unit (Bina and Negba fms. -Turonian-Late Cenomanian); mainly karstic limestone and dolomite, 100-550 m thick.
The Shephela Group (Senonian-Eocene age), which constitutes the top of the aquifer where it is confined, is built of rocks whose dominant components are chalk and marl. The group is 500-600 m thick.
The eastern boundary of the basin is a water divide, which is partly structural as a result anticlinal ridges that form an impermeable boundary, and in part, hydraulic. In the west near the sea, the facies of the conductive change to impermeable chalk and shales (Talme Yafe Gr.) but may be a source of salt.
The northern boundary of the basin runs along the axis of the Menashe syncline up to the Taninim stream, whose springs constitute the lowest natural outlet for the basin's waters. The southern boundary is at the Yeroham-Dimona axis, which constitutes a structural water divide.
The Yarqon-Taninim basin is in direct contact with the coastal basin in the area between the southern part of the northern Sharon region and the Holon region. In the rest of the areas, these basins are separated by impermeable rocks.
In certain areas there is a hydraulic connection between the upper unit of the Yarqon-Taninim basin ("upper Cenomanian aquifer") and the lower unit (“lower Cenomanian aquifer"). In exposures of the upper, water and contaminants leak to the lower unit. In the confined area in the margins of the mountains and in the coastal plain, the waters of the lower aquifer rise to the upper aquifer where there is a hydraulic connection between them.
In the upper unit within the basin, three zones of groundwater salinity can be distinguished:
- The phreatic zone with freshwater (chloride concentration less than
- The confined zone with salinity between 100 and 250 mgCl/L (locally
reaching 400 mg/L)
- The western and southern margins and the Taninim spring waters to the northwest, with a chloride concentration exceeding 1000 mg/L.
In wells pumping from the lower unit in the margins of the mountains, the salinity is markedly lower than that in the upper unit.
The concentration of nitrate in the groundwater is generally low (5-25 mg/L) except in the Qalqilya-Tulkarm area in which high concentrations were found in the upper aquifer within a patchy area parallel to the coastline (some of the wells exceed 50-70 mg/L). Field surveys of non-ionic detergents and nitrate along streams from the Taninim in the north to Soreq in the south found boreholes close to streams contaminated by sewage effluents discharged into the streams.
Groundwater temperatures in the pumping fields range between 23 and 29oC. In the southwestern margins, the temperature of the water fluctuates in the realm of 30-40o, a fact that attests to the water's having risen from depth.
The Yarqon-Taninim basin is one of the two main sources of groundwater exploitation in Israel. The production from the basin (pumping and discharge from springs) varied in the past 30 years between 300 and 500 million m3/yr. The basin is fed by rain that falls on permeable exposures in Samaria and in Judea. The average annual natural recharge is 350 million m3/yr. The basin is exploited via hundreds of wells spread along the margins of the mountain, most of them in the upper unit, supplying water suitable for every purpose. The specific capacity of the wells is particularly high, (hundreds of m3/hr per meter of drawdowm due to the high transmissivity of the aquifer in their vicinity.
This basin includes eight sub-basins that are divided into cells clustered in three geographic groups: (1) Coastal cells; these belong to the coastal aquifer which stretches along the Galilee coast from Rosh haNiqra to the Bay of Haifa. The aquifer is built of sand and sandstone of Neogene-Quaternary age, fed by precipitation, by return flow and by flow from the mountain cells. The groundwater drains to the sea and is pumped from wells; (2) Mountain cells; part of the mountain aquifer extending from the watershed in the center of the Galilee to the Shefar'am syncline. The mountain aquifer in the Galilee coast is confined beneath the coastal aquifer and beneath the younger rocks of the Shephela Group of Senonian and Eocene age. The aquifer is built of limestone and dolostone of Cenomanian-Turonian age. It is fed by precipitation, by return flow, and is drained by pumping, springs, flow to the sea and by upward flow to the coastal cells; (3) Deep cells; these belong to Eocene age aquifer in the Shefar'am syncline and to the western Jezre'el Valley fill aquifer of Neogene-Quaternary age.
The Mount Carmel basin consists of three cells: (1) The Carmel coastal aquifer of Quaternary age fed by rainwater and flow from the western Carmel and drained by pumping and flow to the sea; (2) The western Carmel; Judea Group aquifer (Cenomanian) fed by rainwater and drained by pumping and by flow to the Carmel coast and to the sea; (3) The eastern Carmel; Judea Group aquifer (Cenomanian) fed by rainwater and drained by pumping, flow to the sea and to neighboring cells.
Water production in 2000/01 was 38 million m3, of which 35 million m3 was from pumping and 3 from saline discharge from the Sa'adia spring in the eastern margins of the Carmel. Likewise, there are a number of springs with low discharge flowing mainly from the perched aquifer in the Judea Group.
The long term mean replenishment of the Carmel basin is estimated at 47 million m3/yr. In the year 2000/01 the natural replenishment was 38 million m3 (about 81% of the perennial average). Exploitation of the aquifer in 2000/01 (pumping + flow + losses to the sea and to neighboring cells) was more than the estimated replenishment to the basin. This was manifested in a drop in the levels in the mountain cells of 30-60 cm and of about 13 cm in the coastal cell. The levels in the basin have been in a downward trend since the peak of 1991/92.
The Kinneret basin is the largest groundwater basin in Israel with a production of some 500 million m3/yr (27% of the national production). The groundwater of the sub-basins have a natural common outlet, the Sea of galilee (Lake Kinneret). The hydrogeological units of the sub-basins include aquifers of Jurassic, Cenomanian, Eocene and Quatenary (basalt and alluvial fill) ages. The basin extends from Mount Hermon in the north to Lake Kinneret in the south. Its western limit is the country-wide watershed in the Galilee. The eastern limit is in the Golan, bounded by the Jordan catchment basin. The source of the recharge is direct precipitation: rain and snow, and lateral flow from neighboring basins.
The natural drains are the springs of the Jordan River sources, the margins of the Hula Valley, the Kinneret and Hammat Gader. Most of the waters exploited are spring waters and some of them (10% of the total production of the basin) is pumped from wells. The discharge fluctuated between a peak of 454 million m3 in 1992/93 and a low of 200 million m3 in 2000/2001. The quality of the water pumped is excellent.
The eastern mountain basin is located east of the country-wide groundwater divide, and form seven sub-basins in accordance with hydrogeological homogeneity and geographical spread. The boundaries of the basin are the Yavnel Valley, the Jordan Valley and the Dead Sea, down to Nahal Hemar in the south. The western boundary passes through the mountain backbone along the Hebron and Ramalla anticlinal axes and the margins of the Samaria syncline, and separates the eastern mountain basin from the Yarqon-Taninim basin.
There are five aquifers within the basin. The principal one is the Judea Group aquifer built of limestone and dolostone of Early Cenomanian to Turonian age. The secondary aquifers consist of limestone and chalk of Eocene age in the Samaria syncline, basalt of Neogene-Quaternary age in the lower Galilee area, alluvium fill of Neogene-Quaternary age in the Bet She'an area and the Jordan Valley, and limestone of Jurassic age.
The basin is fed by rainwater and are naturally drained through springs in the margins of the mountains, in the internal valleys and the Jordan Valley and by groundwater flow to the Dead Sea. The average annual discharge of the springs is around 250 million m3, of which around 150 million m3 is saline water in the Jordan Valley springs and the Dead Sea, and about 100 million m3 is fresh water. Pumping from the basin increased from about 50 million m3 /yr at the beginning of the 1970's to 150 million m3 /yr in the last years. In 2000/01, precipitation was about 77% of the perennial average compared to 83% in the previous year. Pumping from the basin in 1999/2000 reached 142 million m3. The discharge from the springs in that same year was about 188 million m3. In 1999/2000, 78% of the water pumped had a chloride concentration of less than 400 mg/L.
The Negev highland and the Arava includes three aquifer units: (1) The Kurnub Group aquifer consisting of sandstone of Early Cretaceous age; (2) The Judea Group aquifer - limestone and dolostone of Cenomanian-Turonian age; (3) The Hazeva Formation and Arava aquifers - clastic rocks (sandstone, conglomerate) of Neogene and Quaternary ages.
Sources: Geological Survey of ISrael