Hydrologic Setting

Figure 2. Map showing major physiographic provinces of the region.

Much of the study area is semiarid to arid. Annual precipitation varies from less than 50 millimeters (mm) in the southern and eastern deserts to about 1,200 mm in local areas of the north-western mountains (Executive Action Team, 1998). Most precipitation occurs during the winter and spring, from mid-November through May. Rainfall is not adequate to sustain non-irrigated agriculture in about 50 percent of the area on the western side and about 90 percent of the area on the eastern side of the Jordan Rift Valley (fig. 2). This creates a demand for irrigation water, which accounted for 66 percent of the total water use in the study area during 1994 (Executive Action Team, 1998).

Population in the study area increased from about 6 million people in 1970 to about 12 million in 1995 (Executive Action Team, 1998; Committee on Sustainable Water Supplies for the Middle East, 1999). During that time, annual water use increased from about 2,000 million cubic meters (MCM) to more than 3,000 MCM. Water managersare concerned about the effect this increase has had on the water supply available in the study area and on the quality of that supply. In 1994, 56 percent of water use in the study area was supplied by wells, 35 percent was supplied by surface water (including discharge from springs), and the remainder was supplied by wastewater reuse. Natural replenishment of groundwater and surface-water supplies can exceed demands during unusually wet periods but often falls far below demands during droughts.

Most of the groundwater supplied in the study area is obtained from Quaternary sand and gravel in the Coastal Plain (referred to as the Coastal aquifer) and the Jordan Valley; Cretaceous lime-stone in the Mountain Belt, Jordan Highland, and escarpments along the Jordan Valley; basalt in the Jordan Highland and Plateau; and sandstone in the South Jordan Desert (fig. 2). The major springs in the area are primarily in the Mountain Belt, Jordan Highland, and escarpments along the Jordan Valley. Much of the surface-water supply originates in the Jordan River watershed, which includes the Jordan, Yarmouk, and Zarqa Rivers as well as Lake Tiberias and King Tallal Reservoir. Smaller amounts of surface water are available in basins that drain the western side of the Mountain Belt and escarpment to the east of the Dead Sea.

GROUNDWATER BASINS AND AQUIFERS

Figure 3. Map showing groundwater basins in the region.

Groundwater resources of the region are sub-divided into groundwater basins (fig. 3) on the basis of natural boundaries (such as structural features, aquifer extent, and groundwater divides) and administrative boundaries which are defined for operative reasons. Because aquifers exist in three dimensions, the natural boundaries can overlap when they are viewed on a two-dimensional map. Thus, a basin might contain several aquifers of different areal extent that lie at different depths. The primary aquifers in the region are in geologic units that range in age from aquifers and geology reported by the Executive Action Team (1998).

QUATERNARY AQUIFERS

Quaternary aquifers occur primarily in the Coastal and Jordan Valley Basins. The aquifer in the Coastal Basin, referred to as the Coastal aquifer, consists of unconsolidated sands and gravel, sandstone, and conglomerate underlain by relatively impervious clay, marl, limestone, and chalk of Pliocene origin

Two Quaternary aquifers occur in the Jordan Valley Basin. The alluvial, or Recent, aquifer consists of sand and gravel deposits from alluvial fans, primarily east of the Jordan River. This aquifer contains about 80 percent of the fresh groundwater in the valley (Executive Action Team, 1998). The Pleistocene aquifer, primarily west of the Jordan River, consists of unconsoli-dated sands and gravel separated by imperme-able layers of saline marls of lacustrine origin. Where sufficient flushing of saline water has occurred, these sand and gravel deposits make good local aquifers potentially usable for agricul-tural or potable supply. In other locations, these deposits contain saline water that is not usable for any purpose.

TERTIARY AQUIFERS

Tertiary aquifers are present in the Northeastern, Azraq, Wadi Hamad, and Wadi Sirhan Basins. The Eocene aquifer (west of the Rift Valley) or B4/B5 aquifer (east of the Rift Valley) consists mainly of nummulitic limestone with chalk. The limestones are thin bedded with chalk and marl intercala-tions. These aquifers are generally thin, shallow, and of limited areal extent; therefore, they are directly affected by rainfall.

CRETACEOUS AQUIFERS

Several Upper Cretaceous aquifers are very extensive on both sides of the Jordan Rift Valley and are important sources of water supply. The B2/A7 aquifer is productive in the Jordan Highland and Plateau. It consists of chalk, chert, limestone, and marl. Well yields are highly vari-able and can be affected by geologic structures and cavernous zones in the limestone. West of the Rift Valley, this aquifer is referred to as the Senonian and is less extensive.

The Turonian and Cenomanian aquifers produce large quantities of water in the Eastern Mountain, Western Mountain, and Northeastern Basins. The Turonian aquifer consists mainly of limestone and dolomites that are generally massive and thick but can be thin bedded locally. Well yields are best where the aquifer is fully submerged below the water table. The Upper Cenomanian aquifer consists mainly of interbedded dolomites and chalky limestone. In some areas, productivity is low due to limited aquifer thickness. The Lower Cenomanian aquifer consists mainly of limestone and dolomitic limestone, thick and thin bedded with marly limestone which is more massive toward the upper part of the unit. The aquifer is particularly productive because it is exposed to recharge by the rainfall along most of the apex of the Mountain Belt anticline.

PALEOZOIC AQUIFERS

Paleozoic aquifers are present in the Wadi Araba Basins and the South Jordan Desert. The Ram Group aquifer is an important source of water in the Disi Basin. It consists of water-bearing lime-stone, dolomite, and sandstone layers inter-bedded with shale, clay, and gypsum. In some areas, high salinity precludes use of the water.

BASALT AQUIFERS

Basalt aquifers ranging in age from Quaternary to Triassic are present in the central area of the Jordan Highland and Plateau. Water is generally good quality, and high well yields are common..