2 Groundwater Resources

Before highlighting the efforts exercised by Libyan authorities to protect groundwater, it is appropriate to give a brief description for the groundwater basins; their geographical distribution, lateral and vertical extent, number of aquifers involved, and most importantly, their quantitative and qualitative characteristics. In general, six groundwater basins exist in the country (Figure 1).

2.1 Gefara Plain basin

This basin occupies the north-western part of Libya. It is bounded by the Mediterranean coast on the north and has a surface area of only 15,000 km*. It has a great economic and social importance, and encompasses the largest Libyan city, Tripoli and more than 40% of the Libyan population.

It contributes more than 50% of the national agricultural output, as well as a wide industrial base.

Hydrogeologically, the Gefara Plain contains four aquifer systems varying in age from Quaternary to Triassic.

The Quaternary aquifer is generally unconfined with thickness ranging from 30 to 150 m, reflecting moderately high values for transmissivities and storage coefficients. Its

127

Mediterranean

Sea J~bal

Gulf of Sirtr

El

K&l

Volcanic rocks Cambra -ordovlcain

I

s* : *.*.* . -. . Nu bian Sandstone

IO. 12. 14.

100 ^{200 Km.}

I I I

10. 20’ 22* 24‘

hlgure 1. Groundwater basins in Libya.

i

i

0.

--L

water table is under continuous decline exceeding 5 mm/

year in areas of intensive irrigation (Figure 2). As a result, seawater intrusion is taking place along a strip of 2 by 10 km wide around the city of Tripoli.

The water quality varies considerably in all directions with salinities ranging from less than 1 g/l to over 5 g/l. The aquifer is recharged through direct rainfall and wadi runoff.

The Miocene aquifer is a confined aquifer underlying the Quaternary and consists of both middle and lower Miocene aquifers. The first has a thickness of 125 to 200 m with relatively good transmissivity and storativity values, but of remarkably poor water quality of 3 to 4 g/l. The second occurs at a greater depth with even lower hydraulic properties and higher salinity.

The Triassic aquifer, consisting of sandstone (Abushayba) and limestone (Azizia) aquifers which are both unconfined at areas of outcropping in the south central part and confined elsewhere. Both aquifers occur at great depth and possess

good hydraulic properties and water quality of less than 2 g/l. They are relatively thicker than the overlying Quaternary and Miocene and have better well yield. Like other aquifers, they also undergo a continuous decline in their piezometric levels.

Total extraction of water from the Gefara plain aquifer is in the order of 1.2 billion m3/year, as compared to an annual recharge of approximately 200 million m3.

2.2 Jabal Akhdar

The Jabal Akhdar is an east-west anticline of Upper Cretaceous and Tertiary Carbonate deposits. The fractured Eocene and Miocene rocks are the main aquifers in the area.

They are karstified aquifers of local importance as suppliers of adequate water for many small communities. They also contribute water to the great number of karstic springs and receive direct recharge from rainfall. Their yield depends mainly on the secondary porosity with salinities generally less than 2.5 g/l, deteriorating rapidly in the southern direction towards discharge zones. In recent years, water 128 Proceedings

of the

Groundwater Protection: The Libyan Experience contamination from industrial and household wastes.

The Jabal Akhdar aquifer system receives around 200 million m3 of annual recharge, whereas abstraction is estimated at 600 million m3/year.

2.3 Hamada basin

The Hamada basin can be defined as a group of sub-basins hydraulically interconnected on a regional scale. It extends over a surface area of about 215,000 km2, with moderate rainfall in the north, along the southern slopes of Jabal Nafusa. The aquifer system is composed of lower Cretaceous sandstone rocks and upper Cretaceous carbonate rocks. The former is a regional aquifer extending westward towards Algeria and Tunisia, where it is known as the villages of the Hamada, Saw El-Jin and central zone areas.

It is characterized by high artesian pressure, good water quality with salainities of 1 to 2 g/l and fairly high hydraulic parameters. In the eastern part, water temperature undergoes a sudden increase and can exceed 7O’C. Little decline in piezometric levels is witnessed in the exploitation zones.

The groundwater flow is generally towards the NE and NW in the direction of the great depressions and coastal sabkhas.

The Upper aquifer system is that of the Upper Cretaceous.

This aquifer or group of local aquifers, is of minor importance due to its relatively poor hydraulic properties, and high salinity, except in the eastern part of the basin where it occurs under artesian condition, with salinities less than 1.5 g/l, and is in direct contact with the underlying Lower Cretaceous and Cambro-Ordovician regional aquifers.

The total abstraction by wells from the Hamada aquifer systems is estimated at 150 million m3/yr. This is still below the total annual recharge of 475 million m’/yr in the form territory of the basin is either a gravely desert or sand dunes.

Two main aquifer systems are known to exist, the Nubian aquifer of Triassic to Lower Cretaceous sandstone and the Palaeozoic aquifer of Cambrian, Ordovician and Devonian sandstone. The upper aquifer (Nubian) is unconfined and

occupies the central part of the basin. It is locally exploited for the water production to several agricultural and domestic uses. Its thickness is maximum at the center where it reaches about 1000 m and reduces to about 300 m at the margins.

The water quality is usually good, with a salinity of less than 0.5 g/l except for the shallow horizons. The aquifer is not receiving current recharge due to lack of rainfall. The water therefore, is considered fossil water and is exploited accordingly. Little decline in water levels is observed in the intensive exploitation zones. Such decline is, however, local and is not spreading far from well fields.

The second aquifer system is that of the Palaeozoic, which outcrops and therefore becomes unconfined at the outer peripheries of the basin. Elsewhere, it lies very deep below the Nubian. The aquifer is being tapped for both agricultural and domestic uses, in the SW and NE corners. It maintains flowing and semi-artesian conditions all over the basin. The water quality is normally good and the salinity is rarely exceeding 1 g/l. A slow decline in the piezometric head is also recorded in observation wells, although very local in nature. As in the case of the Nubian aquifer, the water is considered as fossil water and is dealt with on this basis.

Remarkably, however, it maintains a high hydraulic gradient from SW to NE towards Wadi Shati depression and the coastal sabkhas.

2.5 Sark basin

The Sarir basin is a tertiary basin consisting of two main aquifers of Miocene and Oligocene sandstone. The water quality in both aquifers is generally good and rarely exceeds

1 g/l. The aquifers are characterized by high yield and good hydraulic properties. Their present use is limited to local agricultural development as well as to transporting part of the water northward to close the gap between supply and demand in the densely populated areas. Both aquifers undergo lithofacies changes in the horizontal and vertical directions, with salinity increasing to very high levels at the northern edges, where the aquifers discharge along the EW depressions.

A close monitoring of the piezometric levels in the shallow and deep aquifers is maintained through a network of observation wells. The recorded decline is notably within the acceptable range and is limited to well field areas.

2.6 Kufra basin

The Kufra basin occupies the SE corner of Libya and consists of two aquifer systems; the Nubian and Palaeozoic.

Extending over a surface area of about 250,000 km2, the Kufra basin is a regional basin shared with Egypt, Sudan and Chad. The two aquifers are hydraulically interconnected and form one regional body of huge fresh water reserve.

The thickness of the saturated zone exceeds 3000 m at the center of the basin. The hydraulic parameters, as well as the water quality are excellent. Salinities of less than 0.5 g/

130 Proceedings

of the

I are common. The aquifer is exploited only for local development in the oasis of Kufra. The groundwater flow

is from south to north towards the Sarir basin. Monitoring of groundwater levels in shallow and deep aquifers revealed that drawdown over the last three decades of exploitation is small and does not extend far beyond well field centers.

The aquifer is being considered for future expansion of the Great Man-Made River Project.