Two major sedimentary basins occur in eastern Libya. The uppermost horizons in each—post-Eocene in the Sirte and Cretaceous Nubian in the Kufra basin—constitute regional hydrogeological systems. The deposits of the Sirte basin include mainly fluviatile sands and clays in the S with marine carbonates and sandy carbonates with some evaporites progressively increasing to the north. In addition to a northwards thickening of the deposits there is an ‘inwards increase’ which attains a maximum thickness of about 1800 m along a broad N-S trending axial trough. The Nubian sandstone of the Kufra basin has a maximum thickness of 900 m and includes mainly cross-bedded sandstones, subordinate shales and conglomerates, which are fluvio-glacial and limnic in origin.

The Cretaceous sandstone aquifer in the Kufra basin is essentially phreatic and groundwater flow is to the northeastwards discharging into the main Nubian artesian basin of Egypt. Present development of the aquifer occurs at only a few locations but at these the water quality is very fresh with dissolved solids typically less than 100 mg/1. The basin is bounded to W and S by highlands from which appreciable run-off can be assumed. Although not measured, assumptions of recharge from run-off of some 10–20% of annual rainfall over the elevated areas seems not unreasonable; this is approximately equivalent to the computed order of outflow from the basin of between 70–160 M m3/a based on transmissivity and piezometric gradient. Vertical recharge is unlikely to occur over the main basin where the rainfall is probably less than 3 mm/a. The age of the water at Kufra in the north of the basin is in excess of 30 000 years. The phreatic aquifer cannot therefore be in equilibrium with current recharge but it is believed that an approach to equilibrium may exist in marginal recharge areas.

The groundwater in the S of the Sirte basin is fresh but in the N it becomes brackish to saline in the direction of flow. The known groundwater ages range from 30 000 to 5000 years BP and fall into three main groups. Water from recent recharge may also exist in the uppermost phreatic levels. Various evolutionary trends have been considered and modelling techniques are used to assist elucidation. The results demonstrate that present water levels may be fairly close to equilibrium, with recharge to the aquifer system occurring from several sources including lateral inflow from the Kufra basin, marginal recharge via run-off from the highlands to the W and SW, and direct recharge from precipitation in the N of the basin. Discharge occurs in the N into sabkhat areas between the Gulf of Sirte and Qattara.

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