ABSTRACT
The groundwater reserves of the Eastern Sahara are mainly contained in the Nubian Aquifer System (NAS), which extends over an area of 2 km2 (Thorweihe 1988). 14C groundwater ages of almost >20 kilo years B.P. (ky) and stable isotopic composition (Deuterium and 180) suggest that the East-Saharan groundwater has been formed by local precipitation of humid periods in late pleistocene time (Sonntag 1978). In opposite to other Sahara areas, however, Holocene paleowaters have been missed in the Eastern Sahara, although many paleoclimatic indicators for humid climate in early and mid Holocene time were found like elsewhere. This problem turned out to be an artefact of groundwater sampling. In the Eastern Sahara, particularly in the Western Desert of Egypt, the water wells are deeply cut into the land surface. In these topographic lows, the hydraulic head is close to the depression surface or even above (artesian groundwater). Thus (natural) groundwater discharge occurs either by capillary rise from the groundwater table or by ascend of confined or even artesian groundwater through leaky confining beds followed by water vapour diffusion through top layers in dry soils and by transpiration of wild vegetation. In the vicinity of these discharge centers of NAS, Holocene paleowaters, which were originally there, have already disappeared. They should, however, still exist in the bast desert area outside the depressions. This hypothesis has been proven by isotope dating of unconfined groundwater in the catchment area of the fairly flat Bir Tarfawi/Bir Safsaf depression in the hyperarid Southwest of Egypt, the groundwater balance of which is discussed below.
