Aquifers can occur in some very surprising locations. Because underground water can travel hundreds of kilometers from its source, water supplies are sometimes located beneath the surface of very dry regions. In 1981, the space shuttle Columbia mapped the eastern Sahara desert using radar, which is a form of microwave radiation that can penetrate clouds and dry desert sands but cannot penetrate moist sediments. The radar map reveals an image of ancient topography beneath the desert sands characterized by wide river valleys flanked by rocky hills (Fig. 14D). This image is a powerful illustration of how climate changes and is thought to reflect conditions during the last ice age (approximately 10,000 to 70,000 years ago) when Earth’s average temperature was cooler than today. Many regions that experience desert conditions today had more hospitable climates with significantly higher rainfall then.
Figure 14D: Ancient Rivers in the Sahara.
Satellite image (left) of the Safsaf Oasis in the eastern Sahara desert provides few clues as to the origin of the oasis. But a radar image (right), taken by the space shuttle Columbia in 1981, reveals the rock layer beneath the desert sand, unveiling an ancient topography with wide black channels cut by the meandering of an ancient river that once fed the oasis.
The ancient drainage system was fed by precipitation, primarily in the adjacent highlands in Libya, and developed on top of a very porous and permeable type of rock known as the Nubian sandstone. This sandstone, which occurs beneath most of the Sahara Desert, is about 900 meters (3000 feet) thick and stores enormous quantities of groundwater, estimated to be in excess of 18,000 cubic kilometers (4300 cubic miles). The antiquity of this water has been confirmed by radiocarbon dating (see Chapter 8). These analyses show that the groundwater is about 35,000 years old, indicating that the water actually fell as precipitation during the last ice age.
Locally, the Nubian sandstone is folded upward into anticlines (see Chapter 10) and reaches the surface (Fig. 14E). Where this happens, the water seeps out at the surface to form an oasis, a local spring of artesian water in the desert. In some instances, the sandstone layer is offset by a fault. Like a leak in a plumbing system, water escapes from the sandstone layer and rises up the fault to form an oasis at the surface.
Figure 14E: Nubian Aquifer.
A cross section beneath the Sahara Desert from the mountains of Libya in the west to the Nile River in the east unmasks the Nubian aquifer, which extends beneath the Sahara Desert.
The Nubian aquifer is a major source of water in western Egypt and Libya. However as large as this resource is, it is no longer being renewed by modern rainfall. Its water is therefore a nonrenewable resource and care must be taken to ensure that it is well managed and used efficiently.