Living on Million-Year-Old Water

One recent study demonstrated that the Sahara’s hidden but increasingly hard-pressed supply of groundwater is actually fossil water that fell as rain more than a million years ago. This source of groundwater fell from the sky when the African continent was farther south, the current Mediterranean Sea was a low-lying desert, and the Sahara was a wet grassland. The underground water has been flowing to the north for eons, according to a study by scientists at Argonne National Laboratory published in Geophysical Research Letters.

The Argonne researchers developed an ingenious way to determine the age of ancient water. When cosmic rays spit out by the sun hit molecules of water floating about in the air, the impact creates an unstable isotope of the faintly radioactive element krypton. A regular krypton atom hit by a cosmic ray may absorb the energy and become krypton . This isotope will eventually release the extra energy and revert to normal krypton, although the process takes an average of about 500,000 years. As rain falls through the atmosphere, it picks up these isotopes of krypton. Once the water soaks into the ground, the clock starts ticking on the conversion of krypton into regular krypton. So if physicists can measure the ratio between regular krypton and krypton , they can estimate how long ago the water in the underground water table fell through the sky as raindrops. However, that requires finding a way to detect the almost impossibly rare atoms of krypton in water samples taken from wells in the Sahara. In the atmosphere, only one out of trillion krypton atoms are the rare krypton , and raindrops absorb only a tiny percentage of the free-floating krypton atoms on their plunge toward Earth. Fortunately, the scientists used the ultrasensitive Atom-Trap Trace Analysis to spot a few krypton atoms in the groundwater.

The results showed that almost all of the water now in the underground aquifers in the Sahara fell as rain between 00,000 and million years ago. That means that even when the Sahara was a grassland 0,000 years ago, very little rainwater soaked in deep enough to join the aquifers.