How old is this aquifer?

The Nubian Aquifer, the font of fabled oases in Egypt and Libya, stretches languidly across 770,000 square miles of northern Africa, a pointillist collection of underground pools of water migrating, ever so slowly, through rock and sand toward the Mediterranean Sea.

The aquifer is one of the world’s oldest. But its workings – how it flows and how quickly surface water replenishes it – have been hard to understand, in part because the tools available to study it have provided, at best, a blurry image.

Now, to solve some of the puzzles, physicists at the Department of Energy’s Argonne National Laboratory in Illinois have turned to one of the rarest particles on earth: an elusive radioactive isotope usually ricocheting around in the atmosphere at hundreds of miles an hour.

Their first success was in distilling these elusive isotopes, krypton 81, from the water in the huge Nubian Aquifer, part of which lies two miles below the oases of western Egypt where temples honour Alexander the Great.

Their second was in holding these isotopes still and measuring how much they had decayed since they last saw sunlight. Knowing how long water has been underground helps researchers understand how fast aquifers are recharged by surface water and how fast they move, leading to more accurate geological models. Groundwater is becoming an increasingly crucial component of the world’s available fresh water, and the findings could significantly increase understanding of how it behaves.

Pradeep Aggarwal, who runs the isotope hydrology section of the International Atomic Energy Agency’s water resources programme, said that success in tracking older bodies of water had long been elusive. Carbon 14 dating, so useful in archaeology, reaches back just 50,000 years or so. It is now clear that the Nubian Aquifer has been a million years in the making.

“For decades, we have been looking at different means of fingerprinting water,” Aggarwal said. “We used a bunch of different isotopes – stable isotopes – to trace where the rain comes from. We also used the radioisotopes to figure how quickly groundwater moves.” For years, scientists had relied on carbon 14 dating indicating the aquifer was just 40,000 years old.

They knew that krypton 81, an isotope present in the open air but not underground, would be a better marker for the forensic work of tracking underground water’s movement. When water loses contact with air, the radioactive clock starts; the isotope decays by a factor of two every 230,000 years, and the decay is measurable as far back as two million years.

Elusive krypton 81 isotopes
But the krypton 81 isotopes were devilishly difficult to isolate and even more difficult to catch. Zheng-Tian Lu, a physicist at the Argonne laboratory, and his colleagues have spent 14 years mastering and extending techniques to slow down atoms, the same laser-based techniques that were pioneered by the current energy secretary, Steven Chu, in the 1980s, and for which he won a Nobel Prize. When Lu realised the potential benefit of isolating krypton 81 isotopes, “I got hooked on the problem,” he said. “I tried to use the trapping method I’d already learned to try and solve the radio-krypton dating problem.”

“We are combining the ability to control and manipulate atoms to select krypton 81 out of a million kinds of krypton isotopes,” he added. There is one krypton atom in every million molecules of water; one in a trillion of these krypton atoms is the krypton 81 isotope.
The key, he said, is using lasers to pinpoint the frequency at which atoms oscillate – a loose equivalent of trying to determine the exact pitch of a musical note. Detecting the infinitesimal differences in isotopes’ resonance is hard, but when done, lasers can be tuned to pick up each isotope’s frequency. When krypton 81 atoms go through a laser attuned to them, they glow brightly and slow down, giving scientists an easier target to isolate.

The process begins when water is extracted from the aquifer without any contact with air. Krypton is bled from the water into a vacuum system. Once identified and slowed, the krypton 81 isotopes are trapped by six laser beams focusing on them from the four cardinal points of the compass and from above and below. Then their decay can be measured.

Realistic hydrology model
“From this aging information, you are looking at how the water flowed in the long past,” Lu said. “But it does have implications about how to manage waters today.” He added, “To manage a water resource you need to build a realistic hydrology model.” That is where Neil C Sturchio, a geologist at the University of Illinois at Chicago, comes in. He works with the most accepted model of how water flows through the Nubian Aquifer.

“The reason this model was done,” he said, “is that there is an international agreement among the countries that share this water” – Egypt, Libya, Chad and Sudan. “The issue is if Libya is starting to pump on their water seriously and Egypt is doing the same thing in their oasis areas,” what happens to the rest of the aquifer? If heavy pumping comes too close to a coastline, saltwater may be drawn into the hydrologic depression created by the pumping. The Nubian Aquifer is not exactly running dry; it is filled with the equivalent of more than 500 years of Nile River flow; the groundwater in the Egyptian portion alone is estimated to exceed 10,000 cubic miles.

Nonetheless, Aggarwal of the International Atomic Energy Agency pointed out, “As a result of the drawdown we have dried up the oases in a couple of places. In Libya they have dried up Kufra Lake.” In 1920, he said, National Geographic published a picture of the lake at high water. “Right now it is a dry bed, because they are pumping so heavily,” he said. And even though the aquifer is huge, its recharge rate, at best, “is measured in millimeters per year,” Aggarwal said – tiny compared with what is being pumped out.

Best way to extract water
In addition, Sturchio said, there remains the question of how best to extract water: “where you put wells, how deep, how close to each other.” “If you design it the right way, you can get a lot more water without problems,” he went on. “But if you put all the wells in one spot, you could be causing yourself a lot of trouble.”

In addition to being applied to other aquifers in places like the Philippines and Australia, the krypton 81 techniques are being explored as a way of tracking underground brine in places like southeastern New Mexico, where radioactive waste from ships, submarines and aircraft carriers is stored underground. In the end, management of nuclear waste, like management of water, is a political matter.