By Richard Mertens, courtesy of the University of Chicago Magazine
Photo by Lloyd DeGrane
“ The quality of water is threatened in many parts of the world. And water interacts with other large-scale systems we all depend on.”
Pritzker Director and dean of the Institute for Molecular Engineering
Editor’s note: This story is adapted from May/June 2015 issue of the University of Chicago Magazine. Read it in its entirety here.
Zheng-Tian Lu, SM’91, first heard about krypton 81 dating at a meeting of physicists in Germany in 1996. The use of radioactive isotopes like krypton 81 and the more commonly known carbon 14 to date organic objects goes back to 1907, when Bertram Boltwood first measured the age of rocks containing radioactive uranium. Carbon 14 dating reaches its limit at about 30,000 years, but krypton 81, with a half-life of 229,000 years, has the potential to date much older things—among them subterranean aquifers.
In an age of shrinking water supplies, a better understanding of groundwater is one critical front. Almost half the world’s drinking water comes from underground (rather than from surface sources like lakes and streams). And in many regions where people rely on groundwater, these resources are under threat. But questions about aquifers have been largely matters of guesswork and conjecture, much of it wrong: How long has the water been in the ground? How fast and in which direction is it flowing? And, most important, at what rate is it being replenished from above? With better answers, those resources could be managed more effectively. Krypton 81 dating, Lu and others recognized, might help, but first a number of daunting challenges had to be solved.
The Water Research Initiative, a joint project of UChicago, Argonne National Laboratory, and Ben-Gurion University of the Negev in Israel, is taking on issues like this. The initiative applies the insights of basic research in physics, chemistry, biology, and other disciplines, especially at the molecular level, to developing new technologies for addressing scarcity, pollution, and other real-world water problems. A project of the Institute for Molecular Engineering, it springs from the conviction that water is among the most high-stakes challenges of our time—and likely to grow still more so as the global population climbs and climate change worsens.
The initiative encourages researchers from the three institutions to work together on common projects, bringing to bear a wide range of expertise. The water project team will be housed at the Chicago Innovation Exchange, in a new space that will open in October 2015.
In 2013 the initiative awarded its first grants, totaling more than $1 million, to five research groups; it added a sixth project this year. The teams are designing sophisticated new membranes to filter out viruses, the smallest pathogens, from water; developing catalysts that can destroy dangerous organic pollutants; and figuring out ways to prevent bacteria from fouling the membranes used to desalinate seawater, a technology that water-starved regions around the world are turning to more and more. And Lu’s team, continuing to refine work that began almost 20 years ago, has already unlocked some of the secrets of ancient aquifers by capturing and measuring the elusive krypton 81.
Discovered in 1950 by Argonne’s John Reynolds, SM’48, PhD’50, krypton 81 is formed when cosmic rays strike ordinary krypton atoms high in the atmosphere. Water on the surface absorbs air, including krypton 81, but once underground it is cut off from the atmosphere and no longer absorbs new gases. Over time—a very long time—the krypton 81 decays, turning into bromine. Fill a jar with rainwater, close the lid, and in 229,000 years you will have only half the krypton 81 you started with. In a million years you will be down to one-sixteenth. By extracting the gas from a sample of groundwater and measuring the amount of krypton 81 in it, scientists can determine with considerable accuracy how long the water has been in the ground.
But krypton 81 is exceedingly rare. It is difficult to detect and almost impossible to measure using conventional methods of radioisotope dating. In the air around us, only 1.14 particles in a million are krypton. Of these, only one in a trillion is krypton 81. A liter of air contains about 20,000 atoms of the isotope—and 10 sextillion (10) of everything else. If all the sand on earth were air, four grains would be krypton 81. It’s the needle in the atmospheric haystack.
Lu found it. In 1997 the physicist and part-time professor at UChicago joined Argonne, where he and his colleagues worked for two years to build an atom trap that could detect and measure the isotope. Made of stainless steel, coiled copper, ceramic, and other materials, it stretched the length of a kitchen table. He called it ATTA, for Atom Trap Trace Analysis. It worked by releasing a small amount of krypton gas into a vacuum, sending it down a long tube, and then trapping individual atoms by striking them with lasers on six sides. Set to the right frequency, the lasers could single out krypton 81 atoms and make them glow like fireflies at dusk. And Lu could count them.
Since 1999, when Lu first published the ATTA method, krypton 81 dating has begun to transform scientists’ understanding of aquifers around the world. “Especially now that water is getting more and more precious, with climate change … it becomes more and more important to manage wisely whatever water resources there are,” says Neil Sturchio, a geochemist at the University of Delaware who has worked with Lu. “In many parts of the world, groundwater is the only source of water that’s available, especially in arid regions.”
Last summer Lu and a team of researchers from Argonne and Ben-Gurion spent two weeks traveling across Israel, sampling deep wells. They wanted to determine the age and movements of water in a critical sandstone aquifer 1,000 meters below the Negev desert. Using equipment designed by Reika Yokochi, a UChicago geoscientist, that could fit in the back of a van, they extracted the gases from several hundred liters of groundwater in just an hour. In two weeks they sampled more than 30 wells and sent tanks of compressed gas, each about the size of a scuba tank, to Chicago.
Using another device she designed, a tangle of tubes and aluminum foil, Yokochi extracted the krypton from each sample. Each tank of gas yielded a vial of krypton about the size of a pencil stub, which she sent to the group at Argonne. By measuring the krypton 81 in each vial, they discovered that in some places the water was quite young—in the ground 30,000 years or less—while in others it was as old as 600,000 years. “People don’t expect that,” Lu says. Further analysis showed that water beneath Israel was flowing very slowly, about a meter a year, from the Sinai Desert in the west to the Dead Sea region in the east. “They knew the water level everywhere,” says Lu. “What they didn’t know is how this water moves underneath.”
The method keeps yielding surprises, and critical data. Lu first used it a decade ago to date water from the Nubian Aquifer in western Egypt. Sturchio, who collected the samples, said conventional wisdom had held that the aquifer was 30,000 to 40,000 years old. But Lu discovered that the water had been underground for a million years. “A lot of the old literature that had become gospel about groundwater is pretty much all wrong,” Sturchio says.
The Water Research Initiative dates to the summer of 2012, when two old friends met in Chicago to discuss how their universities might collaborate. Moshe Gottlieb, a chemical engineer from Ben-Gurion, and Matthew Tirrell, the Pritzker Director of UChicago’s just-formed Institute for Molecular Engineering, had known each other since the 1970s, when they were both starting out at the University of Minnesota. Now in their 60s, they spent a warm July weekend discussing research topics that might link UChicago’s strength in basic science with Ben-Gurion’s expertise in engineering.
Over two days, he and Gottlieb considered different possibilities for collaboration. They talked about renewable energy, biomedicine, information technology, and other potential topics, some of which the IME is pursuing today. All were important, all were feasible, and yet none seemed right for this collaboration.
“We didn’t want to do something that people already do and were far in advance,” Gottlieb recalls. “There are lots of people working on green energy. We would be just another one.” On the second day they recognized that the emerging crisis in the supply of fresh water, from California to South Asia to sub-Saharan Africa, was ideally suited to the institutions’ strengths, and in need of new ideas and approaches. “The quality of water is threatened in many parts of the world,” says Tirrell. “And water interacts with other large-scale systems we all depend on, including energy.”
Originally published on July 13, 2015.