How helium - the light noble gas that sends balloons floating in the air - enters the atmosphere has wracked the brains of scientists for generations. Now the mystery has been solved, as an unexpected side benefit of research done by a group of scientists in an IAEA-led project to study groundwater in South America.
The Guarani Aquifer is one of the world’s largest water systems, and Pradeep Aggarwal, Head of the IAEA’s Isotope Hydrology Section and a group of other scientists set out to study this aquifer to learn how it can be better managed and protected.
“In effect, this aquifer, under Argentina, Brazil, Paraguay and Uruguay, is a huge natural laboratory where we were able to infer for the first time that helium from deep in the earth reaches the atmosphere along with the discharging ground water,” said Pradeep Aggarwal.
Helium is produced as uranium and thorium in the earth’s crust decay. Until this study it was unclear how it entered the atmosphere.
The results of the findings, following three years of study, has been published in Nature Geoscience. Aggarwal is the lead author with two other IAEA experts and nine contributors from five institutions in Brazil, the United States and Switzerland who took part in the study. They used a laser-cooling and atom-trapping technique at the Argonne National Laboratory in the United States for measuring the rare, radioactive krypton isotope (Kr-81). In this technique, specific lasers are used to slow down and count individual Kr-81 atoms, which are only a few among more than a trillion atoms of stable krypton (Kr-84). The reduced number of Kr-81 atoms in groundwater compared to the atmospheric krypton allowed the estimation of the age of water, which established the link between groundwater and the passage of helium. Krypton-81 has a half-life of about 229,000 years and is used for dating old (about 50,000 to one million year-old) groundwater.
The IAEA Guarani project aimed to provide more knowledge about the aquifer.
“The Agency is working with its international partners to improve our understanding of groundwater systems so that we can better protect and manage this vital freshwater resource,” said Aggarwal.
“As part of this process we need to continue to better understand earth’s physical systems. In pursuing the Guarani project, we found out more than we expected, but that is the nature of scientific exploration.”
Helium is quite rare on earth but is widely used in industry. The gas is important to the electronics industry and for cooling super-conducting magnets such as those used in magnetic resonance imaging (MRI). Most helium is obtained from natural gas drilling in the United States.