Nuclear Physics Used to probe Floridan Aquifer Threatened by Climate Change

Only one in a million atoms in the atmosphere is krypton. What’s more, only one in a trillion krypton atoms is krypton-81 specifically. This leaves so few atoms to detect in a sample that scientists count them one by one using a technique called Atom Trap Trace Analysis developed at Argonne.

The team collected samples from eight wells tapping the aquifer and extracted the gas dissolved in the water, including the krypton-81. At the TRACER Center, they sent the gas down a beamline where six laser beams come together to create a trap unique to the isotope of interest (in this case, krypton-81). The trapped atoms show up on a camera, and scientists can count them down to the individual atom.

This study is the first application of radiokrypton dating on the Floridan Aquifer.

There’s Good News and Bad News
Some of the samples contained 40,000-year-old saltwater from just before the last glacial maximum at around 25,000 years ago, when much of the water that is now in the ocean was captured in huge glaciers. During this period, the sea level was over 100 meters lower than it is now.

“Because of global warming, the sea level is rising, causing seawater to spoil freshwater sources,” said Reika Yokochi, research professor at the University of Chicago and lead scientist on the study. ​“The presence of the moderately old water means saltwater persists in the aquifer once it gets in. This is bad news. We have to minimize the rate of this pollution.”

While the salty samples are concerning, there is good news, too. The scientists confirmed that the water in the southern part of the Floridan Aquifer was recharged with freshwater during the last glacial period (sometime between 12,000 to 115,000 years ago), bolstering the current understanding of freshwater dynamics.

“We also found a sample with relatively young freshwater, which is good news for Florida because it means that the water is actively flowing and renewable near central Florida,” said Yokochi.

New Technique with Great Potential
Radiokrypton dating is a relatively new technique, and the scientists are just getting started. This tool has incredible potential to drive discovery in physics, geology and beyond.

For example, scientists armed with radiokrypton dating can use the water in coastal aquifers as potential messengers of changes in water cycles and the composition of ancient seawater. The technique can also provide insight about the movement of elements across land-ocean boundaries, which impacts carbon dioxide (CO₂) levels in the atmosphere.

“As water flows on the surface or underground, it reacts with surrounding rock and picks up signatures that tell a story,” said Yokochi. ​“This information can help to improve and validate our models of Earth’s systems and the cycle of the elements, which are tightly linked with global climate.”

Radiokrypton dating also serves as a complement to carbon dating when performed on the same samples. Scientists can use results from radiokrypton dating to calibrate carbon dating analysis. Once corrected, the carbon data can provide additional insight, especially on rates of water-carbonate reactions. 

“When you have a new tool like this and apply it for the first time, even in an aquifer that has been studied a lot, suddenly you get a new perspective and new insight,” said Mueller. ​“The data from just a few samples is rich with opportunity, and this study demonstrates the great potential of krypton-81 in multiple fields of geochemistry.”

The results of the study were published in the September issue of Earth and Planetary Science Letters.