Scientists use nuclear physics to probe Floridan Aquifer threatened by climate change

Peter Mueller, an Argonne scientist, at the TRACER Center. This facility is advancing the science of krypton dating in young and old groundwaters and glacial ice. Credit: Argonne National Laboratories
Scientists are now using a new nuclear dating technique to monitor the flow of water as rising sea levels threaten coastal areas.

Florida is famous for its water. The state is well-watered thanks to its beaches, swamps and storms. The largest freshwater source in the country is located below the state's entire surface.

The Floridan Aquifer is responsible for producing 1.2 trillion gallons annually of water. That's nearly 2 million Olympic-sized swimming pool. It is the primary source of water for more than 10 million people, and it supports irrigation of over 2,000,000 acres. It supplies water to thousands of springs, lakes, and wetlands as well as the environment they create.

Peter Mueller, Argonne National Laboratory scientist, said that the data is only a small sample and showed great potential.

As glaciers melt, rising sea levels pose a threat to this water source and other coastal aquifers with the introduction of saltwater. It is more important than ever to understand the history and behavior water in these aquifers. Florida's dynamic water systems are a great testbed.

A University of Chicago study used a dating method developed by nuclear physicists from the U.S. Department of Energy (DOE) ArgonneNational Laboratory. This radioactive version of the element krypton was used to study the flow and origin of freshwater and saltwater within the Floridan Aquifer. These findings show the potential of the new technique to understand and forecast climate change's effects on coastal aquifers. They also reveal insights into other geological processes.

Counting krypton

The scientists used radiokrypton dating to study the flow of water within the aquifer. The technique is similar to carbon dating in that it uses the amount of an element left in the sample to determine the age. Instead of using carbon, the radioactive isotope Krypton-81 is used.

Krypton-81 can be found in the atmosphere in small amounts and can dissolve in water droplets in clouds or bodies of water. When water is underground, it stops absorption of krypton-81. Over time, what remains slowly transforms into other elements.

Scientists can determine the ratio of the krypton-81 present in water and the atmosphere to calculate the time it has been underground.

Peter Mueller, Argonne's Physics Division said that "this is extremely difficult." "Krypton-81 is a rare substance, so you will need sensitive measuring tools to detect it in a sample."

Krypton is only one in every million atoms in our atmosphere. Furthermore, krypton-81 is the only krypton atom that is found in one trillion kryptons. Scientists can only detect so few atoms in a sample, that they use a technique called Atom Trap Trace Analysis at Argonne to count them one-by-one.

Reika Yokochi, a scientist, collects water samples from Florida's Aquifer. The team took samples from eight wells, and extracted the gas dissolved into the water. This was then sent to Argonnes TRACER Center for analysis. Credit: Argonne National Laboratories

The team took samples from eight wells that tap the aquifer, and extracted the gas in the water. The gas was sent down a beamline at the TRACER Center where six laser beams combine to form a trap that is unique to the particular isotope (in this instance, krypton-81). Scientists can see the trapped atoms on a camera and count them down to their individual atoms.

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

There are good and bad news.

Some samples contained saltwater dating back to 40,000 years ago. This was just before the last glacial maximum, which occurred around 25,000 years ago. The sea level was 100 meters lower during this time than it is today.

Reika Yokochi (research professor at the University of Chicago, and the lead scientist in the study) said, "Global warming is causing the sea level to rise, which causes seawater to spoil freshwater resources." "The moderately old water indicates that saltwater remains in the aquifer after it has gotten in. This is very bad news. This pollution must be reduced.

The salty samples are troubling, but there is also good news. Scientists confirmed that water in the Floridan Aquifer's southern portion was replenished with freshwater by the last glacial period (12,000-115,000 years ago), which further enhances our understanding of freshwater dynamics.

Yokochi stated that a sample of freshwater was also discovered. This is good news for Florida as it indicates that water is still flowing near central Florida.

A new technique that has great potential

Radiokrypton dating, which is still a relatively new technique for dating radiokrypton, is just being developed by scientists. This tool is a powerful tool that can drive discoveries in physics, geology, and other areas.

Radiokrypton dating is a technique that allows scientists to use water from coastal aquifers to detect changes in the water cycle and composition of seawater. This technique can provide information about the movement and composition of elements across land-ocean borders, which has an impact on carbon dioxide (CO 2) levels in the atmosphere.

Yokochi stated, "Water flows on the surface and underground. It reacts with surrounding rocks and picks up signatures which tell a story." This information can be used to validate and improve our models of Earth's systems, the cycle of elements and global climate.

Radiokrypton dating can also be used to complement carbon dating, provided that the same samples are used. Radiokrypton dating results can be used by scientists to calibrate carbon dating analyses. After being corrected, the carbon data will provide further insight, particularly on the rates of water-carbonate reaction.

Mueller stated, "When you have a tool like this, and you apply it for the very first time in an aquifer that's been studied a lot," he said. The data from a small number of samples are rich in opportunity. This study shows the immense potential of krypton-81/81 in many fields of geochemistry.

Further Paper Details Techniques to Date Groundwater

Additional information: Reika Yokochi and colleagues, Origin of water mass in Floridan Aquifer system revealed by 81Kr Earth and Planetary Science Letters (2021). Information for Journal: Earth and Planetary Science Letters Reika Yamakochi, Origin of water mass in Floridan Aquifer System discovered by 81Kr (2021). DOI: 10.1016/j.epsl.2021.117060