As sea level rise threatens coastal areas, scientists use new nuclear dating technique to track the ins and outs of water flow
Florida is famous for water. Between its beaches, swamps, storms and humidity, the state is soaked. And beneath its entire surface is the country’s largest freshwater aquifer.
The Florida aquifer produces 1.2 trillion gallons of water each year, or nearly 2 million Olympic swimming pools. It serves as the primary source of drinking water for over 10 million people and supports the irrigation of over 2 million acres. It also supplies thousands of lakes, springs, and wetlands, and the environments they support.
“The data from just a few samples is rich in opportunity, and this study demonstrates the great potential of krypton-81 in multiple fields of geochemistry,” said Peter Mueller, Argonne scientist.
But as glaciers melt due to global warming, rising sea levels threaten this water source – and other coastal aquifers – with saltwater intrusion. Studying the history and behavior of the water in these aquifers is more crucial than ever, and Florida’s dynamic hydraulic systems make it a premier test bed.
In a study by the University of Chicago, scientists applied a dating technique developed by nuclear physicists at the US Department of Energy’s (DOE) Argonne National Laboratory that uses a radioactive version of the element krypton to study the origin and flow of fresh and salt water. in the Florida aquifer. Their results demonstrate the promise of this new technique to help understand and predict the effects of climate change on coastal aquifers, inform water resource management, and reveal insight into other geological processes.
To study the flow of water in the water table, the scientists used the TRACER Center in Argonne to perform radiokrypton dating. This technique works on the same principles as carbon dating, where the age of something is determined based on how much of a certain element remains in the sample. But instead of carbon, it uses the radioactive isotope krypton-81.
A small amount of krypton-81 is naturally produced in the atmosphere and can dissolve in water droplets in clouds and bodies of water. Once the water goes underground, it stops absorbing krypton-81 from the atmosphere and what remains slowly transforms into other elements over time.
If scientists can determine the ratio of krypton-81 in water to the atmosphere, they can calculate how long it has been underground.
“It’s extremely difficult,” said Peter Mueller, a scientist in Argonne’s physics division. “Because krypton-81 is so rare, you need very sensitive measuring tools to detect the small amount in a sample.”
Only one in a million atoms in the atmosphere is krypton. In addition, only one in a trillion krypton atom is specifically krypton-81. 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 took samples from eight wells capturing the aquifer and extracted gas dissolved in the water, including krypton-81. At the TRACER Center, they sent the gas over a beamline where six laser beams come together to create a unique trap for the isotope of interest (in this case, krypton-81). The trapped atoms appear on a camera and scientists can count them down to the individual atom.
This study is the first radiokrypton dating application on the Florida aquifer.
There is good news and bad news
Some of the samples contained 40,000-year-old salt water just before the last glacial maximum around 25,000 years ago, when much of the water now in the ocean was captured in d ‘huge glaciers. During this period, the sea level was more than 100 meters lower than it is now.
“Due to global warming, sea levels are rising, causing seawater to spoil freshwater sources,” said Reika Yokochi, a research professor at the University of Chicago and senior scientist at the study. “The presence of moderately old water means that salt water persists in the aquifer once it enters it. This is bad news. We need to minimize the rate of this pollution.
While salty samples are cause for concern, there is also good news. Scientists have confirmed that water in the southern part of the Florida Aquifer recharged with fresh water during the last Ice Age (some 12,000 to 115,000 years ago), strengthening understanding current dynamics of freshwater.
“We also found a sample with relatively young freshwater, which is good news for Florida because it means the water is actively flowing and renewable near central Florida,” Yokochi said.
New technique with high potential
Radiokrypton dating is a relatively new technique and scientists are just getting started. This tool has incredible potential to stimulate discovery in physics, geology and beyond.
For example, scientists armed with radiokrypton dating can use water from coastal aquifers as potential messengers of changes in water cycles and ancient seawater composition. The technique can also provide information about the movement of elements across land-ocean boundaries, which impacts carbon dioxide (CO2) levels in the atmosphere.
“When water flows on the surface or underground, it reacts with the surrounding rock and picks up signatures that tell a story,” Yokochi said. “This information can help improve and validate our models of Earth systems and the element cycle, which are closely related to global climate.”
Radiokrypton dating also serves as a complement to carbon dating when performed on the same samples. Scientists can use the results of radiokrypton dating to calibrate the analysis of carbon dating. When corrected, the carbon data can provide additional information, particularly on the rates of water-carbonate reactions.
“When you have a new tool like this and you apply it for the first time, even in an aquifer that has been studied a lot, you suddenly get a new perspective and a new insight,” Mueller said. “The data from only a few samples is rich in opportunity, and this study demonstrates the great potential of krypton-81 in multiple fields of geochemistry.”