Roughly 6 million years after the Grand Canyon's formation, researchers studying the groundwater of the nearby Paradox Basin made a surprising discovery. The groundwater there was much younger than it should have been at that depth.

The methods used to reveal the basin's history could also help determine the age of other bodies of groundwater in the future, as humans continue to drill deeper wells for drinking water.

The Paradox Basin, a deep depression filled in with sediments over millions of years, is found mostly in southeastern Utah and southwestern Colorado within the Colorado Plateau. A University of Arizona-led team sampled the basin's groundwater from the Earth's surface down to the bottom of the basin, nearly 2 miles deep. They discovered that rapid erosion of the Colorado Plateau – which led to the formation of the Colorado River and Grand Canyon – allowed newly recharged groundwater to circulate deep underground and flush away some of the much older and saltier, or saline, groundwater that settled there long ago.

The findings are published in the journal Geophysical Research Letters.

"We were really surprised to find relatively young groundwater so deep in the subsurface," said Jihyun Kim, the paper's first author and a UArizona doctoral student in the Department of Hydrology and Atmospheric Sciences when the research was conducted. "Usually, the deeper the water, the older it is."

Then they found another surprise. About midway down the basin, they discovered ancient seawater from the Paleozoic era, more than 250 million years ago, trapped in impermeable salt deposits in the center of the basin. The sediments of the Paradox Basin were deposited 200 million to 300 million years ago when an ocean still existed in the region. As the seawater receded, it left behind lagoons, which evaporated and left behind a thick salt layer and super salty seawater called brine.

"Since then, groundwater recharge and circulation has been relatively shallow, until about 6 million years ago when the Colorado River started to cut into the landscape and allowed deep circulation of recharge from rain and snow," said Jennifer McIntosh, a professor in the Department Hydrology and Atmospheric Sciences. "No one has ever tried to date the timescales of this process using groundwater before this paper."

McIntosh leads the lab that conducted the research, and she co-authored the paper with Kim and Grant Ferguson, an adjunct faculty member in the Department of Hydrology and Atmospheric Sciences.

Their findings were made possible by a relatively new method for dating groundwater, which determines the age of the water based on how much of the radioactive element krypton-81 is present. Other co-authors on the study developed the technology and method for measuring individual atoms of krypton-81. This is one of the first studies to apply krypton-81 dating to deep saline bodies of water.

There's a set, known amount of krypton-81 in the atmosphere, and it can be transported with groundwater as it moves from the surface through the subsurface. Krypton-81 is radioactive and decays very slowly over time, so scientists can determine the age of the groundwater depending on the amount of krypton-81 it has. The less krypton-81 present, the older the water.

The traditional method of dating old groundwater, which relies on knowing how long it takes for radioactive carbon atoms to decay, is only useful for dating waters less than about 50,000 years old. Krypton-81 on the other hand, takes much longer to decay and as a result can be used to date water as old as 1.2 million years, McIntosh said.

Establishing the age of groundwater can also reveal something about the climate and geologic conditions present when the groundwater is refilled by rain and snow.

"That's what was so exciting about this study," McIntosh said. "We expected to find that groundwater would get progressively older as you go deeper. Instead, we found million-year-old groundwater – which is relatively young – about 10,000 feet beneath the surface in sediments that are hundreds of millions of years old."

The Paradox Basin is important to the story because, like a sandwich, there's an aquifer system on top of the salt and one at the bottom of the basin.

"Prior to the incision of the Colorado River, the Colorado Plateau was relatively flat, and ancient seawater from the Paleozoic era was trapped within the sediments for hundreds of millions of years. Then, suddenly, the Colorado River cut through the landscape and allowed flushing of these ancient saline waters from aquifers above and below the salt deposits at the center of the basin," McIntosh said.

This flushing process dilutes water's salinity, but the water is still much too salty to drink. At nearly 2 miles below the surface, the water is about 10 times saltier than seawater.

"So, people are not drinking this young, less-than-million-year-old groundwater in the Paradox Basin, but there are plenty of aquifer systems around the world that have groundwater old enough to need krypton-81 to date it," McIntosh said.

The krypton-81 dating technique becomes especially important for understanding the conditions of water for drinking or other purposes, as we continue to drill deeper and deeper wells to collect potable water. When scientists know how old the groundwater is, they can also know something about the conditions in which it was deposited.

McIntosh is excited to next study if microbial life hitched a ride with water as it moved from the surface into the depths of the basin. She and her team were awarded a $2.8 million five-year National Science Foundation grant to look into the history of groundwater and microbial life in the deep subsurface.

Understanding the subsurface is also important when planning where and how to store waste such as nuclear waste, human-made carbon dioxide and even alternative energy like hydrogen deep underground.

"The places humans have targeted for subsurface waste disposal are the places we think have ancient saline groundwater," McIntosh said. "So, this is why we were shocked to find such young groundwater nearly 2 miles down in the Paradox Basin, beneath trapped ancient seawater. Showing that we can have flushing of ancient saline groundwater in less than a million years because of landscape change is pretty dramatic – which is about the timescale that people talk about for secure storage of nuclear waste, for example. We should be studying subsurface systems using tracers like krypton-81 that are so informative to let us know how groundwater is circulated deep down."