Since 1989, Utah’s Great Salt Lake has lost some 70% of its surface area, reducing its ecosystem services and creating stretches of drying lake bed (playa) that send toxic dust into the air.
That drying ground has also provided opportunities for scientists to survey what lies below the lake’s floor. In a study published in Geosciences, researchers revealed glimpses of fresh water and salt water, with some fresh water lurking only a few meters below the surface. The work could provide clues for conserving the lake, a crucial resource for both the ecology and the economy of the region.
Salt Lake, Fresh Water
In 2023, Michael Thorne and colleagues began using a technique known as electrical resistivity tomography (ERT), which can reveal the presence of fresh or salty water, at dozens of spots near the southern and eastern edges of the Great Salt Lake. Thorne is a geophysicist at the University of Utah in Salt Lake City and a coauthor of the new study.
The lake’s desiccation allowed the researchers to access areas where “at previous times, you would never be able to do measurements because [they] would be underwater,” said Thorne.
Establishing a network of ERT sensors requires robust fieldwork. Over the course of long days in the field, Mason Jacketta, lead author of the new study, and others placed electrodes into the ground a few meters apart, making lines that stretched hundreds of meters. Between pairs of electrodes, they measured the resistance to electrical current. Salty water, filled with electricity-conducting ions, has lower resistance than fresh water.
Paired with information on the rock and sediment beneath the surface, as well as with measurements from nearby wells, the ERT data allowed the team to work out a profile of how electrical resistance varied with depth and to figure out what kind of water seeped through pores in the ground below. The team shared the results of their work on the southern part of the lake in Geosciences, while more in-depth findings about the eastern shore will appear in an upcoming publication.
“What this is really showing is that [fresh water is] prevalent all over the place.”
At many of the sites, Jacketta and others found fresh water near the surface.
“What this is really showing is that [fresh water is] prevalent all over the place,” said Elliot Jagniecki, a geologist at the Utah Geological Survey who wasn’t part of the work.
That fresh water was often in close proximity to patches of salty groundwater. At one spot in the southeastern part of the lake, the team found a shallow layer of brine. But right below that, at only 5 meters of depth, they encountered fresh water. At the team’s most northern study site, they found fresh water around 2 meters deep. On the southern shore, they found fresh water in some places as shallow as 2.8 meters.
Mysterious Formations
The team’s results also helped explain curious features around the Great Salt Lake, including mounds made of salt and islands made of reeds.
The lacy-looking layers of the lake’s so-called mirabilite mounds form in the winter, when the cold freezes upwelling salty water, concentrating its salts. With measurements taken next to where some mirabilite mounds form, the researchers could visualize the underground conduits that send salty water to the surface.
While mirabilite mounds form close to shore, mounds made of Phragmites reeds appear in the lake’s interior as well as along its periphery. Thorne and his colleague William Johnson first noticed these mysterious circles popping up in Google Maps more than a decade ago. When they went to investigate, they found Phragmites.
“The population of Phragmites around the Great Salt Lake is really not allowing fresh groundwater to go back into the Great Salt Lake.”
In the new work, the team placed a line for electrical resistivity tomography straight through a Phragmites mound. These reeds wouldn’t be able to survive in the lake’s briny water, Thorne said, but the team’s results showed fresh water rising right to where the invasive reeds grew thick.
“The population of Phragmites around the Great Salt Lake is really not allowing fresh groundwater to go back into the Great Salt Lake,” said study coauthor Tonie van Dam, a geophysicist at the University of Utah. The reeds suck up some 70,000 acre-feet of fresh water that could go back into the lake, she said. In “sucking up [fresh water] for their own existence,” van Dam explained, the reeds crowd out native plant species that provide habitat for native birds.
More Than a Beautiful Landscape
Overall, the study provides a new picture of the fresh and salty groundwater beneath the lake and how these resources feed what people observe at the surface.
It’s also helped to prompt other work, Thorne said, including one recent study in which researchers used a helicopter carrying a wire loop to create and sense electrical currents underground. That study, published in Scientific Reports, suggested there could be a large amount of fresh water under one part of the lake.
But that work is a proof of concept, Jagniecki said, and accessing such potential aquifers might not be sufficient to help address the lake’s current desiccation. Even if they could, refilling them could take thousands of years. “I just don’t think that’s a solution,” he said.
Saline lakes are fragile ecosystems sensitive to climate change, Jagniecki said. The Great Salt Lake harbors plenty of life, such as brine shrimp that become food for a host of migratory birds that use the lake as a stopover. Mineral extraction and the use of brine shrimp for feed in aquaculture are important drivers of Utah’s economy.
Getting a better understanding of how saline lake systems function could be helpful in conserving them and maintaining the resources they provide humans, Jagniecki explained.
“It’s actually more than that. It’s a beautiful landscape,” he said.
—Carolyn Wilke, Science Writer