While a potential environmental catastrophe, the lake’s dewatering presents numerous research opportunities for University of Utah geoscientists, including several who are looking to characterize the extent, characteristics, chemistry and flow of a mysterious, mostly freshwater aquifer under the playa.
In a pair of studies coming out this year, a team led by geophysicist Mike Thorne deployed electrical resistivity tomography, or ERT, lines in 30 locations around the lake’s southern and eastern margins to build 2D cross-sectional images of the subsurface.
Patchwork beneath the surface The researchers’ discoveries, including a hidden mirabilite layer near the historic Saltair, are adding to a growing inventory of new clues U scientists are generating into the previously unknown natural processes at play under and in the lake.
“If you go along the southern shore of the Great Salt Lake, it’s much more complex than what you would ever think,” said Thorne, an associate professor in the Department of Geology & Geophysics. “You just walk out across this flat playa, everything looks the same. But underneath it, there’s a lot of lateral heterogeneity, there’s a lot of variability.”
In some places the groundwater is salty at depth, yet freshwater is observed not far below the surface at most other locations. The findings are reported this week in the journal Geosciences, with graduate student Mason Jacketta as the lead author.
The study is part of a larger hydrology research project, involving several U geology faculty and funded by the Utah Department of Natural Resources.
“In the end, we want to know how much freshwater is there. We can see it’s a large volume,” said Bill Johnson, another Utah geology professor and co-author on Thorne’s paper. “What we don’t know is the flux. What can we pull out of it without harming other beneficial impacts of that groundwater?”
Much to learn about what’s below terminal lakes The goal of Thorne and Johnson’s research is to understand the groundwater under the playa, but it’s also exploring new scientific terrain in the process and could advance our general understanding of terminal lakes, which are among the world’s most imperiled landscapes.
“This is the foundational study for doing this kind of geophysical research on terminal lakes. To my knowledge, nobody’s done this,” Thorne said. Such lakes form in basins with no outlet so they become saline over time through evaporation that concentrates the dissolved salts. Terminal lakes are common in the Great Basin and provide important habitat for migratory birds. Around the world these lakes are in serious peril thanks to water diversions that have dried them out, leaving behind environmental disaster zones that unleash wind-borne dust pollution.
Despite the ecological importance of terminal lakes, not much has been done to study the groundwater hidden under them until recent years when Utah geoscientists turned their eye on the receding Great Salt Lake and the endless expanses of playa surrounding what’s left of the storied lake.
In the past few years, dozens of mounds have curiously appeared on the dry lakebed along the east shore, essentially small islands choked with phragmites. These round spots occur at places where artesian groundwater is forced to the surface under pressure through holes in the briny lens immediately below the playa, according to ongoing research led by Johnson.
The mysterious mounds prompted scientific enquiries, including Thorne’s ERT research which began along the causeway connecting Antelope Island and Davis County mainland in 2023.
What electrical resistivity reveals Preliminary findings helped him secure a $60,000 grant from the Utah Division of Forestry, Fire and State Lands, which he used to deploy ERT lines around the lake’s southeastern perimeter with a crew of geology graduate students, including Jacketta.
Electrical resistivity tomography is an advanced ground-based geophysical method for characterizing subsurface lithography, hydrology and morphology.
“You put electrodes into the ground, and then you send a current through the ground and you are measuring the resulting voltage,” explained Jacketta. The current passing through the ground will face different levels of resistance, depending on what it encounters.
Measured in ohm-meters, the electrical resistivity serves as a proxy for groundwater salinity in Jacketta’s study. These measurements leverage water’s increasing conductivity (or reduced resistivity) the more salt it contains. So low resistivity means high salinity, while resistivity exceeding roughly 7 ohm-meters is considered freshwater.
For the latest published study, the team processed data from 16 ERT lines, along with six transient electromagnetic (TEM) surveys, focused on three locations on the lake’s south shore. The westernmost was Burmester, where the Stansbury Mountains meet the lake; Saltair in the middle; and Lee’s Creek Natural Area on the east.
The data revealed a great deal of variability in the salinity of the groundwater. The underground picture here is patchy and complex, shaped by geology, rivers, mountain recharge and the lake’s long history of rise and fall.
At Burmester and Saltair, according to the analysis, a thick layer of very salty groundwater lies just a few meters below the surface. At Saltair, the brine was trapped beneath a hard mineral layer of mirabilite, a sodium sulfate also called Glauber’s salt. Cracks and vertical pathways in this mineral layer allow brine to rise to the surface, forming
Farther east near the Lee’s Creek, the researchers detected signs of fresh groundwater at shallow depths—sometimes as little as 3 meters below ground. This freshwater probably comes from mountain recharge but could be a leftover remnant from ancient Lake Bonneville, which covered the region until 14,000 years ago.
A freshwater aquifer under Farmington Bay Meanwhile the picture is totally different just to the north under Farmington Bay on the east shore, which will be covered in a separate paper that Thorne and Jacketta are preparing for publication. Groundwater under the lake’s eastern margins, the part facing the snow-laden Wasatch Mountains, is almost entirely fresh below four meters under the playa. These data align with findings recently published by Johnson and graduate student Eben Adomako-Mensah, who drilled holes of varying depths in Farmington Bay’s playa and used piezometers and other tools to characterize the groundwater.
But Thorne’s forthcoming study reveals an even more fascinating picture, according to a presentation he gave Feb. 10 to the state’s Great Salt Lake Technical Team. There appears to be a kind of “fingering” of saltwater penetrating downward at particular locations under Farmington Bay.
Because saltwater above is denser than freshwater below, it results in convective instability when the salt sinks. Thorne believes this is just the second time this phenomenon has been observed in the natural environment anywhere on Earth.
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The study appears in the journal Geosciences under the title, “Characterization of Hydrogeologic and Lithologic
Heterogeneity Along the Southern Shore of the Great Salt Lake, Utah, from Electrical Methods.” Funding came from the Utah Department of Natural Resources. Co-authors include Surya Pachhai, Ivan Tochimani-Hernandez, Tonie van Dam and Leif Anderson of the U’s Department of Geology & Geophysics, and Christian Hardwick of the Utah Geological Survey.
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