Utah's other Great Salt Lake is underground, ancient, deep....and fresh

Under the Great Salt Lake playa lies a potentially vast reservoir of pressurized freshwater that has accumulated over thousands of years from mountain-derived snowmelt, according to new research from University of Utah geoscientists. This groundwater occupies the pore spaces in sediments that fill the basin west of the Wasatch Mountains and below a 30-foot-thick salty layer.  Windows through this salt lens exist in the form of odd circular reed-choked mounds or islands that have formed where the freshwater pipes through to the lakebed of Farmington Bay, which is otherwise left dry by receding water levels thanks to drought and diversions.

Funded by the Utah Department of Natural Resources and the Great Salt Lake Commissioner’s Office, a team led by U hydrologists Bill Johnson and Kip Solomon working with the U.S. Geological Survey Water Science Center and the Utah Geological Survey is conducting a multi-prong investigation of this invisible body of water, using an array of instruments and methods to determine its depth and breadth, chemical properties, age, source and other characteristics.

“Fresh water exists under the periphery of the lake and maybe even under the lake itself. And we need to better characterize it to understand how we can use it as a resource,” said Johnson, a professor geology & geophysics. While he does not view the aquifer as a potential water source for replenishing the depleted lake, Johnson believes it could provide water for restoring lakebed crusts that naturally prevent exposed sediments from contributing to dust storms blowing into Wasatch Front cities.

“We proposed that for higher elevation dust spots, you drop a well and flood the area and douse the dust spot. What we’re trying to check out is whether we can use a modest amount of that water without weakening the nice upward pressure,” Johnson said. “That needs to be checked out and we proposed to do some of that work.”

Artesian water under the lake appears to be ancient

Great Salt Lake started taking its present shape about 8,000 years ago as a remnant of the vast freshwater Lake Bonneville that once covered most of northwestern Utah. Johnson suspects the some of water contained in the aquifer dates back to the Bonneville era.

“The age of the water at depth is thousands of years old. It may be left over from the Ice Age,” he said. “That tells us that playa groundwater is not getting to the lake anytime soon. We know from complementary to-be-published studies that there is a big groundwater flux coming off the mountain front, but it seems to get to the lake via seepage to incoming rivers rather than via direct groundwater inflow in the playa.”

The first of several anticipated papers associated with the project was posted last month, reporting the results of  piezometer data collected at mounds forming in Farmington Bay, sandwiched between Great Salt Lake’s southeast shore and Antelope Island. These findings are upending scientists’ understanding of how spring runoff reaches the terminal lake.

About 9 meters below the playa, there is a freshwater-saltwater interface that was previously unknown and researchers now call the “saltwater lens,” according to lead author Ebenezer Adomako-Mensah, a graduate student in Johnson’s lab.

What are these strange round spots?

Johnson and Adomako-Mensah have spent the last two years boring wells of varying depths into the mounds to install piezometers that measure water pressure and flow at depth. They regularly return by airboat or mountain bike to collect data, which now show these mounds occur where freshwater is pushed under pressure through the saltwater lens to the surface of the playa.

“The size or radius of the round spot corresponded with the freshness of the groundwater; the bigger the round spot the fresher the water at the center,” Adomako-Mensah said. “In the center the water tends to be fresh and when you move to the edge it tends to be saline. The round spot is more like a pipe or a channel that is allowing groundwater to flow to the surface.”

This paper is part of a larger state-funded research effort involving several other senior members of the U’s geology faculty, including  Kip Solomon, Mike Thorne and Michael Zhdanov, aimed at fully characterizing the body of freshwater hidden under the salty lake.

Solomon’s lab is using isotope analysis to determine the age of the groundwater and its recharge elevation, or where it originated in the mountains. Thorne is constructing on-ground resistivity profiles. And Zhdanov and Michael Jorgensen are processing the electromagnetic data gathered in airborne geophysical surveys to construct a 3D image of the playa’s deep subsurface.

For Johnson, key questions begin with the mysterious round spots that have been called “phragmites oases”  and “mystery islands” in recent news media accounts.

“Because you have water flowing from the mountains to the basin that can create pressure under Farmington Bay, it’s actually driving the water upwards. Before we got these numbers no one knew there was artesian freshwater under the entire eastern playa, nobody knew how extensive this pressurized water under Farmington Bay was,” Johnson said. “This is a ton of work on Eben’s part, doing tests in the field and tests in the lab to get these numbers. They show that the conductivity range is pretty big, but conductivity is low except in a few special cases such as the ‘phragmites islands’ where the sediment is coarse.”

Why the sediment is coarser in these spots, and why these features are circular, and whether their existence is related to basement structure remain unknown and these questions warrant further investigation, the study states. Also need is more analysis to understand whether the younger lakeward lateral flux of water from the mountains forms a continuous arc with the ancient upward vertical flux under the playa. Accordingly, this U-led project is generating more and more questions that will likely keep curious scientists busy for years.

Banner image: U graduate student Ebenezer Adomako-Mensah collects piezometer data on a mound formed in Farmington Bay where groundwater breaches the surface of the Great Salt Lake playa. Photo credit: Brian Maffly

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The study, titled “Significance of Ancient Artesian Fresh Groundwater Below the Playa of a Hypersaline Terminal Lake of Hemispheric Significance,” appeared online Dec. 19 in the Journal of Hydrology. Co-authors include Hugh Hurlow of the Utah Geological Survey and Samuel Carter, Kip Solomon, William Mace and Scott Hynek of the U’s Department of Geology & Geophysics. Hynek also is a hydrologist with the U.S. Geological Survey. Funding for this research comes from the Utah Department of Natural Resources.