The mystery islands of Great Salt Lake

As Great Salt Lake’s levels continue to sag, yet another strange phenomenon has surfaced, offering Utah scientists more opportunities to plumb the vast saline lake’s secrets. 

Phragmites-covered mounds in recent years have appeared on the drying playa off the lake’s southeast shore. After several years of scratching their heads, University of Utah geoscientists, deploying a network of piezometers and aerial electromagnetic surveys, are now finding out what’s going on under the lakebed that is creating these reed-choked oases.

Bill Johnson, a professor in the Department of Geology & Geophysics, suspects the circular mounds have formed at spots where a subsurface plumbing system delivers fresh groundwater under pressure into the lake and its surrounding wetlands.

“Water in the lake has spent a significant time underground on its way to the lake. But where that happened, we don’t know,” said Johnson during a recent visit to one of the mounds, a research site known as Round Spot 9. “Did that happen somewhere in the uplands where the water spent time in the ground and emerged in the stream before going to the lake? Or was it transmitted directly to the lake?”

On this day, Johnson and graduate student Ebenezer Adomako-Mensah were checking piezometers they had installed there last year to record underground water pressures at various depths and locations around the island.

Mapping the lakebed' subsurface

In February 2025, Johnson hired a Canadian firm, Expert Geophysics, to conduct airborne electromagnetic surveys over Farmington Bay using a circular device hanging under a helicopter. The pilot flew a grid pattern over the bay, collecting data that will help locate freshwater deposits lurking under the lakebed.

The equipment generates current in the loop, which transmits a frequency deep into the lakebed below. A receiver suspended in a ball at the center of the hoop records the electromagnetic signals bouncing back.

“It’ll give you a spectrum, basically, of magnetic fields, and we’ll use that data to create a 3D image of what’s under the earth,” said Jeff Sanderson, a crew leader with Expert Geophysics.

As low lake levels persist, the lakebed will increasingly serve as a source of wind-blown dust affecting Utah’s population centers. Ongoing research by U atmospheric scientists suggests that the disturbed lakebed crusts that keep sediments in place can be regenerated when they are submerged.

One goal of Johnson’s research is to determine whether the groundwater can be tapped to restore broken lakebed crusts, thereby reducing dust pollution.

“It looks like it’s a from a water resource that could be useful in the future, but we need to understand it and not overexploit it to the detriment of the wetlands,” said Johnson, who has served on the Great Salt Lake Strike Team, the university-state agency partnership exploring ways to reverse the lake’s decline.

Armed with new data, Johnson secured preliminary funding from the Utah Department of Natural Resources to investigate to characterize this underground water resource. The research team, which includes other senior geology faculty, including  Kip Solomon, Mike Thorne and Michael Zhdanov, is seeking to discover the breadth and depth of the freshwater under the lake.

For example, 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 the airborne geophysical surveys to construct a 3D image of the subsurface beneath the lake.

“We hope to map out the boundary between fresh water and salt water, and find the location of freshwater springs that are discharging groundwater into the lake,” said Solomon, who is scheduled to present preliminary findings this week at the Geochemical Society’s 2025 Goldschmidt conference in the Czech Republic.

For Johnson, the groundwater mystery began several years ago when he was traveling Great Salt Lake’s North Arm by airboat and observed something strange. Water and gas were roiling the surface in a circle about twice the size of the airboat, suggesting that groundwater was rushing under pressure into the lake at that spot.

Johnson dropped a 30-foot depth gauge into the swirl, but it failed to hit the bottom of the shallow lake.

“I always wondered what the heck that was, because it seemed like groundwater was coming to the system at a huge rate,” he said. Later on, he and others noticed mounds appearing on Google Earth images of the Farmington Bay playa.

Where does the groundwater come from?

Previously, it was believed that direct groundwater discharge accounts for just 3% of the lake’s water budget, but recently gathered data using chemical mass balance methods indicate it may be as much as 12% and new insights are emerging.

For starters, Johnson’s team has found that fresh water at depth in several spots far offshore.

“We didn’t expect that. We expected that fresh water would be coming into the system at the periphery, farther away from the lake,” he said, “and yet there it is, all the way underneath the causeway in Farmington Bay.”

Johnson’s team has located freshwater almost everywhere they looked in tight sediments 30 feet below the surface. The new geophysical data indicate these sediments are up to 10,000 feet deep.

“We don’t know if it’s freshwater that deep, but it is certainly going to be fresh a long way down, and it could be fresh all the way down,” Johnson said.  “The last thing I want to do is get this hyped as a water resource, but it’s very clear, and it’s under pressure. And in my mind, it could help mitigate any dust generation on the exposed playa.”

The focus of Johnson’s research homes in on one of at least 18 mounds detected off the lake’s southeast shore, most of them choked with thickets of phragmites, the water-hogging invasive reed cluttering the lakeshore.

A vast underground plumbing system

On Round Spot 9 in Farmington Bay, Johnson’s team has installed three sets of four piezometers at various distances from the edge of the 250-foot-diameter island. The four instruments are placed at varying depths, 7, 11, 30 and 60 feet, connected to the surface via white PVC pipes.

Accompanied by grad students like Adomako-Mensah, Johnson has regularly visited the site this year by mountain bike or airboat to recover data that is already telling an interesting story.

Near-surface water is the purest measured at the center of the mound and gets progressively more saline as you move closer to the edge, while deep water is fresh at all locations. In other words, the groundwater is not reaching the surface on the periphery of the island, but mostly at the center. Why?

“Those show that the fresh water in the center is pressured the deeper you go; the more hydraulic head it has. It really wants to come up,” Johnson said. “And that’s true also in the perimeter, but it’s freshwater at depth there. It’s not coming up because it’s capped.”

Johnson believes there are hundreds of these groundwater-fed oases scattered across Great Salt Lake’s exposed playa, suggesting the presence of a vast underground reservoir connected to the surface by a plumbing system that is only now getting close study, thanks in part to the lake’s decline.

With his colleagues’ help, the geologist hopes to discover where the water came from, when it fell as snow and most critically, how much is there.

“The last thing we wanted to do is for this to be characterized as a water resource we should be tapping,” he said. “It’s much more fragile than that, and we need to understand it better.”