Beneath Utah's vanishing Great Salt Lake, a hidden patchwork of fresh and salty groundwater

Walk out across the southern shore of Utah's Great Salt Lake and everything looks the same. Flat, cracked, white playa stretching to the horizon. The uniformity is deceptive. Beneath that featureless surface lies one of the more complex hydrological systems anyone has mapped under a terminal lake - and much of it was unknown until the lake started disappearing.

A lake in retreat

Great Salt Lake has shrunk by 70% since 1989, a casualty of upstream water diversions and climate change. The contraction has exposed roughly 800 square miles of playa and mudflats, creating potential environmental hazards - dust storms carrying heavy metals, habitat loss for migratory birds - but also opening terrain that was previously underwater to direct scientific investigation.

A team led by geophysicist Mike Thorne at the University of Utah seized the opportunity. Using electrical resistivity tomography (ERT) - a technique that sends electrical current through the ground and measures the resulting voltage to infer subsurface composition - they deployed survey lines at 30 locations around the lake's southern and eastern margins. The goal was to characterize a mysterious, mostly freshwater aquifer that had been hinted at but never systematically mapped.

Salty on one block, fresh on the next

The findings, published in Geosciences with graduate student Mason Jacketta as lead author, reveal dramatic lateral variation. At the westernmost survey site near Burmester, where the Stansbury Mountains meet the lake, a thick layer of very salty groundwater sits just a few meters below the surface. At Saltair, in the middle of the southern shore, the team found brine trapped beneath a hard mineral layer of mirabilite, a sodium sulfate mineral. Cracks in this mineral layer allow brine to rise to the surface.

But move east toward Lee's Creek Natural Area, and the picture changes entirely. There the researchers detected freshwater at shallow depths - sometimes just three meters below ground. This freshwater likely arrives from mountain recharge, though some of it could be a remnant of ancient Lake Bonneville, which covered the region until roughly 14,000 years ago.

The patchwork defies simple characterization. Geology, river inputs, mountain snowmelt, and the lake's long history of rising and falling have created a subsurface mosaic that changes over short distances.

Freshwater under Farmington Bay

Under Farmington Bay on the east shore, facing the snow-laden Wasatch Mountains, the picture is different again. A forthcoming paper by Thorne and Jacketta will show that groundwater there is almost entirely fresh below about four meters under the playa. This aligns with findings from a separate team led by geologist Bill Johnson, who drilled wells of varying depths in the bay's exposed lakebed.

But Thorne's most striking finding is still more exotic. In certain locations under Farmington Bay, saltwater appears to be penetrating downward in narrow columns into the freshwater below - a phenomenon called convective instability, driven by the density difference between the overlying brine and the lighter freshwater beneath it. Thorne believes this is only the second time this process has been observed in a natural environment anywhere on Earth.

How much freshwater is there?

The practical question driving much of this research is water supply. Utah's population is growing, its rivers are over-allocated, and climate change is reducing mountain snowpack. A substantial freshwater aquifer beneath the Great Salt Lake playa could be a resource - but only if withdrawing from it would not cause other problems.

"We can see it's a large volume," said Bill Johnson. "What we don't know is the flux. What can we pull out of it without harming other beneficial impacts of that groundwater?" Those impacts include maintaining the lake's remaining ecosystem, supporting the artesian springs that create small vegetated mounds on the exposed lakebed, and preserving the complex geochemical balance of the subsurface.

A new field of study

Terminal lakes - lakes with no outlet that become saline through evaporation - are among the world's most imperiled landscapes. They are common across the Great Basin and provide critical habitat for migratory birds. Around the globe, water diversions have drained many of them, creating environmental disaster zones that unleash dust pollution.

Despite their ecological importance, the groundwater beneath terminal lakes has received remarkably little scientific attention. Thorne's work is foundational - the first systematic geophysical characterization of subsurface hydrology under a terminal lake playa.

The research is funded by the Utah Department of Natural Resources and involves several University of Utah geology faculty. It will not resolve the Great Salt Lake's decline, which ultimately requires reducing upstream water diversions. But it is filling in a map that nobody had drawn - revealing that beneath one of America's most visible environmental crises lies a hidden hydrological world far more complex than the flat, empty surface above it would suggest.