For years, the water level at the Great Salt Lake has been slowly dropping, and as it does so, the shoreline continues to pull back. As the lakebed emerged, green circular patches began to appear on the white salt flat.
Soon, however, researchers determined that these circles were more than just some random type of plant life.
The circles appeared to point toward a submerged structure beneath the drying lakebed.
How did enormous freshwater reservoirs remain hidden beneath one of North America’s most saline lakes?
Strange circles showed up when the water left
The Great Salt Lake is extremely saline and can support little or no freshwater plant life. Yet green circular patches covered by tall reeds appeared in several areas of Farmington Bay. These green circles stood out against the otherwise light-colored salt flat.
Prior studies provided the first clue to how plants could survive in such conditions. Instruments placed under each mound indicated water was rising under high pressure toward the surface.
This led them to believe that a larger underground system might exist beneath the playa.
However, it was still unclear how large the body of water was that lay underneath the lake.
The circles appeared only after the lake level dropped far enough to expose the surrounding lakebed.
This raised the possibility that additional circles existed underwater but could not be seen yet.
Investigating the lakebed without digging
To explore the lakebed without drilling through it, researchers from the University of Utah used an airborne survey method. The equipment recorded differences in the materials beneath the surface rather than identifying visible signs of water.
Each variation produced unique signals that reflected the materials beneath the surface. Using those signals, researchers were able to create detailed maps of the subsurface sediments that lie beneath the lakebed.
NASA Science reported that many of the circular features only became visible after falling water levels exposed portions of the lakebed. At that point, researchers did not know if the freshwater was isolated or if it was part of a much greater whole.
When comparing their new maps with prior data collected during site visits, a larger scale pattern began to emerge.
The green circles were no longer appearing to be singular features.
Airborne electromagnetic surveys helped determine how far the freshwater-filled sediments extended beneath Farmington Bay.
Instead, they were beginning to look like they were part of something much larger under the lake.
What the aerial surveys revealed
The aerial surveys identified large volumes of pressurized freshwater beneath a shallow layer of hypersaline water and sediment. This salty layer served as a cap over the freshwater reservoir. Because of this cap, the groundwater remained hidden under the lakebed for thousands of years.
Only where pressure pushed the freshwater upward did it reach the surface. Freshwater-supported mounds of reeds occurred at those discharge points.
These eventually grew into visible structures when the shoreline retreated from them. Researchers also identified freshwater-filled sediments extending approximately 3-4 km, or 1.9-2.5 miles, beneath portions of Farmington Bay.
They believe that the extent of this underground reservoir may exceed their present understanding.
The green circles serve as “windows” into the previously hidden groundwater system.
Had the shoreline not retreated, researchers say those windows may have never been visible.
The discovery changed how researchers view the lake
The discovery has altered how researchers think about the Great Salt Lake’s total water budget. Freshwater flows into the lake may be significantly higher than scientists thought.
Initially, the green circles looked like simply a couple of isolated patches of reeds growing across a dry lakebed.
However, what they ultimately proved was that an extensive freshwater system had lain hidden beneath thousands of years’ worth of salt and sediment.
Researchers are still working to understand just how large that system may be.
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