Jana Lasser et al, ‘Salt Polygons and Porous Media Convection’, Phys. Rev. X 13, 011025, February 24, 2023
In salt flats across the world, the salt on the surface forms ridges that join together in a patchwork of pentagons and hexagons. These captivating patterns have been photographed as far apart as Bolivia, Chile, China, India (in the Rann of Kutch), Iran, Tunisia, and the U.S. These shapes are also invariably a metre or two across. That the salt always crenellates in these shapes and at these sizes, irrespective of the local environmental conditions, the mineral chemistry, the soil type, and other factors has puzzled researchers.
In a new study, researchers from Austria, Germany, and the U.K. may finally have an explanation. Using a combination of ground sampling and computer models, they have pointed their fingers at the way salt flows up and down in the soil below this formation. It is important to know the underlying mechanism because salt flats have significant effects on both humans and the climate.
A salt flat is a natural landscape in which a large area of flat land is covered by salt. Perhaps the world’s most well-known salt flat is the Salar de Uyuni in Bolivia. It is the largest in the world of its kind, and also contains more than half of the planet’s lithium reserves.
A salt flat forms from a natural water body whose recharge rate is lower than the evaporation rate. Over time, all the water evaporates, leaving behind the dissolved minerals, usually salts. They reflect sunlight strongly and thus appear bright. The underlying soil is highly saline — even if the water table is shallow. The groundwater is too salty for humans to drink.
The study and its findings
The researchers began with the hypothesis that the salt on the surface is influenced by the salt flowing through the soil below.
Imagine the soil in a salt flat: there are some ridges on the top, followed by a layer of salt, then the topmost layer of the soil, and finally the rest of the soil. The groundwater in the soil is saline but the distribution of salt is not uniform. The salinity is highest near the top of the soil and decreases towards the bottom.
The researchers found that the salt penetrated deeper into the soil exactly below the ridges, and remained shallow under the flat areas. That is, if you removed the topmost layer and looked directly down at the soil, you would see that the salty groundwater is flowing deeper into the soil along vertical sheets, not throughout.
The surface of a salt flat has a layer of salt that has been deposited over time. So just under the surface, the groundwater is highly saline and denser than the groundwater further below. If any water reaches and rises above the surface, it evaporates to leave more salt behind. The researchers found that if the rate of evaporation is sufficiently high, that is if the rate of salt deposition on the surface is sufficiently high, the denser groundwater will sink down and the less-saline, less-dense groundwater will rise to the top. This body of descending and ascending water is called a convection cell.
Over time, there will be more saline groundwater rising up towards the surface through the convection cells than through other parts of the soil – simply because the less-dense water within the column is being displaced upwards. As a result, the salt this water carries will accumulate on the surface, forming the narrow ridges that make up the polygons.
The mathematical equations the researchers assembled for their computer model indicated that the “subsurface convection … is relatively insensitive to salt chemistry”, as they wrote in their paper, and that over time, the convection columns naturally grew to have a stable width of 1-2 m – just like the dimensions of the polygons on the surface.
Implications of the results
Since at least the early 1960s, scientists have offered different explanations for why the surface of dried salt lakes becomes covered with this pattern of polygonal shapes. Most of them have either considered above-the-surface dynamics or below-the-surface dynamics, whereas the new study shows that the polygons are formed when these two realms interact.
The theory and the results matter because when winds blow over salt flats, they carry some of the salt with them as particulate matter. When this air mass reaches the ocean, it deposits the salts there. Such sea salt can enter the atmosphere and go on to swirl at the centre of cyclones. When a salt-bearing air mass reaches an inhabited area, the particles cause significant respiratory problems. A 1996 study characterised the salt flat of what was once Owens Lake in California the “single greatest source of particulate matter in North America”.
To mitigate the deleterious effects of salt flats, experts have recommended covering them in a shallow layer of water, so that the salt is deposited on the surface more uniformly and less salt is carried away by winds.
Salt suspensions are also an important group of aerosols (suspensions of fine solids in air) that reflect sunlight. We have also known for some time that saline lakes around the world are shrinking, including due to agriculture. So more accurate climate models will need to better understand the sources of salt, and the new findings describe one such source.
