Salt flats around the world are covered in a similar pattern of ridges

Salt flats around the world are covered in a similar pattern of ridges

Subject: Environment

Section: Msc

Context: 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.

Salt flats:

• 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.

How do the shapes form?

• The researchers began with the hypothesis that the salt on the surface is influenced by the salt flowing through the soil below.
• 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 the topmost layer is removed and looked directly down at the soil that the salty groundwater is flowing deeper into the soil along vertical sheets, not throughout is visible.

What explains this finding?

• 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, i.e. 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.
• 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.
• 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.