Context: 

In a new study, researchers from Austria, Germany, and the U.K. may finally have an explanation on why the salt on the surface of salt flats forms ridges that join together in a patchwork of pentagons and hexagons.

Image Source: The Hindu

What are Salt Flats? 

• A salt flat is a natural landscape in which a large area of flat land is covered by salt. 
• The Salar de Uyuni in Bolivia is the largest salt flat  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: 

• Hypothesis: The researchers began with the hypothesis that the salt on the surface is influenced by the salt flowing through the soil below. 
• Soil structure: There are some ridges on the top of the soil in a salt flat, followed by a layer of salt, then the topmost layer of the soil, and finally the rest of the soil. 
• Salinity: 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. 
• Salt penetration: The researchers found that the salt penetrated deeper into the soil exactly below the ridges, and remained shallow under the flat areas. 
• Groundwater flow: If the topmost layer is removed, it will be observed 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. 
• Salt deposition: 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. 
• Accumulation of salt: 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. 

Implications of Salt Accumulation in Salt Flats:

• Wind-borne salt: The theory and the results matter because when winds blow over salt flats, they carry some of the salt with them as particulate matter. 
• Sea salt in atmosphere: 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. 
• Health Effect: When a salt bearing air mass reaches an inhabited area, the particles cause significant respiratory problems.
• Mitigation: 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: Salt suspensions are also an important group of aerosols (suspensions of fine solids in air) that reflect sunlight. 
• Shrinking saline lakes: 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.

News Source: The Hindu