How do salts build up in the centre of a flooded playa basin?

In a flooded playa basin, salts build up in the center through the process of evaporation, which exceeds the rate of water inflow from rainfall, rivers, or streams. As the ponded floodwater gradually evaporates, the remaining brine becomes increasingly concentrated, leading to the precipitation of various minerals.123

This process creates a distinct zoning pattern, often described as "bathtub rings," based on the solubility of the minerals:4

• Outer Margins: Less-soluble minerals, such as calcium carbonate and magnesium carbonate, precipitate first and form the outer rings.4
• Intermediate Zones: Sulfates of calcium and sodium, such as gypsum or thenardite, typically form the next layer.4
• Center: Highly soluble minerals, particularly halite (sodium chloride), precipitate last from the remaining brines of exceptionally high salinity, resulting in their accumulation at the center of the basin.4

Additionally, groundwater flow can transport dissolved salts to the lowest portions of the playa, where they form saline crusts through rigorous evaporative concentration.35