Effects of Salt Precipitation in the Topmost Soil Layer Investigated by NMR

The drying of highly concentrated aqueous salt solutions in sand and soil has been investigated by one-dimensional spatially resolved low-field relaxation measurements of 1 H nuclei in water as well as high-field MRI of 1 H and 23 Na nuclei of water and sodium ions. Water evaporates until the solutions in the solid matrix reach saturation conditions, when salt begins to crystallize. Depending on salt type and conditions, such as actual soil water content and air humidity, this crystallization can occur above (efflorescent) or below (subflorescent) the soil surface. Both effects occur in nature and affect the evaporation behavior of water. The formation of salt precipitate domains is demonstrated by MRI, where the precipitate domains remain penetrable to water. Complete drying is achieved in the top 2 mm of soil with the exception of strongly hygroscopic perchlorates which maintain a constant amount of liquid water under ambient laboratory conditions and dry air. This situation is considered similar to the co-existence of perchlorates and water in strongly eutectic mixtures on Mars.