Effect of Sodium Sulfate Extraction on Aeolian Process (A Case Study of South of Eyvankey City)

Lowlands, playas, and downstream portions  of rivers in desert areas contain vast reserves of fine-grained sediments, such as silt and clay, as well as soluble materials including a variety of salts. Surfaces created by the combination of these materials can become periodically susceptible to wind erosion. Thus,they are considered to be major dust sources on a global scale.Depending on the spatio-temperial distribution and composition of salts and fine materials, some proportions of these areas areusually either covered by an evaporite salt crust or disperedsoil.Crust is a relatively thin consolidated soil surface layer or seal that is more compact and cohesive than the material immediately below it. When crusts are formed, particles are bound together and become less susceptible to abrasion by blowing soils compared tothe less stable material below the crust. Both crusted surfaces and dispersed soils are morphologically and geochemically dynamic and can respond rapidly to changes in the local environmental conditions.These changes can be natural, such as the frequencies of surfacedrying and flooding by rainwater or the changing groundwater levels, or can be the result ofanthroponic activitiesproviding salt resources for economic use.Over time, the continued operations of both mechanical and chemical processes on lowland surfaces ultimately lead to the decay of salt crust integrity.Crusts usually provide a protectionagainstan underlying ‘fluffy’ layer of sedimentsrepresentingas salty sediments of dust-size fractions with notably low bulk densities.Wind erosion activity occurs particularly when the crust is disturbed or broken by different activities, such as  salt extraction or vehicular traffic flow.In all desert areas of Iransodium sulphate (Na2SO4) salts are deposited based on humidityand temperatureconditions, as well as groundwater levels and degrees of salt solubility (concentration). These areas usually occur between downstream of covered pediments and upstream of playas.Traditionally, these areas arevalued forsodium sulphate salt extraction,whichcontributes to the economy of the local population in several ways.The study area  was the lowland area of Ivanki, which was one of the areas undergoing wide sodium sulfate extraction. According to the residents, this area providedsand sources forwind erosion and air pollution.It was often a source of emission made by the existingmaterials not only because of wind erosion, but also due tosodium sulfate extraction.This paper investigated the effect of sodium sulfate extraction on creating or exacerbating wind erosion through a collection of sediment samples taken at the sodium sulfate extraction site and their grain-size testing.

The sodium sulfate extraction sites were identified based on local information and interpretation of satellite images. The areawas located in the southwest of Eyvankey City between covered pediments and internetworks of playa.It occupied an area of approximately 5000 ha. The sampling points were identified based on geological and geomorphological studies.SThe sampling wascarried out at the summer season.  4Four sites were considered for sampling;two sites asthe control sites and two sites forsodium sulfate extraction. In the control sites, only one sample was taken from the topsoil (natural land) without manipulation and extraction, whilethe samples in the other two sites were taken from 3horizons: a) soil samples fromthe degraded surfaces; b) samplesoriginated fromthe extraction horizon; and c) samples from the lower layers (without manipulation and extraction).Thus, 8 samples were totally collected.The obtained samples were granulated by the common dry-sieving method.Granulometric statistical analysis wasdone for each sample by using GRADISTAT software. 

According to the ambrothermic diagram, drought conditions prevailed in the region for about 7 months of the year. This drought couldthe aggravating wind erosionparameters, such as soil moisture and vegetation cover. The warm period corresponded to the warm seasons (spring and summer).Anemometer measurements showed that the study area was affected by erosive and strong winds blowing from the north, northwest, and east.Land cover studies revealedthat more than 60% of the soil surface in the control samples was preserved by the crust with particles larger than 2000 microns. However, after crust destruction for sodium sulfateextraction, the effect of crust cover was less than 45%. In other words, the soil surface lost 25% resistance to wind erosion.In the process of sodium sulfate extraction, the soil under the crust, which contained soil particles, along with a significant amount of powdered sodium sulfate particles, was exposed to wind erosion.Our studyshowed that the frequency percentage of vulnerable particles changedfrom about 10% in the surface layer in the control samples to about 50% in the middle and lower layers of the extracted areas. This meant that the region was about 5 times more sensitive to wind erosion.Studies on the statistical parameters of the samples demonstratedthat the average particle diameters significantly and regularly changed from very coarse sands (surface layers of the control samples or natural lands) to coarse sands (degraded surface layers),fine sands (middle layers), and finally very fine sands (bottom layers), which indicatedincreasedsensitivity to the wind erosion process from the surface layer (crust) to the bottom layer. 

In this research, field observations, mechanical analysis of soil particle granulation, and investigation of wind characteristics showed that a very high potential ofdust emission from degraded crusts triggered by open extraction. Spatial changes and displacement of removal areas caused bysodium sulfate reduction duringthe extraction periodled tofurther environmental destruction and wind erosion intensification. The results revealeda significant complexity in the relationships ofthe flux of dust emitted from thecrust degraded by sodium sulfate extraction and natural surface crust withthe threshold wind speed required for wind erosion, which suggests furtherresearch to be conducted in this regard in the future. Keywords:sodium sulphate, wind erosion, evaporative crust, granulometry, soil conservation .