Soil ion components and soil salts transport in frozen layer in seasonal freezing-thawing areas

Abstract: To understand the salt movement mechanism in the freezing-thawing soil and the salt transport in the frozen layer, an experiment is carried out in Yonglian experiment station, Hetao Irrigation District, China. The soil temperature, frozen layer depth, water table depth, groundwater salinity and the ion components, soil water content and soil salinity and its ion components during the freezing-thawing period were observed and measured. The data were analyzed to demonstrate the soil salt transport variations in the soil and groundwater as well as its impact factors. The correlation analysis was implemented to calculate the relationship between the soil content and the ion components and the relationship among those ions, and then to obtain the major ions and major salt composition of the soil soluble salts to control soil salt change. The transport mechanism of soil soluble salts in the freezing-thawing soil was discussed by using the binary phase diagram of water-salt system. The experiment results show that the soil temperature change within the depth of 0-1.0m. There are three stages during the freezing-thawing period in the study area as quickly freezing period (from December 6th, 2017 to February 4th, 2018), slowly freezing period (from February 4th, 2018 to March 12th, 2018) and thawing period (from March 12th, 2018 to April, 13th, 2018). The water table depth showed the change as quick increase-slow increase-decrease in the 3 stages, while the average groundwater salinity significantly decreased. Na+, Cl- and SO42- had the greatest variability among all the 8 ions both in the groundwater. These demonstrated that the ions change in the groundwater was caused by the advection of ions in the soil solute. The correlation coefficients of Na+, Cl-, SO42- with soil salt content were larger than 0.9, and the correlation coefficients among Na+, Cl-, SO42- were larger than 0.9, which showed the major ion components of soil salt were Na+, Cl- and SO42-, and the major soil soluble salts were sodium sulfate and sodium chloride. The soil salt transport in the frozen layer depended on the initial soil salt gradient and co-saturated points of sodium sulfate and sodium chloride. The max mass fractions of sodium sulfate and sodium chloride were 2.01% and 1.55%, smaller than their co-saturated points. In this case, the salt would accumulate in the frozen layer when the initial soil salt gradient was positive (the salt content increase along depth). Otherwise, the soil salt would decrease in the frozen layer. This work is significant important to illustrate the relationship between the ion components with the soil salt change and the salt accumulation mechanism in the frozen layer.