季节性冻融土壤盐分离子组成与冻结层盐分运移规律研究

该文对冻融过程中土壤盐分离子组成及冻结层盐分运移规律进行研究,在内蒙古河套灌区永联试验站开展了冻融期土壤水盐及其离子成分监测试验,分析了冻融期地温、冻结层深度、地下水埋深与水质、土壤含水率、土壤盐分及离子组成的变化规律,通过离子相关性分析确定了土壤盐分运移的主控离子成分和盐分类型,进一步利用二元水盐体系相图探讨了冻融期主控盐分的运移规律。结果表明:冻融期地温梯度变化主要发生在0～1.0 m范围土层中,地下水埋深在冻融期变化趋势为快速增大-缓慢增大-减少,地下水矿化度均值在融化期显著降低;研究区地下水中变异性最大的离子为Na~+、Cl~–和SO_4~(2–),土壤盐分运移和扩散是地下水矿化度变化的主要原因;土壤中Na~+、Cl~–与SO_4~(2–)与含盐量相关系数高于0.9,冻融期土壤盐分浓度变化的主控盐分类型为氯化钠和硫酸钠;冻结层积盐或者脱盐取决于土壤盐分梯度和不同盐分的共饱和点,研究区最大氯化钠浓度(质量分数1.55%)和最大硫酸钠浓度(2.01%)均低于各自的共饱和点,当冻结前土壤溶液浓度梯度为正(从上到下浓度增大)时,冻结层易积盐,反之冻结层主要表现为脱盐。研究对阐明冻融期冻结层盐分累积规律的成因具有重要意义。

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.