Distribution and migration characteristics of salinity in ice-water-sediment in Lake Ulansuhai

Abstract

盐分是参与湖泊物质循环的重要成分之一，湖泊盐度增加对湖泊生态系统健康造成了严重的威胁.乌梁素海总溶解性固体（TDS）和盐度均处于较高的水平，为揭示盐分在冰-水-沉积物中的分布及迁移规律，冰封期在乌梁素海7个采样点采集冰、冰下水和不同深度沉积物样品，分析样品的TDS、Na⁺和Cl^-浓度，得到各自在冰-水间浓度的比值，即分配系数K，并对水-沉积物界面Na⁺和Cl^-的扩散通量进行估算.结果显示，TDS、Na⁺和Cl^-在冰-水中分配系数K的均值分别为0.02、0.03和0.01，表明在湖水结冰形成冰盖的过程中，随着冰晶的析出，TDS、Na⁺和Cl^-逐渐在水体中浓缩，水体中Na⁺和Cl^-在浓度梯度驱动力作用下，向沉积物间隙水中扩散，估算其扩散通量均值分别为-229和-676 mg/（m²·d）.总之，湖水在冻结过程中，由于冰晶的析出，盐分向冰下水体中迁移，使得盐分浓度在冰下水体中浓缩增加，继而向沉积物中迁移，对湖泊水生态环境构成胁迫.;Salinity is an important substances involved in the material cycle of lakes. Salinity increase in lakes poses a serious threat to the health of lake ecosystems. Lake Ulansuhai is one of the eight largest freshwater lakes in China, the largest lake in the Yellow River Basin, and also a typical shallow grassland lake in the cold and arid zone in Inner Mongolia. The lake water usually starts freezing in the early November and thawing in the end of March or in the early April of the following year. Total dissolved solids(TDS)and salinity in the lake have been increasing gradually and are at a high level at present. In order to reveal the distribution and migration characteristics of the TDS and salinity in the ice-water-sediment interface, samples of the ice, water, and sediments at different depths were collected at seven sampling sites during the ice-sealing period. The concentrations of TDS, Na⁺ and Cl^- were analyzed. The distribution coefficient (K) of TDS, Na⁺ and Cl^- between the ice and water were analyzed. The diffusion fluxes of Na⁺ and Cl^- between the water-sediment interfaces were estimated. The results showed that the mean K of TDS, Na⁺, and Cl^- were 0.02, 0.03 and 0.01, respectively. This indicated that the salinity was gradually concentrated in the water body during the process of ice crystal precipitation. As a result, the salinity would be further transported into sediment due to the concentration gradient between the water-sediment interfaces. The estimated mean diffusion fluxes of Na⁺ and Cl^- between the water-sediment interfaces were -229 mg/(m²·d) and -676 mg/(m²·d), respectively. In short, during the lake's freezing process, the salinity migrated to the liquid phase due to the precipitation of ice crystals. The concentration of salinity in the water body increased gradually and then migrated into sediment. This would constitute a stress on the lake's ecological environment.