因长期累积效果,盐碱地排水沟内盐分含量不断升高,危及排水沟系统的生态功能,利用淡水定期进行稀释可延缓盐分累积过程。该文以陕西富平县卤泊滩盐碱地改良区为例,通过实验室土柱试验,研究了排水沟蓄集淡水、水体保持静态条件下,底泥与上覆水体的盐分交换规律,揭示了底泥盐分释放的内在机理。研究结果表明,在分子扩散作用下底泥-上覆水界面之上10 cm 的范围内存在一个高盐分渐变区(即扩散边界层),10 cm 以上的水体盐分基本一致；分子扩散是边界层内盐分运移的主要机制；扩散边界层对底泥盐分释放具有一定的阻滞作用,并且随着边界层厚度的增加,阻滞效应也显著增强；底泥盐分释放通量符合负的幂指数形式。%In arid and semi-arid agricultural regions, artificial drainage is provided to maintain salt balance of the saline farmland. As a result of poor drainage outlet in some sites, salt accumulation in the drainage ditches often leads to salinity rising to a critical level that threatens ecological functions of the ditch system. Periodic flushing with fresh water has been suggested to slow down the process of salinity growth in such ditch system. But the effect of the slow moving freshwater in drainage ditches on salt balance is unclear. In order to investigate the internal mechanism of salt release in saline drainage ditches when freshwater is added, and to examine the relationship of salt exchange between sediment and the overlying water under hydrostatic condition, we conducted a laboratory study to measure spatial and temporal variations of salinity in sediment and water interface with two experimental plexiglass columns. Each column is 15 cm in (inner) diameter and 100 cm high. The column has a top cover to prevent the evaporation loss of water. Sediments were filled to 30 cm thick at the bottom and covered by a freshwater layer of 65 cm. Room temperature was kept at (15±1)℃ during the experiment. The sediment salinity was measured by extracting the pore water through four sampling holes on each column wall. Electrical conductivity of the sediment pore water and the overlying water were measured every 24 hours during the monitoring period. The sediments were taken from a saline agricultural drainage ditches in Lubotan reclamation area in Shaanxi, China, where salinity elevation in the drainage ditches presents a problem to the ecological functions of the ditch system, and freshwater is available for salt flushing in the end of the irrigation season. We measured salinity variations in water and sediments in the experimental columns continuously for a period of 648 hours, then calculated salt fluxes in the water and sediment interface based on measured sediment properties. The monitoring data showed that there existed a linear salinity variation zone (or the diffusion boundary layer) within 10 cm of the sediment and water interface, salinity of water remained stable above 10 cm. The results also showed that salt diffusion within the boundary layer slowed the salt release from the sediment, and the slowing effect increased with the thickness of the boundary layer. Flux calculation results showed that salt fluxes released from the sediment can be predicted with a power function, i.e., salt fluxes decreased rapidly initially and then stabilized with time. Observed salinity change in sediment indicates that salt release through molecular diffusion only in drainage ditches is a slow process, which may have only limited impact on salt balance in drainage ditches when the flushing water moves slowly. And hydrodynamic dispersion is required to speed up release of salt from the ditch sediment in order to remove accumulated salts in the drainage ditches in saline environment.
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