扩散边界层对沉积物盐分释放的阻滞影响
Diffusive boundary layer and its resistance on salt release from deposited sediments in a polder reservoir
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摘要: 扩散边界层是水环境沉积物与上覆水之间物质交换的必经区。通过无风和有风条件下两组沉积物盐分释放实验,研究海湾水库砂质沉积物-水界面扩散边界层的形成情况,并通过水动力扰动下穿过扩散边界层的实测盐分通量与Fick第一定律理论计算通量的比较,以及扰动与无扰动时通量的比较,探讨扩散边界层对沉积物盐分释放的影响。研究结果表明,在稳定风场条件下,沉积物-水界面处存在厚度约3.5 cm的扩散边界层,有风条件下的界面盐分交换通量略大于无风时的通量;分子扩散是边界层内盐分运移的主要机制,扩散边界层对沉积物盐分释放具有阻滞作用,并且边界层越厚,阻滞效应越显著。另外,由于扩散边界层的存在,限制了水动力对沉积物与上覆水之间物质交换的增强作用,所以海湾水库沉积物中蓄积盐分对上覆水的影响将是长期的过程。Abstract: The diffusive boundary layer (DBL) is a zone for exchanging matter across sediment-water interface between the mixture of solid sediment and interstitial water from an overlying body of water.The exchange of substances between deposited sediments and overlying water must also pass through the DBL.In this study,two series of flume experiments with and without wind blowing conditions were conducted to study the salt release process from sandy sediments in a polder reservoir.The thickness of DBL was directly measured.The experimental salt fluxes through the DBL under the two wind conditions were inter-compared and compared with the estimations using the Fick's first law in an effort to explore the role of DBL in contact inhibition of salt release.Results indicate that the observed salt fluxes under the two wind conditions show a marginal difference.Under the steady wind condition,a DBL with a constant thickness of 3.5 cm is observed right above the deposited sediment.The molecular diffusion is found to be the dominant mechanism for salt transport within the DBL.The latter can suppress the hydrodynamic enhancement on the exchange of substances between deposited sediments and overlying water.The thicker the DBL is,the stronger the inhibition will be.Thus,the presence of DBL in a polder reservoir can result in a delay of natural clearing process of polluted water.The effect of salt release from deposited sediments on the water quality will be a long lasting process.