Tan Bo, Liu Chao, Tan Xiao, You Xueji, Dai Chaomeng, Liu Shuguang, Li Jun, Li Naiwen
State Key Laboratory of Hydraulics and Mountain River Engineering, College of Water Resource & Hydropower, Sichuan University, 610065 Chengdu, Sichuan, China.
Department of Hydraulic Engineering, College of Civil Engineering, Tongji University, 1239 Siping Road, Shanghai, 200092, China.
Water Res. 2022 Jun 15;217:118370. doi: 10.1016/j.watres.2022.118370. Epub 2022 Mar 26.
Co-transport of colloidal substances and pollutants is a pivotal link that significantly affects the environment of coastal groundwater. The effect of colloid mobilization and aquifer pore structure change on heavy metal transport driven by seawater-freshwater interface dynamics is not fully understood. In this study, packed column experiments were conducted to model the seawater intrusion (SWI) and freshwater replenishment (FWR) processes using a sampled medium from a coastal sandy aquifer. Hydrodynamic, hydrochemical variables, and heavy metal (Pb, Cu, Cd) transport during the propagation of the seawater-freshwater interface were tested and analyzed. During the SWI stage, cation exchange induced heavy metal liberations, and it developed peak concentrations synchronized with the seawater-freshwater interface at the pore volume of 1.00. The colloid-facilitated transport for heavy metals was the predominant mechanism in the FWR stage, characterized by a peak release lagging the interface propagation by approximately 0.5 pore volumes. Because the colloidal fraction was mobilized during aquifer desalination, it lagged behind the decline of the salinity gradient. Furthermore, Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations explained that the replenishment decreased the depth of the secondary energy minimum of the colloids; meanwhile, the thickness of the electrical double layer increased from 0.63 nm to 10.14 nm, resulting in a repulsive energy barrier up to 3,213 kT. The transport of colloids led to a reduction in porosity from 18.16% to 2.28% of the total immobile domain. At these times, the dimension of the transported colloids evolved, showing a size-selective transport and therefore regulating the total emission fluxes of the heavy metals. These mechanisms were proposed to be incorporated in colloid filtration theory for targeting the coastal scenario.
胶体物质与污染物的共运移是显著影响滨海地下水环境的关键环节。胶体运移和含水层孔隙结构变化对由海水 - 淡水界面动态驱动的重金属运移的影响尚未完全明晰。本研究通过使用取自滨海砂质含水层的采样介质进行填充柱实验，模拟海水入侵（SWI）和淡水补给（FWR）过程。对海水 - 淡水界面传播过程中的水动力、水化学变量以及重金属（Pb、Cu、Cd）运移进行了测试与分析。在SWI阶段，阳离子交换引发重金属释放，并在孔隙体积为1.00时形成与海水 - 淡水界面同步的峰值浓度。在FWR阶段，胶体促进的重金属运移是主要机制，其特征是峰值释放滞后于界面传播约0.5个孔隙体积。由于在含水层脱盐过程中胶体部分被 mobilized，它滞后于盐度梯度的下降。此外，Derjaguin - Landau - Verwey - Overbeek（DLVO）计算表明，补给降低了胶体二次能量最小值的深度；同时，双电层厚度从0.63 nm增加到10.14 nm，导致排斥能垒高达3213 kT。胶体的运移导致总固定域孔隙度从18.16%降至至2.28%。在这些时刻，运移胶体的尺寸发生演变，呈现出尺寸选择性运移，从而调节重金属的总排放通量。这些机制被提议纳入针对滨海场景的胶体过滤理论中。 （注：原文中“mobilized”一词在中文语境下较难准确对应，这里保留英文未译，因为直接翻译可能影响整体语句通顺度，需结合上下文理解其含义，大致是指胶体部分在含水层脱盐过程中有某种动态变化的意思。）
