Numerical Simulation of Chlorobenzene Multiphase Migration in a Coastal Aquifer
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摘要: 为揭示海水入侵所驱动的变密度流场对重质非水相液体（dense non-aqueous phase liquids，DNAPL）多相运移路径与相间质量交换的调控机制，本研究以氯苯为例构建变密度流-多相流-溶质运移耦合数值模型，系统模拟其入渗、再分布及溶解羽演化过程.研究结果表明：（1）NAPL相氯苯在重力主导下垂直入渗并在隔水底板形成非对称污染池；（2）咸水楔显著改变溶解相氯苯的运移路径，密度梯度驱动其沿咸淡水界面爬升，并在咸淡水过渡带积聚，且排海通量峰值较无海水入侵情景降低62%；（3）渗透系数K的增大加速NAPL相和溶解相氯苯向海洋边界迁移，同时强化溶解相在咸淡水过渡带的累积.本研究阐明了咸淡水过渡带对DNAPL迁移的截留机制，证实该区域是滨海地下水环境风险评估中不可忽视的长期次生污染源.Abstract: To reveal the regulatory mechanism of the variable-density flow field induced by seawater intrusion on the multiphase migration pathways and interphase mass exchange of dense non-aqueous phase liquids (DNAPL), this study developed a coupled variable-density flow, multiphase flow, and solute transport numerical model using chlorobenzene as an example, systematically simulating its infiltration, redistribution, and dissolved plume evolution. The results indicate follows: (1) The NAPL-phase chlorobenzene vertically infiltrates under gravity-dominated conditions, forming an asymmetric contaminant pool above the aquitard. (2) The saltwater wedge significantly alters the migration pathway of dissolved-phase chlorobenzene, with density gradients driving its upward movement along the freshwater-saltwater interface and accumulation in the mixing zone, while the peak discharge flux to the sea decreased by 62% compared to the scenario without seawater intrusion. (3) An increase in hydraulic conductivity (K) accelerates the migration of both NAPL-phase and dissolved-phase chlorobenzene toward the marine boundary, while enhancing the accumulation of the dissolved phase within the mixing zone. This study elucidates the retention mechanism of the mixing zone for DNAPL migration and confirms that this zone represents a non-negligible long-term secondary pollution source in coastal groundwater environmental risk assessments.
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图 2 泄漏阶段NAPL相氯苯饱和度时空演化
a. 泄漏阶段NAPL相氯苯饱和度Sn随时间变化分布（红线、绿线、白线分别表示90%、50%、10%等盐线）; b. 泄漏点垂向剖面X=300 m处Sn分布; c. 隔水底板界面Z=0 m处Sn分布
Fig. 2. Spatiotemporal evolution of NAPL-phase chlorobenzene saturation during the leakage stage
图 3 再分布阶段NAPL相氯苯饱和度时空演化
a. 再分布阶段NAPL相氯苯饱和度随时间变化分布（红线、绿线、白线分别表示90%、50%、10%等盐线）; b. 泄漏点垂向剖面X=300 m处Sn分布; c. 隔水底板界面Z=0 m处Sn分布
Fig. 3. Spatiotemporal evolution of chlorobenzene NAPL saturation during the redistribution stage
图 4 溶解相氯苯浓度时空演化
箭头表示地下水流向及相对流速; a. 泄漏阶段溶解相氯苯浓度C2分布随时间变化; b. 再分布阶段溶解相氯苯浓度C2分布随时间变化
Fig. 4. Spatiotemporal evolution of dissolved chlorobenzene concentration
图 5 溶解相氯苯运移归趋的关键指标时空演化
a. 有/无海水入侵条件下污染羽质心横坐标随时间变化图; b. 有/无海水入侵条件下污染羽质心纵坐标随时间变化图; c. 溶解氯苯空间质量分配随时间变化图; d. 有/无海水入侵条件下溶解氯苯边界通量随时间变化图
Fig. 5. Spatiotemporal evolution of key indicators for dissolved chlorobenzene transport and fate
图 6 渗透系数(K)对NAPL相氯苯迁移的影响
a. 再分布末期隔水底板Z=0 m处Sn分布; b. DNAPL污染池前缘位置随时间变化
Fig. 6. Effect of hydraulic conductivity (K) on the migration of NAPL-phase chlorobenzene
图 7 渗透系数(K)对溶解相氯苯迁移的影响
a. 边界通量; b. 系统内总质量; c. 再分布末期空间质量分配; d. 再分布末期咸水楔内累积质量及占比
Fig. 7. Effect of hydraulic conductivity (K) on the migration of dissolved chlorobenzene
表 1 模型参数
Table 1. Parameter values
参数 值 单位 含水层特性 孔隙度φ 0.4 - 渗透系数K 1，2，3，4，5 m·d-1 纵向弥散度αL 1 m 横向弥散度αT 0.1 m Brooks-Corey模型入口毛细压力Pd 30 kPa Brooks-Corey模型孔隙大小分布指数λ 2 - 流体特性 淡水密度ρf 1 000 kg·m-3 海水密度ρs 1 025 kg·m-3 DNAPL(氯苯)密度ρn 1 100 kg·m-3 水相动力黏度μw 0.001 Pa·s DNAPL(氯苯)动力黏度μn 0.000 8 Pa·s 水相残余饱和度Sr, w 0.05 - NAPL相残余饱和度Sr, n 0.1 - 溶质与反应参数 氯苯溶解度S 0.45 kg·m-3 氯苯的分子扩散系数DF 8.8e-10 m2·s-1 氯苯溶解传质速率系数k 2.9e-8 s-1 注：加粗表示基准案例参数. 
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