饱和黏性土中重质非水相有机污染物纵向迁移数值模拟

NumericalAnalysisofVertical Migration of Dense Nonaqueous-phase Liquids in Saturated Clay

基于COMSOL Multiphysics自动有限元软件开发了考虑两相流和对流弥散作用的重质非水相有机污染物(DNAPL)迁移分析模块。以氯苯为例,选取典型参数值,进行了均质饱和黏性土中DNAPL一维纵向迁移的数值分析。单一的对流弥散分析结果表明,在静水条件下,以分子扩散作用为主的溶解态的DNAPL的纵向迁移速度较为缓慢(每年约几厘米)。DNAPL-水两相流和对流弥散的耦合分析结果表明,对于渗透系数k达到1×10^-6cm·s^-1的饱和粉质黏土,受自重驱动的两相流所造成的自由相DNAPL的纵向迁移平均速率达到每年几十厘米;自由相DNAPL的溶解进一步导致土中溶解相DNAPL的浓度迅速增大到溶解度,直接影响了孔隙水中溶解相DNAPL的浓度。研究表明,两相流很可能是造成生产时间达几十年的旧农药厂的饱和黏性土地层深部出现DNAPL污染物的主要机理。

An analytical module based on the COMSOL Multiphysics software was developed to consider the effects of two-phase flow and convective dispersion.Using chlorobenzen as the material and choosing typical parameter values, the numerical analysis of vertical migration of DNAPL in homogeneous saturated clay was conducted. The convective diffusion analysis shows that the migration speed of the dissolved DNAPL by molecular diffusion is very slow,only about several centimeter per year. The DNAPL-water two-phase flow coupled convective diffusion analysis indicates that for saturated cohesive soil with a larger permeability coefficient(k=1×10^-6 cm·s^-1),The DNAPL migration velocity caused by the two-phase flow is large,about tens of centimeters per year. The dissolution of liquid DNAPL leads to the concentration of dissolved DNAPL in pore-liquid quickly and achieves solubility. These results suggest that two-phase flow is likely to be the main mechanism for the deep migration of DNAPL in saturated clay observed in some sites.