重金属砷在饱和土壤中迁移过程模拟

Simulation of Migration Process of Heavy Metal Arsenic in Saturated Soil

重金属污染预测和评估是土壤与地下水修复的重点与难点,土壤中重金属迁移过程模拟能够为污染防治和水资源保护提供科学指导。基于有限元方法,建立了三维地下水重金属溶质运移模型,考察南京某化工场地重金属As(V)的迁移运动规律。数值模拟通过改变渗透系数、弥散度、分配系数、孔隙率,对比分析不同参数条件下,重金属As(V)溶质在土壤中的迁移规律。引入敏感性指数作为量化指标,比较各参数对重金属砷迁移的影响。数值分析结果表明:污染晕中心砷质量浓度随污染物持续通入逐渐增加到7.12 mg/L,污染范围不断扩大,纵向迁移距离远大于横向;7年后污染物不再注入,污染范围在水流的作用下整体往下游迁移,污染晕中心浓度不断下降,第40年污染晕中心往下游迁移6.5 m,浓度已下降为0.33 mg/L。连续注入污染物10年情况下,增加渗透系数与弥散度都会导致污染晕中心As(V)浓度下降,纵向迁移距离增加;分配系数增加则出现污染晕中心浓度与纵向迁移距离都减少的情况;由于土壤对重金属As(V)的吸附较强,在参数±50%的变化中,分配系数对重金属的迁移距离影响较大,与渗透系数、弥散度的影响程度相当,敏感性指数分别为18.0%、18.0%与16.3%;孔隙率敏感性指数为0.006%,本文模型下对重金属As(V)迁移的影响几乎可忽略。

Prediction and evaluation of heavy metal pollution is a main difficulty for soil and groundwater remediation.Simulation of heavy metal migration in soil can provide a scientific guidance for pollution control and water resources protection.Based on the finite element method,a three-dimensional solute transport model was established to investigate the migration and movement of heavy metal As(V)in a chemically contaminated site in Nanjing.The migration of heavy metal As(V)was analyzed under different parameters via changing hydraulic conductivity,dispersion,distribution coefficient and porosity.The sensitivity index was introduced as a quantitative index to compare the effects of various parameters on the migration of heavy metal arsenic.Numerical analysis results show that the concentration of As(V)in the center of the pollution halo gradually increases to 7.12 mg/L.with the continuous injection of pollutants,the pollution range continues to expand,and the vertical migration distance is much larger than the horizontal migration distance.Seven years later,the injection of pollutants stops,the pollution range migrates downstream with water flow,and the concentration of the pollution halo center continues to decline.After 40 year,the pollution halo center migrates downstream by 6.5 m,and the concentration decreases to 0.33 mg/L.In the case of continuous injection of pollutants for 10 years,increasing the hydraulic conductivity and dispersion leads to the decrease of As(V)concentration in the center of pollution halo and the increase of longitudinal migration distance.With the increasement of partition coefficient,the concentration of pollution halo center and longitudinal migration distance decrease.Due to the strong adsorption of heavy metal As(V),the distribution coefficient has a great influence on the migration distance of heavy metals in the change of parameters±50%,which is equivalent to the influence of permeability coefficient and dispersion.The sensitivity indexes are 18.0%,16.3%and 18.0%respectively.In the proposed model,the porosity sensitivity index is 0.006%,and its effect on the migration of heavy metal As(V)can be ignored.