ABSTRACT

The clay soil in the middle and lower reaches of the Yangtze River in China is widely distributed and the groundwater table is shallow. In order to accurately analyze the distribution and assess the potentially harmful effects of the pollutants in this region, it’s necessary to study the diffusion rate and subsequent concentrations of organic pollutants. Our study area, located in Kunshan in the Jiangsu Province, represents a typical contaminated site consisting of a silty clay layer located in a shallow water table. Our model for the horizontal diffusion, vertical diffusion, and concentration distribution of organic pollutants through this clay layer over 30 years. The simulation results were compared to the measured horizontal distribution of the pollutant concentration at a depth of 2 m and the electrical resistivity tomography data reflecting the longitudinal changes in the polluted area. Our model generated accurate pollutant concentration and diffusion depth results, but the modelled horizontal diffusion distance of the pollution plume did not agree with observational data. After increasing the soil permeability coefficient at a depth of 2 m by a factor of 20,000, the numerical simulation results more closely match those of the measured results. Based on model results, we propose that the seasonal fluctuation of the groundwater table controls the vertical diffusion distance, and that the wide horizontal pollutant diffusion distance is a consequence of the sharp increase in the cohesive soil permeability (caused by repeated water gain and loss).

摘要

我国长江中下游黏土分布广泛，地下水位浅。为了准确分析该地区污染物的分布情况和评估其潜在的危害影响，有必要研究有机污染物的扩散速率和后续浓度。我们的研究区位于江苏省昆山市，是一个典型的污染场地，由位于浅水位的粉质粘土层组成。30 年来有机污染物通过该粘土层的水平扩散、垂直扩散和浓度分布模型。将模拟结果与2 m深度处测得的污染物浓度水平分布和反映污染区纵向变化的电阻率层析成像数据进行了比较。我们的模型生成了准确的污染物浓度和扩散深度结果，但模拟的污染羽流水平扩散距离与观测数据不一致。将2 m深度的土壤渗透系数增加20,000倍后，数值模拟结果与实测结果更加吻合。基于模型结果，我们提出地下水位的季节性波动控制垂直扩散距离，