Using MODFLOW/MT3DMS and electrical resistivity tomography to characterize organic pollutant migration in clay soil layer with a shallow water table

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).