Simulation Of Chromium Pollution Migration In Soil And Groundwater Based On MODFLOW

Heavy metal pollution is one of the most severe soil and groundwater pollution problems,especially hexavalent chromium pollution,which poses a threat to the utilization of natural resources and the safety of the ecological environment.Chromium pollution’s migration and transformation processes in soil and groundwater systems are complex.Many processes such as convection,dispersion,and biogeochemical reaction interact.At present,numerical simulation has become an essential means to study the migration and remediation process of groundwater pollutants at home and abroad.The numerical simulation of chromium pollution migration studies the dynamics and trend of chromium pollution migration and transformation through model prediction,which helps analyze and study the law of chromium pollution migration and carry out pollution remediation.MODFLOW is a commonly used software in soil and groundwater modeling.Based on MODFLOW,this manuscript numerically simulates the migration and transformation of chromium pollution in a small-scale area.The simulation results analyzed the diffusion and migration law of pollutants in the study area.The leading research and results include:(1)The MODFLOW model in steady-state mode is first used to model the hydraulic head and using the parameter estimation(PEST)tool,based on the calibration method of pilot point,two pilot point schemes with different densities are designed,namely,the regular pilot point placement and the middle head measurement lower gradient(MHMDG)placement.The pilot point calibration method in the PEST model was used to obtain the head model of groundwater in the study area,and the hydraulic head simulation research was carried out;(2)Based on the data of 37 observation wells in the study area,the three-dimensional structure model of the site was established by the GOCAD SGrid.Based on the obtained initial concentration data and the parameters such as permeability coefficient and dispersion coefficient obtained by experiment,the three-dimensional attribute digital model was constructed based on Kriging interpolation;(3)The experimental simulation was conducted in the confined aquifer based on the chromium pollution dynamic model of convection,dispersion,and adsorption in MODFLOW.Based on the MT3 DMS numerical simulation program,the migration process of hexavalent chromium pollution was simulated by considering the calibrated groundwater flow model in the small-scale area.The results show that the experimental area shows apparent heterogeneity in horizontal hydraulic conductivity.MHMDG pilot point case with the more significant density gave the best R-squared 0.901 and minimum residual error of 0.0053 m compared to 0.880 and 0.078 m,respectively,for the regular placement.The calibration accuracy depended on the pilot point’s frequency and emplacement.The simulation of pollution migration shows that the location of the pollution source is the critical point influencing the diffusion.Secondly,the migration of pollutants in unsaturated soil depends on the diffusion coefficient and permeability coefficient,and the porosity of the crossing layer.In addition,the convection of pollutants is consistent with the velocity direction of groundwater in the saturation zone,and the silty clay layer works as a retaining layer for the contaminants,which makes the convection of pollutants slow.The research in this thesis will contribute to the test of remediation schemes in the experimental area and the monitoring of pollutant distribution.Based on the migration law and distribution characteristics of hexavalent chromium in soil and groundwater,the hydrogeological parameters of surrounding groundwater pollution are experimentally obtained to support future research on the remediation of chromium pollution in soil and groundwater.