Migration mechanisms and pump-and-treat remediation simulation for benzene and halogenated hydrocarbons in groundwater at a pesticide-contaminated site

With advancing urbanization in China， legacy contaminated sites resulting from discontinued or relocated chemical plants are posing increasingly severe environmental risks to groundwater. This contaminated groundwater presents a substantial threat to human health. In this study， a pesticide-contaminated site was investigated using the Groundwater Modeling System （GMS） software to construct a three-dimensional geological structure model， a groundwater flow model， and a solute transport model. These models were used to simulate the migration mechanisms of three representative contaminants （benzene， chloroform， and vinyl chloride） in aquifers and to evaluate the remediation performance of pump-and-treat technology. The results showed that during the initial contaminant transport phase （within 100 d）， contaminants spread uniformly from leakage sources. Subsequently， between 500 and 2000 days， diffusion evolved into elliptical patterns along the southeast-northwest groundwater flow direction， with maximum migration distances increasing to 154.5 m （benzene）， 130.5 m （chloroform）， and 165.2 m （vinyl chloride）， and diffusion areas reaching 18117.4 m²， 13928.5 m²， and 25043.2 m²， respectively. The peak concentrations decreased over time and distance， with benzene decreasing from 53.3 mg/L to 13.0 mg/L （a 75.6% reduction）， chloroform dropping from 112.0 mg/L to 20.0 mg/L （an 82.1% reduction）， and vinyl chloride declining from 128.0 mg/L to 24.0 mg/L （an 81.3% reduction）. For the pump-and-treat remediation approach， the dynamic pumping scheme was simulated. At a simulation time of 400 days， the contaminant plume areas of benzene， chloroform， and vinyl chloride were reduced by 85.8%， 78.0%， and 85.9%， respectively， while their peak concentrations decreased by approximately 90.2%， 82.1%， and 93.8%. Within the 500-day remediation period， all contaminant concentrations dropped below remediation target values. The total pumping volumes were 94500 m³， 60000 m³， and 72000 m³， respectively. Combining pump-and-treat with Fenton reagent （3% dosage） effectively treated the extracted groundwater. This study demonstrates that numerical simulation optimizes contamination delineation and pumping strategies， offering a cost-efficient technical pathway for similar sites.