Remediation of typical NAPL contaminated sites in Shanghai using recharge-extraction and dewatering-extraction:A comparison
Shanghai is characterized by low-permeability soft soil that cannot be easily remediated once the soil is contaminated by non-aqueous phase liquid(NAPL).If the typical in-situ extraction remediation technology is adopted,deciding whether to dewater or not is challenging in engineering practice.Therefore,there are two options:recharge-extraction and dewatering-extraction.Aiming at the characteristics of layered substrata in Shanghai,this study established three-dimensional recharge-extraction and dewatering-extraction numerical models using COMSOL software,and compared the in-situ remediation efficiencies of the two technologies in a low-permeability NAPL contaminated site.The former mainly considered NAPL-water phase mass transfer and solute transport,while the latter mainly considered the water phase saturation change,gas phase component migration and three-phase non-equilibrium mass transfer.The results showed that the dewatering-extraction scenario had the lowest removal rate of contaminant due to the low relative water/gas flow in the low-permeability contaminated site,slow reduction of water saturation and hysteresis of NAPL-gas phase mass transfer.However,if the water can be rapidly pumped,the soil vapour extraction was more efficient than the recharge-extraction method.In addition,when the permeability coefficient of the contaminated site increased by one order of magnitude,the removal rate of rapid dewatering-soil vapour extraction increased from 5.8％to 54.0％,which was 24.8％higher than that of the recharge-extraction scenario.Therefore,the technologies of fracturing extraction can be a better choice for the low-permeability contaminant sites.To shorten the construction period,it was also possible to couple thermal enhancement and surfactant solubilization in localized areas of high contamination based on the precise delineation of the contamination plume.
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