上海典型NAPL污染场地补水-抽提与降水-抽提效率对比

Remediation of typical NAPL contaminated sites in Shanghai using rechargeextractionand dewatering- extraction: A comparison

上海是典型软土低渗透场地,一旦被非水相液体(NAPL)污染,修复难度极大,若采用典型的抽提原位修复技术,是否降水是工程实践中权衡的难点,因此可分为补水-抽提和降水-抽提两种方式。针对上海地区分层土的特点,本研究利用COMSOL软件建立了补水-抽提与降水-抽提三维数值模型,对比两种技术在低渗透NAPL污染场地的原位修复效率,前者主要考虑NAPL-水相传质和溶质运移,后者主要考虑水相饱和度变化、气相组分迁移和三相非平衡传质。结果表明,低渗透NAPL污染地层中水/气相对流速度缓慢,水相饱和度降低时间较长且NAPL-气传质滞后,导致降水-抽提工况的污染物去除率最低,但是在快速降水的基础上气相抽提效果会高于补水-抽提。此外,若污染地层的渗透系数增大一个量级,快速降水-气相抽提的污染物去除率可以从5.8%增加到54.0%,且比补水-抽提工况的高24.8%,因此在低渗透NAPL污染地层中引入压裂增渗技术可得到更好的修复效果,为缩短工期还可基于污染羽流的精准刻画,在局部高污染区域耦合热强化、表面活性剂增溶等技术。

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 lowpermeability 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.