氯代烃污染场地原位热脱附降温阶段土壤气相污染富集与分布特征

原位热脱附是近年来我国兴起和大规模应用的修复技术，为明确修复中不同介质污染物浓度水平，解决原位热脱附修复后全面效果评估问题，以某氯代烃污染场地原位热脱附修复工程为案例，采集修复加热周期结束进入降温阶段时土壤和土壤气体剖面样品进行检测分析，识别修复后期土壤和土壤气中目标污染物的浓度水平、空间分布特征以及影响因素，并提出对于原位热解吸修复后土壤修复效果评估和二次污染防控建议. 结果表明:①案例场地土壤中氯代挥发性有机污染物仅1%痕量检出，所有样品均达到修复目标值. ②土壤气中污染物有不同浓度检出，其中三氯乙烯最大浓度为2 310 μg/m3，有潜在健康风险. ③低渗透层对气相污染物迁移具有阻滞作用，地表的水泥层下积聚了不同浓度的污染物. ④三氯乙烯、四氯乙烯和顺式-1,2-二氯乙烯的沸点低，土壤有机碳分配系数(K_OC)低，垂向迁移效率高，它们在土壤气中的浓度最大值均出现在顶层；六氯丁二烯相对沸点高，K_OC高，其浓度最大值出现在深层粉质黏土低渗透地层处. 研究显示，原位热脱附技术对于土壤中高浓度氯代烃污染具有较好的去除效果，但是研究时段内土壤达到修复标准后土壤气中污染物仍有不同程度检出. 因此，建议原位热脱附修复后，应同时对土壤和土壤气两种介质进行采样评估，同时加强原位热脱附区域低渗透层识别，优化气相抽提方案. 

Enrichment and Distribution Characteristics of Soil Gas-Phase Contamination in Cooling Stage of In-situ Thermal Desorption at Chlorinated Hydrocarbon Contaminated Site

In-situ thermal desorption (ISTD) is a remediation technology that has emerged and been applied on a large scale in China in recent years. In order to clarify the concentration levels of pollutants in different media during remediation and to carry out verification scientifically after remediation, the ISTD remediation project of a chlorinated hydrocarbon contaminated site was used as a case study. At the end of the remediation heating cycle into the cooling stage, soil and soil gas profile samples were collected for testing and analysis to finely identify the concentration levels and spatial distribution characteristics of pollutants in soil and soil gas. The effects of pollutant physicochemical properties, stratigraphic structure, especially the low permeability layer on gas migration and pollution distribution were analyzed. In view of this, the verification of soil remediation and the secondary pollution prevention and control after ISTD were proposed. The results showed that: (1) Only 1% of soil samples were detected of chlorinated volatile organic pollutants in trace amounts in this case, and all samples met the remediation target values. (2) Pollutants in soil gas were detected at different concentrations, among which trichloroethylene was detected at the highest concentration of 2310 μg/m3, which has a potential health risk. (3) The low permeable layer has a blocking effect on the migration of gas-phase pollutants, and high concentrations of pollutants accumulated under the surface cement layer. (4) Trichloroethylene, tetrachloroethylene and cis-1,2-dichloroethylene have low boiling points, low soil organic carbon partition coefficients (K_OC), high vertical migration efficiency, and the highest gas-phase pollutant concentrations in the top layer. Hexachlorobutadiene has a high relative boiling point and high K_OC, and the maximum value occurs in the deep low-permeability stratum. The study showed that the ISTD technology has a good removal effect for high concentration of chlorinated hydrocarbon pollution, but the contaminants were still detected in soil gas after the soil reached the standard in this case study. Therefore, it is recommended that both multi-media verification of soil and soil gas should be conducted after ISTD remediation for a comprehensive assessment. The identification of low permeability layers in the ISTD area should be enhanced to optimize the gas phase extraction scheme.