蒸气入侵暴露情景下土壤气筛选值推导与比较

采用J&E模型推导了典型蒸气入侵暴露情形下土壤气中ρ(苯)、ρ(甲苯)、ρ(氯仿)及ρ(1,1-二氯乙烯)的筛选值,并与US EPA(美国国家环境保护局)及美国各州的颁布值进行比较. 结果表明,具有致癌效应的苯、氯仿相同暴露情形下的筛选值低于非致癌效应的甲苯、1,1-二氯乙烯3~4个数量级,表明VOCs污染场地应重点关注致癌性污染物. 其中,浅层土壤气居住暴露情形下ρ(苯)、ρ(甲苯)、ρ(氯仿)及ρ(1,1-二氯乙烯)的筛选值分别为9.6×102、2.7×102、1.1×107、4.0×105μg/m3,工商业暴露情形下分别为4.6×103、1.3×103、6.3×107、2.4×106μg/m3. 深层土壤气居住暴露情形下ρ(苯)、ρ(甲苯)、ρ(氯仿)及ρ(1,1-二氯乙烯)的筛选值分别为1.1×103、3.1×102、1.2×107、4.5×105μg/m3,工商业暴露情形下分别为5.2×103、1.5×103、7.1×107、2.7×106μg/m3. 筛选值大小的决定因素包括污染物的室内允许浓度、土壤气衰减系数及建筑物参数. 浅层与深层土壤气中各污染物筛选值无明显差异,但与US EPA及美国各州的颁布值差异较大,这主要是由污染物室内允许浓度及衰减系数确定方法的不同所致. 浅层土壤气平均衰减系数为2.3×10-4,与深层土壤气平均衰减系数(2.0×10-4)无明显差异,但均低于US EPA对应经验值〔0.1(浅层)、0.01(深层)〕2~3个数量级. 在不考虑吸附及生物降解时,污染源上方清洁土壤对污染物的衰减作用不明显. 

Derivation of Soil Gas Screening Value for Vapor Intrusion Scenario

Soil gas screening values for benzene, toluene, chloroform and 1,1-dichloroethylene were derived by the J&E model according to typical vapor intrusion scenarios, and compared with values issued by US EPA and its member states. The results revealed that the screening values of carcinogenic benzene and chloroform are three to four orders of magnitude lower than the values of non-carcinogenic toluene and 1,1-dichloroethylene. This result indicated that more attention should be paid to carcinogenic contaminants in VOCs sites. For the shallow soil gas, the screening values for benzene, chloroform, toluene and 1,1-dichloroethylene were 9.6×102,2.7×102, 1.1×107and 4.0×105μg/m3under the residential scenario, while were 5.2×103,1.5×103,7.1×107and 2.7×106μg/m3for the corresponding pollutant under the industrial/commercial scenario. For the deep soil gas, the corresponding values were 1.1×103,3.1×102,1.2×107and 4.5×105μg/m3according to residential scenario, while were 4.6×103,1.3×103,6.3×107and 2.4×106μg/m3based on the industrial/commercial scenario. The maximum allowable indoor air concentrations, soil gas attenuation factors and building parameters, are the main factors in determining the values. Difference between screening values of shallow and deep soil gas is negligible. However, it is significant between the values issued by US EPA and its member states. For shallow soil gas, the difference for benzene, chloroform, toluene and 1,1-dichloroethylene ranges between 3-309,0.5-245.0,4-2,5and 27-96,0times, respectively. In addition, for the deep soil gas, the difference ranges between 0.7-35.0,3-28,4.6-323.0and 4.5-10,0.0times, respectively. Different methods applied to derivate the maximum allowable indoor air concentrations and attenuation factors were the main reason contributing to this difference. The corresponding average attenuation factors for the shallow and deep soil gas were 2.3×10-4 and 2.0×10-4, indicating the difference between them is not significant, but both were lower than the corresponding empiric values, 0.1for the shallow soil gas and 0.01for the deep soil gas, applied by US EPA by two-three orders of magnitude. The attenuation effect contributed by clean soil laid above contaminate source is not significant if adsorption and biodegradation were not considered.