VOCs污染场地风险管理策略的筛选及评估

以某VOCs(volatile organic compounds,挥发性有机化合物)污染场地为例,结合实地调查,将健康风险评估用于场地风险管理策略的筛选. 结果表明:①该场地不同深度土壤均受到氯仿、二氯甲烷和苯的污染,污染物垂向迁移特征明显,最大迁移深度达25.8 m,其中深度≤15.0 m的土壤污染较重. ②基于保守的通用场地概念模型对将其规划为居住用地时的健康风险进行评估显示,氯仿、二氯甲烷和苯的致癌风险分别达6.0×10-2、2.9×10-4、7.4×10-5,均超过可接受风险水平(1.0×10-6),三者修复目标分别为0.22、12.00和0.64 mg/kg. 如采取策略一,即将场地内超过修复目标的土壤进行清除,需修复的土壤深度达24.0 m,修复土方量为33.4×104 m3. ③结合污染物垂向分布及场地未来地下空间开发规划,提出策略二,即对0~15.0 m深度范围内重污染土壤进行清除异位修复、>15 m深度范围内土壤采取工程控制措施. 实施策略二后的风险评估结果显示,虽然>15.0 m深度范围内土壤中依然存在w(氯仿)超过修复目标的采样点,但致癌风险(8.3×10-8)远低于可接受水平；概率风险评估显示,该风险值对应的累计频率为99.5%,考虑各参数取值的不确定性后,风险模拟结果最大值也仅为1.06×10-7. 可见,策略二足够保守,能够保障未来居民的身体健康；与策略一相比,策略二可减少修复土方量6.4×104 m3,因此更具经济性,为风险管理策略的优选方案. 

Screening and Evaluation of Risk Management Strategies for a Site Contaminated by VOCs

Health risk assessment, in combination with contamination investigation, was applied in screening of risk management strategies for a site contaminated by VOCs. The results revealed that the soil was contaminated by chloroform, dichloromethane and benzene as deep as 25.8 m below ground level. Distinct changes in the vertical profiles of the three contaminants were observed; the most contaminated soil was within the depth of 15.0 m. Health risk assessment was carried out based on the conservative generic conceptual model that the site would be redeveloped as a residential area. The assessment revealed that the carcinogenic risks of chloroform, dichloromethane and benzene were 6.0×10-2, 2.9×10-4 and 7.4×10-5, respectively, all far above the acceptable level of 1.0×10-6. To meet the acceptable risk level, the corresponding remediation targets of the three contaminants were 0.22 mg/kg, 12.00 mg/kg and 0.64 mg/kg, respectively. If the removal of all contaminated soil exceeding the remediation targets (Strategy One) was chosen as the risk management strategy for the site, soil as deep as 24.0 m, with a total volume of 334,0 m3, should be remediated. By considering the vertical distributions of the three contaminants and basement development plans of future buildings, removal of heavy contaminated soil within 15.0 m depth in combination with engineering control measures on the remaining contaminated soil was another potential alternative strategy (Strategy Two). The risk assessment of Strategy Two revealed that the residual health risk of chloroform was 8.3×10-8, even though concentrations of some samples below 15.0 m were still above the remediation targets derived in Strategy One. Further probabilistic risk assessment indicated that the residual risk corresponding to the value of 99.5% cumulative percentile was 1.06×10-7, which means that the strategy was conservative enough in protecting the health of future residents, and remediated soil volume would be reduced by 64,0 m3. Therefore, Strategy Two was preferred due to the reduction of remediation cost and less environmental impact.