基于地下水数值模型的蒸气入侵健康风险评估案例研究

Case study for the health risk assessment due to the vapor intrusion based on the groundwater simulation model

某化工厂搬迁后拟开发为住宅用地,但其浅层地下水中存在的乙苯污染可能通过蒸气入侵途径进入人体,从而导致人体健康风险。建立了研究区的地下水污染数值模型,对各关注点进行了分时段蒸气入侵健康风险评估;并将所得评估结果与常规健康风险评估结果进行比较。结果表明,较常规健康风险评估方法,采用基于地下水数值模型的分时段健康风险评估方法所计算的致癌及非致癌健康风险水平,在中心点低1个数量级,在上游点及下游点低3~4个数量级。基于地下水数值模型的健康风险评估方法考虑了污染物的自然降解作用,因而避免了常规健康风险评估方法将现状质量浓度作为长期暴露质量浓度而导致的评估结果过于保守的缺点;基于地下水数值模型的健康风险评估还可以看出位于污染羽中心点、上游点及下游点各关注点在不同时段内的健康风险水平变化情况,因而可更好地指导风险控制决策。

The present paper is to bring about a case study for the health risk assessment due to the vapor intrusion based on the groundwater simulation model. For the said purpose,we have conducted a health risk assessment to clarify if the identified contamination may impose the unacceptable health risk to the future residents through the vapor intrusion via the shallow groundwater aggravation at a historical chemical plant site contaminated by ethyl benzene. Based on the site groundwater contaminant fate and transport model,we have simulated the average concentration rate of the ethyl benzene at different points under study in the different periods of time. Since the concentration rates of ethyl benzene in the room-in air environment can be calculated through a vapor intrusion model, we have conducted a time-dependent room-in health risk assessment,with the results of our study compared with the general risk assessment. Needless to say,for a long run,the concentration of ethyl benzene is likely to change significantly due to the natural attenuation,which calls for a timedependent health risk assessment to be conducted at the site under study to identify the actual risks likely to be imposed to the actual would-be residents. The results of our case study sample have shown that at the point under study in the central area of the plume,the risk level worked out by the time-dependent risk assessment model is equal to 1 order of magnitude lower than the risk level calculated by using the general risk assessment method.For example,at the points under study in upgrade and downgrade area of the plume,the risk levels tend to be 3- 4 orders of magnitude lower than the general risk assessment results. The reason is nothing else but the naturally degradable contaminant like ethyl benzene can get rid of the shortage of general health risk assessment by using the current concentration rate as the long term exposure one,which may result in too conservative conclusions.However,the time-dependent risk assessment can gain more accurate risk levels of different points under study for different periods of time,whose results can be used to improve the risk control initiatives and measures. Therefore,we propose that for volatile organic compounds contaminated site that may impose a vapor intrusion risk,time-dependent risk assessment should be adopted to gain more incredible and more precise risk assessment levels,so as to better guide the actual risk control measures.