MOE Key Lab of Environmental Remediation and Ecosystem Health, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou, 310058, China; Research Center for Air Pollution and Health, Zhejiang University, Hangzhou, 310058, China; Institute of Environmental Health, Zhejiang University, Hangzhou, 310058, China.
MOE Key Lab of Environmental Remediation and Ecosystem Health, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou, 310058, China; Research Center for Air Pollution and Health, Zhejiang University, Hangzhou, 310058, China; Institute of Environmental Health, Zhejiang University, Hangzhou, 310058, China.
Water Res. 2019 Mar 1;150:111-119. doi: 10.1016/j.watres.2018.11.038. Epub 2018 Nov 20.
At contaminated sites, indoor inhalation of volatile organic compounds from groundwater contamination, known as vapor intrusion (VI), is an important exposure pathway to determine groundwater cleanup level. Based on empirical analysis, US EPA concluded that there is a low probability for vapors from fuel hydrocarbons dissolved in groundwater to induce indoor concentrations that exceed risk-based standards, and recommended 6 feet vertical building-source separation distance as the risk screening tool for such cases. In this study, we examine this recommendation by performing numerical modeling to investigate the detailed effects of the capillary fringe on petroleum vapor biodegradation and attenuation. First, the numerical model is validated by comparison with laboratory data and field measurements in US EPA's database. Then the verified model is used to simulate two scenarios involving the capillary fringe effect, one with a groundwater source at various depth and the other with a soil gas source located above the groundwater level. For a groundwater contaminant source, the capillary fringe plays a significant role in VI by controlling the soil moisture content and oxygen availability, thus affecting the soil gas concentration biodegradation and attenuation. Specifically, the capillary fringe effect can significantly decrease the indoor air concentration by decreasing upward diffusion rates of hydrocarbon, increasing the thickness of the aerobic zone, and enhancing aerobic biodegradation. As a result, it is highly unlikely for sources located at groundwater level to induce unacceptable vapor intrusion risks, supporting US EPA's recommendation. Moreover, the simulations suggest that the vertical smear zone of residual light non-aqueous liquid contamination, induced by temporal fluctuations of groundwater level, may lead to a potential threat to indoor air quality for a short vertical source-building separation distance, and thus requires more attention. The sensitivity test of the numerical model also indicates that it is the vertical separation distance between building foundation and the top of the smear zone instead of the smear zone thickness that should be given more attention during the investigation.
在污染场地，室内吸入受地下水污染影响的挥发性有机化合物，即蒸气入侵（VI），是确定地下水清理水平的一个重要暴露途径。基于实证分析，美国环保署得出结论，溶解在地下水中的燃料烃蒸气不太可能导致室内浓度超过基于风险的标准，因此建议将 6 英尺的垂直建筑物源分离距离作为此类情况的风险筛选工具。在这项研究中，我们通过数值模拟来研究毛细带对石油蒸气生物降解和衰减的详细影响，以此检验这一建议。首先，通过与美国环保署数据库中的实验室数据和现场测量进行比较，对数值模型进行验证。然后，使用验证后的模型模拟两种涉及毛细带效应的情况，一种是不同深度的地下水源，另一种是位于地下水位以上的土壤气体源。对于地下水污染物源，毛细带通过控制土壤湿度和氧气供应来影响土壤气体浓度的生物降解和衰减，从而在 VI 中发挥重要作用。具体来说，毛细带效应可以通过降低碳氢化合物的向上扩散率、增加好氧区的厚度以及增强好氧生物降解，显著降低室内空气浓度。因此，位于地下水位的源不太可能引发不可接受的蒸气入侵风险，这支持了美国环保署的建议。此外，模拟结果表明，由于地下水水位的时变波动而产生的残留轻质非水相液体污染的垂直弥散带可能会对短距离垂直源-建筑物分离的室内空气质量构成潜在威胁，因此需要给予更多关注。数值模型的敏感性测试还表明，在调查过程中，应更加关注建筑物基础和弥散带顶部之间的垂直分离距离，而不是弥散带的厚度。
