Vapor intrusion (VI) occurs when volatile contaminants in the subsurface migrate through the vadose zone into overlying buildings. The 2015 U.S. EPA petroleum VI guidance recommends that additional investigation of the VI risk from gasoline hydrocarbons at the underground storage tank (UST) sites is not necessary where the vertical distance between a building and a vapor source exceeds a recommended vertical screening distance. However, due to the lack of soil-gas data on the attenuation of ethylene dibromide (EDB), additional VI investigations to evaluate VI risk from EDB are recommended at UST sites with leaded gasoline releases containing EDB. We analyzed soil-gas and groundwater concentrations of EDB from eight petroleum UST sites using a new analytical method with soil-gas detection limit <0.16 μg/m³ EDB (VI screening level at the 10⁻⁶ risk level). The analysis included (1) assessing the frequency of EDB detections ≤0.16 μg/m³ at various vertical separation distances and (2) predicting vertical screening distances for EDB using the U.S. EPA PVIScreen model for different soil types in the vadose zone above dissolved-phase and LNAPL sources. Ranges of estimated aerobic biodegradation rate constants for EDB, air exchange rates for residential buildings, and source vapor concentrations for other constituents were combined with conservative estimates of EDB source concentrations as model inputs. Concentrations of EDB in soil-gas indicated that the U.S. EPA recommended vertical screening distances are protective of VI risk from EDB. Conversely, vertical screening distances predicted by modeling were >6 ft (1.8 m) for sites with sand and loam soil above dissolved phase sources and >15 ft (4.6 m) for sites with sand soil above LNAPL sources. This predicted dependence on the vapor source type and soil type in the vadose zone highlights the importance of soil characterization for VI screening at sites with EDB sources.

中文翻译：

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垂直筛选距离法评估二溴乙烷的蒸气入侵风险

当地下的挥发性污染物通过渗流带迁移到上覆的建筑物中时，就会发生蒸气入侵（VI）。2015年美国EPA石油VI指南建议，如果建筑物与蒸气源之间的垂直距离超过建议的垂直筛查距离，则无需对地下储油罐（UST）场所的汽油碳氢化合物进行VI风险的进一步调查。但是，由于缺乏有关二溴乙烷（EDB）衰减的土壤气体数据，建议在UST地点使用含EDB的含铅汽油释放，对VI进行额外的VI研究以评估EDB的VI风险。我们使用一种新的分析方法，分析了土壤气检测极限<0.16μg/ m ^3的八个石油UST站点中EDB的土壤气体和地下水浓度EDB（风险筛查水平为10 ^-6风险水平）。分析包括（1）评估EDB检测频率≤0.16μg/ m ³（2）使用美国EPA PVIScreen模型预测溶解相和LNAPL源上方渗流带中不同土壤类型的EDB的垂直筛选距离。将EDB的需氧生物降解率常数范围，住宅建筑的空气交换率以及其他成分的源蒸气浓度范围与EDB源浓度的保守估计值结合起来作为模型输入。土壤气体中EDB的浓度表明，美国EPA建议的垂直筛查距离可保护EDB避免VI风险。相反，通过建模预测的垂直筛分距离对于高于溶解相源的砂壤土和壤土站点，> 6 ft（1.8 m）；对于高于LNAPL源的砂土土壤，站点的垂直筛查距离，> 15 ft（4.6 m）。