A value of 0.001 is recommended by the United States Environmental Protection Agency (USEPA) for its groundwater-to-indoor air Generic Attenuation Factor (GAFG), used in assessing potential vapor intrusion (VI) impacts to indoor air, given measured groundwater concentrations of volatile chemicals of concern (e.g., chlorinated solvents). The GAFG can, in turn, be used for developing groundwater screening levels for VI given target indoor air quality screening levels. In this study, we examine the validity and applicability of the GAFG both for predicting indoor air impacts and for determining groundwater screening levels. This is done using both analysis of published data and screening model calculations. Among the 774 total paired groundwater-indoor air measurements in the USEPA's VI database (which were used by that agency to generate the GAFG) we found that there are 427 pairs for which a single groundwater measurement or interpolated value was applied to multiple buildings. In one case, up to 73 buildings were associated with a single interpolated groundwater value and in another case up to 15 buildings were associated with a single groundwater measurement (i.e, that the indoor air contaminant concentrations in all of the associated buildings were influenced by the concentration determined at a single point). In more than 70% of the cases (390 of 536 paired measurements in which horizontal building-monitoring well distance was recorded) the monitoring wells were located more than 30 meters (and some up to over 200 meters) from the associated buildings. In a few cases, the measurements in the database even improbably implied that soil gas contaminant concentrations increased, rather than decreased, in an upward direction from a contaminant source to a foundation slab. Such observations indicate problematic source characterization within the dataset used to generate the GAFG, and some indicate the possibility of a significant influence of a preferential contaminant pathway. While the inherent value of the USEPA database itself is not being questioned here, the above facts raise the very real possibility that the recommended groundwater attenuation factors are being influenced by variables or conditions that have not thus far been fully accounted for. In addition, the predicted groundwater attenuation factors often fall far beyond the upper limits of predictions from mathematical models of VI, ranging from screening models to detailed computational fluid dynamic models. All these models are based on the same fundamental conceptual site model, involving a vadose zone vapor transport pathway starting at an underlying uniform groundwater source and leading to the foundation of a building of concern. According to the analysis presented here, we believe that for scenarios for which such a "traditional" VI pathway is appropriate, 10-4 is a more appropriately conservative generic groundwater to indoor air attenuation factor than is the EPA-recommended 10-3. This is based both on the statistical analysis of USEPA's VI database, as well as the traditional mathematical models of VI. This result has been validated by comparison with results from some well documented field studies.

中文翻译：

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检查 USEPA 通用衰减因子在确定地下水蒸气入侵筛选水平中的使用情况。

美国环境保护署 (USEPA) 建议其地下水到室内空气通用衰减系数 (GAFG) 的值为 0.001，用于评估潜在的蒸汽入侵 (VI) 对室内空气的影响，假设测得的地下水浓度为值得关注的挥发性化学品（例如氯化溶剂）。反过来，GAFG 可用于在给定目标室内空气质量筛查水平的情况下制定 VI 的地下水筛查水平。在这项研究中，我们检验了 GAFG 在预测室内空气影响和确定地下水筛选水平方面的有效性和适用性。这是通过对已发布数据的分析和筛选模型计算来完成的。在 USEPA VI 数据库（该机构用于生成 GAFG）中总共 774 个成对的地下水-室内空气测量中，我们发现有 427 对将单个地下水测量或插值应用于多个建筑物。在一种情况下，多达 73 座建筑物与单一内插地下水值相关，而在另一种情况下，多达 15 座建筑物与单一地下水测量值相关（即，所有相关建筑物中的室内空气污染物浓度均受到浓度在单点测定）。在超过 70% 的案例中（536 次配对测量中的 390 次记录了水平建筑物-监​​测井距离），监测井距离相关建筑物超过 30 米（有些甚至超过 200 米）。在少数情况下，数据库中的测量结果甚至不可能暗示土壤气体污染物浓度从污染物源到基础板向上方向增加而不是减少。这些观察结果表明用于生成 GAFG 的数据集中存在问题的源特征，并且一些观察结果表明优先污染物途径可能产生重大影响。虽然 USEPA 数据库本身的内在价值在这里并没有受到质疑，但上述事实提出了一种非常现实的可能性，即建议的地下水衰减因子受到迄今为止尚未得到充分考虑的变量或条件的影响。此外，预测的地下水衰减因子往往远远超出VI数学模型（从筛选模型到详细的计算流体动力学模型）预测的上限。所有这些模型都基于相同的基本概念场地模型，涉及包气带蒸汽传输路径，从底层统一地下水源开始，通向相关建筑物的地基。根据此处提供的分析，我们认为对于适合这种“传统”VI 途径的场景，10-4 是比 EPA 推荐的 10-3 更为保守的通用地下水对室内空气的衰减系数。这是基于USEPA VI 数据库的统计分析以及VI 的传统数学模型。通过与一些有据可查的实地研究的结果进行比较，这一结果得到了验证。