A comprehensive understanding of the transport and fate of per- and poly-fluoroalkyl substances (PFAS) in the subsurface is critical for accurate risk assessments and design of effective remedial actions. A multi-process retention model is proposed to account for potential additional sources of retardation for PFAS transport in source zones. These include partitioning to the soil atmosphere, adsorption at air-water interfaces, partitioning to trapped organic liquids (NAPL), and adsorption at NAPL-water interfaces. An initial assessment of the relative magnitudes and significance of these retention processes was conducted for two PFAS of primary concern, perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), and an example precursor (fluorotelomer alcohol, FTOH). The illustrative evaluation was conducted using measured porous-medium properties representative of a sandy vadose-zone soil. Data collected from the literature were used to determine measured or estimated values for the relevant distribution coefficients, which were in turn used to calculate retardation factors for the model system. The results showed that adsorption at the air-water interface was a primary source of retention for both PFOA and PFOS, contributing approximately 50% of total retention for the conditions employed. Adsorption to NAPL-water interfaces and partitioning to bulk NAPL were also shown to be significant sources of retention. NAPL partitioning was the predominant source of retention for FTOH, contributing ~ 98% of total retention. These results indicate that these additional processes may be, in some cases, significant sources of retention for subsurface transport of PFAS. The specific magnitudes and significance of the individual retention processes will depend upon the properties and conditions of the specific system of interest (e.g., PFAS constituent and concentration, porous medium, aqueous chemistry, fluid saturations, co-contaminants). In cases wherein these additional retention processes are significant, retardation of PFAS in source areas would likely be greater than what is typically estimated based on the standard assumption of solid-phase adsorption as the sole retention mechanism. This has significant ramifications for accurate determination of the migration potential and magnitude of mass flux to groundwater, as well as for calculations of contaminant mass residing in source zones. Both of which have critical implications for human-health risk assessments.

中文翻译：

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评估额外的保留过程对地下PFAS阻滞的潜在贡献

全面了解地下的全氟烷基物质和多氟烷基物质（PFAS）的运输和命运对于准确的风险评估和有效的补救措施设计至关重要。提出了一种多过程保留模型，以考虑源区中PFAS传输的潜在额外延迟源。这些措施包括分配到土壤大气，在空气-水界面吸附，分配到捕集的有机液体（NAPL）和在NAPL-水界面吸附。对两个主要关注的PFAS，全氟辛烷磺酸（PFOS）和全氟辛酸（PFOA）以及示例前体（氟调聚物醇，FTOH）进行了对这些保留过程的相对强度和重要性的初步评估。使用代表沙质渗流带土壤的测得的多孔介质特性进行了说明性评估。从文献中收集的数据用于确定相关分布系数的测量值或估计值，进而将其用于计算模型系统的延迟因子。结果表明，空气-水界面的吸附是PFOA和PFOS保留的主要来源，在所采用的条件下，约占总保留量的50％。吸附至NAPL-水界面和分配至大量NAPL也是保持力的重要来源。NAPL分配是FTOH保留的主要来源，约占总保留的98％。这些结果表明，在某些情况下，这些额外的过程可能是 PFAS地下运输的重要滞留源。各个保留过程的具体大小和重要性将取决于所关注的特定系统的特性和条件（例如PFAS成分和浓度，多孔介质，水性化学，流体饱和度，共污染物）。在这些额外的保留过程很重要的情况下，源区中PFAS的延迟可能会大于通常基于固相吸附作为唯一保留机制的标准假设所估计的延迟。这对于准确确定向地下水的迁移潜力和质量通量的大小以及计算源区中的污染物质量具有重要意义。