ペルフルオロアルキル物質およびポリフルオロアルキル物質の部位特異的土壌スクリーニングレベルの開発に関する概念と方法

Soils are a primary long-term reservoir for PFAS, which poses risks to groundwater via leaching through the vadose zone. The application of soil screening levels (SSLs) is one standard, common means to determine risk-based soil concentrations that are anticipated to be protective of groundwater. As such they serve an important role in site characterization, risk assessment, and decision-making concerning the implementation of mitigation or remediation efforts. Despite their significance and widespread use, the determination of SSLs has received minimal research focus. The objective of this work was to examine the determination of SSLs specifically for PFAS-impacted sites. The conceptual basis of SSLs and of the different application methods was presented. It was demonstrated that the magnitudes of porewater concentrations in soil are more complex for unsaturated conditions, and that the difference between values for unsaturated and saturated conditions is a function of the specific PFAS as well as other factors. Methods for determining required input parameters for the SSL calculations were discussed in detail, including estimation methods, the use of laboratory measurements, and the application of field-based data. The applications and limitations of using batch leach tests and field-measured soil and porewater concentrations were specifically examined. Issues associated with data and parameter uncertainty, background concentrations, and other factors were discussed. The application of the methods is illustrated with examples for a fictitious data set and for PFAS-impacted field sites. The outcomes of this study are anticipated to provide clarity on both the conceptual and practical elements of SSL determinations, thereby leading to more robust implementations.