PFAS Mass Discharge at an AFFF‐Impacted Site: 2. Evaluation of SPLP Test Reliability

There is a critical need to understand if synthetic precipitation leaching procedure (SPLP) or equivalent tests are sufficiently representative for the purpose of estimating soil‐to‐groundwater mass discharge of per‐ and polyfluoroalkyl substances (PFAS). A mass balance approach was derived to define the complex relationship between SPLP leachate and in situ porewater concentrations. Differences in SPLP leachate and porewater are expected to occur due to PFAS adsorption to air–water interfaces and the large liquid‐to‐liquid dilution factor (LLDF) associated with SPLP tests. This study analyzed 24 PFAS analyte results for co‐located soil, SPLP, and porewater samples collected at 15 lysimeters at Ellsworth Air Force Base, an aqueous film‐forming foam (AFFF)‐impacted site. SPLP tests overestimated perfluorooctanesulfonic acid concentrations in porewater by up to an order of magnitude because SPLP does not account for adsorption to the air–water interface. SPLP also underestimated short‐chain PFAS concentrations in porewater by up to two to three orders of magnitude due to the large LLDF associated with SPLP tests. Porewater concentrations calculated based on SPLP leachate concentrations, and accounting for the LLDF and estimated air–water interface partitioning, differ from observed porewater concentrations by up to an order of magnitude. Visualization of PFAS using radial diagrams and stacked bar maps was effective for demonstrating these trends on a single figure, and for showing that the PFAS composition in SPLP leachate samples was different than the composition in porewater samples. Differences observed in PFAS concentration between SPLP leachate and porewater concentrations may occur at a wide range of other AFFF‐impacted sites. Based on the results of this study, caution should be employed when considering the use of SPLP leachate concentrations for estimating PFAS mass discharge or soil screening levels in the vadose zone. Porewater sampling is the most direct and authoritative metric for estimating PFAS mass discharge in the vadose zone at sites such as Ellsworth AFB.