PFAS Vadose Zone Mass Discharge at an AFFF-Impacted Site: 1. Validation Based on a Multiple Lines-of-Evidence Approach

Questions have recently been raised about the suitability of using porewater samples collected with suction lysimeters to estimate per- and polyfluoroalkyl substances (PFAS) mass discharge in the vadose zone. Multiple lines of evidence were evaluated to determine if porewater samples collected during a data gaps investigation in the former fire training area (FFTA) at Ellsworth Air Force Base were reproducible and representative. Lines of evidence used include supplemental geologic and hydrogeologic data, which show the presence of a semi-continuous silt/clay layer directly above the water table throughout much of the FFTA, and gravimetric moisture content results that demonstrate vertical water flow at a depth of 15 ft is substantially limited due to semi-arid climatic conditions. Radial diagrams were used to visually demonstrate that there was negligible variability in porewater concentrations caused by varying sample yields in the deep zone. Additional lysimeters installed in the FFTA during the remedial investigation did not result in a significant difference in the estimated vertical mass discharge, which demonstrates that the number of lysimeter sampling points in this area is sufficient. Strong attenuation in PFAS soil concentrations with depth validates the order(s) of magnitude reduction between shallow and deep porewater concentrations observed near the former burn pit. Vadose zone PFAS mass discharge is shown to be 4%–19% of groundwater mass discharge below the water table for regulated constituents. Recommendations for field data collection and estimation of PFAS mass discharge in the vadose zone are presented.