Advancing the Use of Suction Lysimeters to Inform Soil Leaching and Remediation of PFAS Source Zones

Porewater in soil vadose zones is an integrator of the fundamental processes governing the transport and partitioning of per and poly-fluoroalkyl substances (PFAS) as they move from source zones to groundwater. Suction lysimeters are being advanced as a method to provide reliable and representative PFAS porewater samples, to inform PFAS leaching and for monitoring remedial approaches. We report outcomes of lysimeter investigations across 3 sites and 18 lysimeters within fine-textured soil profiles. Soil cores were recovered from the same locations, and PFAS concentrations in soils and lysimeter porewater were compared and compared with prior laboratory investigations. Variable concentration distributions with depth of PFAS in soils were found with a maximum sum of PFAS of ~56 mg/kg dominated by perfluorooctane sulfonic acid (PFOS). The maximum sum of PFAS in porewater was 13.5 mg/L. Comparison across all collocated soil and porewater concentrations did not provide consistent trends. PFAS mass fractions within lysimeter porewater samples were much higher for most PFAS than mass fractions determined from laboratory investigations, but the fraction was lower for PFOS. The results indicate preferential recovery of individual shorter chain PFAS via leaching at lower liquid: soil ratios such as those experienced under suction during recovery of porewater by lysimeters. Suggestions are offered to advance the use of suction lysimeters in promoting porewater PFAS concentrations as an alternative for regulatory compliance, and in closing the gap between field and laboratory approaches. There is merit in using lysimeters at PFAS field sites with improvements and considerations embraced.