As our attention turns from site characterization to remediation for the emerging contaminant class, per- and polyfluoroalkyl substances (PFAS), we are struck pondering not only the treatment challenges these recalcitrant compounds present, but the scale of the problem and lack of consensus on how clean is clean. With guidelines and standards for aqueous media established at low parts per trillion, the handful of default standards for soil leaching are low. Further, standard leaching procedures such as synthetic precipitation leaching procedure (SPLP) yield detectable PFAS from soils with very low PFAS concentrations, single digit parts per billion. But are these technically justifiable approaches to determining remediation liability, or are they overly conservative methods that would result in bankrupting responsible parties if enforced? PFAS release areas vary widely in mechanism and source. Many are aged and may be at a point of steady state with regards to leaching to groundwater. Many site-specific factors influence the complex behavior of PFAS in release areas. PFAS retention is controlled by PFAS properties, compound chain length, functional group, co-contaminants, stratigraphy/geology, depth to water, and soil properties including grain size, cation/anion exchange capacity, organic carbon content and pH. The analysis of soil physiochemical properties is used to evaluate how clay content, presence of organic matter, cation and anion exchange capacities, and pH affects PFAS sorption to soil. Calculation of site-specific soil screening levels (SSLs) using the US Environmental Protection Agency’s Regional Screening Level calculator can result in very low SLLs, below laboratory detection limits; any detection of PFAS may require mitigation to protect groundwater. Direct measurement of soil leaching by lysimetry may provide the most accurate, site-specific measure of soil leaching. Porewater data from lysimeters can provide a definitive measurement of the spatially integrated in-situ mass discharge of PFAS to groundwater, and account for site-specific soil retention processes and rates. Relevant retention processes for PFAS include hydrophobic interactions with soil organic matter, electrostatic interactions with soil mineral phases, and air-water interfacial partitioning. These processes are non-linear, irreversible to some extent, and depend on the composition of the soil solution (both the cation/anion composition and total dissolved solids). This presentation will present two case-study examples of developing site-specific soil leaching criteria. The first is a recently completed assessment at a fire training area (FTA) where site-specific data was used to calculate SSLs. This example suggests that well-drained soils at a decommissioned FTA may no longer be contributing to groundwater impacts. The second case study presents a methodology for using lysimetry as a tool to understand the site-specific mass discharge of PFAS to groundwater at two aqueous film forming foam source areas. The sites will be instrumented with shallow/deep lysimeter pairs and porewater will be collected over four quarters for analysis of 16 PFAS compounds. As part of the investigation, the vertical soil profile will be characterized through use of a hydraulic profiling tool to determine the location of transmissive zones and for visual soil logging prior to installation. Soil samples will be collected at the same depth as the lysimeter is installed and analyzed for 16 PFAS compounds. Soil samples from each area will also be for analyzed for total organic carbon, grainsize analysis, permeability, pH, and anion and cation exchange capacity. The goal of these studies is to advance technically robust assessment methods for soil leaching that will focus remediation efforts on soils that actually present an ongoing risk to groundwater. Nathan Hagelin, Principal and Global Technical Leader, Environmental Remediation, Wood Nathan Hagelin is a Principal and a Global Technical Leader for Environmental Remediation at Wood. He is the remediation technology leader in Wood’s Emerging Contaminants Work Group. He is a Certified Geologist, Licensed Environmental Professional, and Board Certified Environmental Scientist working for 30 years on the remediation of contaminated industrial properties and military installations. He has prior experience as a Hydrologist with the U.S. Geological Survey Water Resource Division.