Bethany Flynn is a Principal Geologist with Wood Environment and Infrastructure Solutions. She has been managing Site Inspections at AFFF Areas for the last five years. She also managed mitigation responses for private wells where PFOS and PFOA exceeded the LHA
Using Lysimetry to Determine Mass Discharge of PFAS from AFFF Source Areas
We all know the drill on how to investigate potential source areas. Collect soil samples at the source area and analyze for suspected compounds. Then compare the results to groundwater protection soil screening levels (SSL). Proceed to collect more samples until you reach the magic sample that shows you are below the SSL. Now you have characterized soil to a concentration below the SSL, but what does the data mean? Soil screening levels (SSLs) protective of groundwater are typically derived based on hydrophobic interactions with soil organic matter (kd =koc (foc)); assumes equilibrium and linearity (koc partitioning coefficient); spatially averages the vertical profile from where the sample was collected; and assumes a static groundwater domain. Using the EPA RSL Calculator, SSLs for PFOS and PFOA are 0.38 µg/kg and 0.17 µg/kg which are at or below the analytical detection limits for soil. Is using SSLs for evaluating potential impacts from PFAS compounds to groundwater the best approach? Is it the only approach? Understanding the fate and migration of PFAS from soil to groundwater is complex, but necessary for evaluating fate and transport and updating the CSM. Collection of site-specific soil porewater data using lysimetry may be a viable alternative to using SSLs. Porewater data from lysimeters can provide a definitive measurement of the spatially integrated in situ mass discharge of PFAS to groundwater, and account for all 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 composition and TDS). To test lysimetry as a tool to understand the site-specific mass discharge of PFAS to groundwater, five AFFF source areas will be instrumented with shallow/deep lysimeter pairs and porewater will be collected over 4 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 TOC, grainsize analysis, permeability, pH, and anion and cation exchange capacity. Project work plans are currently in progress. Lysimeter installation and sampling is schedule to begin in spring 2020.