Dr. Alauddin Khan, Ph.D.

Quantifying the Migration of PFAS From Soil to Groundwater

Dr. Alauddin Khan, Ph.D., Senior Environmental Engineer, Leidos

Background/Objectives:     

This presentation provides a comparative analysis of methods to evaluate the soil to groundwater exposure pathway for PFAS. While EPA Regional Screening Levels (RSLs) and/or state specific screening values are available for several PFAS compounds, these values may be overly conservative for some scenarios. A common concern is that these generic screening values do not account for site-specific conditions or PFAS accumulation at the air-water interface. Management of sites with PFAS-impacted soil requires a thorough understanding of the concentrations that represent unacceptable risks for the site groundwater. To address this challenge, this study uses groundwater, soil, and porewater data collected at four areas impacted by aqueous film-forming foam at a Department of Defense site. Three methods were utilized to develop protection of groundwater site soil screening levels (SSSLs) and these SSSLs were compared against the generic RSLs for the soil to groundwater pathway. The methods included EPA’s Development of Pathway Specific Soil Screening Levels (EPA 1996); PFAS-LEACH developed by University of Arizona’s Dr. Bo Guo; and ESCI LLC’s SESOIL and AT123D modeling software.    The objectives of this study include: determining SSSLs which are protective of the groundwater; comparing SSSLs to generic RSLs to assess how the differences may impact site decision making; exploring the total costs required to generate SSSLs; and evaluating situations where developing SSSLs may be beneficial.    

Approach/Activities:     

Our approach to evaluate PFAS leaching used soil, lysimeter (porewater), and groundwater data collected during a CERCLA PFAS Remedial Investigation at a facility in the Midwestern US. The facility includes multiple areas of concern (AOC) where high PFAS concentrations are present in soils, porewater, and groundwater. Our analysis included soil data screening followed by fate and transport analysis via EPA Soil Screening Guidance and a combination of mathematical models (SESOIL/AT123D and PFAS-LEACH). Fate and transport modeling was used to simulate vertical transport of PFAS from vadose zone soils to groundwater, as well as horizontal transport within the groundwater system from the source area to receptor locations.    PFAS-LEACH, accounts for air–water interfacial adsorption and rate-limited solid-phase sorption and solves the partial differential equations for PFAS leaching through the vadose zone. Estimated leachate concentrations are then coupled with a simple groundwater dilution factor model which computes PFAS concentrations at downgradient groundwater receptors to derive SSSLs. The SESOIL model estimates contaminant concentrations leached from soil and predicts the contaminant mass flux over time that is discharging to groundwater. AT123D imports the flux values and computes the transient spread of PFAS through the aquifer and at downgradient receptors.    SSSLs were developed for each PFAS of concern at each AOC using the three methods discussed above. The results were utilized to explore whether detected soil concentrations represent continuing sources to the groundwater system. Resulting SSSLs were reviewed to explore the applicability of SSSLs for various sites and project scenarios.    

Results/Lessons Learned:     

SSSLs calculated based on EPA’s Soil Screening Guidance were lower than SSSLs calculated via the modeling methods. In comparison to generic RSLs, the magnitude of difference in the calculated SSSLs varied by PFAS compound. Compounds with shorter chain lengths generally had less difference between generic RSLs and calculated SSSLs. Perfluoroalkyl carboxylic acids (PFCAs) tended to have less difference between generic RSLs and calculated SSSLs, whereas perfluoroalkane sulfonic acids (PFSAs) tended to have greater differences between generic RSLs and calculated SSSLs. Comparison of the modeling approaches indicated that PFAS-Leach tends to overpredict the impact of adsorption at the air-water interface and produce significantly higher SSSLs, particularly for longer chain PFSAs. SESOIL/AT123D generally produced SSSLs that were less conservative than generic values, but more conservative than PFAS-Leach values.     Development of SSSLs provides a method to provide a rigorous understanding of site risks. This deeper understanding may prove beneficial for certain projects and/or project phases, however in some cases, it may be more expedient to rely on default values. For example, at a site impacted by short-chain PFCAs, development of SSSLs may require significant costs but only lead to minimal difference in screening levels. Whereas for sites impacted by long chain PFSAs, development of SSSLs may provide a one to two order of magnitude difference in screening levels.   

Dr. Khan has more than 30 years of environmental engineering experience in characterization, design, remedial action, remedial construction, and optimization for DoD, DOE, USACE, USAF, ANG, NASA, and more. He has extensive experience and expertise in groundwater and surface water flow and contaminant fate and transport modeling. Dr. Khan leads contaminant fate and transport modeling in simple to highly complex hydrogeologic settings using diverse software packages for numerous CERCLA and RCRA-based projects. He develops CSMs, selects computer codes, analyzes sampling data, builds models, prepares reports, and addresses stakeholder comments. His extensive expertise in groundwater flow and contaminant transport modeling has allowed Leidos to make efficient and cost-effective decisions regarding contaminant plume extents, site characterizations, and future remedial actions. Dr. Khan is a member of Leidos’ PFAS Technical Team, and he leads efforts to evaluate and simulate PFAS leaching from soil to groundwater and subsequent fate and transport in groundwater.