Aqueous Film-Forming Foams Exhibit Greater Interfacial Activity than PFOA, PFOS, or FOSA

Simulating PFAS transport in effluent-irrigated farmland using PRZM5, LEACHM, and HYDRUS-1D models

Application of wastewater effluent to agricultural lands can serve as a sustainable approach to meet irrigation and nutrient needs for crop production. While nitrogen and phosphorous loadings can be effectively managed, concerns have been raised regarding the fate of emerging contaminants, including per- and polyfluoroalkyl substances (PFAS), which are widely detected in wastewater effluent. The objective of this paper was to evaluate the ability of three unsaturated flow and transport models, Pesticide Root Zone Model 5 (PRZM5), LEACHM, and HYDRUS-1D, to predict the distribution of PFAS in the soil profile at the Pennsylvania State University Living Filter site, which has received daily wastewater effluent applications for several decades. The models were modified to include adsorption at the air-water interface (AWI), which has been shown to be an important factor governing PFAS transport and phase distribution in the vadose zone. Simulations showed that PRZM5 did not accurately reproduce the observed perfluorooctanesulfonic acid (PFOS) behavior, which was attributed to the "tipping bucket" approach used for water flow that results in the disappearance of AWI during water flow. In contrast, both LEACHM and HYDRUS-1D captured the observed retention of PFOS and perfluorooctanoic acid (PFOA) over a 50-year simulation period. Due to differences in the approach used to calculate the AWI area, LEACHM predicted greater accumulation of PFOS and PFOA at the AWI compared to HYDRUS-1D. These findings indicate that mathematical models that directly account for unsaturated water flow and adsorption at the AWI are able to provide reasonable predictions of long-term PFAS leaching resulting from land application of wastewater effluent.

Fate, distribution, and transport dynamics of Per- and Polyfluoroalkyl Substances (PFASs) in the environment

Per- and Polyfluoroalkyl Substances (PFASs) are persistent organic pollutants with significant environmental and health impacts due to their widespread occurrence, bioaccumulation potential, and resistance to degradation. This paper comprehensively reviews current knowledge of PFAS fate and transport mechanisms by correlating PFAS leaching, retention, and movement to their physicochemical properties and environmental factors based on observing PFAS fate and transport in unsaturated zones, surface water, sediments, plants, and atmosphere. The complex and unique physiochemical properties of PFASs, such as their carbon-fluorine bonds and amphiphilic nature, determine their environmental behavior and persistence. Recent studies emphasize that concentration-dependent affinity coefficients predict the transport of diverse PFAS mixtures by considering the impact of the Air-Water Interface (AWI). These studies highlight the complex interactions that influence PFAS behavior in environmental systems and the need for refined modeling techniques to account for transport dynamics. Competitive adsorption at the AWI, influenced by PFAS physicochemical properties and environmental factors, is crucial. PFAS chain length profoundly affects PFAS volatility and mobility, i.e., longer chains show higher solid matrix adsorption, while shorter chains exhibit greater atmospheric deposition potential. Solution chemistry, encompassing pH and ionic strength, variably alters PFAS sorption behaviors. Mathematical models, such as the Leverett Thermodynamic Model (LTM) and Surface Roughness Multipliers (SRM), effectively predict PFAS retention, offering enhanced accuracy for surface-active solutes through empirical adjustments. Co-contaminants' presence influences the transport behavior of PFASs in the environment. Microbial activity alters PFAS retention, while microplastics, especially polyamide, contribute to their adsorption. These complex interactions govern PFAS fate and transport in the environment. The paper identifies critical gaps in current understanding, including the fate of PFASs, analytical challenges, ecological risk assessment methods, and the influence of episodic events on PFAS transport dynamics. This paper also investigates the research gap in refining current models and experimental approaches to predict PFAS transport accurately and enhance risk mitigation efforts. Addressing these gaps is crucial for advancing remediation strategies and regulatory frameworks to mitigate PFAS contamination effectively.

Transport and competitive interfacial adsorption of PFOA and PFOS in unsaturated porous media: Experiments and modeling

Among emerging contaminants, per- and polyfluoroalkyl substances (PFAS) have captured public attention based upon their environmental ubiquity and potential risks to human health. Due to their typical surface release conditions and amphiphilic properties, PFAS tend to sorb to soil and accumulate at the air-water interface within the vadose zone. These processes can result in substantial plume attenuation. Although there is a growing body of literature on vadose zone transport, few studies have explored PFAS mixture transport, particularly under conditions where nonlinear sorption processes are important. The present study aims to advance our understanding of PFAS transport in variably saturated porous media through integration of experiments and mathematical modeling. Experiments include batch studies to quantify sorption to the solid phase, interfacial tension (IFT) measurements to estimate adsorption at the air-water interface (AWI), and column studies with F-70 Ottawa sand at 100 % and ca. 50 % water saturation to explore transport mechanisms. Employed PFAS solutions encompass individual solutes and binary mixtures of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) at concentration levels spanning four orders of magnitude to assess competitive and nonlinear sorption at the AWI. Observations demonstrate that concentration levels and competitive effects substantially influence PFAS transport in unsaturated systems. In the presence of PFOS, PFOA experienced less retention than would be anticipated based on single-solute behavior, and effluent breakthrough curves exhibited chromatographic peaking. The presented mathematical model for simultaneous flow and transport of PFAS was able to capture experimental observations with a consistent set of parameters and minimal curve fitting. These results demonstrate the robustness of the model formulation that included rate-limited interfacial mass transfer, an extended Langmuir-Szyszkowski model for adsorption at the AWI, and a scaled Leverett thermodynamic model to predict the AWI specific area. Overall, the results of this work underscore the importance of the AWI in PFAS transport and highlight the relevance of competition effects in adsorption formulations.

Influence of chemical structures on reduction rates and defluorination of fluoroarenes during catalytic reduction using a rhodium-based catalyst

Continuous growth in fluoroarene production has led to environmental pollution and health concerns owing to their persistence, which is attributed to the stable C-F bond in their structures. Herein, we investigated fluoroarene decomposition via hydrodefluorination using a rhodium-based catalyst, focusing on the effects of the chemical structure and functional group on the defluorination yield. Most compounds, except (pentafluoroethyl)benzene, exhibited full or partial reduction with pseudo-first-order rate constants in the range of 0.002-0.396 min-1 and defluorination yields of 0%-100%. Fluoroarenes with hydroxyl, methyl, and carboxylate groups were selected to elucidate how hydrocarbon and oxygen-containing functional groups influence the reaction rate and defluorination. Inhibition of the reaction rate and defluorination yield based on functional groups increased in the order of hydroxyl < methyl < carboxylate, which was identical to the order of the electron-withdrawing effect. Fluoroarenes with polyfluoro groups were also assessed; polyfluoro groups demonstrated a different influence on catalyst activity than non-fluorine functional groups because of fluorine atoms in the substituents undergoing defluorination. The reaction kinetics of (difluoromethyl)fluorobenzenes and their intermediates suggested that hydrogenation and defluorination occurred during degradation. Finally, the effects of the type and position of functional groups on the reaction rate and defluorination yield were investigated via multivariable linear regression analysis. Notably, the electron-withdrawing nature of functional groups appeared to have a greater impact on the defluorination yield of fluoroarenes than the calculated C-F bond dissociation energy.

Modeling 1-D aqueous film forming foam transport through the vadose zone under realistic site and release conditions

Aqueous film forming foams (AFFFs) have been used to extinguish fires since the 1960s, leading to widespread subsurface contamination by per- and polyfluoroalkyl substances (PFAS), an essential component of AFFF. This study presents 1-D simulations of PFAS migration in the vadose zone resulting from AFFF releases. Simulation scenarios used soil profiles from three US Air Force (USAF) installations, encompassing a range of climatic conditions and hydrogeologic environments. A three-component mixture, representative of major constituents of AFFF, facilitated the exploration of competitive and synergistic effects of co-constituents on PFAS migration. To accurately capture unsaturated transport of PFAS in porous media, the model considers (1) surfactant-induced flow, (2) non-linear sorption to the solid phase, (3) competitive accumulation at the air-water interface, and (4) the moisture-dependence of the air-water interfacial area. Defined PFAS releases were consistent with fire training exercises, emergency responses, and accidental spills of record. Simulation results illustrate the importance of hydrogeologic, climatic, geochemical, and AFFF release conditions on PFAS transport and retention. Comparison of field observations and model simulations for Ellsworth AFB indicate that much of the PFOA and PFOS mass is associated with the air-water interface and the solid phase, which limits their migration potential in the vadose zone. Results also show that rates of migration in the aqueous phase are largely controlled by hydrogeologic properties, including recharge rates and hydraulic conductivity. AFFF spill scenarios varying in volume, concentration, and frequency reveal the importance of release characteristics in determining rates of PFAS migration and concentration peaks. Variability is attributed to non-linear sorption processes, where, contrary to simple linear partitioning formulations, transport is strongly affected by the concentration of PFAS species. Simulations also demonstrate the importance of modeling the AFFF as a mixture since competitive interfacial accumulation effects are shown to enhance the mobility of less surface-active PFAS compounds.

Enhanced aggregation and interfacial adsorption of an aqueous film forming foam (AFFF) in high salinity matrices

Per- and polyfluoroalkyl substances (PFAS) exist in contaminated groundwater, surface water, soil, and sediments from use of aqueous film forming foams (AFFFs). Under these conditions PFAS exhibit unusual behavior due to their surfactant properties, namely, aggregation and surface activity. Environmental factors such as salinity can affect these properties, and complicate efforts to monitor PFAS. The effect of high salinity matrices on the critical micelle concentration (CMC) of a AFFF formulation manufactured by 3M and the surface accumulation of PFAS was assessed with surface tension isotherm measurements and bench-scale experiments quantifying PFAS at the air-water interface. Conditions typical of brackish and saline waters substantially depressed the CMC of the AFFF by over 50% and increased the interfacial mass accumulation of PFAS in the AFFF mixture by up to a factor of 3, relative to values measured in ultrapure water. These results indicate that high salinity matrices increase the aggregation and surface activity of PFAS in mixtures, which are key properties affecting their transport.

A systematic study of the competitive sorption of per- and polyfluoroalkyl substances (PFAS) on colloidal activated carbon

Treatment of environmental media contaminated with per- and polyfluoroalkyl substances (PFAS) is crucial to mitigate mounting health risks associated with exposure. Colloidal activated carbon (CAC) has shown promise in treating contaminated soils, but understanding the interaction among PFAS during sorption is necessary for optimal remediation. This study investigated the extent to which PFAS of varying chain lengths and functional groups compete for sorption to CAC. Batch tests were conducted with natural soil and spiked water, using CAC at 0.2% w/w to remove seven PFAS with individual starting concentrations up to 0.05 mmol L-1. PFAS sorption to CAC was evaluated in three systems: a composite mixture of all studied compounds, a binary-solute system, and a single-solute system. The sorption experiments exhibited strong PFAS affinity to CAC, with removal rates between 41% and 100%, and solid/liquid partition coefficients (Kd) between 10 and 104 L kg-1. Differences were noticed among the various spiking mixtures, based on perfluorocarbon chain length, functional group, and the starting PFAS concentrations. Competition effects were detected when PFAS were in a multi-solute system, with an average 10% drop in removal, which can evidently become more relevant at higher concentrations, due to the observed non-linearity of the sorption process. The PFAS most vulnerable to competition effects in multi-solute systems were the short-chain perfluoropentanoic acid (PFPeA) and perfluorobutane sulfonic acid (PFBS), with an up to 25% reduction in removal. In bi-solute systems, perfluorooctane sulfonamide (FOSA) dominated over its ionisable counterparts, i.e. perfluorooctane sulfonic acid (PFOS) and perfluorononanoic acid (PFNA), indicating the importance of hydrophobic effects or layer formation in the sorption process. These results underscore the importance of considering competition in PFAS sorption processes when designing and implementing remediation techniques for PFAS-contaminated media.

Predicting Interfacial Tension and Adsorption at Fluid-Fluid Interfaces for Mixtures of PFAS and/or Hydrocarbon Surfactants

Many per- and polyfluoroalkyl substances (PFAS) are surface-active and adsorb at fluid-fluid interfaces. The interfacial adsorption controls PFAS transport in multiple environmental systems, including leaching through soils, accumulation in aerosols, and treatment methods such as foam fractionation. Most PFAS contamination sites comprise mixtures of PFAS as well as hydrocarbon surfactants, which complicates their adsorption behaviors. We present a mathematical model for predicting interfacial tension and adsorption at fluid-fluid interfaces for multicomponent PFAS and hydrocarbon surfactants. The model is derived from simplifying a prior advanced thermodynamic-based model and applies to nonionic and ionic mixtures of the same charge sign with swamping electrolytes. The only required model inputs are the single-component Szyszkowski parameters obtained for the individual components. We validate the model using literature interfacial tension data of air-water and NAPL (non-aqueous phase liquid)-water interfaces covering a wide range of multicomponent PFAS and hydrocarbon surfactants. Application of the model to representative porewater PFAS concentrations in the vadose zone suggests competitive adsorption can significantly reduce PFAS retention (up to 7 times) at some highly contaminated sites. The multicomponent model can be readily incorporated into transport models to simulate the migration of mixtures of PFAS and/or hydrocarbon surfactants in the environment.

Impacts of Nanobubbles in Pore Water on Heavy Metal Pollutant Release from Contaminated Soil Columns

This study investigated the release of heavy metals from polluted soil under the pore water flow containing nanobubbles (NBs) to simulate natural ebullition. Three types of NBs (CH₄, H₂, and CO₂) were generated in water and characterized, including bubble size, zeta potential, liquid density, and tension. The flow rate used in column tests was optimized to achieve proper soil fluidization and metal desorption or release. The leachate chemistries were monitored to assess the effect of NBs on conductivity, pH, oxidation-reduction potential (ORP), and dissolved oxygen (DO). The results showed that NBs in the pore water flow were significantly more effective in releasing Pb compared to DI water, with CO₂ NB water being the most effective and H₂ NB water being the least effective. CO₂ NB water was also used to rinse column soil contaminated with four different metals (Pb, Cu, Zn, and Cr), which exhibited different leaching kinetics. Moreover, a convective-dispersion-deposition equation (CDDE) model accurately simulated the leaching kinetics and explained the effects of NBs on the key parameters, such as the deposition rate coefficient (K_d), that affect the released metal transport. The findings could provide new insights into soil pollutant release under ebullition and soil remediation using water wash containing NBs.

Predicting Concentration- and Ionic-Strength-Dependent Air-Water Interfacial Partitioning Parameters of PFASs Using Quantitative Structure-Property Relationships (QSPRs)

Air-water interfacial retention of poly- and perfluoroalkyl substances (PFASs) is increasingly recognized as an important environmental process. Herein, column transport experiments were used to measure air-water interfacial partitioning values for several perfluoroalkyl ethers and for PFASs derived from aqueous film-forming foam, while batch experiments were used to determine equilibrium K_ia data for compounds exhibiting evidence of rate-limited partitioning. Experimental results suggest a Freundlich isotherm best describes PFAS air-water partitioning at environmentally relevant concentrations (10¹-10⁶ ng/L). A multiparameter regression analysis for K_ia prediction was performed for the 15 PFASs for which equilibrium K_ia values were determined, assessing 246 possible combinations of 8 physicochemical and system properties. Quantitative structure-property relationships (QSPRs) based on three to four parameters provided predictions of high accuracy without model overparameterization. Two QSPRs (R² values of 0.92 and 0.83) were developed using an assumed average Freundlich n value of 0.65 and validated across a range of relevant concentrations for perfluorooctane sulfonate (PFOS), perfluorooctanoate (PFOA), and hexafluoropropylene oxide-dimer acid (i.e., GenX). A mass action model was further modified to account for the changing ionic strength on PFAS air-water interfacial sorption. The final result was two distinct QSPRs for estimating PFAS air-water interfacial partitioning across a range of aqueous concentrations and ionic strengths.

High density polyethylene (HDPE) and thermoplastic polyurethane (TPU) wristbands as personal passive samplers monitoring per- and polyfluoroalkyl substances (PFASs) exposure to postgraduate students

Per- and polyfluoroalkyl substances (PFASs) present adverse effects for human health, which result in strong needs for reliable tools monitoring personal exposure to PFASs. This study manufactured two wristbands of high density polyethylene (HDPE) and thermoplastic polyurethane (TPU), and used the wristbands to monitor PFASs personal exposure. The analytical method was developed to measure 32 PFASs in the paired HDPE and TPU wristbands worn by 60 postgraduates. Twenty-nine of 32 PFASs were detected and hexafluoropropylene oxide dimer acid (HFPO-DA) was predominant individual PFASs with median concentrations of 337 and 554 pg/g for HDPE and TPU wristbands respectively. The gender and grade of students had moderate effects on PFASs distribution in the wristbands. Higher PFASs levels were determined in the two wristbands worn by the male students compared to the females, and the greatest PFASs concentration was observed in the wristbands worn by the first-year postgraduates, follow by second- and third-year postgraduates. Additionally, significant correlations between paired HDPE and TPU wristbands were observed for perfluorobutanoic acid (PFBA), perfluorohexane sulfonic acid (PFHxS), perfluoroheptane sulfonic acid (PFHpS), perfluorooctane sulfonic acid (PFOS), and HFPO-DA. These results suggest that HDPE and TPU wristbands can be used as effective tools for monitoring personal PFAS exposure.

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) in groundwater: current understandings and challenges to overcome

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) have been frequently detected in groundwater globally. With the phase-out of perfluorooctane sulfonate (PFOS) and perfluorooctanate (PFOA) due to their risk to the ecosystem and human population, various novel PFASs have been used as replacements and detected in groundwater. In order to summarize the current understanding and knowledge gaps on PFASs in groundwater, we reviewed the studies about environmental occurrence, transport, and risk of legacy and novel PFASs in groundwater published from 1999 to 2021. Our review suggests that PFOS and PFOA could still be detected in groundwater due to the long residence time and the retention in the soil-groundwater system. Firefighting training sites, industrial parks, and landfills were commonly hotspots of PFASs in groundwater. More novel PFASs have been detected via nontarget analysis using high-resolution mass spectrometry. Some novel PFASs had concentrations comparable to that of PFOS and PFOA. Both legacy and novel PFASs can pose a risk to human population who rely on contaminated groundwater as drinking water. Transport of PFASs to groundwater is influenced by various factors, i.e., the compound structure, the hydrochemical condition, and terrain. The exchange of PFASs between groundwater and surface water needs to be better characterized. Field monitoring, isotope tracing, nontarget screening, and modeling are useful approaches and should be integrated to get a comprehensive understanding of PFASs sources and behaviors in groundwater.

Influence of Residual Nonaqueous-Phase Liquids (NAPLs) on the Transport and Retention of Perfluoroalkyl Substances

Per- and polyfluoralkyl substances (PFAS) are known to accumulate at interfaces, and the presence of nonaqueous-phase liquids (NAPLs) could influence the PFAS fate in the subsurface. Experimental and mathematical modeling studies were conducted to investigate the effect of a representative NAPL, tetrachloroethene (PCE), on the transport behavior of PFAS in a quartz sand. Perfluorooctanesulfonate (PFOS), perfluorononanoic acid (PFNA), a 1:1 mixture of PFOS and PFNA, and a mixture of six PFAS (PFOS, PFNA, perfluorooctanoic acid (PFOA), perfluoroheptanoic acid (PFHpA), perfluorohexanesulfonate (PFHxS), and perfluorobutanesulfonate (PFBS)) were used to assess PFAS interactions with PCE-NAPL. Batch studies indicated that PFAS partitioning into PCE-NAPL (K_nw < 0.1) and adsorption on 60-80 mesh Ottawa sand (K_d < 6 × 10^-5 L/g) were minimal. Column studies demonstrated that the presence of residual PCE-NAPL (∼16% saturation) delayed the breakthrough of PFOS and PFNA, with minimal effects on the mobility of PFBS, PFHpA, PFHxS, and PFOA. Breakthrough curves (BTCs) obtained for PFNA and PFOS alone and in mixtures were nearly identical, indicating the absence of competitive adsorption effects. A mathematical model that accounts for NAPL-water interfacial sorption accurately reproduced PFAS BTCs, providing a tool to predict PFAS fate and transport in co-contaminated subsurface environments.

Estimation of Transport Parameters of Perfluoroalkyl Acids (PFAAs) in Unsaturated Porous Media: Critical Experimental and Modeling Improvements

Predicting the transport of perfluoroalkyl acids (PFAAs) in the vadose zone is critically important for PFAA site cleanup and risk mitigation. PFAAs exhibit several unusual and poorly understood transport behaviors, including partitioning to the air-water interface, which is currently the subject of debate. This study develops a novel use of quasi-saturated (residual air saturation) column experiments to estimate chemical partitioning parameters of both linear and branched perfluorooctane sulfonate (PFOS) in unsaturated soils. The ratio of linear-to-branched air-water interfacial partitioning constants for all six experiments was 1.62 ± 0.24, indicating significantly greater partitioning of linear PFOS isomers at the air-water interface. Standard breakthrough curve analysis and numerical inversion of HYDRUS models support the application of a Freundlich isotherm for PFOS air-water interfacial partitioning below a critical reference concentration (CRC). Data from this study and previously reported unsaturated column data on perfluorooctanoate (PFOA) were reevaluated to examine unsaturated systems for transport nonidealities. This reanalysis suggests both transport nonidealities and Freundlich isotherm behavior for PFOA below the CRC using drainage-based column methods, contrary to the assertions of the original authors. Finally, a combined Freundlich-Langmuir isotherm was proposed to describe PFAA air-water interfacial partitioning across the full range of relevant PFAA concentrations.

Unification of surface tension isotherms of PFOA or GenX salts in electrolyte solutions by mean ionic activity

The surface tension isotherms of soluble salts of per- and polyfluoroalkyl substances (PFAS) in electrolyte solutions are typically reported as functions of the PFAS concentration. However, for univalent salts and electrolytes, the Langmuir-Szyszkowski equation is a function of the mean ionic activity a*. Using previously reported data, we show that for salts of perfluorooctanoic acid (PFOA) or hexafluoropropylene oxide dimer acid (GenX™), use of a* rather than concentration provides a unified surface tension isotherm, independent of the electrolyte concentration. This suggests that the electrolyte dependence of the isotherm arises purely from its effect on PFAS activity, rather than an intrinsic surface property. This finding has important implications for the understanding of PFAS retention in saline unsaturated soils, and for PFAS extraction from saline waters by foam fractionation.

On Per- and Polyfluoroalkyl Substances: Suggested Resources and Considerations for Groundwater Professionals

Per- and polyfluoroalkyl substances, a synthetic class of chemicals comprising thousands of compounds, are receiving increased attention over their presence in the environment, their potential effects on human health, and evolving regulation. This article summarizes suggested resources and key considerations for groundwater professionals wishing to familiarize themselves with this class of compounds. Background information, current groundwater-related regulations, risk considerations, comparison to other groundwater contaminants, and mitigation options are discussed, and a broad selection of references is supplied as a resource.

Influence of aqueous film forming foams on the solubility and mobilization of non-aqueous phase liquid contaminants in quartz sands

At sites where aqueous film forming foams (AFFFs) are used for fire suppression or training activities, interactions between dissolved foam constituents and organic liquids could alter contaminant migration in the subsurface. In this study, batch reactor and column experiments were conducted to investigate the potential for AFFF solutions to enhance the solubility and mobility of three representative non-aqueous phase liquid (NAPLs), JP-4 jet fuel, trichloroethene (TCE), and tetrachloroethene (PCE). For AFFF concentrations up to 5% wt. (50,000 mg/L), aqueous solubilities of TCE and PCE increased by less than 50%, indicating the absence of micellar solubilization. However, NAPL-water interfacial tensions were reduced to less than 1.5 mN/m and resulted in accumulation of up to 2.25 mg/m² of AFFF at the NAPL-water interface. To assess the potential for AFFF to mobilize residual (entrapped) NAPL at a field application rate of 3% wt. (30,000 mg/L), columns were packed with two size fractions of Ottawa sands (20-30 mesh and 60-80 mesh) that yielded residual NAPL saturations ranging from 11.7 to 17.6%. Following injection of 3 pore volumes of the 3% wt. AFFF solution, partial mobilization of residual NAPL was observed for PCE, TCE, and JP-4, with saturation reductions of 0.7 to 2% in 20-30 mesh and 0.3% to 1.3% in 60-80 mesh Ottawa sand. The columns were then flushed with an ultralow-IFT surfactant solution consisting of 4% wt. 1:1 Aerosol AY/OT, which resulted in nearly complete mobilization of the remaining residual NAPL. When NAPL desaturation curves were expressed in terms of the total trapping number (N_T), the threshold value of N_T required for NAPL mobilization by 3% wt. AFFF was approximately 2 × 10^-5, consistent with previous studies. These findings demonstrate that AFFF solutions can lead to partial mobilization of residual NAPL, and that the total trapping number concept can be used to predict such behavior a priori. In addition, the observed IFT reductions are sufficient to alter NAPL accumulation and redistribution behavior in the subsurface through enhanced spreading above the water table (e.g., JP-4) or penetration into lower-permeability media (e.g., TCE, PCE).