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Cited by 224Per- and polyfluoroalkyl substances researchGroundwater flow and contamination studiesSoil and Unsaturated Flow

A Mathematical Model for the Release, Transport, and Retention of Per‐ and Polyfluoroalkyl Substances (PFAS) in the Vadose Zone

Open access
Guo, Bo, Zeng, Jicai, Brusseau, Mark L.·2020·Water Resources Research
DOI 10.1029/2019wr026667PMC PMC7673302
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Abstract

Per‐ and polyfluoroalkyl substances (PFAS) are emerging contaminants of critical concern. As surfactants, PFAS tend to accumulate at air‐water interfaces and may stay in the vadose zone for long times before contaminating groundwater. Yet not well understood, the extent of retention in the vadose zone has critical implications for risk management and remediation strategies. We present the first mathematical model that accounts for surfactant‐induced flow and solid‐phase and air‐water interfacial adsorption. We apply the model to simulate PFOS (a PFAS compound of primary concern) transport in the vadose zone at a model fire‐training area site impacted by aqueous film‐forming foam (AFFF). Air‐water interfacial adsorption is shown to have a significant impact—amplified by the low water content due to gravity drainage—total retardation factors range from 233 to 1,355 for the sand and 146 to 792 for the soil used in the study. The simulations illustrate it can take several decades or longer for PFOS to reach groundwater. Counterintuitively, the lower water content in the sand—due to stronger drainage and weaker capillary retention—leads to retardation factors greater than for the soil. Also, most PFOS is adsorbed at air‐water interfaces with only 1–2% in the aqueous phase. The implications include (1) fine‐texture materials could have lower retardation factors than sand due to higher retained water content, (2) soil PFAS concentrations are likely to be orders of magnitude higher than those in groundwater at source zones. Both implications are consistent with recent field observations at hundreds of AFFF‐impacted sites.

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Crossref · 10.1029/2019wr026667

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Claims the paper makes

0 of 39 cited sources checked · 0 supported · 0 flagged

  • Findingreasoning

    The lower water content in the sand, caused by stronger drainage and weaker capillary retention, produced retardation factors greater than those for the soil.

    “Counterintuitively, the lower water content in the sand—due to stronger drainage and weaker capillary retention—leads to retardation factors greater than for the soil.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: Capillary pressure head scales with surface tension according to Leverett scaling.
    • The authors state this as an assumption: The contact angle does not change when PFAS is dissolved.
    • The authors state this as an assumption: All precipitation is treated as rainfall, with no distinction between rain and snow.
    • The authors state this as an assumption: Evapotranspiration data are approximated as potential evapotranspiration to determine surface evaporation fluxes.
    • The authors state this as an assumption: All applied AFFF solution infiltrates into the vadose zone.
  • Findingown data

    Including interfacial adsorption increased the time for PFOS to reach groundwater by one to two orders of magnitude, from 1-2 years to several decades or longer.

    “The overall time scale for PFOS to reach groundwater has increased by one to two orders of magnitude from 1 or 2 years to several decades or longer.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: Capillary pressure head scales with surface tension according to Leverett scaling.
    • The authors state this as an assumption: The contact angle does not change when PFAS is dissolved.
    • The authors state this as an assumption: All precipitation is treated as rainfall, with no distinction between rain and snow.
    • The authors state this as an assumption: Evapotranspiration data are approximated as potential evapotranspiration to determine surface evaporation fluxes.
    • The authors state this as an assumption: All applied AFFF solution infiltrates into the vadose zone.
  • Findingreasoning

    Among the four scenarios, PFOS moved slowest with low PFOS concentration under a semi-arid climate, with the front migrating only 2 meters after 40 years.

    “Among the four scenarios, PFOS propagates the slowest with the low PFOS concentration under a semi-arid climate—the front has only migrated down 2 meters after 40 years.”
    Quote in the paperNot a substantial claim
    • The authors state this as an assumption: Capillary pressure head scales with surface tension according to Leverett scaling.
    • The authors state this as an assumption: The contact angle does not change when PFAS is dissolved.
    • The authors state this as an assumption: All precipitation is treated as rainfall, with no distinction between rain and snow.
    • The authors state this as an assumption: Evapotranspiration data are approximated as potential evapotranspiration to determine surface evaporation fluxes.
    • The authors state this as an assumption: All applied AFFF solution infiltrates into the vadose zone.
  • Findingown data

    The wetting front reached the groundwater table in about 60 days in the Vinton soil.

    “For Vinton, the wetting front propagation is considerably slower—it takes ~ 60 days for the wetting front to reach the groundwater table (Fig. 4(b)).”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: Capillary pressure head scales with surface tension according to Leverett scaling.
    • The authors state this as an assumption: The contact angle does not change when PFAS is dissolved.
    • The authors state this as an assumption: All precipitation is treated as rainfall, with no distinction between rain and snow.
    • The authors state this as an assumption: Evapotranspiration data are approximated as potential evapotranspiration to determine surface evaporation fluxes.
    • The authors state this as an assumption: All applied AFFF solution infiltrates into the vadose zone.
  • Findingown data

    The wetting front reached the groundwater table in about 5 days in the Accusand.

    “The results show that it takes ~ 5 days for the wetting front to reach the groundwater table in the Accusand (Fig. 4(a)).”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: Capillary pressure head scales with surface tension according to Leverett scaling.
    • The authors state this as an assumption: The contact angle does not change when PFAS is dissolved.
    • The authors state this as an assumption: All precipitation is treated as rainfall, with no distinction between rain and snow.
    • The authors state this as an assumption: Evapotranspiration data are approximated as potential evapotranspiration to determine surface evaporation fluxes.
    • The authors state this as an assumption: All applied AFFF solution infiltrates into the vadose zone.
  • May combine two claims. 34
  • Classified as probably not a substantial claim. 9
  • The quote only partly states this claim. 1

  • May combine two claims.

    Findingown data

    For most of the 5-day Accusand simulation, water saturation stayed below 0.25 and air-water interfacial area stayed above 400 cm2/cm3.

    “For the majority of the time during the 5-day simulation, the water saturation stays below 0.25 (corresponding to a water content of 0.074) and the air-water interfacial area stays above 400 cm2/cm3 (Fig. 4(c)) despite the fact that the conditions represent the largest rainfall event.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: Capillary pressure head scales with surface tension according to Leverett scaling.
    • The authors state this as an assumption: The contact angle does not change when PFAS is dissolved.
    • The authors state this as an assumption: All precipitation is treated as rainfall, with no distinction between rain and snow.
    • The authors state this as an assumption: Evapotranspiration data are approximated as potential evapotranspiration to determine surface evaporation fluxes.
    • The authors state this as an assumption: All applied AFFF solution infiltrates into the vadose zone.

  • May combine two claims.

    Findingown data

    For most of the Vinton simulation, water saturation stayed below 0.35 and air-water interfacial area stayed above 700 cm2/cm3.

    “For the majority of the time, the water saturation is below 0.35 (corresponding to a water content of 0.138) and the air-water interfacial area is above 700 cm2/cm3 (Fig. 4(d)).”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: Capillary pressure head scales with surface tension according to Leverett scaling.
    • The authors state this as an assumption: The contact angle does not change when PFAS is dissolved.
    • The authors state this as an assumption: All precipitation is treated as rainfall, with no distinction between rain and snow.
    • The authors state this as an assumption: Evapotranspiration data are approximated as potential evapotranspiration to determine surface evaporation fluxes.
    • The authors state this as an assumption: All applied AFFF solution infiltrates into the vadose zone.

  • May combine two claims.

    Findingnone given

    Most PFOS was adsorbed at air-water interfaces, with only 1-2% in the aqueous phase.

    “Also, most PFOS is adsorbed at air-water interfaces with only 1–2% in the aqueous phase.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: Capillary pressure head scales with surface tension according to Leverett scaling.
    • The authors state this as an assumption: The contact angle does not change when PFAS is dissolved.
    • The authors state this as an assumption: All precipitation is treated as rainfall, with no distinction between rain and snow.
    • The authors state this as an assumption: Evapotranspiration data are approximated as potential evapotranspiration to determine surface evaporation fluxes.
    • The authors state this as an assumption: All applied AFFF solution infiltrates into the vadose zone.

  • May combine two claims.

    Findingreasoning

    Higher PFOS concentration and a more humid climate both speed up PFOS migration.

    “A higher PFOS concentration and a more humid climate both accelerate the migration of PFOS.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: Capillary pressure head scales with surface tension according to Leverett scaling.
    • The authors state this as an assumption: The contact angle does not change when PFAS is dissolved.
    • The authors state this as an assumption: All precipitation is treated as rainfall, with no distinction between rain and snow.
    • The authors state this as an assumption: Evapotranspiration data are approximated as potential evapotranspiration to determine surface evaporation fluxes.
    • The authors state this as an assumption: All applied AFFF solution infiltrates into the vadose zone.

  • May combine two claims.

    Findingown data

    In the semi-arid case with air-water interfacial adsorption included, about 1.5% of PFOS mass remaining in the vadose zone is in the aqueous phase and about 5.5% is in solid-phase adsorption.

    “Approximately 1.5% is in the aqueous phase, and the remaining 5.5% is in the solid-phase adsorption.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    Simulated water saturation above the groundwater table is generally small, and for Accusand and Vinton soil, saturation and air-water interfacial area are not sensitive to capillary pressure head at low saturations.

    “the simulated water saturation above the groundwater table is generally small, and for Accusand and Vinton soil, saturation (and thus air-water interfacial area) is not sensitive to capillary pressure head (or water pressure head) at low saturations”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The majority of PFOS in the vadose zone is retained at air-water interfaces, with only one or two percent in the aqueous phase.

    “The other critical finding from our simulations is that the majority of the PFOS in the vadose zone is retained at the air-water interfaces—only one or two percent is in the aqueous phase.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    In Vinton soil when air-water interfacial adsorption is turned off, about 60% of total released PFOS remains in the vadose zone in the semi-arid climate and about 20% in the humid climate, almost 10 times greater than Accusand.

    “Approximately 60% and 20% of the total released PFOS remain in the vadose zone for the semi-arid and humid climates respectively, which are almost 10 times greater than those in the Accusand scenarios.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    In the humid-climate case with air-water interfacial adsorption included, about 97% of PFOS mass remaining in the vadose zone is adsorbed at air-water interfaces, about 1% is in the aqueous phase, and about 2% is in solid-phase adsorption.

    “For the humid-climate case, approximately 97% is adsorbed at the air-water interfaces, 1% is in the aqueous phase, and 2% is in the solid-phase.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    At the maximum plume concentration, air-water interfacial adsorption accounts for about 99% of total PFOS in the low PFOS concentration scenario and about 95% in the high PFOS concentration scenario.

    “at the maximum concentration of the plume, the air-water interfacial adsorption accounts for roughly 99% and 95% of the total PFOS in the low and high PFOS concentration scenarios, respectively.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    When air-water interfacial adsorption is not considered, about 7% of total released PFOS remains in the vadose zone at 40 years in the semi-arid climate and about 2% in the humid climate.

    “When air-water interfacial adsorption is not considered, the total mass left in the vadose zone is substantially lower—approximately 7% and 2% remain in the vadose zone for the semi-arid and humid climates, respectively.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    When air-water interfacial adsorption is included, about 58% of total released PFOS remains in the vadose zone at 40 years in the semi-arid climate and about 45% in the humid climate.

    “When air-water interfacial adsorption is considered, approximately 58% and 45% of the total released PFOS remains in the vadose zone at 40 years, for the semi-arid and humid climates, respectively.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    In Vinton soil, solid-phase adsorption accounts for about 30% of total mass in the vadose zone in the semi-arid climate and about 17% in the humid climate.

    “The solid-phase adsorption accounts for about 30% and 17% of the total mass in the vadose zone for the semi-arid and humid climates, respectively (Fig. 10), which are much higher than the corresponding Accusand values.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). May combine two claims.

    Findingnone given

    A humid climate leads to higher water content and lower air-water interfacial area than a semi-arid climate.

    “a humid climate leads to a higher water content and a lower air-water interfacial area than a semi-arid climate.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). The quote only partly states this claim.

    Findingnone given

    The humid climate leads to higher average water content than the semi-arid climate.

    “The humid climate leads to higher average water content as expected.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingreasoning

    In Vinton soil simulations including air-water interfacial adsorption, average retardation factors are on the order of several hundreds and lower than Accusand retardation factors.

    “The retardation factors are on the order of several hundreds—though still very large—they are lower compared to the Accusand retardation factors.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingown data

    The results show that the majority of PFOS in the vadose zone is retained at air-water interfaces (70% or more) and solid surfaces (5% to 30%), leaving only one or two percent in the aqueous phase.

    “the results show that the majority of the PFOS in the vadose zone is retained at the air-water interfaces (70% or more) and the solid surfaces (5% to 30%) leaving only one or two percent in the aqueous phase.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Costanza-Robinson and Henry (2017) held adsorption coefficients constant within each simulation, which underestimated nonlinear adsorption responses under transient flow.

    “The adsorption coefficients—though varying among simulations—were kept constant during each simulation, and thus have underestimated the nonlinear responses of the adsorption processes under transient flow.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Costanza-Robinson and Henry (2017)

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    Under a more humid climate, PFOS migrates faster because more water flows through the vadose zone with higher pore water velocities and because higher water content reduces air-water interfacial adsorption.

    “Under a more humid climate, it is expected that PFOS will migrate faster because: 1) more water is flowing through the vadose zone to groundwater with higher pore water velocities leading to more “leaching”, and 2) the water content is likely higher leading to less air-water interfacial adsorption.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: Capillary pressure head scales with surface tension according to Leverett scaling.
    • The authors state this as an assumption: The contact angle does not change when PFAS is dissolved.
    • The authors state this as an assumption: All precipitation is treated as rainfall, with no distinction between rain and snow.
    • The authors state this as an assumption: Evapotranspiration data are approximated as potential evapotranspiration to determine surface evaporation fluxes.
    • The authors state this as an assumption: All applied AFFF solution infiltrates into the vadose zone.

  • May combine two claims.

    Premisecitation

    Air-water interfacial area depends on capillary pressure head, water saturation, and drainage/imbibition history.

    “The functional relationship between air-water interfacial area and water saturation will in general depend on the drainage and imbibition history (Hassanizadeh & Gray, 1993; Reeves & Celia, 1996; Schaefer et al., 2000).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Hassanizadeh & Gray, 1993

      Not checked in the free run.

    • cites Reeves & Celia, 1996

      Not checked in the free run.

    • cites Schaefer et al., 2000

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    PFAS can be released from fire-training areas, fire response sites, industrial sites, landfills, and wastewater treatment plants.

    “PFAS can be released from various types of sources including fire-training area (FTA) sites, fire response sites, industrial sites, landfills, and wastewater treatment plants (ITRC, 2018).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites ITRC, 2018

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    A 1:100 dilution of an AFFF concentrate gives a PFOS concentration of 100 mg/L, and PFOS makes up 88.7% of total PFAS.

    “Høisæter et al. (2019) reported that 1:100 dilution of an AFFF concentrate leads to a PFOS concentration of 100 mg/L; PFOS is the dominant PFAS accounting for 88.7% of the total PFAS.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Høisæter et al., 2019

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    The lower Vinton retardation factors are counterintuitive because Vinton has larger interfacial areas and stronger solid-phase adsorption than Accusand.

    “This is rather counterintuitive because the Vinton soil has larger interfacial areas and also stronger solid-phase adsorption.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    Anderson et al. (2019) showed that the ratio between PFAS concentration in soil and groundwater decreases with increasing clay content, which they hypothesized is due to higher water saturation with higher clay content.

    “Anderson et al. (2019) showed that the ratio between the PFAS concentration in the soil and groundwater decreases with increasing clay content, which they hypothesized is due to the higher water saturation in the presence of higher clay content.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Anderson et al., 2019

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    PFOS adsorbed at air-water interfaces is released to the aqueous phase during large infiltration, and most of it is then adsorbed to solid surfaces.

    “The PFOS adsorbed at the air-water interfaces is released to the aqueous phase and the majority of that is then adsorbed to the solid surface.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Average water content is slightly lower for higher PFOS concentration, possibly because of greater surfactant-induced drainage caused by a greater decrease in surface tension at higher aqueous PFOS concentration.

    “It is slightly lower for higher PFOS concentration because of possibly greater surfactant-induced drainage caused by a greater decrease of surface tension at a higher aqueous PFOS concentration.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    Anderson et al. (2019) analyzed 324 Air Force sites across the continental US impacted by AFFF and observed higher soil PFAS concentrations in 87% of the data collected.

    “the study by Anderson et al. (2019) was based on an analysis of 324 Air Force sites across the continental US impacted by AFFF, and higher soil PFAS concentrations were observed in the vast majority (87%) of the data collected.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Anderson et al., 2019

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductnone given

    Hysteresis effects are not included in the model.

    “Hysteresis effects are not considered.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationnone given

    A shallower groundwater table may increase overall water content if the porous medium has a thick capillary transition zone.

    “A 530 shallower groundwater table may increase the overall water content if the porous medium 531 has a thick capillary transition zone.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    The reason Vinton has lower retardation factors is that although it has larger interfacial area than Accusand for the same water saturation, it retains much higher water saturation due to stronger capillary forces.

    “The reason is that though the Vinton soil has a larger interfacial area than the Accusand for the same water saturation, it retains much higher water saturation than the Accusand does in the vadose zone due to stronger capillary forces.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Study design and conductnone given

    The model has not been compared directly to experimental data because such data does not yet exist.

    “Though we have not compared our model directly to experimental data (such data does not yet exist)”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Low water content increases the retardation factor because it is in the denominator of the retardation expression and because it leads to a large interfacial area and thus more interfacial adsorption.

    “A low water content increases the retardation factor because: 1) it is in the denominator of the expression for the retardation factor, and 2) it leads to a large interfacial area and thus more interfacial adsorption.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Study design and conductown data

    The numerical implementation was verified against HYDRUS-1D for water infiltration and solute transport, and the two codes agreed excellently.

    “To verify our implementation of the Richards’ and advection-dispersion equations, we have compared our code with HYDRUS-1D for a simulation of water infiltration and solute transport (see the supporting information (SI)). Excellent agreement between the results from the two codes is observed (Fig. S1 in the SI).”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Study design and conductown data

    For all simulations, relative global mass conservation error at the end of 40 years is less than 0.1% for water and less than 0.005% for PFOS.

    “For all simulations performed in the present work, the relative global mass conservation error (at the end of 40 years) is less than 0.1% for water and less than 0.005% for PFOS, respectively.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Accusand has much lower water content than Vinton soil (more than 3 times lower), which is the main reason for Accusand's greater retardation factors.

    “A closer inspection reveals that the Accusand has much lower water content than the Vinton soil (more than 3 times lower), which is the main reason responsible for the Accusand’s greater retardation factors.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationown data

    The solid-phase adsorption contribution to retardation in Vinton soil is about 4 times that in Accusand because Vinton has almost 9 times greater solid-phase adsorption capacity.

    “When looking at the contributions to the retardation factors associated with the solid-phase adsorption alone (f¯spa), they are ~ 4 times that of the Accusand f¯spa values because Vinton has a solid-phase adsorption capacity that is almost 9 times greater.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S4 (based).

    Study design and conductnone given

    The simulations converged with respect to grid size and time step.

    “Our simulations have converged for Δx and Δt.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationnone given

    Seasonal and interannual variability of the groundwater table may influence water saturation and air-water interfaces in the vadose zone, especially when the vadose zone is shallow.

    “the seasonal and interannual variability of the groundwater table may influence the water saturation in the vadose zone and hence the air-water interfaces, especially when the vadose zone is relatively shallow.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    Not accounting for transpiration fluxes due to plant uptake will likely overestimate water content and underestimate the time-scale of retention in the vadose zone.

    “not accounting for transpiration fluxes due to plant uptake will likely overestimate the water content and thus underestimate the time-scale of retention in the vadose zone.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Premisenone given

    This implication is consistent with recently reported data at hundreds of AFFF-impacted FTA sites in the US and elsewhere.

    “which is consistent with recently reported data at hundreds of AFFF-impacted FTA sites in the US and elsewhere.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    Dauchy et al. (2019) showed that PFAS were detected 15 m below ground despite clay layers, implying that retardation factors in clay may not be as great as usually thought for other contaminants.

    “Dauchy et al. (2019) showed that PFAS were detected 15m below ground despite the presence of clay layers implying that the retardation factors in clay may not be as great as what is usually thought for other contaminants.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Dauchy et al., 2019

      Not checked in the free run.

41 of 41

  • The authors state this as an assumption.

    Capillary pressure head scales with surface tension according to Leverett scaling.

    “Here we assume that the capillary pressure head (−h) scales with surface tension σ following Leverett et al. (1941) and Smith and Gillham (1994)”

  • The authors state this as an assumption.

    The contact angle does not change when PFAS is dissolved.

    “For simplicity, the contact angle is assumed unchanged with dissolved PFAS.”

  • The authors state this as an assumption.

    All precipitation is treated as rainfall, with no distinction between rain and snow.

    “For simplicity, we treat all precipitation as rainfall and do not distinguish between rain and snow in our simulations.”

  • The authors state this as an assumption.

    Evapotranspiration data are approximated as potential evapotranspiration to determine surface evaporation fluxes.

    “In addition, we approximate the evapotranspiration data as potential evapotranspiration to determine the surface evaporation fluxes.”

  • The authors state this as an assumption.

    All applied AFFF solution infiltrates into the vadose zone.

    “Assuming all of the AFFF solutions infiltrate into the vadose zone, it gives a total infiltration of 0.0458cm per training session.”

  • The paper relies on this without support.

    The time scale over which PFAS may remain in the vadose zone due to fluid-fluid interfacial adsorption and the potential for long-term release to groundwater are unknown.

    “In particular, the time scale that PFAS may stay in the vadose zone due to fluid-fluid interfacial adsorption and the potential long-term release to groundwater are unknown.”

  • The paper relies on this without support.

    PFAS behave as surfactants and tend to accumulate at air-water interfaces, potentially remaining in the vadose zone for long periods.

    “when released to the environment they tend to accumulate at air-water interfaces and may stay in the vadose zone for long periods of time (Fig. 1(b,c)).”

  • The paper relies on this without support.

    The implications are consistent with recent field observations at hundreds of AFFF-impacted sites.

    “Both implications are consistent with recent field observations at hundreds of AFFF-impacted sites.”

  • The paper relies on this without support.

    Laboratory-measured parameters used in the simulations are representative of field-scale vadose zone conditions.

    “though our simulations used laboratory measured parameters”
9 of 9

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • The AWI sorption capability of the extended LEACHM code has been successfully benchmarked against Guo et al. (2020) and Zeng et al. (2021).

    “The AWI sorption capability of the extended LEACHM code has been successfully benchmarked against Guo et al. (2020) and Zeng et al. (2021”

    From Model-based identification of vadose zone controls on PFAS mobility under semi-arid climate conditions (2022) · cites this paper as Guo et al. (2020)

  • Prior field studies and theoretical models suggest that retention of PFAS in the vadose zone due to partitioning at the air-water interface and biotransformation of precursors may sustain long-term downstream contamination of the more mobile perfluoroalkyl sulfonates and perfluoroalkyl carboxylates.

    “These studies suggest that retention of PFAS in the vadose zone due to partitioning at the air–water interface and biotransformation of precursors may sustain long-term downstream contamination of the more mobile perfluoroalkyl sulfonates (PFSA) and PFCA.”

    From Centurial Persistence of Forever Chemicals at Military Fire Training Sites (2023) · cites this paper as [14]

  • These results agree with mathematical and experimental evidence showing much larger solids retention of PFAS under unsaturated compared to saturated conditions.

    “These results agree with mathematical and experimental evidence showing much larger solids retention of PFAS under unsaturated compared to saturated conditions.14,33”

    From Centurial Persistence of Forever Chemicals at Military Fire Training Sites (2023) · cites this paper as [14]

  • Results from theoretical models and laboratory experiments in prior work suggest that PFAS retention at the air-water interface in the vadose zone is likely to be a rate-limiting step mediating attenuation at AFFF-contaminated sites.

    “Results from theoretical models and laboratory experiments in prior work suggest that PFAS retention at the air–water interface in the vadose zone is likely to be a rate-limiting step mediating attenuation at AFFF-contaminated sites.14,49”

    From Centurial Persistence of Forever Chemicals at Military Fire Training Sites (2023) · cites this paper as [14]

  • The simulation code represented transient flow under variably saturated conditions, surfactant effects on flow, and both nonlinear and rate-limited sorption to solids and air-water interfaces.

    “The mathematical model that was employed to conduct the numerical simulations accounts for transient variably saturated flow, surfactant-induced flow, nonlinear and rate-limited solid-phase sorption, and nonlinear and rate-limited air-water interfacial adsorption (Guo et al., 2020).”

    From PFAS concentrations in soil versus soil porewater: Mass distributions and the impact of adsorption at air-water interfaces (2022) · cites this paper as Guo et al., 2020

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • First-model claim conflicts with the paper's account of earlier work

    Minor

    The abstract calls this the first model to include surfactant-induced flow and both adsorption processes. The introduction describes an earlier model that included those same processes, although it treated the adsorption coefficients as constant during each simulation.

1 of 1

Cites

Flagged first, then by citations

Research works printed but not linked: 2

  1. 10.1021/es981355 · DOI
  2. Simunek J., 2008, The HYDRUS‐1D software package for simulating the one‐dimensional movement of water, Heat, and Multiple Solutes in Variably‐Saturated Media, Version, 4, 281

Cited by

Sorted by citations

Related

Authors
Guo, B., Zeng, J., & Brusseau, M. L.
Affiliations
University of Arizona (US)
Corresponding author
Guo, Bo
ORCID
Venue
Water Resources Research
Publisher
American Geophysical Union (AGU)
Published
2020-02
Volume
56
Issue
2
ISSN
  • 0043-1397 (ISSN-L)
  • 1944-7973
DOI
10.1029/2019wr026667
PMC
PMC7673302
OpenAlex
W2999663693
Cited by
224
References
74
Citations by year
2026: 45 · 2025: 37 · 2024: 38 · 2023: 27 · 2022: 31 · 2021: 37 · 2020: 9
FWCI
9.64
Type
journal article
Language
en
Open access
Licence
onlinelibrary.wiley.com/termsAndConditions#vor (vor, 6 d)
Locations
  • Water Resources Research (publishedVersion)
  • PubMed (publishedVersion)
  • Europe PMC (PubMed Central) (acceptedVersion)
  • PubMed Central (acceptedVersion)
  • UA Campus Repository (The University of Arizona) (acceptedVersion)
Funders
National Institute of Environmental Health Sciences (P42ES004940)
Topics
  • Physical Sciences › Environmental Science › Environmental Chemistry › Per- and polyfluoroalkyl substances research
  • Physical Sciences › Environmental Science › Environmental Engineering › Groundwater flow and contamination studies
  • Physical Sciences › Engineering › Civil and Structural Engineering › Soil and Unsaturated Flow
Keywords
per- and polyfluoroalkyl substances, PFOS, vadose zone, air-water interface adsorption, solid-phase adsorption, retention factor, groundwater contamination, aqueous film-forming foam, gravity drainage, capillary retention, source zone

About these checks

Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.

Claims read by deepseek-v4.1-flash under definition version 3 (extract-v3) on 2026-10-06.

Classified by jev-1.13-20260917 (gold-guideline-v0.2) on 2026-10-06.

Statistics and arithmetic are recomputed, not judged.

Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.

What counts as a claim ›How we measure confidence ›How we check this ›

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