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Advection and Groundwater Flow
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==PFAS Transport and Fate==
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The transport and fate of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]] in the environment is controlled by the nature of the PFAS source, characteristics of the individual PFAS, and environmental conditions where the PFAS are present.  Transport, partitioning, and transformation are the primary processes controlling PFAS fate in the environment. PFAS compounds can also be taken up by both plants and animals, and in some cases, bioaccumulate through the food chain.
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Understanding PFAS transport and fate is necessary for evaluating the potential risk from a PFAS release and for predictions about PFAS occurrence, migration, and persistence, and about the potential vectors for exposure. This knowledge is important for site characterization, identification of potential sources of PFAS to the site, development of an appropriate conceptual site model (CSM), and selection and predicted performance of remediation strategies.
  
Groundwater migrates from areas of higher [[wikipedia: Hydraulic head | hydraulic head]] (a measure of pressure and gravitational energy) toward lower hydraulic head, transporting dissolved solutes through the combined processes of [[wikipedia: Advection | advection]] and [[wikipedia: Dispersion | dispersion]].  Advection refers to the bulk movement of solutes carried by flowing groundwater.  Dispersion refers to the spreading of the contaminant plume from highly concentrated areas to less concentrated areas.  Dispersion coefficients are calculated as the sum of molecular [[wikipedia: Diffusion | diffusion]], mechanical dispersion, and macrodispersion. In many groundwater transport models, solute transport is described by the advection-dispersion-reaction equation. 
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<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
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'''Related Article(s): '''
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* [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
  
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
 
  
'''Related Article(s):'''
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'''Contributor(s): '''  
*[[Dispersion and Diffusion]]
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Dr. Hunter Anderson and Dr. Mark L. Brusseau
*[[Sorption of Organic Contaminants]]
 
*[[Plume Response Modeling]]
 
  
'''CONTRIBUTOR(S):'''
 
*[[Dr. Charles Newell, P.E.]]
 
*[[Dr. Robert Borden, P.E.]]
 
  
'''Key Resource(s):'''
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'''Key Resource(s): '''
*[http://hydrogeologistswithoutborders.org/wordpress/1979-english/ Groundwater]<ref name="FandC1979">Freeze, A., and Cherry, J., 1979. Groundwater, Prentice-Hall, Englewood Cliffs, New Jersey, 604 pages. Free download from [http://hydrogeologistswithoutborders.org/wordpress/1979-english/ Hydrogeologists Without Borders].</ref>, Freeze and Cherry, 1979.
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*[https://pfas-1.itrcweb.org/wp-content/uploads/2020/04/ITRC_PFAS_TechReg_April2020.pdf  Per- and Polyfluoroalkyl Substances (PFAS), PFAS-1. ITRC 2020.]<ref name="ITRC2020">Interstate Technology and Regulatory Council (ITRC), 2020. Technical/Regulatory Guidance: Per- and Polyfluoroalkyl Substances (PFAS), PFAS-1. ITRC, PFAS Team, Washington DC. [https://pfas-1.itrcweb.org/wp-content/uploads/2020/04/ITRC_PFAS_TechReg_April2020.pdf  Free Download from ITRC].&nbsp;&nbsp; [[Media: ITRC_PFAS-1.pdf | Report.pdf]]</ref>
*[https://gw-project.org/books/hydrogeologic-properties-of-earth-materials-and-principles-of-groundwater-flow/ Hydrogeologic Properties of Earth Materials and Principals of Groundwater Flow]<ref name="Woessner2020">Woessner, W.W., and Poeter, E.P., 2020. Properties of Earth Materials and Principals of Groundwater Flow, The Groundwater Project, Guelph, Ontario, 207 pages. Free download from [https://gw-project.org/books/hydrogeologic-properties-of-earth-materials-and-principles-of-groundwater-flow/ The Groundwater Project].</ref>, Woessner and Poeter, 2020.
 
  
==Groundwater Flow==
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*[[Media: Brusseau2018manuscript.pdf | Assessing the Potential Contributions of Additional Retention Processes to PFAS Retardation in the Subsurface. Brusseau 2018 (manuscript).]]<ref name="Brusseau2018">Brusseau, M.L., 2018. Assessing the Potential Contributions of Additional Retention Processes to PFAS Retardation in the Subsurface. Science of the Total Environment, 613-614, pp. 176-185. [https://doi.org/10.1016/j.scitotenv.2017.09.065 DOI: 10.1016/j.scitotenv.2017.09.065]&nbsp;&nbsp; [[Media: Brusseau2018manuscript.pdf | Author’s Manuscript]]</ref>
[[File:Newell-Article 1-Fig1r.JPG|thumbnail|left|400px|Figure 1. Hydraulic gradient (typically described in units of m/m or ft/ft) is the difference in hydraulic head from Point A to Point B (ΔH) divided by the distance between them (ΔL). In unconfined aquifers, the hydraulic gradient can also be described as the slope of the water table (Adapted from course notes developed by Dr. R.J. Mitchell, Western Washington University).]]
 
Groundwater flows from areas of higher [[wikipedia: Hydraulic head | hydraulic head]] toward areas of lower hydraulic head (Figure 1). The rate of change (slope) of the hydraulic head is known as the hydraulic gradient. If groundwater is flowing and contains dissolved contaminants it can transport the contaminants from areas with high hydraulic head toward lower hydraulic head zones, or “downgradient”.
 
  
==Darcy's Law==
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==Introduction==
{| class="wikitable" style="float:right; margin-left:10px;text-align:center;"
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The transport and fate of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]] is a rapidly evolving field of science, with many questions that are not yet resolvedMuch of the currently available information is based on a few well-studied PFAS compoundsHowever, there is a large number and variety of PFAS with a wide range of physical and chemical characteristics that affect their behavior in the environment. The transport and fate of some PFAS could differ significantly from the compounds studied to date. Nevertheless, information about the behavior of some PFAS in the environment can be ascertained from the results of currently available research.
|+ Table 1.  Representative values of total porosity (''n''), effective porosity (''n<sub>e</sub>''), and hydraulic conductivity (''K'') for different aquifer materials<ref name="D&S1997">Domenico, P.A. and Schwartz, F.W., 1997. Physical and Chemical Hydrogeology, 2nd Ed. John Wiley & Sons, 528 pgs. ISBN 978-0-471-59762-9.  Available from: [https://www.wiley.com/en-us/Physical+and+Chemical+Hydrogeology%2C+2nd+Edition-p-9780471597629 Wiley]</ref><ref>McWhorter, D.B. and Sunada, D.K., 1977. Ground-water hydrology and hydraulics. Water Resources Publications, LLC, Highlands Ranch, Colorado, 304 pgs. ISBN-13: 978-1-887201-61-2 Available from: [https://www.wrpllc.com/books/gwhh.html Water Resources Publications]</ref><ref name="FandC1979"/>
 
|-
 
! Aquifer Material
 
! Total Porosity<br/><small>(dimensionless)</small>
 
! Effective Porosity<br/><small>(dimensionless)</small>
 
! Hydraulic Conductivity<br/><small>(meters/second)</small>
 
|-
 
| colspan="4" style="text-align: left; background-color:white;"|'''Unconsolidated'''
 
|-
 
| Gravel || 0.25 — 0.44 || 0.13 — 0.44 || 3×10<sup>-4</sup> — 3×10<sup>-2</sup>
 
|-
 
| Coarse Sand || 0.31 — 0.46 || 0.18 — 0.43 || 9×10<sup>-7</sup> — 6×10<sup>-3</sup>
 
|-
 
| Medium Sand || — || 0.16 — 0.46 || 9×10<sup>-7</sup> — 5×10<sup>-4</sup>
 
|-
 
| Fine Sand || 0.25 — 0.53 || 0.01 — 0.46 || 2×10<sup>-7</sup> — 2×10<sup>-4</sup>
 
|-
 
| Silt, Loess || 0.35 — 0.50 || 0.01 — 0.39 ||  1×10<sup>-9</sup> — 2×10<sup>-5</sup>
 
|-
 
| Clay || 0.40 — 0.70 || 0.01 — 0.18 ||  1×10<sup>-11</sup> — 4.7×10<sup>-9</sup>
 
|-
 
| colspan="4" style="text-align: left; background-color:white;"|'''Sedimentary and Crystalline Rocks'''
 
|-
 
| Karst and Reef Limestone || 0.05 — 0.50 || — ||  1×10<sup>-6</sup> — 2×10<sup>-2</sup>
 
|-
 
| Limestone, Dolomite || 0.00 — 0.20 || 0.01 — 0.24 ||  1×10<sup>-9</sup> — 6×10<sup>-6</sup>
 
|-
 
| Sandstone || 0.05 — 0.30 || 0.10 — 0.30 ||  3×10<sup>-10</sup> — 6×10<sup>-6</sup>
 
|-
 
| Siltstone || — || 0.21 — 0.41 ||  1×10<sup>-11</sup> — 1.4×10<sup>-8</sup>
 
|-
 
| Basalt || 0.05 — 0.50 || — ||  2×10<sup>-11</sup> — 2×10<sup>-2</sup>
 
|-
 
| Fractured Crystalline Rock || 0.00 — 0.10 || — ||  8×10<sup>-9</sup> — 3×10<sup>-4</sup>
 
|-
 
| Weathered Granite || 0.34 — 0.57 || — ||  3.3×10<sup>-6</sup> — 5.2×10<sup>-5</sup>
 
|-
 
| Unfractured Crystalline Rock || 0.00 — 0.05 || — ||  3×10<sup>-14</sup> — 2×10<sup>-10</sup>
 
|}
 
In unconsolidated geologic settings (gravel, sand, silt, and clay) and highly fractured systems, the rate of groundwater movement can be expressed using [[wikipedia: Darcy's law | Darcy’s Law]]. This law is a fundamental mathematical relationship in the groundwater field and can be expressed this way:
 
  
[[File:Newell-Article 1-Equation 1rr.jpg|center|500px]]
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PFAS transport and fate in the environment is controlled by the nature of the PFAS source, characteristics of the individual PFAS, and environmental conditions where the PFAS are present.  Perfluoroalkyl acids (PFAAs) (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]] for nomenclature) are strong acids and are anionic in the environmentally-relevant pH range.  They are extremely persistent in the environment and do not degrade or transform under typical environmental conditions. Polyfluoroalkyl substances (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]] for nomenclature) include compounds that have the potential to degrade to PFAAs.  These compounds are commonly referred to as PFAA precursors or just ‘precursors’.  Because some polyfluoroalkyl substances can degrade into PFAA via biotic or abiotic degradation pathways, PFAAs are sometimes referred to as “terminal PFAS” or “terminal degradation products”.
::Where:
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The most important molecular properties controlling PFAA transport are the carbon chain length and functional moieties of the headgroups (e.g., sulfonate, carboxylate). The molecular properties of PFAA precursors are more varied, with different carbon chain lengths, headgroups and ionic states<ref name="Buck2011">Buck, R.C., Franklin, J., Berger, U., Conder, J.M., Cousins, I.T., de Voogt, P., Jensen, A.A., Kannan, K., Mabury, S.A., and van Leeuwen, S.P.J., 2011. Perfluoroalkyl and Polyfluoroalkyl Substances in the Environment: Terminology, Classification, and Origins. Integrated Environmental Assessment and Management, 7(4): pp. 513-541.  [ https://doi.org/10.1002/ieam.258  DOI: 10.1002/ieam.258]&nbsp;&nbsp; [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.258 Open Access Article]</ref><ref name="Wang2017">Wang, Z., DeWitt, J.C., Higgins, C.P., and Cousins, I.T., 2017. A Never-Ending Story of Per- and Polyfluoroalkyl Substances (PFASs)? Environmental Science and Technology, 51(5), pp. 2508-2518. American Chemical Society.  [https://doi.org/10.1021/acs.est.6b04806 DOI: 10.1021/acs.est.6b04806]&nbsp;&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/acs.est.6b04806 Free Download from ACS]</ref> (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]]). All of these properties can influence transport and fate of PFAA precursors in the environment.
:::''Q'' = Flow rate (Volume of groundwater flow per time, such as m<sup>3</sup>/yr)
 
:::''A'' = Cross sectional area perpendicular to groundwater flow (length<sup>2</sup>, such as m<sup>2</sup>)
 
:::''V<sub>D</sub>'' = “Darcy Velocity”; describes groundwater flow as the volume of flow through a unit of cross-sectional area (units of length per time, such as ft/yr)
 
:::''K'' = Hydraulic Conductivity (sometimes called “permeability”) (length per time)
 
:::''ΔH'' = Difference in hydraulic head between two lateral points (length)
 
:::''ΔL'' = Distance between two lateral points (length)
 
  
[https://en.wikipedia.org/wiki/Hydraulic_conductivity Hydraulic conductivity] (Table 1 and Figure 2) is a measure of how easily groundwater flows through a porous medium, or alternatively, how much energy it takes to force water through a porous medium. For example, fine sand has smaller pores with more frictional resistance to flow, and therefore lower hydraulic conductivity (Figure 2) compared to coarse sand, which has larger pores with less resistance to flow.  
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Important environmental characteristics include the nature of the source (mode of input into the environment), the length of time that the source was active, and the magnitude of the input, as well as precipitation and infiltration rates, depth to groundwater, surface water and groundwater flow rates and interactions, prevailing atmospheric conditions, the properties of the porous-media (e.g., soil and sediment) and aqueous solution, microbiological factors, and the presence of additional fluid phases such as air and non-aqueous phase liquids [[Wikipedia: Non-aqueous phase liquid | (NAPLs)]] in the vadose zone and water-saturated source.  In the subsurface, soil characteristics (texture, organic carbon content, clay mineralogy, metal-oxide content, solid surface area, surface charge, and exchange capacity) and solution characteristics (pH, redox potential, major ion chemistry, and co-contaminants) can influence PFAS transport and fate.  
  
[[File:AdvectionFig2.PNG|400px|thumbnail|left|Figure 2. Hydraulic conductivity of selected rocks<ref>Heath, R.C., 1983. Basic ground-water hydrology, U.S. Geological Survey Water-Supply Paper 2220, 86 pgs. [[Media:Heath-1983-Basic_groundwater_hydrology_water_supply_paper.pdf|Report pdf]]</ref>.]]
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==PFAS Transport and Fate Processes==
Darcy’s Law was first described by Henry Darcy (1856)<ref>Brown, G.O., 2002. Henry Darcy and the making of a law. Water Resources Research, 38(7), p. 1106. [https://doi.org/10.1029/2001wr000727 DOI: 10.1029/2001WR000727] [[Media:Darcy2002.pdf | Report.pdf]]</ref> in a report regarding a water supply system he designed for the city of Dijon, France. Based on his experiments, he concluded that the amount of water flowing through a closed tube of sand (dark grey box in Figure 3) depends on (a) the change in the hydraulic head between the inlet and outlet of the tube, and (b) the hydraulic conductivity of the sand in the tube. Groundwater flows rapidly in the case of higher pressure (ΔH) or more permeable materials such as gravel or coarse sand, but flows slowly when the pressure is lower or the material is less permeable, such as fine sand or silt.
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[[File:AndersonBrusseau1w2Fig1.png | thumb | 600px | Figure 1. Illustration of PFAS partitioning and transformation processes. Source: D. Adamson, GSI, used with permission.]]
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Transport, partitioning, and transformation are the primary processes controlling PFAS fate in the environment (Figure 1). PFAS compounds can also be taken up by both plants and animals, and in some cases, bioaccumulate through the food chain. However, PFAS uptake and bioaccumulation is not discussed in this article (see “Environmental Concern” section of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]).
  
[[File:Newell-Article 1-Fig3..JPG|500px|thumbnail|right|Figure 3. Conceptual explanation of Darcy’s Law based on Darcy’s experiment (Adapted from course notes developed by Dr. R.J. Mitchell, Western Washington University).]]Since Darcy’s time, Darcy’s Law has been adapted to calculate the actual velocity that the groundwater is moving in units such as meters traveled per year. This quantity is called “interstitial velocity” or “seepage velocity” and is calculated by dividing the Darcy Velocity (flow per unit area) by the actual open pore area where groundwater is flowing, the “effective porosity” (Table 1):
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* '''Transport:''' PFAS can be transported substantial distances in the atmosphere<ref name="Ahrens2012">Ahrens, L., Harner, T., Shoeib, M., Lane, D.A. and Murphy, J.G., 2012. Improved Characterization of Gas–Particle Partitioning for Per- and Polyfluoroalkyl Substances in the Atmosphere Using Annular Diffusion Denuder Samplers. Environmental Science and Technology, 46(13), pp. 7199-7206. [https://doi.org/10.1021/es300898s DOI: 10.1021/es300898s]&nbsp;&nbsp; Free download available from [https://www.researchgate.net/profile/Tom_Harner/publication/225046057_Improved_Characterization_of_Gas-Particle_Partitioning_for_Per-_and_Polyfluoroalkyl_Substances_in_the_Atmosphere_Using_Annular_Diffusion_Denuder_Samplers/links/5cc730c4299bf12097893fdc/Improved-Characterization-of-Gas-Particle-Partitioning-for-Per-and-Polyfluoroalkyl-Substances-in-the-Atmosphere-Using-Annular-Diffusion-Denuder-Samplers.pdf ResearchGate].</ref>, surface water<ref name="Taniyasu2013">Taniyasu, S., Yamashita, N., Moon, H.B., Kwok, K.Y., Lam, P.K., Horii, Y., Petrick, G. and Kannan, K., 2013. Does wet precipitation represent local and regional atmospheric transportation by perfluorinated alkyl substances? Environment International, 55, pp. 25-32. [https://doi.org/10.1016/j.envint.2013.02.005 DOI: 10.1016/j.envint.2013.02.005]</ref>, soil<ref name="Braunig2017">Bräunig, J., Baduel, C., Heffernan, A., Rotander, A., Donaldson, E. and Mueller, J.F., 2017. Fate and redistribution of perfluoroalkyl acids through AFFF-impacted groundwater. Science of the Total Environment, 596, pp. 360-368. [https://doi.org/10.1016/j.scitotenv.2017.04.095 DOI: 10.1016/j.scitotenv.2017.04.095]</ref>, and groundwater<ref name="Weber2017">Weber, A.K., Barber, L.B., LeBlanc, D.R., Sunderland, E.M. and Vecitis, C.D., 2017. Geochemical and Hydrologic Factors Controlling Subsurface Transport of Poly- and Perfluoroalkyl Substances, Cape Cod, Massachusetts. Environmental Science and Technology, 51(8), pp. 4269-4279. [https://doi.org/10.1021/acs.est.6b05573 DOI: 10.1021/acs.est.6b05573]&nbsp;&nbsp; [https://bgc.seas.harvard.edu/assets/weber2017_final.pdf Free Download]</ref>. The primary mechanisms controlling PFAS transport are [[Wikipedia:Advection | advection]] and [[Wikipedia:Dispersive_mass_transfer | dispersion]], similar to other dissolved compounds. For additional information on transport in groundwater, see [[Advection and Groundwater Flow]] and [[Dispersion and Diffusion]]. 
  
[[File:Newell-Article 1-Equation 2r.jpg|400px]]
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* '''Partitioning:''' Partitioning of PFAS between the mobile and immobile phases is one of the most important processes controlling the rate of migration in the environment. The primary mobile phases are typically air and water.  Relatively immobile phases include stream sediments, soils, aquifer material, NAPLs, and interfaces between different phases (air-water, NAPL-water). Partitioning of a significant portion of the PFAS mass into an immobile phase increases the amount of material stored in the system and slows the apparent rate of migration in the mobile phase – a phenomenon that has been observed in field metadata<ref name="Anderson2019">Anderson, R.H., Adamson, D.T. and Stroo, H.F., 2019. Partitioning of poly-and perfluoroalkyl substances from soil to groundwater within aqueous film-forming foam source zones. Journal of Contaminant Hydrology, 220, pp. 59-65. [https://doi.org/10.1016/j.jconhyd.2018.11.011 DOI: 10.1016/j.jconhyd.2018.11.011]&nbsp;&nbsp; Manuscript available from [https://www.researchgate.net/profile/Hans_Stroo3/publication/329227107_Partitioning_of_poly-_and_perfluoroalkyl_substances_from_soil_to_groundwater_WITHIN_aqueous_film-forming_foam_source_zones/links/5e56996b299bf1bdb83e2f69/Partitioning-of-poly-and-perfluoroalkyl-substances-from-soil-to-groundwater-WITHIN-aqueous-film-forming-foam-source-zones.pdf ResearchGate]</ref>.
:Where:
 
::V<sub>S</sub> = “interstitial velocity” or “seepage velocity” (units of length per time, such as m/sec)<br />
 
::V<sub>D</sub> = “Darcy Velocity”; describes groundwater flow as the volume of flow per unit area (units of length per time)<br />
 
::n<sub>e</sub> = Effective porosity (unitless)
 
  
Effective porosity is smaller than total porosity. The difference is that total porosity includes some dead-end pores that do not support groundwater. Typically values for total and effective porosity are shown in Table 1.
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* '''Transformation:''' Transformation of PFAS is controlled by the molecular structure of the individual compounds. Perfluorinated compounds, including PFAAs, are resistant to abiotic and biotic transformation reactions under typical conditions and highly persistent in the environment. In contrast, precursors can be transformed by both abiotic and biotic processes, often resulting in the production of so-called “terminal” PFAA daughter products.
  
[[File:Newell-Article 1-Fig4.JPG|500px|thumbnail|left|Figure 4Difference between Darcy Velocity (also called Specific Discharge) and Seepage Velocity (also called Interstitial Velocity).]]
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==Transport and Partitioning in the Atmosphere==
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Air serves as a transport media for PFAS, particularly for uncharged polyfluorinated PFAS.  Airborne PFAS transport contributes to global distribution and can lead to localized deposition to soils and surface water in the vicinity of emission sources<ref name="Simcik2005">Simcik, M.F. and Dorweiler, K.J., 2005. Ratio of Perfluorochemical Concentrations as a Tracer of Atmospheric Deposition to Surface Waters. Environmental Science and Technology, 39(22), pp.  8678-8683. [https://doi.org/10.1021/es0511218 DOI: 10.1021/es0511218]&nbsp;&nbsp; Free download available from [https://www.researchgate.net/profile/Matt_Simcik/publication/7444956_Ratio_of_Perfluorochemical_Concentrations_as_a_Tracer_of_Atmospheric_Deposition_to_Surface_Waters/links/5f035861299bf1881603c3be/Ratio-of-Perfluorochemical-Concentrations-as-a-Tracer-of-Atmospheric-Deposition-to-Surface-Waters.pdf ResearchGate]</ref><ref name="Prevedouros2006">Prevedouros, K., Cousins, I.T., Buck, R.C. and Korzeniowski, S.H., 2006. Sources, Fate and Transport of Perfluorocarboxylates. Environmental Science and Technology, 40(1), pp. 32-44. [https://doi.org/10.1021/es0512475 DOI: 10.1021/es0512475]&nbsp;&nbsp; Free download available from [https://d1wqtxts1xzle7.cloudfront.net/39945519/Sources_Fate_and_Transport_of_Perfluoroc20151112-1647-19vcvbf.pdf?1447365456=&response-content-disposition=inline%3B+filename%3DSources_Fate_and_Transport_of_Perfluoroc.pdf&Expires=1605023809&Signature=Z6KqgaDN6lKdAazoe6qoASoCtVystG5i~5EnrTcb~qMg3xZPz4O49Kghh62WmMzqEKE788~6EwrnlBVo9o6cM0hjf2vymFYxg4mx-eSIOEonfFjk6RonSaWp5gRbA6m~SNjwsjaKXID3OQyWIlLVpUd2LzAdI5rLGFA~gIXXtNPyCArLuGn-kbPYUIcBUg5TIkTZ6TDLXF~ujmzK9tNv~55UYabsJL4pmwIGC2sNGkEyJrYMfU577fbactdrmQXTJH7XbgpfDSfd4-xWkDZTdvVf~TypDDqUCZdtCkY8wINdpqtfe1KEzLrAj7rxxALAHUYxlVbPB45XTkLAGe5qww__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA Academia]</ref><ref name="Ahrens2011">Ahrens, L., Shoeib, M., Harner, T., Lane, D.A., Guo, R. and Reiner, E.J., 2011. Comparison of Annular Diffusion Denuder and High Volume Air Samplers for Measuring Per- and Polyfluoroalkyl Substances in the Atmosphere." Analytical Chemistry, 83(24), pp. 9622-9628. [https://doi.org/10.1021/ac202414w DOI: 10.1021/ac202414w]&nbsp;&nbsp; Free download available from [https://www.informea.org/sites/default/files/imported-documents/UNEP-POPS-POPRC11FU-SUBM-PFOA-Canada-2-20151211.En.pdf Informea].</ref><ref name="Rauert2018">Rauert, C., Shoieb, M., Schuster, J.K., Eng, A. and Harner, T., 2018. Atmospheric concentrations and trends of poly-and perfluoroalkyl substances (PFAS) and volatile methyl siloxanes (VMS) over 7 years of sampling in the Global Atmospheric Passive Sampling (GAPS) network. Environmental Pollution, 238, pp. 94-102. [https://doi.org/10.1016/j.envpol.2018.03.017 DOI: 10.1016/j.envpol.2018.03.017]&nbsp;&nbsp; Open access article available from [https://reader.elsevier.com/reader/sd/pii/S0269749117352521?token=4C770E6E8AEDB0B3BA6A1D5B2C20ED5385F81823612551FA3380AAA1DA7A978F9CB36834AF6B7F91F35FF57E32013252 ScienceDirect]&nbsp;&nbsp; [[Media:Rauert2018.pdf | Report.pdf]]</ref>.
  
==Darcy Velocity and Seepage Velocity==
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PFAAs, which are ionic and possess a negative charge under ambient environmental conditions, are far less volatile than many other groundwater contaminants.  An online database of vapor pressures and Henry’s Law constants for different PFAS, including PFAAs, is maintained by the Interstate Technology Regulatory Council<ref name="ITRC2020"/>.  In general, vapor pressures of PFAS are low and water solubilities are high, limiting partitioning from water to air<ref name="ITRC2020"/>. However, under certain conditions, particularly within industrial stack emissions, PFAS can be transported through the atmosphere in both the gas phase and associated with fugitive particulates.  In particular, volatile compounds including fluorotelomer alcohols (FTOHs) may be present in the gas phase, whereas, PFAAs can aerosolize and be transported as particulates<ref name="Ahrens2012"/>. In addition, precursors can be transformed to PFAAs in the atmosphere, which can result in PFAA deposition.
In groundwater calculations, Darcy Velocity and Seepage Velocity are used for different purposes. For any calculation where the actual flow rate in units of volume per time (such as liters per day or gallons per minute) is involved, then the original Darcy Equation should be used (calculate V<sub>D</sub>; Equation 1) without using effective porosity. When calculating solute travel time, then the seepage velocity calculation (V<sub>S</sub>; Equation 2) must be used and an estimate of effective porosity is required. Figure 4 illustrates the differences between Darcy Velocity and Seepage Velocity.
+
Short-range atmospheric transport and deposition can result in PFAS contamination in terrestrial and aquatic systems near points of significant emissions, impacting soil, groundwater, and other media of concern<ref name="Fang2018">Fang, X., Wang, Q., Zhao, Z., Tang, J., Tian, C., Yao, Y., Yu, J. and Sun, H., 2018. Distribution and dry deposition of alternative and legacy perfluoroalkyl and polyfluoroalkyl substances in the air above the Bohai and Yellow Seas, China. Atmospheric Environment, 192, pp. 128-135. [https://doi.org/10.1016/j.atmosenv.2018.08.052 DOI: 10.1016/j.atmosenv.2018.08.052]</ref><ref name="Brandsma2019">Brandsma, S.H., Koekkoek, J.C., van Velzen, M.J.M. and de Boer, J., 2019.  The PFOA substitute GenX detected in the environment near a fluoropolymer manufacturing plant in the Netherlands. Chemosphere, 220, pp. 493-500. [https://doi.org/10.1016/j.chemosphere.2018.12.135 DOI: 10.1016/j.chemosphere.2018.12.135]&nbsp;&nbsp; Open access article available from [https://reader.elsevier.com/reader/sd/pii/S0045653518324706?token=E541D5C4B200C8626A86F41049FE9DCA92652BC9A8BA7D9E47832C08070AB5AF256F4872474C50B5C4908F5CA4C24947 ScienceDirect].&nbsp;&nbsp; [[Media: Brandsma2019.pdf | Report.pdf]]</ref>.  Releases of ionic PFAS from factories are likely tied to particulate matter, which settle to the ground in dry weather and are also wet-scavenged by precipitation<ref name="Barton2006">Barton, C.A., Butler, L.E., Zarzecki, C.J., Flaherty, J. and Kaiser, M., 2006. Characterizing Perfluorooctanoate in Ambient Air near the Fence Line of a Manufacturing Facility: Comparing Modeled and Monitored Values. Journal of the Air and Waste Management Association, 56(1), pp.  48-55. [https://doi.org/10.1080/10473289.2006.10464429 DOI: 10.1080/10473289.2006.10464429]&nbsp;&nbsp; Free access article available from [https://www.tandfonline.com/doi/pdf/10.1080/10473289.2006.10464429?needAccess=true Taylor and Francis Online]&nbsp;&nbsp; [[Media: Barton2006.pdf | Report.pdf]]</ref>.  The impact of other potential sources, such as combustion emissions or wind-blown fire-fighting foam from fire training and fire response sites, on the fate and transport of PFAS in air may need to be assessed.
  
==Mobile Porosity==
+
Long-range transport processes are responsible for the wide distribution of neutral and ionic PFAS across the Earth as evidenced by their occurrence in biota, surface snow, ice cores, seawater, and other environmental media in regions as remote as the Arctic and Antarctic<ref name="Bossi2016">Bossi, R., Vorkamp, K. and Skov, H., 2016. Concentrations of organochlorine pesticides, polybrominated diphenyl ethers and perfluorinated compounds in the atmosphere of North Greenland. Environmental Pollution, 217, pp. 4-10. [https://doi.org/10.1016/j.envpol.2015.12.026 DOI: 10.1016/j.envpol.2015.12.026]</ref><ref name="Ahrens2010">Ahrens, L., Gerwinski, W., Theobald, N. and Ebinghaus, R., 2010. Sources of polyfluoroalkyl compounds in the North Sea, Baltic Sea and Norwegian Sea: Evidence from their spatial distribution in surface water. Marine Pollution Bulletin, 60(2), pp. 255-260. [https://doi.org/10.1016/j.marpolbul.2009.09.013 DOI: 10.1016/j.marpolbul.2009.09.013]</ref>
{| class="wikitable" style="float:right; margin-left:10px; text-align:center;"
+
Distribution of PFAS to remote regions far removed from direct industrial input is believed to occur from both: a) long-range atmospheric transport and subsequent degradation of volatile precursors; and b) transport via ocean currents and release into the air as marine aerosols (sea spray)<ref name="DeSilva2009">De Silva, A.O., Muir, D.C. and Mabury, S.A., 2009. Distribution of perfluorocarboxylate isomers in select samples from the North American environment. Environmental Toxicology and Chemistry: An International Journal 28(9), pp. 1801-1814. [https://doi.org/10.1897/08-500.1 DOI: 10.1897/08-500.1]</ref><ref name="Armitage2009">Armitage, J.M., 2009. Modeling the global fate and transport of perfluoroalkylated substances (PFAS). Doctoral Dissertation, Institutionen för tillämpad miljövetenskap (ITM), Stockholm University. [[Media: Armitage2009.pdf | Report.pdf]]</ref>.
|+ Table 2. Mobile porosity estimates from 15 tracer tests<ref name="Payne2008">Payne, F.C., Quinnan, J.A. and Potter, S.T., 2008. Remediation Hydraulics. CRC Press. ISBN 9780849372490  Available from: [https://www.routledge.com/Remediation-Hydraulics/Payne-Quinnan-Potter/p/book/9780849372490 CRC Press]</ref>
 
|-
 
! Aquifer Material
 
! Mobile Porosity<br/><small>(volume fraction)</small>
 
|-
 
| Poorly sorted sand and gravel || 0.085
 
|-
 
| Poorly sorted sand and gravel || 0.04 — 0.07
 
|-
 
| Poorly sorted sand and gravel || 0.09
 
|-
 
| Fractured sandstone || 0.001 — 0.007
 
|-
 
| Alluvial formation || 0.102
 
|-
 
| Glacial outwash || 0.145
 
|-
 
| Weathered mudstone regolith || 0.07 — 0.10
 
|-
 
| Alluvial formation || 0.07
 
|-
 
| Alluvial formation || 0.07
 
|-
 
| Silty sand || 0.05
 
|-
 
| Fractured sandstone || 0.0008 — 0.001
 
|-
 
| Alluvium, sand and gravel || 0.017
 
|-
 
| Alluvium, poorly sorted sand and gravel || 0.003 — 0.017
 
|-
 
| Alluvium, sand and gravel || 0.11 — 0.18
 
|-
 
| Siltstone, sandstone, mudstone || 0.01 — 0.05
 
|}
 
  
Payne et al. (2009) reported the results from multiple short-term tracer tests conducted to aid the design of amendment injection systems<ref name= "Payne2008"/>. In these tests, the dissolved solutes were observed to migrate more rapidly through the aquifer than could be explained with typically reported values of n<sub>e</sub>. They concluded that the heterogeneity of unconsolidated formations results in a relatively small area of an aquifer cross section carrying most of the water, and therefore solutes migrate more rapidly than expected. Based on these results, they recommend that a quantity called “mobile porosity” should be used in place of n<sub>e</sub> in equation 2 for calculating solute transport velocities. Based on 15 different tracer tests, typical values of mobile porosity range from 0.02 to 0.10 (Table 2).   
+
==Transport and Partitioning in Aqueous Systems==
 +
PFAS adsorb from water to a variety of solid materials including organic materials, clay minerals, metal oxides, and granular activated carbon<ref name="Du2014">Du, Z., Deng, S., Bei, Y., Huang, Q., Wang, B., Huang, J. and Yu, G., 2014. Adsorption behavior and mechanism of perfluorinated compounds on various adsorbents – A review. Journal of Hazardous Materials, 274, pp. 443-454. [https://doi.org/10.1016/j.jhazmat.2014.04.038 DOI: 10.1016/j.jhazmat.2014.04.038]</ref>. This process is thought to occur through two primary mechanisms: 1) sorption to organic-carbon components of the solids; and 2) electrostatic (and other) interactions with inorganic constituents of the solids, including clay minerals and metal-oxides<ref name="Guelfo2013">Guelfo, J.L. and Higgins, C.P., 2013. Subsurface Transport Potential of Perfluoroalkyl Acids at Aqueous Film-Forming Foam (AFFF)-Impacted Sites. Environmental Science and Technology, 47(9), pp. 4164-4171. [https://doi.org/10.1021/es3048043 DOI: 10.1021/es3048043]&nbsp;&nbsp; [https://mountainscholar.org/bitstream/handle/11124/80055/Guelfo_mines_0052E_10298.pdf?sequence=1#page=64 Doctoral Dissertation]</ref><ref name="Zhao2014">Zhao, L., Bian, J., Zhang, Y., Zhu, L. and Liu, Z., 2014. Comparison of the sorption behaviors and mechanisms of perfluorosulfonates and perfluorocarboxylic acids on three kinds of clay minerals. Chemosphere, 114, pp. 51-58. [https://doi.org/10.1016/j.chemosphere.2014.03.098 DOI: 10.1016/j.chemosphere.2014.03.098]&nbsp;&nbsp; Free download available from [https://www.researchgate.net/profile/Lixia_Zhao8/publication/262148355_Comparison_of_the_sorption_behaviors_and_mechanisms_of_perfluorosulfonates_and_perfluorocarboxylic_acids_on_three_kinds_of_clay_minerals/links/5b1be5dca6fdcca67b681a4f/Comparison-of-the-sorption-behaviors-and-mechanisms-of-perfluorosulfonates-and-perfluorocarboxylic-acids-on-three-kinds-of-clay-minerals.pdf ResearchGate].</ref>. The relative contribution of each mechanism varies depending on surface chemistry and other geochemical factors, as well as the molecular properties of the PFAS.  In general, the impact of electrostatic interactions with charged soil constituents is more important for PFAS than non-polar, hydrophobic organic contaminants (e.g. hydrocarbons, chlorinated solvents).  Adsorption of PFAS by solids is often nonlinear, with greater sorption at lower solute concentrations.  The impacts of adsorption kinetics and their potential reversibility on PFAS transport have not yet been examined for most PFAS compounds.   
  
A data mining analysis of 43 sites in California by Kulkarni et al. (2020) showed that on average 90% of the groundwater flow occurred in about 30% of cross sectional area perpendicular to groundwater flowThese data provided “moderate (but not confirmatory) support for the mobile porosity concept.<ref name="Kulkarni2020">Kulkarni, P.R., Godwin, W.R., Long, J.A., Newell, R.C., Newell, C.J., 2020. How much heterogeneity? Flow versus area from a big data perspective. Remediation 30(2), pp. 15-23. [https://doi.org/10.1002/rem.21639 DOI: 10.1002/rem.21639]  [[Media:Kulkarni2020.pdf | Report.pdf]]</ref>
+
Sorption of hydrocarbons, chlorinated solvents and other hydrophobic organics is often controlled the by organic-carbon components of the solid phase (see [[Sorption of Organic Contaminants]]).  However, studies of PFAS sorption to solid phase organic carbon have reported conflicting results.  In a study of field sites with aqueous film-forming foam (AFFF, a type of fire-fighting foam) releases, solid phase organic carbon content was found to significantly influence PFAS soil-to-groundwater concentration ratiosStatistical modeling was then used to derive apparent organic carbon partition coefficients for 18 different PFAS<ref name="Anderson2019"/>. A recent compilation of published organic carbon partition coefficients found a good correspondence to PFAS molecular structure<ref name="Brusseau2019a">Brusseau, M.L., 2019. Estimating the relative magnitudes of adsorption to solid-water and air/oil-water interfaces for per-and poly-fluoroalkyl substances. Environmental Pollution, 254B, p. 113102. [https://doi.org/10.1016/j.envpol.2019.113102 DOI: 10.1016/j.envpol.2019.113102]</ref>. However, other studies have shown a general lack of correlation between solid phase partition coefficients and organic carbon<ref name="Li2018">Li, Y., Oliver, D.P. and Kookana, R.S., 2018. A critical analysis of published data to discern the role of soil and sediment properties in determining sorption of per and polyfluoroalkyl substances (PFASs). Science of the Total Environment, 628, pp. 110-120. [https://doi.org/10.1016/j.scitotenv.2018.01.167 DOI: 10.1016/j.scitotenv.2018.01.167]</ref>. It is possible that greater variability may be observed for broader data sets that incorporate different ranges of PFAS concentrations, different solution conditions, different measurement methods, and field-based data which often have less well-defined conditions and may also be influenced by other retention processes<ref name="Anderson2019"/><ref name="Brusseau2019a"/>.
  
==Advection-Dispersion-Reaction Equation==
+
[[File:AndersonBrusseau1w2Fig2.png | thumb | 500px | Figure 2. Example of expected orientation and accumulation of PFAS at air-water interface. Source: D. Adamson, GSI, used with permission.]]
The transport of dissolved solutes in groundwater is often simulated using the Advection-Dispersion-Reaction (ADR) equation. As shown below (Equation 3), the ADR equation describes the change in dissolved solute concentration (''C'') over time (''t'') where groundwater flow is oriented along the ''x'' direction.
+
Most solids present in the environment contain both fixed-charged (negative) and variably charged surfaces.  At neutral to high pH, variably charged clay minerals have a net-negative charge.  As a result, negatively charged PFAAs do not strongly interact electrostatically in most soils, although as the soil pH decreases electrostatic sorption would be expected to increase in soils with variably charged clay minerals.  Cationic and zwitterionic precursors are expected to be more strongly sorbed than anionic PFAAs in most environments due to well-established cation exchange reactions. Other factors, including ionic strength, composition, and the presence of co-solutes, can affect adsorption of PFAS<ref name="Higgins2006">Higgins, C.P. and Luthy, R.G., 2006. Sorption of Perfluorinated Surfactants on Sediments. Environmental Science and Technology, 40(23), pp. 7251-7256. [https://doi.org/10.1021/es061000n DOI: 10.1021/es061000n]</ref><ref name="Chen2009">Chen, H., Chen, S., Quan, X., Zhao, Y. and Zhao, H., 2009. Sorption of perfluorooctane sulfonate (PFOS) on oil and oil-derived black carbon: Influence of solution pH and [Ca2+]. Chemosphere, 77(10), pp. 1406-1411. [https://doi.org/10.1016/j.chemosphere.2009.09.008 DOI: 10.1016/j.chemosphere.2009.09.008]</ref><ref name="Pan2009">Pan, G., Jia, C., Zhao, D., You, C., Chen, H. and Jiang, G., 2009. Effect of cationic and anionic surfactants on the sorption and desorption of perfluorooctane sulfonate (PFOS) on natural sediments. Environmental Pollution, 157(1), pp.325-330. [https://doi.org/10.1016/j.envpol.2008.06.035 DOI: 10.1016/j.envpol.2008.06.035]&nbsp;&nbsp; Free download available from [https://www.researchgate.net/profile/Gang_Pan2/publication/23189567_Effect_of_cationic_and_anionic_surfactants_on_the_sorption_and_desorption_of_perfluorooctane_sulfonate_PFOS_on_natural_sediments/links/5be19d23a6fdcc3a8dc2550d/Effect-of-cationic-and-anionic-surfactants-on-the-sorption-and-desorption-of-perfluorooctane-sulfonate-PFOS-on-natural-sediments.pdf ResearchGate]</ref><ref name="Guelfo2013"/><ref name="Zhao2014"/>.
  
[[File:AdvectionEq3r.PNG|center|650px]]
+
Most PFAS compounds act as surface-active agents (or [[Wikipedia:Surfactant | surfactants]]) due to the presence of a hydrophilic headgroup and a hydrophobic tail. The hydrophilic headgroup will preferentially partition to the aqueous phase and the hydrophobic tail will preferentially partition to the non-aqueous phase (air or organic material).  As a result, PFAS tend to accumulate at interfaces (air-water, water-NAPL, water-solid) (Figure 2).  This tendency to accumulate at interfaces can influence transport in the atmosphere (on water droplets and hydrated aerosols), in the vadose or unsaturated zone at air-water interfaces, in the presence of NAPLs, and in wastewater treatment systems<ref name="Brusseau2018"/><ref name="Brusseau2019b">Brusseau, M.L., 2019. The Influence of Molecular Structure on the Adsorption of PFAS to Fluid-Fluid Interfaces: Using QSPR to Predict Interfacial Adsorption Coefficients. Water Research, 152, pp. 148-158.  [https://doi.org/10.1016/j.watres.2018.12.057 DOI: 10.1016/j.watres.2018.12.057]&nbsp;&nbsp; [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6374777/ Author’s Manuscript]</ref>.
:Where:
+
 
::''R'' is the linear, equilibrium retardation factor (see [[Sorption of Organic Contaminants]]),
+
In theoretical and experimental studies of transport in unsaturated porous media, adsorption at the air-water interface increased PFOS and PFOA retention<ref name="Brusseau2018"/><ref name="Lyu2018">Lyu, Y., Brusseau, M.L., Chen, W., Yan, N., Fu, X., and Lin, X., 2018. Adsorption of PFOA at the Air-Water Interface during Transport in Unsaturated Porous Media. Environmental Science and Technology, 52(14), pp. 7745-7753.  [https://doi.org/10.1021/acs.est.8b02348 DOI: 10.1021/acs.est.8b02348]&nbsp;&nbsp; [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6312111/ Author’s Manuscript]</ref><ref name="BrusseauEtAl2019">Brusseau, M.L., Yan, N., Van Glubt, S., Wang, Y., Chen, W., Lyu, Y., Dungan, B., Carroll, K.C., and Holguin, F.O., 2019. Comprehensive Retention Model for PFAS Transport in Subsurface Systems. Water Research, 148, pp. 41-50.  [https://doi.org/10.1016/j.watres.2018.10.035 DOI: 10.1016/j.watres.2018.10.035]&nbsp;&nbsp; [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6294326/ Author’s Manuscript]</ref>, contributing approximately 20% to 80% of total retention in sands and soil. The impact of oil-water interfacial adsorption on PFAS transport was also quantitatively characterized in recent studies and shown to contribute to total retention on a similar scale as air-water interfacial adsorption<ref name="Brusseau2018"/><ref name="BrusseauEtAl2019"/>.  These processes may result in increased PFAS mass retained in NAPL source zones, increased PFAS sorption with the resulting retardation of transport, and greater persistence of dissolved PFAS in the environment.  
::''D<sub>x</sub>, D<sub>y</sub>, and D<sub>z</sub>'' are hydrodynamic dispersion coefficients in the ''x, y'' and ''z'' directions (L<sup>2</sup>/T),  
 
::''v''  is the advective transport or seepage velocity in the ''x'' direction (L/T), and  
 
::''λ''  is an effective first order decay rate due to combined biotic and abiotic processes (1/T).
 
  
The term on the left side of the equation is the rate of mass change per unit volume. On the right side are terms representing the solute flux due to dispersion in the ''x, y'', and ''z'' directions, the advective flux in the ''x'' direction, and the first order decay due to biotic and abiotic processes. Dispersion coefficients (''D<sub>x,y,z</sub>'') are commonly estimated using the following relationships:
+
==Transformation==
 +
[[File:AndersonBrusseau1w2Fig3.png | thumb | 600px | Figure 3. Conceptual model of precursor transformation resulting in the formation of PFAAs. Source L. Trozzolo, TRC and C. Higgins, Colorado School of Mines, used with permission.]]
 +
Certain polyfluorinated substances have the potential to transform to other PFAS, with PFAAs as the typical terminal daughter products. These polyfluorinated substances are often referred to as “precursors”. The transformation potential of polyfluorinated precursors is influenced by the presence, location, and number of carbon-hydrogen (C-H) bonds and potentially carbon-oxygen (C-O) bonds throughout the carbon chain. Specifically, PFAS with C-H bonds are subject to a variety of biotic and abiotic reactions that ultimately result in the formation of PFAAs with perfluorinated carbon chains of the same length or shorter than the initial polyfluorinated precursor<ref name="Houtz2013">Houtz, E.F., Higgins, C.P., Field, J.A. and Sedlak, D.L., 2013. Persistence of perfluoroalkyl acid precursors in AFFF-impacted groundwater and soil. Environmental Science and Technology, 47(15), pp.  8187-8195.  [https://doi.org/10.1021/es4018877 DOI: 10.1021/es4018877]&nbsp;&nbsp; Free download from [https://www.researchgate.net/profile/Erika_Houtz/publication/252323955_Persistence_of_Perfluoroalkyl_Acid_Precursors_in_AFFF-Impacted_Groundwater_and_Soil/links/59dbddeeaca2728e2018336d/Persistence-of-Perfluoroalkyl-Acid-Precursors-in-AFFF-Impacted-Groundwater-and-Soil.pdf ReseqarchGate]</ref><ref name="McGuire2014">McGuire, M.E., Schaefer, C., Richards, T., Backe, W.J., Field, J.A., Houtz, E., Sedlak, D.L., Guelfo, J.L., Wunsch, A., and Higgins, C.P., 2014. Evidence of Remediation-Induced Alteration of Subsurface Poly- and Perfluoroalkyl Substance Distribution at a Former Firefighter Training Area. Environmental Science and Technology, 48(12) pp. 6644-6652.  [https://doi.org/10.1021/es5006187 DOI: 10.1021/es5006187]&nbsp;&nbsp; Manuscript available from [https://ir.library.oregonstate.edu/downloads/td96k706f Oregon State University]</ref><ref name="Anderson2016">Anderson, R.H., Long, G.C., Porter, R.C. and Anderson, J.K., 2016. Occurrence of select perfluoroalkyl substances at US Air Force aqueous film-forming foam release sites other than fire-training areas: Field-validation of critical fate and transport properties. Chemosphere, 150, pp. 678-685.  [https://doi.org/10.1016/j.chemosphere.2016.01.014 DOI: 10.1016/j.chemosphere.2016.01.014]</ref><ref name="Weber2017"/>.
  
[[File:AdvectionEq4.PNG|center|350px]]
+
Transformation studies published to date have tested only a small subsample of possible precursors and, therefore, much uncertainty exists regarding 1) the extent to which precursor transformation occurs on a global scale, 2) which environmental compartments represent the majority of transformation, 3) relevant environmental conditions that affect transformation processes, and 4) transformation rates and pathways. Nevertheless, a portion of the precursors are expected to transform to PFAAs over time as shown in Figure 3.
:Where:
 
::''D<sub>m</sub>''  is the molecular diffusion coefficient (L<sup>2</sup>/T), and  
 
::''&alpha;<sub>L</sub>, &alpha;<sub>T</sub>'', and ''&alpha;<sub>V</sub>''  are the longitudinal, transverse and vertical dispersivities (L).  
 
  
The ADR equation can be solved to find the spatial and temporal distribution of solutes using a variety of analytical and numerical approachesThe design tools BIOSCREEN<ref name="Newell1996">Newell, C.J., McLeod, R.K. and Gonzales, J.R., 1996. BIOSCREEN: Natural Attenuation Decision Support System - User's Manual, Version 1.3. EPA/600/R-96/087 [[Media: https://www.enviro.wiki/index.php?title=File:Newell-1996-Bioscreen_Natural_Attenuation_Decision_Support_System.pdf| Report.pdf]]  [https://www.epa.gov/water-research/bioscreen-natural-attenuation-decision-support-system BIOSCREEN website]</ref>, BIOCHLOR<ref name="Aziz2000">Aziz, C.E., Newell, C.J., Gonzales, J.R., Haas, P.E., Clement, T.P. and Sun, Y., 2000. BIOCHLOR Natural Attenuation Decision Support System. User’s Manual - Version 1.0. US Environmental Protection Agency, EPA/600/R-00/008.  [[Media:Aziz2000.pdf | Report.pdf]] [https://www.epa.gov/water-research/biochlor-natural-attenuation-decision-support-system  BIOCHLOR website]</ref>, and REMChlor (Falta et al. 2007) employ an analytical solution of the ADR equation (Domenico, 1987)MT3DMS (Zheng and Wang, 1999) uses a numerical method to solve the ADR equation using the head distribution generated by the groundwater flow model MODFLOW (McDonald and Harbaugh, 1988).
+
Precursors can be transformed by a variety of abiotic processes including hydrolysis, photolysis, and oxidation. Hydrolysis of some precursors, followed by subsequent biotransformation, can produce perfluoroalkyl sulfonates (PFSAs)An important example is the production of PFOS from perfluorooctane sulfonyl fluoride (POSF)<ref name="Martin2010">Martin, J.W., Asher, B.J., Beesoon, S., Benskin, J.P. and Ross, M.S., 2010. PFOS or PreFOS? Are perfluorooctane sulfonate precursors (PreFOS) important determinants of human and environmental perfluorooctane sulfonate (PFOS) exposure? Journal of Environmental Monitoring, 12(11), pp.1979-2004. [https://doi.org/10.1039/C0EM00295J DOI: 10.1039/C0EM00295J]&nbsp;&nbsp; Free download from [https://www.researchgate.net/profile/Matthew_Ross3/publication/47415684_PFOS_or_PreFOS_Are_perfluorooctane_sulfonate_precursors_PreFOS_important_determinants_of_human_and_environmental_perfluorooctane_sulfonate_PFOS_exposure/links/00b7d520a6132da945000000.pdf ResearchGate]</ref>. Other hydrolysis reactions produce perfluoroalkyl carboxylates (PFCAs). At neutral pH, the hydrolysis of fluorotelomer-derived polymeric precursors results in the formation of monomeric precursors of PFOA and other PFAAs with half-lives of 50 to 90 years)<ref name="Washington2010">Washington, J.W., Ellington, J.J., Jenkins, T.M. and Yoo, H., 2010. Response to Comments on “Degradability of an Acrylate-Linked, Fluorotelomer Polymer in Soil”. Environmental Science and Technology, 44(2), pp. 849-850. [https://doi.org/10.1021/es902672q DOI: 10.1021/es902672q]&nbsp;&nbsp; [https://pubs.acs.org/doi/pdf/10.1021/es902672q Free Download from ACS].</ref>.  Oxidation of precursors by hydroxyl radicals can occur in natural waters, with the fluorotelomer-derived precursors being oxidized relatively rapidly<ref name="Gauthier2005">Gauthier, S.A. and Mabury, S.A., 2005. Aqueous photolysis of 8: 2 fluorotelomer alcohol. Environmental Toxicology and Chemistry, 24(8), pp.1837-1846.  [https://doi.org/10.1897/04-591R.1 DOI: 10.1897/04-591R.1]&nbsp;&nbsp; Free download from [https://www.researchgate.net/profile/Suzanne_Gauthier/publication/7609648_Aqueous_photolysis_of_8_2_fluorotelomer_alcohol/links/5ec16c4792851c11a86d9438/Aqueous-photolysis-of-8-2-fluorotelomer-alcohol.pdf ResearchGate].</ref><ref name="Plumlee2009">Plumlee, M.H., McNeill, K. and Reinhard, M., 2009. Indirect Photolysis of Perfluorochemicals: Hydroxyl Radical-Initiated Oxidation of N-Ethyl Perfluorooctane Sulfonamido Acetate (N-EtFOSAA) and Other Perfluoroalkanesulfonamides. Environmental Science and Technology, 43(10), pp.3662-3668.  [https://doi.org/10.1021/es803411w DOI: 10.1021/es803411w]&nbsp;&nbsp; Free download from [https://www.researchgate.net/profile/Megan_Plumlee/publication/26309488_Indirect_Photolysis_of_Perfluorochemicals_Hydroxyl_Radical-Initiated_Oxidation_of_N-Ethyl_Perfluorooctane_Sulfonamido_Acetate_N-EtFOSAA_and_Other_Perfluoroalkanesulfonamides/links/5aac0437a6fdcc1bc0b8d002/Indirect-Photolysis-of-Perfluorochemicals-Hydroxyl-Radical-Initiated-Oxidation-of-N-Ethyl-Perfluorooctane-Sulfonamido-Acetate-N-EtFOSAA-and-Other-Perfluoroalkanesulfonamides.pdf ResearchGate].</ref>.
 +
Evidence of aerobic biotransformation is provided from studies of PFAS composition throughout the continuum of wastewater treatments<ref name="Arvaniti2015">Arvaniti, O.S. and Stasinakis, A.S., 2015. Review on the occurrence, fate and removal of perfluorinated compounds during wastewater treatment. Science of the Total Environment, 524, pp. 81-92[https://doi.org/10.1016/j.scitotenv.2015.04.023 DOI: 10.1016/j.scitotenv.2015.04.023]</ref>, from field studies at AFFF-impacted sites<ref name="Houtz2013"/><ref name="McGuire2014"/><ref name="Anderson2016"/><ref name="Weber2017"/>, and from microcosm experiments. In general, the literature on aerobic biotransformation collectively demonstrates or indirectly supports the following conclusions as summarized in ITRC 2020<ref name="ITRC2020"/>:
 +
* Numerous aerobic biotransformation pathways exist with relatively rapid kinetics
 +
* All polyfluorinated precursors studied to date have the potential to aerobically biotransform to PFAAs
 +
* Aerobic biotransformation of various fluorotelomer-derived precursors exclusively results in the formation of PFCAs, including PFOA, without necessarily the conservation of chain-length
 +
* Aerobic biotransformation of various electrochemical fluorination-derived precursors primarily results in the formation of PFAAs, including PFOS, with the conservation of chain-length
 +
Precursor transformation can complicate CSMs (and risk assessments) and should be considered during comprehensive site investigations.  For example, atmospheric emissions of volatile precursors can result in long-range transport where subsequent transformation and deposition can result in detectable levels of PFAAs in environmental media independent of obvious point-sources<ref name="Vedagiri2018">Vedagiri, U.K., Anderson, R.H., Loso, H.M. and Schwach, C.M., 2018. Ambient levels of PFOS and PFOA in multiple environmental media. Remediation Journal, 28(2), pp. 9-51.  [https://doi.org/10.1002/rem.21548 DOI: 10.1002/rem.21548]</ref>.  With respect to site-related precursors, transformation of otherwise unmeasured PFAS into detectable PFAAs is obviously relevant to site investigations to the extent transformation occurs after initial site characterization efforts or if past remedial efforts have accelerated ''in situ'' transformation rates<ref name="McGuire2014"/>.  Additionally, differential transport rates between precursor PFAS and the corresponding terminal PFAA could also confound CSMs if transformation rates are slower than transport rates as has been suggested<ref name="Weber2017"/>. 
 +
To account for otherwise unmeasurable precursors, several surrogate analytical methods have been developed. See [[PFAS Sampling and Analytical Methods]] for additional detail.
  
 
==References==
 
==References==
 
 
<references/>
 
<references/>
  
==See Also==
+
==See Also:==
*[http://iwmi.dhigroup.com/solute_transport/advection.html International Water Management Institute Animations]
 
*[http://www2.nau.edu/~doetqp-p/courses/env303a/lec32/lec32.htm NAU Lecture Notes on Advective Transport]
 
*[https://www.youtube.com/watch?v=00btLB6u6DY MIT Open CourseWare Solute Transport: Advection with Dispersion Video]
 
*[https://www.youtube.com/watch?v=AtJyKiA1vcY Physical Groundwater Model Video]
 
*[https://www.coursera.org/learn/natural-attenuation-of-groundwater-contaminants/lecture/UzS8q/groundwater-flow-review Online Lecture Course - Groundwater Flow]
 

Revision as of 19:52, 13 November 2020

PFAS Transport and Fate

The transport and fate of Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) in the environment is controlled by the nature of the PFAS source, characteristics of the individual PFAS, and environmental conditions where the PFAS are present. Transport, partitioning, and transformation are the primary processes controlling PFAS fate in the environment. PFAS compounds can also be taken up by both plants and animals, and in some cases, bioaccumulate through the food chain. Understanding PFAS transport and fate is necessary for evaluating the potential risk from a PFAS release and for predictions about PFAS occurrence, migration, and persistence, and about the potential vectors for exposure. This knowledge is important for site characterization, identification of potential sources of PFAS to the site, development of an appropriate conceptual site model (CSM), and selection and predicted performance of remediation strategies.

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Contributor(s): Dr. Hunter Anderson and Dr. Mark L. Brusseau


Key Resource(s):

Introduction

The transport and fate of Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) is a rapidly evolving field of science, with many questions that are not yet resolved. Much of the currently available information is based on a few well-studied PFAS compounds. However, there is a large number and variety of PFAS with a wide range of physical and chemical characteristics that affect their behavior in the environment. The transport and fate of some PFAS could differ significantly from the compounds studied to date. Nevertheless, information about the behavior of some PFAS in the environment can be ascertained from the results of currently available research.

PFAS transport and fate in the environment is controlled by the nature of the PFAS source, characteristics of the individual PFAS, and environmental conditions where the PFAS are present. Perfluoroalkyl acids (PFAAs) (see PFAS for nomenclature) are strong acids and are anionic in the environmentally-relevant pH range. They are extremely persistent in the environment and do not degrade or transform under typical environmental conditions. Polyfluoroalkyl substances (see PFAS for nomenclature) include compounds that have the potential to degrade to PFAAs. These compounds are commonly referred to as PFAA precursors or just ‘precursors’. Because some polyfluoroalkyl substances can degrade into PFAA via biotic or abiotic degradation pathways, PFAAs are sometimes referred to as “terminal PFAS” or “terminal degradation products”. The most important molecular properties controlling PFAA transport are the carbon chain length and functional moieties of the headgroups (e.g., sulfonate, carboxylate). The molecular properties of PFAA precursors are more varied, with different carbon chain lengths, headgroups and ionic states[3][4] (see PFAS). All of these properties can influence transport and fate of PFAA precursors in the environment.

Important environmental characteristics include the nature of the source (mode of input into the environment), the length of time that the source was active, and the magnitude of the input, as well as precipitation and infiltration rates, depth to groundwater, surface water and groundwater flow rates and interactions, prevailing atmospheric conditions, the properties of the porous-media (e.g., soil and sediment) and aqueous solution, microbiological factors, and the presence of additional fluid phases such as air and non-aqueous phase liquids (NAPLs) in the vadose zone and water-saturated source. In the subsurface, soil characteristics (texture, organic carbon content, clay mineralogy, metal-oxide content, solid surface area, surface charge, and exchange capacity) and solution characteristics (pH, redox potential, major ion chemistry, and co-contaminants) can influence PFAS transport and fate.

PFAS Transport and Fate Processes

Figure 1. Illustration of PFAS partitioning and transformation processes. Source: D. Adamson, GSI, used with permission.

Transport, partitioning, and transformation are the primary processes controlling PFAS fate in the environment (Figure 1). PFAS compounds can also be taken up by both plants and animals, and in some cases, bioaccumulate through the food chain. However, PFAS uptake and bioaccumulation is not discussed in this article (see “Environmental Concern” section of Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)).

  • Partitioning: Partitioning of PFAS between the mobile and immobile phases is one of the most important processes controlling the rate of migration in the environment. The primary mobile phases are typically air and water. Relatively immobile phases include stream sediments, soils, aquifer material, NAPLs, and interfaces between different phases (air-water, NAPL-water). Partitioning of a significant portion of the PFAS mass into an immobile phase increases the amount of material stored in the system and slows the apparent rate of migration in the mobile phase – a phenomenon that has been observed in field metadata[9].
  • Transformation: Transformation of PFAS is controlled by the molecular structure of the individual compounds. Perfluorinated compounds, including PFAAs, are resistant to abiotic and biotic transformation reactions under typical conditions and highly persistent in the environment. In contrast, precursors can be transformed by both abiotic and biotic processes, often resulting in the production of so-called “terminal” PFAA daughter products.

Transport and Partitioning in the Atmosphere

Air serves as a transport media for PFAS, particularly for uncharged polyfluorinated PFAS. Airborne PFAS transport contributes to global distribution and can lead to localized deposition to soils and surface water in the vicinity of emission sources[10][11][12][13].

PFAAs, which are ionic and possess a negative charge under ambient environmental conditions, are far less volatile than many other groundwater contaminants. An online database of vapor pressures and Henry’s Law constants for different PFAS, including PFAAs, is maintained by the Interstate Technology Regulatory Council[1]. In general, vapor pressures of PFAS are low and water solubilities are high, limiting partitioning from water to air[1]. However, under certain conditions, particularly within industrial stack emissions, PFAS can be transported through the atmosphere in both the gas phase and associated with fugitive particulates. In particular, volatile compounds including fluorotelomer alcohols (FTOHs) may be present in the gas phase, whereas, PFAAs can aerosolize and be transported as particulates[5]. In addition, precursors can be transformed to PFAAs in the atmosphere, which can result in PFAA deposition. Short-range atmospheric transport and deposition can result in PFAS contamination in terrestrial and aquatic systems near points of significant emissions, impacting soil, groundwater, and other media of concern[14][15]. Releases of ionic PFAS from factories are likely tied to particulate matter, which settle to the ground in dry weather and are also wet-scavenged by precipitation[16]. The impact of other potential sources, such as combustion emissions or wind-blown fire-fighting foam from fire training and fire response sites, on the fate and transport of PFAS in air may need to be assessed.

Long-range transport processes are responsible for the wide distribution of neutral and ionic PFAS across the Earth as evidenced by their occurrence in biota, surface snow, ice cores, seawater, and other environmental media in regions as remote as the Arctic and Antarctic[17][18]. Distribution of PFAS to remote regions far removed from direct industrial input is believed to occur from both: a) long-range atmospheric transport and subsequent degradation of volatile precursors; and b) transport via ocean currents and release into the air as marine aerosols (sea spray)[19][20].

Transport and Partitioning in Aqueous Systems

PFAS adsorb from water to a variety of solid materials including organic materials, clay minerals, metal oxides, and granular activated carbon[21]. This process is thought to occur through two primary mechanisms: 1) sorption to organic-carbon components of the solids; and 2) electrostatic (and other) interactions with inorganic constituents of the solids, including clay minerals and metal-oxides[22][23]. The relative contribution of each mechanism varies depending on surface chemistry and other geochemical factors, as well as the molecular properties of the PFAS. In general, the impact of electrostatic interactions with charged soil constituents is more important for PFAS than non-polar, hydrophobic organic contaminants (e.g. hydrocarbons, chlorinated solvents). Adsorption of PFAS by solids is often nonlinear, with greater sorption at lower solute concentrations. The impacts of adsorption kinetics and their potential reversibility on PFAS transport have not yet been examined for most PFAS compounds.

Sorption of hydrocarbons, chlorinated solvents and other hydrophobic organics is often controlled the by organic-carbon components of the solid phase (see Sorption of Organic Contaminants). However, studies of PFAS sorption to solid phase organic carbon have reported conflicting results. In a study of field sites with aqueous film-forming foam (AFFF, a type of fire-fighting foam) releases, solid phase organic carbon content was found to significantly influence PFAS soil-to-groundwater concentration ratios. Statistical modeling was then used to derive apparent organic carbon partition coefficients for 18 different PFAS[9]. A recent compilation of published organic carbon partition coefficients found a good correspondence to PFAS molecular structure[24]. However, other studies have shown a general lack of correlation between solid phase partition coefficients and organic carbon[25]. It is possible that greater variability may be observed for broader data sets that incorporate different ranges of PFAS concentrations, different solution conditions, different measurement methods, and field-based data which often have less well-defined conditions and may also be influenced by other retention processes[9][24].

Figure 2. Example of expected orientation and accumulation of PFAS at air-water interface. Source: D. Adamson, GSI, used with permission.

Most solids present in the environment contain both fixed-charged (negative) and variably charged surfaces. At neutral to high pH, variably charged clay minerals have a net-negative charge. As a result, negatively charged PFAAs do not strongly interact electrostatically in most soils, although as the soil pH decreases electrostatic sorption would be expected to increase in soils with variably charged clay minerals. Cationic and zwitterionic precursors are expected to be more strongly sorbed than anionic PFAAs in most environments due to well-established cation exchange reactions. Other factors, including ionic strength, composition, and the presence of co-solutes, can affect adsorption of PFAS[26][27][28][22][23].

Most PFAS compounds act as surface-active agents (or surfactants) due to the presence of a hydrophilic headgroup and a hydrophobic tail. The hydrophilic headgroup will preferentially partition to the aqueous phase and the hydrophobic tail will preferentially partition to the non-aqueous phase (air or organic material). As a result, PFAS tend to accumulate at interfaces (air-water, water-NAPL, water-solid) (Figure 2). This tendency to accumulate at interfaces can influence transport in the atmosphere (on water droplets and hydrated aerosols), in the vadose or unsaturated zone at air-water interfaces, in the presence of NAPLs, and in wastewater treatment systems[2][29].

In theoretical and experimental studies of transport in unsaturated porous media, adsorption at the air-water interface increased PFOS and PFOA retention[2][30][31], contributing approximately 20% to 80% of total retention in sands and soil. The impact of oil-water interfacial adsorption on PFAS transport was also quantitatively characterized in recent studies and shown to contribute to total retention on a similar scale as air-water interfacial adsorption[2][31]. These processes may result in increased PFAS mass retained in NAPL source zones, increased PFAS sorption with the resulting retardation of transport, and greater persistence of dissolved PFAS in the environment.

Transformation

Figure 3. Conceptual model of precursor transformation resulting in the formation of PFAAs. Source L. Trozzolo, TRC and C. Higgins, Colorado School of Mines, used with permission.

Certain polyfluorinated substances have the potential to transform to other PFAS, with PFAAs as the typical terminal daughter products. These polyfluorinated substances are often referred to as “precursors”. The transformation potential of polyfluorinated precursors is influenced by the presence, location, and number of carbon-hydrogen (C-H) bonds and potentially carbon-oxygen (C-O) bonds throughout the carbon chain. Specifically, PFAS with C-H bonds are subject to a variety of biotic and abiotic reactions that ultimately result in the formation of PFAAs with perfluorinated carbon chains of the same length or shorter than the initial polyfluorinated precursor[32][33][34][8].

Transformation studies published to date have tested only a small subsample of possible precursors and, therefore, much uncertainty exists regarding 1) the extent to which precursor transformation occurs on a global scale, 2) which environmental compartments represent the majority of transformation, 3) relevant environmental conditions that affect transformation processes, and 4) transformation rates and pathways. Nevertheless, a portion of the precursors are expected to transform to PFAAs over time as shown in Figure 3.

Precursors can be transformed by a variety of abiotic processes including hydrolysis, photolysis, and oxidation. Hydrolysis of some precursors, followed by subsequent biotransformation, can produce perfluoroalkyl sulfonates (PFSAs). An important example is the production of PFOS from perfluorooctane sulfonyl fluoride (POSF)[35]. Other hydrolysis reactions produce perfluoroalkyl carboxylates (PFCAs). At neutral pH, the hydrolysis of fluorotelomer-derived polymeric precursors results in the formation of monomeric precursors of PFOA and other PFAAs with half-lives of 50 to 90 years)[36]. Oxidation of precursors by hydroxyl radicals can occur in natural waters, with the fluorotelomer-derived precursors being oxidized relatively rapidly[37][38]. Evidence of aerobic biotransformation is provided from studies of PFAS composition throughout the continuum of wastewater treatments[39], from field studies at AFFF-impacted sites[32][33][34][8], and from microcosm experiments. In general, the literature on aerobic biotransformation collectively demonstrates or indirectly supports the following conclusions as summarized in ITRC 2020[1]:

  • Numerous aerobic biotransformation pathways exist with relatively rapid kinetics
  • All polyfluorinated precursors studied to date have the potential to aerobically biotransform to PFAAs
  • Aerobic biotransformation of various fluorotelomer-derived precursors exclusively results in the formation of PFCAs, including PFOA, without necessarily the conservation of chain-length
  • Aerobic biotransformation of various electrochemical fluorination-derived precursors primarily results in the formation of PFAAs, including PFOS, with the conservation of chain-length

Precursor transformation can complicate CSMs (and risk assessments) and should be considered during comprehensive site investigations. For example, atmospheric emissions of volatile precursors can result in long-range transport where subsequent transformation and deposition can result in detectable levels of PFAAs in environmental media independent of obvious point-sources[40]. With respect to site-related precursors, transformation of otherwise unmeasured PFAS into detectable PFAAs is obviously relevant to site investigations to the extent transformation occurs after initial site characterization efforts or if past remedial efforts have accelerated in situ transformation rates[33]. Additionally, differential transport rates between precursor PFAS and the corresponding terminal PFAA could also confound CSMs if transformation rates are slower than transport rates as has been suggested[8]. To account for otherwise unmeasurable precursors, several surrogate analytical methods have been developed. See PFAS Sampling and Analytical Methods for additional detail.

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See Also: