PFAS in Groundwater
A nontechnical description of the composition, origin, hazard, and movement of PFAS in groundwater
Introduction to PFAS Webpage
PFAS is the acronym for perfluoroakyl and polyfluoroalkyl substances. This webpage describes PFAS and its behavior in groundwater. It is written in nontechnical language with related technical comments in parentheses or endnotes referenced in parentheses. Some of the technical terms and concepts are used after they are introduced.
Composition of PFAS
PFAS are a family of man-made chemicals that contain carbon, fluorine and other elements. The PFAS family includes more than 4000 chemicals. All PFAS contain a chain of carbon atoms and some of the carbon atoms are bonded to fluorine atoms. For example, PFOA (perfluorooctanoic acid), which is a member of PFAS is diagrammed in Figure 1.
Figure 1. Diagram of PFOA (From NIEHS)
The carbon chains of the perfluoroalkyl substances are fully fluorinated (all carbons except the last one are attached to fluorines). Perfluoroalkayls can be linear or branched. The carbon chains of polyfluoroalkyl substances are not fully fluorinated, but at least one carbon is attached to a fluorine atom. PFAS in groundwater that have received much attention are the perfluoroalkyl substances PFOA and PFOS which are diagrammed in Figure 2. The structural formulas do not depict the three-dimensional structure. They just show a two-dimensional approximation. Both PFOA and PFOS have more than one molecular structure (isomeric). There are many molecular structures in which the carbon chain is branched.
Figure 2. Molecular and structural formulas for PFOA and PFOS.
Properties of PFOA and PFOS
The fluorine atoms are very strongly bonded to the carbon atoms in perfluor0alkyl substances such as PFOA and PFOS.(1) The fluorine atoms shield the carbon chain from being degraded by microorganisms or reaction with other substances in groundwater or in soil above the water table. The functional group at the end of the molecule that contains hydrogen (H) and oxygen (O) is called the head of the molecule and the remainder of the molecule is called the tail.
When PFOA and PFOS (strong acids) are dissolved in water the positively charged nucleus (proton) of the hydrogen molecule in the head dissociates and passes into the water. Thus he head becomes negatively charged, and the molecule becomes an ion. The tail also presents as negative to the surrounding water. The negative character of the tail is due to the negative electrons that are shared in the carbon-fluorine bond being more strongly attracted to the fluorine.(1)
Origin of PFAS
PFAS have been manufactured and used in a variety of industries since the 1940s. They have been found in food, fabrics, nonstick products, polishes, waxes, paints, cleaning products and firefighting foams. PFOA and PFOS are no longer manufactured in the United States but they can be imported in consumer goods.
PFAS Health Hazard
There is evidence that exposure to PFAS can lead to adverse human health effects. PFOA and PFOS have been the most extensively produced and studied of these chemicals. Both are very stable, so that once they enter a groundwater system they do not break down. In the past they have been extensively produced, and they are persistent in the environment and in the human body. They have caused adverse health effects in laboratory animals and may cause adverse health effects in humans if the human exposure is great enough.
In June 2022 EPA issued health advisories for PFOA of 0.004 parts per trillion (ppt) and PFOS of 0.02 ppt. EPA also issued health advisories of 10 ppt for GenX chemicals and 2,000 ppt for PFBS. GenX and PFBS are high performance perfluoralkyl substances that have been used since PFOA and PFOS manufacturing ceased in the United States. Health advisories are non-enforceable guidelines.
Uncertainties are associated with the application of such low concentration in health advisories. Samples can be contaminated during collection, and some laboratories may not be capable of producing accurate analyses at such low concentrations (Cook and O'Reilly, 2023).
PFAS Movement in Groundwater
The concentrations and movement of PFOA and PFOS in PFAS plumes in groundwater downgradient from PFAS sources is affected by:
The rate of inflow of the PFAS into the groundwater and its temporal variation,
The size and shape of the source area,
The direction and rate of movement of the groundwater,
Adsorption and desorption partition of the PFAS with solids in the groundwater system (2),
Attraction to water-napl (non-aqueous phase liquid) interfaces such as oil, and
Generation of PFOA and PFOS from polyfluoroalkyl precursors.
Soruces of PFAS in groundwater include but are not limited to:
Firefighting foams at airports, refineries, and military facilities,
Industrial facilities where PFAS were produced or used,
Landfills (carpets, textiles),
Wastewater treatment plant sludge and biosolids in infiltration beds and applied on fields.
Endnotes for PFAS in Groundwater
The tendency of an atom in an organic molecule to attract the shared pair of electrons in a covalent bond towards itself is called electronegativity. In general, the greater the electronegativity difference between two atoms, the stronger the covalent bond between them, although other factors may also have an effect. This tendency is caused by the more electronegative atom pulling the shared electrons closer to its nucleus. The C-F bond is the strongest bond in organic molecules. Fluorine has the greatest electronegativity of all of the elements (3.98). Carbon electronegativity is 2.55.
Both fluorine and carbon have their valence electrons in the second energy shell, but fluorine has a greater positive charge in its nucleus and a smaller atomic radius. The shared electron is attracted more strongly to the fluorine nucleus than to the carbon nucleus and is pulled more closely to it. This causes the greatest negative charge to be displaced outward in the tail of the PFAS molecule.
(2) PFOA and PFOS Adsorption
The sorption mechanism of PFOA and PFOS in groundwater systems is yet to be fully understood due to the complex processes of the interaction with aquifer solids. (Fargbaygbo,2022). Partitioning between different phases in an aquifer system is controlled by total free energy. Factors that affect sorption of PFAS include:
Presence of divalent cations such as calcium ions in the groundwater,
Ionic strength of the groundwater,
PFAS concentration in the groundwater,
pH of the groundwater,
Amount of organic carbon and protein in the aquifer (but distribution coefficients cannot be reliably estimated based on organic carbon alone),
Amount and species of clay in the aquifer (which generally have negatively charged surfaces),
Amount and species of iron oxides in the aquifer (for example goethite and ferrihydrite),
Concentration of non-aqueous phase liquids (NAPLs) in the groundwater,
Relative proportions of other minerals in the aquifer,
Induration and degree of fracturing of the aquifer,
Porosity of the matrix in fractured aquifers,
Adsorption /desorption rate (equilibrium assumption not necessarily valid and the rates may be hysteretic).
References for PFAS in Groundwater
Cook, L. and K. O'Reilly (2023): Regulating PFAS at the Edge of Detection; American Bar Association Committee Article, June 21, 2023.
Fargbaygbo, B. O. (and others):Sorption and Partitioning of Perfluorooctanioc Acid (PFOA) and Perfluorooctane Sulfonate (PFOS) Onto Sediments of Diep and Plankenburg River Systems Western Cape, South Africa; Environmental Technology & Innovation, Volume 25.
Posted September 29, 2023.