How Defensible Is PFAS Precursor Transformation Modeling?
Lumped reactive-transport & Multiple mechanisms
PFAS Precursor Transformation
At many PFAS sites, particularly those impacted by aqueous film-forming foam (AFFF) and various mixed PFAS sources, some of the contaminant mass may be present as precursor compounds that can transform into terminal perfluoroalkyl acids (PFAAs). Given that these precursors can maintain future daughter-PFAS loading even after the initial release diminishes, the transformation of precursors may play a crucial role in the long-term prediction of vadose-zone leaching and its impact on groundwater.
PFAS precursor transformation modeling is defensible, but only under certain conditions. The strongest technical approach is to treat it as a lumped reactive-transport approximation of net precursor loss and daughter-PFAS generation, rather than a universal mechanistic equation governing all PFAS precursor chemistry. In real site applications, the model is frequently attempting to answer a site-scale specific question such as: How much terminal PFAS can be generated from the precursor reservoir over time, and how does this effect groundwater loading? That usage is often more plausible than asserting that the model resolves the entire biochemical pathway network.
PFAS precursor transformation can be represented at different levels of complexity depending on the modeling objective. Recent PFAS vadose-zone modeling work explicitly incorporates first-order precursor transformation into broader nonequilibrium transport formulations, while literature review reveals that real precursor transformation can involve multiple intermediates, branching pathways, and environment-dependent behavior. Chen and Guo (2025) utilized a lumped first-order transformation approach within a vadose-zone reactive-transport model to estimate the net effect of precursor depletion and daughter-PFAS generation on long-term leaching and groundwater loading.
In contrast, Zhang et al. (2021) examine the wider literature on environmental biochemical transformation, demonstrating that precursor degradation may involve various precursor classes, multiple intermediate compounds, branching pathways, and condition-dependent mechanisms such as oxidation, dealkylation, and defluorination. Together, the two papers support a balanced technical position: first-order precursor terms are useful and defensible at site scale, but they should be presented as lumped approximations rather than universal mechanistic laws governing PFAS precursor reactions.
- Chen and Guo (2025) supports using a first-order precursor-loss term in a vadose-zone transport model when the goal is practical site-scale forecasting of PFAS leaching and daughter-PFAS loading.
- Zhang et al. (2021) supports the caution that such a term should be framed as a simplification, because real precursor transformation can involve several precursor classes, intermediates, reaction branches, and environmental controls.
Why precursor transformation matters
Substantial portion of PFAS mass at AFFF-impacted and mixed-source sites may exist as primary precursors, secondary precursors, and terminal PFAAs. Reviews of AFFF-impacted sites indicate that AFFF is often a major source of primary precursors, secondary precursors, and PFAAs, and that transformation plays an important role in explaining why PFAS profiles in the environment differ from the original source mixture. Ignoring precursors can understate future groundwater loading where the source mixture contains transformable PFAS species. Thus, precursors should be evaluated during site characterization, and TOP assay can determine if oxidizable precursors are present and potentially available for PFAA transformation in a plume or vadose-zone source area. Recent PFAS vadose-zone modeling work considers precursor transformation within transport formulations, reflecting the recognition that precursor mass can materially affect long-term mass discharge to groundwater.
Lumped precursor transport equation
A practical simplified precursor transport equation for vadose-zone modeling can be written as:
$$
\frac{\partial (\theta C_p)}{\partial t}
=
\frac{\partial}{\partial z}
\left(
\theta D_p \frac{\partial C_p}{\partial z}
\right)
–
\frac{\partial}{\partial z}(q C_p)
–
\lambda_p \theta C_p
$$
This equation indicates that precursor mass can be transported through advection, spread by dispersion, and simultaneously depleted by transformation.
The corresponding PFAS-production source term may be written as:
$$
R_{p \to i} = Y_i \lambda_{p,i} \theta C_p
$$
For multiple PFAS generation or multiple precursor classes:
$$
\sum_{j} Y_{ij}\,\lambda_{p,j}\,\theta\,C_{p,j}
$$
- = precursor concentration in pore water
- = volumetric water content
- = hydrodynamic dispersion coefficient for the precursor
- = Darcy flux
- = lumped first-order precursor transformation coefficient
- = production rate of daughter species i
- Yi or Yij = effective yield term
- λp,i = lumped transformation coefficient for the precursor-to-daughter pathway
PFAS Precursor Transformation Modeling
PFAS precursor transformation modeling is defensible, but requires careful qualification. It is best supported when presented as a lumped reactive-transport approximation that illustrates the net conversion of precursor compounds into terminal PFAS, rather than as a universal mechanistic law governing all PFAS precursor chemistry. That distinction holds significance as published PFAS literature indicates that precursor transformation can involve multiple intermediate compounds, branching pathways, and variable products depending on precursor class and environmental conditions.
In practical site modeling, the objective is typically not to replicate every biochemical step in the precursor degradation network. The real question is whether a simplified formulation can reasonably estimate how much precursor mass may transform into regulated daughter PFAS over time and space under site-relevant conditions. Recent vadose-zone PFAS modeling efforts supports this approach by incorporating first-order transformation within broader nonequilibrium PFAS transport formulations.
Lumped precursor transport formulation is most effectively understood as a net source–sink approximation within a transport model. This approach is beneficial as it links precursor loss to daughter formation in a mathematically coherent manner; however, it does not suggest that all PFAS precursor transformations occur in a single-step, first-order, or pathway-invariant way.
That caution is not merely theoretical. Reviews of PFAS precursor biotransformation indicate that transformation can occur via oxidation, dealkylation, defluorination, and various other pathways, often resulting in the formation of multiple intermediates and shorter-chain daughter compounds. In essence, real precursor chemistry frequently exhibits more complexity than the simplified first-order term employed in site-scale transport models.
For that reason, precursor transformation equations are most justifiable when they are framed as practical modeling constructs. Their foundation lies in standard reactive-transport theory, modified for PFAS systems to estimate the overall environmental dynamics of precursor loss and daughter generation. They ought not to be portrayed as universally accepted PFAS reaction laws that apply to every source mixture, all media, and every site condition.
The significance of this issue is heightened at AFFF-impacted sites. Recent reviews indicate that AFFF often serves as a source of primary precursors, secondary precursors, and terminal PFAAs simultaneously. In addition, environmental PFAS profiles may vary considerably from original source formulations due to ongoing transformation post-release. Precursor transformation may significantly influence long-term plume evolution and groundwater loading, necessitating its inclusion in the conceptual site model when supported by source history.
Precursor modeling becomes more defensible when there is actual evidence that precursors are present. Research indicates that the TOP assay can be utilized to determine the presence of oxidizable precursors that may be available for transformation to PFAAs. It is also advisable to evaluate the trends of precursors and PFAAs throughout the plume. In that context, precursor transformation modeling is not speculative add-on chemistry; it becomes a reasonable attempt to represent an identified site process.
The geochemical setting of the site is also important. The potential for transformation varies significantly among different environments. Aerobic precursor biotransformation might be hindered in anaerobic environments, while more oxidizing conditions could promote the conversion to terminal PFAAs. This implies, redox conditions, oxygen availability, residence time, and microbial activity must be included in the defensibility argument whenever precursor transformation is modeled.
Laboratory and column studies additionally lend support to the notion that precursor transformation may be incorporated into transport models, although with caution. For instance, Yan et al. demonstrated that the biotransformation of 6:2 FTS in flow-through soil columns was influenced by seepage velocity and residence time, and that the yields of daughter products varied with hydraulic conditions. This result underscores the importance of incorporating precursor transformation in transport modeling, highlighting why a single universal field rate constant is often insufficient.
Consequently, In real consulting practice, precursor transformation modeling is usually most defensible for screening, bounding, scenario testing, and long-term mass-discharge forecasting. The argument becomes less tenable when it suggests an exceedingly accurate reconstruction of species-by-species pathway chemistry without direct site evidence. A lumped model can answer the practical question, “What quantity of daughter PFAS may be generated over time from the precursor reservoir?” It cannot, on its own, demonstrate a distinct mechanistic pathway.
What makes precursor transformation modeling defensible?
1. A credible precursor source term
The robustness of defensibility is significantly enhanced when the conceptual site model is grounded in a credible foundation for the presence of precursors, including AFFF releases, fluorotelomer-based inputs, landfill leachate, or the application of biosolids. Recent AFFF reviews underscore the significance of precursor occurrence and transformation in understanding the evolution of PFAS sources and their downgradient profiles.
2. Evidence that precursors are actually present
Precursor modeling is significantly enhanced when the presence of precursors is corroborated by analytical evidence, source fingerprinting, or forensic indicators. Research indicates that the TOP assay is effective for determining the presence and availability of precursors for transformation into PFAAs, and advises the assessment of spatial trends of precursors and PFAAs along the flow path.
3. A geochemically plausible transformation setting
Transformation is not uniformly probable throughout the site. Studies show that in anaerobic environments, the biotransformation of aerobic precursors is likely to be hindered, while in oxidizing and aerobic conditions, there is a greater chance for precursors to convert into PFAAs. Redox, oxygen availability, and site geochemistry must be integral to the defensibility argument, rather than merely an afterthought.
4. Transparent acknowledgment that kinetics are lumped
The model should clearly state that λp is a lumped effective rate for the selected precursor class, medium, and model scale. It should not suggest that a single rate constant serves as a universal intrinsic constant applicable to all field conditions. The literature underscores this caution, as transformation rates and pathways differ across precursor types and environments.
5. Calibration or bounding against site-relevant evidence
The most robust precursor models are not based solely on assumptions. The calibration, bounding, verification of precursor-to-daughter ratios, changes in plume profiles, results from TOP assays, column studies, microcosm findings, and site-wide mass-balance trends are presented. Research suggests the comparison of precursor and PFAA patterns along the plume, as well as the estimation of transformation effects by analyzing the remaining precursor mass in relation to the end-product mass.
Conclusion
The essential conclusion is this: incorporate precursor transformation when the conceptual site model allows for it, but refer to it as a lumped reactive-transport approximation rather than a universally established PFAS chemical reaction. The approach outlined is the most technically sound and professionally prudent method for employing precursor transformation modeling in vadose-zone and groundwater fate-and-transport analyses.
References
Chen, S., & Guo, B. (2025). Semi-analytical solutions for nonequilibrium transport and transformation of PFAS and other solutes in heterogeneous vadose zones with structured porous media. Advances in Water Resources, 206, 105099.
Interstate Technology & Regulatory Council. (2023). PFAS guidance document: Environmental fate and transport processes and Site characterization.
U.S. Environmental Protection Agency. (2025). Environmental forensic tools for understanding PFAS fate and transport (RPM bulletin).
Yan, P.-F., Dong, S., Woodcock, M. J., Manz, K. E., Garza-Rubalcava, U., Abriola, L. M., & Pennell, K. D. (2024). Biotransformation of 6:2 fluorotelomer sulfonate and microbial community dynamics in water-saturated one-dimensional flow-through columns. Water Research.
Zhang, W., Pang, S., Lin, Z., Mishra, S., Bhatt, P., & Chen, S. (2021). Biotransformation of perfluoroalkyl acid precursors from various environmental systems: Advances and perspectives. Environmental Pollution, 272, 115908.