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Dissertation
Numerical modeling of fate and transport of per- and polyfluoroalkyl substances (PFAS) through vadose zone using HYDRUS software packages
Doctor of Philosophy (Ph.D.), Drexel University
Jun 2025
DOI:
https://doi.org/10.17918/00010963
Abstract
Per- and polyfluoroalkyl substances (PFAS) have become a major water pollution issue over the past two decades. The unsaturated zone (vadose zone) is located above the water table (from the root zone to the capillary fringe). Understanding PFAS fate and transport through this zone is crucial, as it acts as a key pathway for contaminants to reach groundwater resources. This dissertation aims to evaluate the fate and transport of PFAS through the vadose zone via numerical modeling. Numerical modeling serves as a valuable tool for simulating the physical and chemical processes that play a critical role in the movement and behavior of PFAS within the vadose zone. Organic carbon (OC) and air-water interface (AWI) sorption are crucial mechanisms affecting PFAS migration in the vadose zone, yet few numerical models account for both. This dissertation identifies HYDRUS 1D as an appropriate tool to address this gap. The first study aimed to use ColThetaTr as an add-on to the standard version of HYDRUS 1D, V.4 to consider the AWI phenomena in the fate and transport of PFAS through the vadose zone. ColThetaTr was initially intended for modeling virus accumulation at the AWI in Bradford et al.'s 2015 study. Kinetic adsorption and desorption rate coefficients were determined through inverse modeling to match breakthrough curves closely, and these coefficients were then used for forward modeling. The study's forward modeling reveals that PFAS migration rates and breakthroughs at the water table are highly influenced by the depth to the water table, with shallower depths yielding higher concentrations over a 100-year simulation period. The implementation of ColThetaTr within HYDRUS 1D for PFAS transport modeling highlights the significant role of AWI partitioning in the vadose zone. By calibrating with inverse modeling, the study obtained adsorption and desorption rate coefficients for PFAS sorption to both organic carbon and the AWI, aligning closely with observed breakthrough curves from column experiments. Results show that AWI sorption substantially contributes to PFAS retention, limiting downward migration, and resulting in notable stagnation of PFOS and PFOA within upper soil layers. This finding indicates that PFAS compounds exhibit limited mobility once sorbed to the AWI, underscoring the importance of AWI interactions in long-term retention and transport predictions. However, ColThetaTr's adaptation for PFAS also reveals limitations that constrain its effectiveness for comprehensive vadose zone modeling. Initially developed for virus transport, ColThetaTr lacks tailored mechanisms for PFAS's amphiphilic and persistent properties, leading to incomplete representations of complex, non-linear, and potentially irreversible sorption behaviors. The model also struggles to simulate PFAS desorption under varying soil moisture, creating a "stagnation effect" where PFAS at the AWI fails to respond dynamically to saturation changes. Additionally, the model does not account for interactions among PFAS compounds at the AWI, which can alter sorption efficiency, especially at higher concentrations. These limitations highlight the need for advanced models that incorporate non-linear sorption, dynamic desorption, and multi-PFAS interactions to improve accuracy and reliability in predicting PFAS transport in the vadose zone. Biosolids are crucial for agricultural practices and have gained more attention because of their potential as a source of PFAS. Thus, the application loading rate of biosolids to soils can impact the release and transport of PFAS through the vadose zone. Despite this, few studies have been done regarding PFAS fate and transport through the unsaturated zone, particularly in settings where biosolids are applied. Therefore, as part of this dissertation, the second study utilized the HYDRUS 1D model with a PFAS module to assess PFAS leaching from land-applied biosolids in agriculture, aiming to understand its impact on groundwater. Focused on two sites in the West U.S. with varied soil textures, the simulations span 200 years post-biosolids application, considering actual Weather data conditions. Results show that soil equilibrium processes dominate PFAS transport to groundwater, and that AWI adsorption is a minor factor in biosolids-amended soils but is more important in homogenous vadose zone soils than in the heterogeneous ones considered here. Such an approach is critical to developing site-specific PFAS soil screening levels and leachate levels, and for improved best management practices in land application of biosolids. PFAS raises serious worries because of their tendency to contaminate groundwater since they persist in the environment. Knowledge of their transport behavior in the vadose zone – the unsaturated soil zone above the water table – may help predict long-term environmental impacts and develop effective remediation strategies. Comparing the extensions of ColThetaTr to HYDRUS 1D Version 5.1 and its PFAS add-on module allows users to understand better the various capabilities and limitations of the different modeling approaches in model simulations under various conditions. Each module offers unique algorithms and features, such as how they handle adsorption-desorption processes, degradation pathways, and water flow dynamics, all of which are critical to simulating PFAS behavior accurately in unsaturated soils. An appreciation of the subtleties of the differences between the various HYDRUS 1D software packages will lead to more nuanced models and, in turn, more precise predictions of the mobility, persistence, and potential for contamination of PFAS in groundwaters. These comparisons could help pinpoint which model best captures what we observe in the real world, establish which models should be used to inform regulatory and remediation strategies and allow us to determine the risk associated with PFAS contamination. The Third study of this dissertation specifically compared the algoritums, capabilities, and limitations of using the extensions of ColThetaTr to HYDRUS 1D Version 5.1 and its PFAS add-on to model PFAS transport in the vadoze zone. The ColThetaTr module provides a simplified framework for simulating PFAS transport, focusing on basic sorption and advection-dispersion processes. However, it cannot accurately represent the complex behavior of PFAS, particularly at the air-water interface and under nonlinear sorption conditions. In contrast, HYDRUS 5.1 with the PFAS Add-on module introduces significant advancements, including nonlinear and irreversible sorption dynamics, improved modeling of PFAS interactions at the air-water interface, and long-term retention mechanisms. Because of these enhancements, greater predictive capacity is available using the PFAS Add-on module compared to ColThetaTr, especially for predicting PFAS behavior in environmental scenarios where sorption and retention become the primary factors in understanding the long-term risks associated with contamination. The chapter illustrates how the modeling capabilities of HYDRUS 5 now provide a more realistic depiction of the fate and transport of PFASs and, as such, offer better tools for environmental risk assessment and remediation planning. The findings from this dissertation underscore the significance of accounting for AWI interactions, biosolid application dynamics, and complex sorption mechanisms in modeling PFAS persistence in the vadose zone. Each study's results support a more comprehensive framework for predicting PFAS behavior, emphasizing the need for advanced models that reflect both transient environmental conditions and PFAS-specific properties. These models ultimately could aid in better risk assessment and management of PFAS contamination in the subsurface environment.
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Details
- Title
- Numerical modeling of fate and transport of per- and polyfluoroalkyl substances (PFAS) through vadose zone using HYDRUS software packages
- Creators
- Nona Jesmani
- Contributors
- Christopher Sales (Advisor)
- Awarding Institution
- Drexel University
- Degree Awarded
- Doctor of Philosophy (Ph.D.)
- Publisher
- Drexel University; Philadelphia, Pennsylvania
- Number of pages
- xv, 247 pages
- Resource Type
- Dissertation
- Language
- English
- Academic Unit
- Civil (and Architectural) Engineering [Historical]; College of Engineering (1970-2026); Drexel University
- Other Identifier
- 991022055640304721