Bo Guo

Interests

Teaching

HWRS 504 Numerical Methods for Environmental Transport ProblemsHWRS 505 Vadose Zone HydrologyHWRS 170A1 Earth: Our Watery HomeHWRS495/595 Hydrology and Atmospheric Sciences Weekly Colloquium

Research

My research aims to: 1) advance our fundamental understanding of fluid flow and transport processes in porous materials at microscale (i.e., pore scale) by developing computational models that explicitly represent the pore structures and the physical and chemical processes, and 2) then take the microscale understanding and develop predictive computational models at field scale (often involves model reduction via upscaling or multiscale formulations) to address practical engineering problems for energy and environmental systems in the subsurface including contaminant transport in soil and groundwater especially emerging contaminants such as PFAS, shale gas/oil production, and geological CO₂ storage.

Courses

Scholarly Contributions

Journals/Publications

• Baluja, R., Guo, B. o., Howden, W., Langer, A., & Lemoine, D. (2025). PFAS-contaminated drinking water harms infants. Proceedings of the National Academy of Sciences, 122(50), e2509801122.
• Baluja, R., Guo, B., Howden, W., Langer, A. A., & Lemoine, D. M. (2025).

  PFAS-contaminated drinking water harms infants

  . Proceedings of the National Academy of Sciences.
• 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. doi:10.1016/j.advwatres.2025.105099
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  We present screening-type semi-analytical models for quantifying the fate and transport of PFAS, including perfluoroalkyl acids (PFAAs) and their precursors (i.e., polyfluoroalkyl substances that can transform to PFAAs), in a heterogeneous vadose zone. The models employ one-dimensional multi-continuum representations with varying complexities (dual-porosity, dual-permeability, or triple-porosity). They account for PFAS-specific transport processes, including multi-site rate-limited adsorption at solid–water and air–water interfaces, and first-order biochemical transformation. Assuming steady-state infiltration, we derive semi-analytical solutions for all models under arbitrary initial and boundary conditions. We validate these new solutions using literature experimental breakthrough curves of PFAS and other solutes for various soils and wetting conditions. Furthermore, we demonstrate the models’ capability by analyzing the long-term leaching and mass discharge of two example PFAS (PFOS and a precursor PFOSB) in a heterogeneous vadose zone beneath a model PFAS-contaminated site. The results demonstrate that the precursor undergoes significant transformation and adds additional PFOS mass discharge to groundwater. Additionally, the simulations suggest that, due to strong retention in the vadose zone (i.e., large residence time), the PFAS in the high- and low-conductivity transport pathways can be considered as in equilibrium. Taking advantage of this result, we illustrate that the multi-continuum models may be simplified to an effective single-porosity model for simulating the transport of longer-chain PFAS in a heterogeneous vadose zone. Overall, the semi-analytical models provide practical tools for assessing long-term fate and transport of PFAS in the vadose zone and mass discharge to groundwater in the presence of precursor transformations.
• Chen, S., & Guo, B. o. (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, 105099.
• Divine, C., Guo, B. o., Brusseau, M., Kinser, B., & Shepherd, C. (2025). Practical PFAS Immobilization in the Vadose Zone by Extreme Soil Vapor Extraction: Conceptual Understanding, Modeling, and Cost Analysis. Groundwater Monitoring & Remediation, 45(3), 69--76.
• Divine, C., Guo, B., Brusseau, M., Kinser, B., & Shepherd, C. (2025). Practical PFAS Immobilization in the Vadose Zone by Extreme Soil Vapor Extraction: Conceptual Understanding, Modeling, and Cost Analysis. Groundwater Monitoring and Remediation, 45(Issue 3). doi:10.1111/gwmr.12722
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  Practical and cost-effective technologies are needed for per- and polyfluoroalkyl substance (PFAS) sources in the vadose zone to prevent continued migration of these contaminants from soil to groundwater. Many PFAS are characterized by high air–water interfacial adsorption coefficient (Kaw) values, and therefore, the air–water interface exerts a strong control on their transport. As soil moisture decreases in the vadose zone, air–water interfacial area generally increases. As a result, the effective retention of some PFAS can be increased by 100-fold or more in some cases with relatively modest reductions in soil moisture content. Quantitative modeling and conceptual costing analysis confirm the viability of a two-pronged PFAS immobilization strategy where (1) a surface cap is installed which is intended to prevent water infiltration, and (2) extreme soil vapor extraction (XSVE) is applied to dry the soil, which reduces or eliminates downward water flux and increases PFAS retention. Modeling results show that water flux and PFAS mass discharge to groundwater can be essentially eliminated using this approach. Even if recharge is not completely prevented (due to a leaking cap and/or insufficient soil drying), simulations show PFAS mass discharge to groundwater will still be greatly reduced due to the significantly enhanced PFAS retention. The equipment required for this approach is commercially available, and installation costs are modest and predictable. Based on this analysis, future pilot testing and field demonstrations are warranted.
• Liu, Y., Zheng, T., Guo, B. o., Tao, Y., Jiang, S., Cao, M., Zheng, X., & Luo, J. (2025). Reactive transport of different dissolved organic nitrogen components in an unconfined aquifer. Journal of Hazardous Materials, 493, 138259.
• Liu, Y., Zheng, T., Guo, B., Tao, Y., Jiang, S., Cao, M., Zheng, X., & Luo, J. (2025). Reactive transport of different dissolved organic nitrogen components in an unconfined aquifer. Journal of Hazardous Materials, 493(Issue). doi:10.1016/j.jhazmat.2025.138259
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  Dissolved organic nitrogen (DON) is often an overlooked form of nitrogen that can leach from the soil into aquifers. The reactive transport and dispersion of DON in aquifers can contribute to regional nitrogen contamination. The current body of research has primarily focused on the vertical leaching process of DON through the vadose zone. However, these studies have largely ignored the broader reactive transport of DON within aquifers under the influence of groundwater flow. In this study, we investigate the reactive transport of DON under groundwater flow conditions. Utilizing molecular biological technologies, we aim to reveal DON's intrinsic role in the nitrogen cycle within aquifers. Our findings reveal that urea exhibits greater mobility compared to amino acids and proteins. The transport of amino acids and proteins reduces the NO3--N concentrations (44.6 % and 89.6 %) compared to the blank control, while urea leads to the accumulation of NO3--N in groundwater (10.1 %). Amino acid and protein columns show higher relative abundances of Pseudomonas (10.1 % and 7.3 %) and Thermomonas (3.9 % and 5.1 %) with denitrification functions, facilitating denitrification in groundwater. Conversely, the presence of urea increases the relative abundances of Nitrosomonadaceae and Nitrophilus (0.33 % and 0.67 %), posing a potential NO3--N contamination risk. Biotransformation has the greatest effect on protein transport (19.6 %), while adsorption mainly influences amino acid transport (12.4 %). The study provides fundamental insights into the reactive transport of different DON components in aquifers, which holds important implications for regional groundwater environment protection.
• Russell, R., Guo, B. o., Zeng, J., Brusseau, M. L., Schaefer, C. E., Shea, S., Higgins, C. P., & Ferre, T. P. (2025). Combining field datasets and mathematical modeling to quantify PFAS leaching and mass discharge at an AFFF-impacted site. Water Research, 124063.
• Russell, R., Guo, B., Zeng, J., Brusseau, M. L., Schaefer, C. E., Shea, S., Higgins, C. P., & Ferre, T. P. (2025). Combining field datasets and mathematical modeling to quantify PFAS leaching and mass discharge at an AFFF-impacted site. Water Research, 286. doi:10.1016/j.watres.2025.124063
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  Vadose zones serve as significant reservoirs of per- and polyfluoroalkyl substances (PFAS) at contaminated sites, posing risks to the groundwater underneath. Partitioning of PFAS to the solid–water and air–water interfaces in soils complicates PFAS leaching in the vadose zone. We apply mathematical models representing PFAS-specific retention and transport processes to simulate vadose-zone leaching and mass discharge at a PFAS-contaminated field site. The mathematical models are constrained by detailed datasets collected at the site under both ambient rainfall and artificial flushing conditions. Predicted porewater concentrations generally agree with those sampled by suction lysimeters over a period of 2 months. Model-based analysis suggests: (1) minimal downward migration of PFOS and PFOA occurred over the 2-month period, (2) variations in observed porewater concentrations were caused by mass redistribution among the different phases in response to dynamic changes in soil moisture content and air–water interfacial area, (3) accounting for rate-limited solid-phase desorption reduces discrepancies between simulated and sampled porewater concentrations for PFOS and PFOA, particularly for the shallowest depth interval, and (4) porewater concentration and moisture data may be used to estimate air–water interfacial area. Additional 40-year long-term simulations indicate that the simulated leaching is consistent with field observations for PFOS, but it generally overestimates the leaching for PFOA, PFHxS, and PFBS, which appears to be caused by the simulations not accurately representing desorption kinetics, underestimating solid-phase adsorption, and/or not accounting for precursor transformation. Our results suggest that accurate quantification of source strength and mass discharge at a PFAS-contaminated site requires characterizing hydraulic and transport parameters especially kinetic solid-phase desorption behaviors, PFAS soil concentration profiles, precursor transformation, and site-specific infiltration rates.
• Shi, B., Rong, J., Jiang, H., Guo, B. o., Hassanizadeh, S. M., & Qin, C. (2025). The pore-network-continuum modeling of two-phase flow properties for multiscale digital rocks. Advances in Water Resources, 105138.
• Shi, B., Rong, J., Jiang, H., Guo, B., Hassanizadeh, S. M., & Qin, C. Z. (2025). The pore-network-continuum modeling of two-phase flow properties for multiscale digital rocks. Advances in Water Resources, 206(Issue). doi:10.1016/j.advwatres.2025.105138
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  Many subsurface formations and reservoirs exhibit multiscale and heterogeneous pore structures, such as soils, carbonate rocks, shales and tight sandstones. Understanding and predicting their two-phase flow properties are crucial to underground applications including contamination remediation, oil and gas recovery, and geological storage of carbon dioxide. For a multiscale digital rock, pores with a wide pore-size distribution spanning several orders of magnitude cannot be visualized in one image, due to the trade-off between image resolution and field of view. However, a large number of unresolved pores (i.e. microporosity) can challenge the modeling of flow and transport. We develop an efficient pore-network-continuum model (PNCM) for quasi-static two-phase flow in multiscale digital rocks. The resolved pores and microporosity are represented by a pore network and continuum grids, respectively. Instead of costly CT-based characterization, we propose to use the bimodal van Genuchten model of mercury intrusion capillary pressure to infer the pore-size distribution of heterogeneous microporosity. The PNCM is applied to a laminated sandstone with synthesized homogeneous microporosity and an Estaillades carbonate rock with heterogeneous microporosity. Both single-phase and two-phase flow properties including absolute permeability, formation factor, resistivity index, capillary pressure, and relative permeability are predicted and compared with experimental data. The good agreement demonstrates the robustness and reliability of the developed PNCM. Using the case studies, we illustrate how microporosity influences and determines two-phase flow properties.
• Wang, X., Qin, C., Guo, B. o., Pop, S., & Tian, J. (2025). Experimental validation of an image-based dynamic pore-network model for spontaneous imbibition in sandstones. Advances in Water Resources, 195, 104859.
• Wang, X., Qin, C., Guo, B., Pop, S., & Tian, J. (2025). Experimental validation of an image-based dynamic pore-network model for spontaneous imbibition in sandstones. Advances in Water Resources, 195(Issue). doi:10.1016/j.advwatres.2024.104859
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  Spontaneous imbibition (SI) in porous media driven by capillary action is pivotal to many subsurface and industrial applications. The pore-scale modeling has been playing a vital role in unraveling wetting dynamics in pore spaces, which will eventually determine flow parameters and behaviors. In this paper, we mainly contribute to validating an image-based dynamic pore-network model (PNM) for SI. For the scenario of water imbibing into dry porous media, we measured imbibition rates and residual saturations of three types of sandstones, namely, Nubian, Bentheimer and Upper Berea as the validation data. Then, we extracted the pore networks of the μCT images of the same core samples used in the lab experiments, to reduce heterogeneity uncertainties. We demonstrate that using either a uniform or a lognormal distribution of effective contact angles that is consistent with experimental measurements in the literature, the dynamic PNM can accurately predict experimental imbibition rates and residual saturations. Given the challenge of experimentally determining effective contact angles, we further investigate the effects of these two plausible contact angle distributions on the predictions of pore-scale wetting events, relative permeability, capillary pressure, and imbibition rates for more viscous nonwetting fluids. Although uncertainties remain in the preset of effective contact angles, we show that the validated dynamic PNM can provide quantitative and valuable insights into pore-scale wetting dynamics.
• Zheng, T., Li, S., Gao, S., Guo, B. o., Zheng, X., & Luo, J. (2025). Expanding freshwater lenses in circular islands using cut-off walls: a three-dimensional model. ACS ES&T Water, 5(11), 6811--6820.
• Bigler, M. C., Brusseau, M. L., Guo, B. o., Jones, S. L., Pritchard, J. C., Higgins, C. P., & Hatton, J. (2024). High-resolution depth-discrete analysis of PFAS distribution and leaching for a vadose-zone source at an AFFF-impacted site. Environmental Science \& Technology, 58(22), 9863--9874.
• Bigler, M., Brusseau, M., Guo, B., Jones, S., Pritchard, J., Higgins, C., & Hatton, J. (2024). High-Resolution Depth-Discrete Analysis of PFAS Distribution and Leaching for a Vadose-Zone Source at an AFFF-Impacted Site. Environmental Science and Technology, 58(22). doi:10.1021/acs.est.4c01615
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  The long-term leaching of polyfluoroalkyl substances (PFAS) within the vadose zone of an AFFF application site for which the depth to groundwater is approximately 100 m was investigated by characterizing the vertical distribution of PFAS in a high spatial resolution. The great majority (99%) of PFAS mass resides in the upper 3 m of the vadose zone. The depths to which each PFAS migrated, quantified by moment analysis, is an inverse function of molar volume, demonstrating chromatographic separation. The PFAS were operationally categorized into three chain-length groups based on the three general patterns of retention observed. The longest-chain (>∼335 cm3/mol molar volume) PFAS remained within the uppermost section of the core, exhibiting minimal leaching. Conversely, the shortest-chain (
• Brusseau, M. L., & Guo, B. o. (2024). Vapor-phase transport of per and polyfluoroalkyl substances: Processes, modeling, and implications. Science of The Total Environment, 947, 174644.
• Brusseau, M., & Guo, B. (2024). Vapor-phase transport of per and polyfluoroalkyl substances: Processes, modeling, and implications. Science of the Total Environment, 947(Issue). doi:10.1016/j.scitotenv.2024.174644
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  An increasing number of studies have demonstrated the presence of per and polyfluoroalkyl substances (PFAS) in the vapor phase. It is therefore important to consider the potential for vapor-phase transport of PFAS in soil and the vadose zone and to investigate the processes impacting the retention and transport of volatile PFAS in soil. It is also critically important to evaluate existing models and develop new models as needed for their application to PFAS vapor-phase transport. The objectives of the present work were to provide an overview of vapor-phase transport processes and modeling, with a specific focus on their relevance for PFAS, and to discuss implications for mass discharge to groundwater, vapor intrusion, and soil vapor extraction. Decades of research have been devoted to the retention and transport of legacy volatile organic contaminants in the vadose zone. This work provides an abundant source of information concerning the many factors and processes of relevance, and insights into the development and application of mathematical modeling. However, given the unique properties of PFAS, there is a need to conduct research to investigate vapor-phase transport of PFAS and to develop PFAS-specific models. We highlight with illustrative examples that vapor-phase transport can be significantly more rapid than aqueous-phase advective transport, which can result in enhanced mass discharge to groundwater.
• Divine, C., Hasbrouck, K., Guo, B., Brusseau, M., Zeng, J., Wright, J., Fortner, E., Chapman, S., Munn, J., Parker, B., & Packer, B. (2024). Dynamic Storage, Release, and Enrichment of some Per- and Polyfluoroalkyl Substances in the Groundwater Table Fluctuation Zone: Transport Processes Requiring Further Consideration. Groundwater Monitoring and Remediation, 44(4). doi:10.1111/gwmr.12694
• Guo, B. o., & Brusseau, M. L. (2024). Challenges and opportunities for porous media research to address PFAS groundwater contamination. InterPore Journal, 1(2), ipj240824--2.
• Liu, L., Zheng, T., Ma, H., Hao, Y., Liu, G., Guo, B., Shi, Q., & Zheng, X. (2024). Nitrate and nitrite reduction by adsorbed Fe(II) generated from ligand-promoted dissolution of biogenic iron minerals in groundwater. Science of the Total Environment, 951(Issue). doi:10.1016/j.scitotenv.2024.175635
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  Chemical denitrification by redox-active Fe(II) species is pivotal in the coupled iron and nitrogen cycles. The reductive dissolution of ferric minerals by ligand can generate Fe(II)-ligand complexes, but their reducing capability for electrophilic pollutants like nitrate and nitrite remains uncertain. Here, biogenic secondary iron minerals (SIM) after dissimilatory iron reduction were reductively dissolved by oxalate and the siderophore desferrioxamine B, and subsequently the partially-dissolved SIM (SIMD) effectively removed NO2− from groundwater via reduction, while exhibiting much lower reactivity towards NO3−. The dissolution and removal processes were well-fitted with the Kabai model and the pseudo-second-order adsorption model, respectively. The equilibrium NO2− removal capacity (qe) of SIMD reached 0.146–0.223 mmol/g, accompanied with the rate constants as 0.433–0.810 g/(mmol·h). The emission of N2O and NO verified the occurrence of chemical denitrification during NO2− removal by SIMD. From the perspective of Fe(II) reactivity, SIMD exhibited higher densities of surface Fe(II) and more negative Eh values than SIM, and these two indicators showed linear correlations with the removal rates. Combined with microscopic, electrochemical and spectral analysis, our results indicated the redox reaction of adsorbed Fe(II)-complexes with NO2− on SIMD surface. The concurrent substance biochar was also considered, as it indirectly influenced dissolution and pollutant removal by shifting the iron mineral phase in SIM from magnetite to goethite. These findings highlight the significant role of reductive dissolution of iron mineral in N transformation, expand the electron pool available to support chemical denitrification, and have implications for Fe and N cycling coupling with pollutant reduction.
• Liu, Y., Zheng, T., Guo, B. o., Jiang, S., Cao, M., & Zheng, X. (2024). Adsorption Characteristics of Dissolved Organic Nitrogen on Aquifer Porous Media: The Role of Media Particle Size. ACS ES\&T Water, 4(5), 2170--2180.
• Liu, Y., Zheng, T., Guo, B., Jiang, S., Cao, M., & Zheng, X. (2024). Adsorption Characteristics of Dissolved Organic Nitrogen on Aquifer Porous Media: The Role of Media Particle Size. ACS ES and T Water, 4(5). doi:10.1021/acsestwater.3c00815
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  The adsorption of dissolved organic nitrogen (DON) bears the potential to exert a profound influence on the migration ability and groundwater quality. However, the effects of aquifer porous media, characterized by diverse particle dimensions, on the capacity of DON adsorption remain uncertain. The present study clarifies the characteristics and intricate machinations of DON adsorption on porous media with different particle sizes. Our findings suggest that DON adsorption has the potential to achieve immediate equilibrium, and the rate of kinetic adsorption increases as the particle size decreases. Fine sand exhibits a higher adsorption rate (0.030-0.058 kg (mg h)−1) and a greater adsorption capacity (137.73-486.67 mg N kg-1). Besides, adsorption capacities of amino acids and proteins on porous media are higher, caused by the forces of hydrogen bonding and ester groups. Urea’s adsorption capacity exhibits the lowest values, and the main adsorption mechanism is the reactivity of carbonyl groups with porous media. The impact of environmental factors (DOM, metal oxides, and salinity) on adsorption pattern is mainly linked to DON types while being slightly affected by variations in the particle size of porous media. The study highlights that the capacity of DON adsorption on porous media is substantial enough to warrant consideration in site characterization and design of remediation strategies.
• Shi, B., Jiang, H., Guo, B. o., Tian, J., & Qin, C. (2024). Modeling of flow and transport in multiscale digital rocks aided by grid coarsening of microporous domains. Journal of Hydrology, 633, 131003.
• Shi, B., Jiang, H., Guo, B., Tian, J., & Qin, C. (2024). Modeling of flow and transport in multiscale digital rocks aided by grid coarsening of microporous domains. Journal of Hydrology, 633(Issue). doi:10.1016/j.jhydrol.2024.131003
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  Many subsurface porous media such as soils, carbonate rocks, and mudstones possess multiscale porous structures that play an important role in regulating fluid flow and transport therein. A pore-network-continuum hybrid model is promising for numerical studies of a multiscale digital rock. It is, however, still prohibitive to the REV-size modeling because tens of millions of microporosity voxels may exist. In this work, we develop a novel and robust algorithm for coarsening microporosity voxels of a multiscale digital rock. Then, we combine coarsened microporosity grids with the pore network of resolved macropores to form efficient computational meshes. Furthermore, a pore-network-continuum simulator is developed to simulate flow and transport in both a synthesized multiscale digital rock and a realistic Estaillades carbonate rock. We show that the coarsening algorithm can reduce computational grids by about 90%, which substantially reduces computational costs. Meanwhile, coarsening microporosity has a minor impact on the predictions of absolute permeability, gas production curves, and breakthrough curves of solute transport. We illustrate the mechanisms of flow and transport in multiscale porous media induced by microporosity. Finally, the efficient hybrid model is used to predict the absolute permeability of an Estaillades digital rock. The numerical prediction matches well with the reported experimental data. We highlight the importance of characterizing mean pore-size distributions in microporosity for the prediction of rock permeability and local flow fields. The developed pore-network-continuum hybrid model aided by grid coarsening of microporosity serves as a useful numerical tool to study flow and transport in multiscale porous media.
• Smith, J., Brusseau, M. L., & Guo, B. o. (2024). An integrated analytical modeling framework for determining site-specific soil screening levels for PFAS. Water Research, 252, 121236.
• Yu, W., Zheng, T., Guo, B. o., Tao, Y., Liu, L., Yan, N. i., & Zheng, X. (2024). Coupling of polyhydroxybutyrate and zero-valent iron for enhanced treatment of nitrate pollution within the Permeable Reactive Barrier and its downgradient aquifer. Water Research, 250, 121060.
• Yu, W., Zheng, T., Guo, B., Tao, Y., Liu, L., Yan, N., & Zheng, X. (2024). Coupling of polyhydroxybutyrate and zero-valent iron for enhanced treatment of nitrate pollution within the Permeable Reactive Barrier and its downgradient aquifer. Water Research, 250(Issue). doi:10.1016/j.watres.2023.121060
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  Permeable Reactive Barriers (PRBs) have been utilized for mitigating nitrate pollution in groundwater systems through the use of solid carbon and iron fillers that release diverse nutrients to enhance denitrification efficiency. We conduct laboratory column tests to evaluate the effectiveness of PRBs in remediating nitrate pollution both within the PRB and in the downgradient aquifer. We use an iron-carbon hydrogel (ICH) as PRB filler, which has different weight ratios of polyhydroxybutyrate (PHB) and microscale zero-valent iron (mZVI). Results reveal that denitrification in the downgradient aquifer accounts for at least 19.5 % to 32.5 % of the total nitrate removal. In the ICH, a higher ratio of PHB to mZVI leads to higher contribution of the downgradient aquifer to nitrate removal, while a lower ratio results in smaller contribution. Microbial community analysis further reveals that heterotrophic and mixotrophic bacteria dominate in the downgradient aquifer of the PRB, and their relative abundance increases with a higher ratio of PHB to mZVI in the ICH. Within the PRB, autotrophic and iron-reducing bacteria are more prevalent, and their abundance increases as the ratio of PHB to mZVI in the ICH decreases. These findings emphasize the downgradient aquifer's substantial role in nitrate removal, particularly driven by dissolved organic carbon provided by PHB. This research holds significant implications for nutrient waste management, including the prevention of secondary pollution, and the development of cost-effective PRBs.
• Zeng, J., Brusseau, M. L., & Guo, B. o. (2024). Modeling PFAS subsurface transport in the presence of groundwater table fluctuations: The impact on source-zone leaching and exploration of model simplifications. Water Resources Research, 60(11), e2024WR037707.
• Zeng, J., Brusseau, M., & Guo, B. (2024). Modeling PFAS Subsurface Transport in the Presence of Groundwater Table Fluctuations: The Impact on Source-Zone Leaching and Exploration of Model Simplifications. Water Resources Research, 60(11). doi:10.1029/2024WR037707
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  Air–water interfacial adsorption represents a major source of retention for many per- and poly-fluoroalkyl substances (PFAS). Therefore, transient hydrological fluxes that dynamically change the amount of air–water interfaces are expected to strongly influence PFAS retention in their source zones in the vadose zone. We employ mathematical modeling to study how seasonal groundwater table (GWT) fluctuations affect PFAS source-zone leaching. The results suggest that, by periodically collapsing air–water interfaces, seasonal GWT fluctuations can lead to strong temporal variations in groundwater concentration and significantly enhance PFAS leaching in the vadose zone. The enhanced leaching is more pronounced for longer-chain PFAS, coarser-textured porous media, drier climates, and greater amplitudes of fluctuations. GWT fluctuations and lateral migration above the GWT introduce a downgradient persistent secondary source zone for longer-chain PFAS. However, the enhanced leaching and the secondary source zone are greatly reduced when subsurface heterogeneity is present. In highly heterogeneous source zones, GWT fluctuations may even lead to overall slower leaching due to lateral flow (in the GWT fluctuation zone and above the GWT) moving PFAS into local regions with greater retention capacities. Model simplification analyses suggest that the enhanced source-zone leaching due to GWT fluctuations may be approximated using a static but shallower GWT. Additionally, while vertical 1D models underestimate source-zone leaching due to not representing lateral migration, they can be revised to account for lateral migration and provide lower- and upper-bound estimates of PFAS source-zone leaching under GWT fluctuations. Overall, our study suggests that representing GWT fluctuations is critical for quantifying source-zone leaching of PFAS, especially the more interfacially active longer-chain compounds.
• Zhang, L. i., Guo, B. o., Qin, C., & Xiong, Y. (2024). A hybrid pore-network-continuum modeling framework for flow and transport in 3D digital images of porous media. Advances in Water Resources, 190, 104753.
• Zhang, L., Guo, B., Qin, C., & Xiong, Y. (2024). A hybrid pore-network-continuum modeling framework for flow and transport in 3D digital images of porous media. Advances in Water Resources, 190(Issue). doi:10.1016/j.advwatres.2024.104753
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  Understanding flow and transport in multiscale porous media is challenging due to the presence of a wide range of pore sizes. Recent imaging advances offer high-resolution characterization of the multiscale pore structures. However, simulating flow and transport in 3D digital images requires models to represent both the resolved and sub-resolution pore structures. We develop a hybrid pore-network-continuum modeling framework. The hybrid framework treats the smaller pores below the image resolution as a continuum using the Darcy-scale formalism and explicitly represents the larger pores resolved in the images employing a pore network model. We validate the hybrid model against direct numerical simulations for single-phase flow and solute transport and further demonstrate its applicability for simulating two-component gas transport in a shale rock sample. The results indicate that the new hybrid model represents the flow and transport process in multiscale porous media while being much more computationally efficient than direct numerical simulation methods for the range of simulated conditions.
• Zhang, W., & Guo, B. (2024). Anomalous Adsorption of PFAS at the Thin‐Water‐Film Air‐Water Interface in Water‐Unsaturated Porous Media. Water Resources Research, 60(3). doi:10.1029/2023wr035775
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  Per- and poly-fluoroalkyl substances (PFAS) are interfacially-active contaminants that adsorb at air-water interfaces (AWIs). Water-unsaturated soils have abundant AWIs, which generally consist of two types: one is associated with the pendular rings of water between soil grains (i.e., bulk AWI) and the other arises from the thin water films covering the soil grains. To date, the two types of AWIs have been treated the same when modeling PFAS retention in vadose zones. However, the presence of electrical double layers of soil grain surfaces and the subsequently modified chemical potential of PFAS at the AWI may significantly change the PFAS adsorption at the thin-water-film AWI relative to that at the bulk AWI. Given that thin water films contribute to over 90% of AWIs in the vadose zone under many field-relevant wetting conditions, it is critical to quantify the potential anomalous adsorption of PFAS at the thin-water-film AWI. We develop a thermodynamic-based mathematical model to quantify this anomalous adsorption. The model couples the chemical equilibrium of PFAS with the Poisson-Boltzmann equation that governs the distribution of electrical potential in a thin water film. Our model analyses suggest that PFAS adsorption at thin-water-film AWI can deviate significantly (up to 82%) from that at bulk AWIs. The deviation increases for lower porewater ionic strength, thinner water film, and higher soil grain surface charge. These results highlight the importance of accounting for the anomalous adsorption of PFAS at the thin-water-film AWI when modeling PFAS fate and transport in the vadose zone.
• Zhang, W., & Guo, B. o. (2024). Anomalous adsorption of PFAS at the thin-water-film air-water interface in water-unsaturated porous media. Water Resources Research, 60(3), e2023WR035775.
• Zhang, X., Fang, Y., Niu, G., Troch, P. A., Guo, B. o., Leung, L. R., Brunke, M. A., Broxton, P., & Zeng, X. (2024). Impacts of Topography-Driven Water Redistribution on Terrestrial Water Storage Change in California Through Ecosystem Responses. Water Resources Research, 60(2), e2023WR035572.
• Zhang, X., Fang, Y., Niu, G., Troch, P. A., Guo, B., Leung, L. R., Brunke, M. A., Broxton, P., & Zeng, X. (2024). Impacts of Topography‐Driven Water Redistribution on Terrestrial Water Storage Change in California Through Ecosystem Responses. Water Resources Research, 60(2). doi:10.1029/2023wr035572
• Zhang, X., Fang, Y., Niu, G., Troch, P., Guo, B., Leung, L., Brunke, M., Broxton, P., & Zeng, X. (2024). Impacts of Topography-Driven Water Redistribution on Terrestrial Water Storage Change in California Through Ecosystem Responses. Water Resources Research, 60(2). doi:10.1029/2023WR035572
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  Lateral subsurface flow plays an essential role in sustaining the terrestrial ecosystem, but it is not explicitly represented in most Earth System Models. In this study, we implemented an explicit lateral saturated flow model into the E3SM land model (ELM). The model explicitly describes lateral flow in the saturated zone by representing, for each model grid, an idealized hillslope consisting of five hydrologically connected soil columns. We conducted three model experiments driven by 0.125° atmospheric forcing data during 1980–2015 over California using models of the default ELM, a modified version of ELM to enhance infiltration, and the model with the lateral saturated flow model. The simulated runoff, evapotranspiration, and terrestrial water storage anomaly (TWSA) from the three simulations were evaluated against available observations, and the model explicitly representing lateral flow performs best. The new model produces greater gridcell-averaged evapotranspiration especially over the mountainous regions with moderate relief and seasonally dry climates. Most importantly, it improves the modeled seasonal variations, interannual variabilities, and the recent decadal decline of TWSA. Many of these improvements can be attributed to the enhanced ecosystem resilience to droughts as demonstrated by transpiration increases caused by lateral flow. Model sensitivity experiments suggest that subsurface runoff is most sensitive to the ratio between horizontal and vertical saturated hydraulic conductivity, followed by hillslope planforms (convergent, divergent, and uniform), number of columns, and lower boundary conditions. Future work should effectively characterize hillslopes in global models and explore the long-term influences of lateral water movement on modeled biogeochemical cycle.
• Brusseau, M. L., & Guo, B. (2023). Revising the EPA dilution-attenuation soil screening model for PFAS. Journal of Hazardous Materials Letters, 4.
• Chen, S., & Guo, B. (2023). Pore-Scale Modeling of PFAS Transport in Water-Unsaturated Porous Media: Air-Water Interfacial Adsorption and Mass-Transfer Processes in Thin Water Films. Water Resources Research, 59(8).
• Zeng, J., & Guo, B. (2023). Reduced Accessible Air-Water Interfacial Area Accelerates PFAS Leaching in Heterogeneous Vadose Zones. Geophysical Research Letters, 50(8).
• Becker, B., Guo, B. o., Buntic, I., Flemisch, B., & Helmig, R. (2022). An Adaptive Hybrid Vertical Equilibrium/Full-Dimensional Model for Compositional Multiphase Flow. Water Resources Research, 58(1), e2021WR030990.
• Brusseau, M. L., & Guo, B. (2022). PFAS concentrations in soil versus soil porewater: Mass distributions and the impact of adsorption at air-water interfaces. Chemosphere, 302, 134938.
• Fang, Y., Zheng, T., Guo, B. o., Zhan, H., Wang, H., Zheng, X., & Walther, M. (2022). Transformation in the Stability of Tide-Induced Upper Saline Plume Driven by Transient External Forcing. Water Resources Research, 58(6), e2021WR031331.
• Guo, B. o., Zeng, J., Brusseau, M. L., & Zhang, Y. (2022). A screening model for quantifying PFAS leaching in the vadose zone and mass discharge to groundwater. Advances in Water Resources, 160, 104102.
• Huang, D., Saleem, H., Guo, B. o., & Brusseau, M. L. (2022). The impact of multiple-component PFAS solutions on fluid-fluid interfacial adsorption and transport of PFOS in unsaturated porous media. Science of The Total Environment, 806, 150595.
• Lyu, Y., Wang, B., Du, X., Guo, B. o., & Brusseau, M. L. (2022). Air-water interfacial adsorption of C4-C10 perfluorocarboxylic acids during transport in unsaturated porous media. Science of the Total Environment, 831, 154905.
• Qin, C., Wang, X., Hefny, M., Zhao, J., Chen, S., & Guo, B. o. (2022). Wetting dynamics of spontaneous imbibition in porous media: From pore scale to darcy scale. Geophysical Research Letters, 49(4), e2021GL097269.
• Brusseau, M. L., & Guo, B. (2021). Air-water interfacial areas relevant for transport of per and poly-fluoroalkyl substances. Water research, 207, 117785.
• Brusseau, M. L., Guo, B., Huang, D., Yan, N., & Lyu, Y. (2021). Ideal versus Nonideal Transport of PFAS in Unsaturated Porous Media. Water research, 202, 117405.
• Canez, T. T., Guo, B. o., McIntosh, J. C., & Brusseau, M. L. (2021). Perfluoroalkyl and Polyfluoroalkyl substances (PFAS) in Groundwater at a Reclaimed Water Recharge Facility. Science of The Total Environment, 147906.
• Chen, S., Jiang, J., & Guo, B. (2021). A pore-network-based upscaling framework for the nanoconfined phase behavior in shale rocks. Chemical Engineering Journal.
• El, O. A., Guo, B. o., Zhong, H., & Brusseau, M. L. (2021). Testing the validity of the miscible-displacement interfacial tracer method for measuring air-water interfacial area: Independent benchmarking and mathematical modeling. Chemosphere, 263, 128193.
• Guo, B., Zeng, J., Brusseau, M. L., & Zhang, Y. (2021). A screening model for quantifying PFAS leaching in the vadose zone and mass discharge to groundwater. Advances in Water Resources.
• Huang, D., Saleem, H., Guo, B., & Brusseau, M. L. (2021). The impact of multiple-component PFAS solutions on fluid-fluid interfacial adsorption and transport of PFOS in unsaturated porous media. The Science of the total environment, 806(Pt 2), 150595.
• Ji, Y., Yan, N., Brusseau, M. L., Guo, B., Zheng, X., Dai, M., Liu, H., & Li, X. (2021). Impact of a Hydrocarbon Surfactant on the Retention and Transport of Perfluorooctanoic Acid in Saturated and Unsaturated Porous Media. Environmental science & technology, 55(15), 10480-10490.
• Zeng, J., & Guo, B. (2021). Multidimensional simulation of PFAS transport and leaching in the vadose zone: Impact of surfactant-induced flow and subsurface heterogeneities. Advances in Water Resources.
• Zeng, J., Brusseau, M. L., & Guo, B. (2021). Model validation and analyses of parameter sensitivity and uncertainty for modeling long-term retention and leaching of PFAS in the vadose zone. Journal of Hydrology, 603. doi:10.1016/j.jhydrol.2021.127172
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  PFAS are emerging contaminants widespread in the environment. As surfactants, PFAS tend to accumulate at solid–water and air–water interfaces in the vadose zone, which may pose long-term threats to groundwater. The primary factors that control the long-term retention of PFAS in the vadose zone remain poorly understood. To address this knowledge gap, we first use multiple datasets from transport experiments to validate a state-of-the-art mathematical model that incorporates transient variably saturated flow, surfactant-induced flow, and rate-limited and nonlinear solid-phase and air–water interfacial adsorption. We then employ the validated model to simulate and analyze the primary processes and parameters controlling the retention and leaching of PFAS in the vadose zone at a model fire-training-area site. Our simulations show that adsorption at solid–water and air–water interfaces leads to strong retention of PFAS in the vadose zone. The strength of retention increases with PFAS chain length and porewater ionic strength, while it decreases at greater PFAS concentrations due to nonlinear adsorption. Comprehensive parameter sensitivity analyses reveal that model predictions are most sensitive to parameters related to the air–water interfacial area and PFAS interfacial properties when air–water interfacial adsorption (AWIA) is more important than solid-phase adsorption (SPA). Predicted PFAS leaching rates vary by a wide range resulting from uncertainties in the input parameters, but the uncertainty range is much greater for longer-chain PFAS than that of their shorter-chain counterparts. The simulated arrival times to groundwater were found to follow log-normal distributions. Finally, model complexity analysis reveals that nonlinearity in AWIA and kinetic SPA and kinetic AWIA have a minimal impact on the long-term retention of PFAS under the wide range of field conditions examined in the present study.
• Zeng, J., Brusseau, M. L., & Guo, B. (2021). Model validation and analyses of parameter sensitivity and uncertainty for modeling long-term retention and leaching of PFAS in the vadose zone. Journal of Hydrology.
• Zheng, T., Guo, B. o., & Shao, H. (2021). A hybrid multiscale framework coupling multilayer dynamic reconstruction and full-dimensional models for CO2 storage in deep saline aquifers. Journal of Hydrology, 600, 126649.
• Brusseau, M. L., Lyu, Y., Yan, N., & Guo, B. (2020). Low-Concentration Tracer Tests to Measure Air-Water Interfacial Area in Porous Media. Chemosphere.
• Guo, B., Zeng, J., & Brusseau, M. L. (2020). A Mathematical Model for the Release, Transport, and Retention of Per- and Polyfluoroalkyl Substances (PFAS) in the Vadose Zone. Water Resources Research.
• Jiang, H., Guo, B., & Brusseau, M. L. (2020). Characterization of the micro-scale surface roughness effect on immiscible fluids and interfacial areas in porous media using the measurements of interfacial partitioning tracer tests. ADVANCES IN WATER RESOURCES, 146.
• Jiang, H., Guo, B., & Brusseau, M. L. (2020). Pore-scale modeling of fluid-fluid interfacial area in variably saturated porous media containing micro-scale surface roughness. Water Resources Research.
• Bandilla, K. W., Guo, B. o., & Celia, M. A. (2019). A guideline for appropriate application of vertically-integrated modeling approaches for geologic carbon storage modeling. INTERNATIONAL JOURNAL OF GREENHOUSE GAS CONTROL, 91.
• Guan, K. M., Nazarova, M., Guo, B., Tchelepi, H., Kovscek, A. R., & Creux, P. (2019). Effects of Image Resolution on Sandstone Porosity and Permeability as Obtained from X-Ray Microscopy. Transport in Porous Media, 1--13.
• Guo, B., Mehmani, Y., & Tchelepi, H. (2019). Multiscale formulation for pore-scale compressible Darcy-Stokes flow. Journal of Computational Physics.
• Qin, C., Guo, B., Celia, M., & Wu, R. (2019). Dynamic pore-network modeling of air-water flow through thin porous layers. Chemical Engineering Science.
• Tao, Y., Guo, B., Bandilla, K., & Celia, M. (2019). Vertically-integrated dual-continuum models for CO2 sequestration in fractured reservoirs. Computational Geosciences. doi:10.1007/s10596-018-9805-x
• Becker, B., Guo, B., Bandilla, K., Celia, M. A., Flemisch, B., & Helmig, R. (2018). An Adaptive Multiphysics Model Coupling Vertical Equilibrium and Full Multidimensions for Multiphase Flow in Porous Media. WATER RESOURCES RESEARCH, 54(7), 4347-4360.
• Guo, B., Ma, L., & Tchelepi, H. A. (2018). Image-based micro-continuum model for gas flow in organic-rich shale rock. ADVANCES IN WATER RESOURCES, 122, 70-84.
• Liu, Q., Sun, L., Tang, X., & Guo, B. (2018). Modelling hydraulic fracturing with a point-based approximation for the maximum principal stress criterion. Rock Mechanics and Rock Engineering.
• Becker, B., Guo, B., Bandilla, K., Celia, M., Flemisch, B., & Helmig, R. (2017). A Pseudo-Vertical Equilibrium Model for Slow Gravity Drainage Dynamics. Water Resources Research, 53(12). doi:10.1002/2017wr021644
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  Vertical equilibrium (VE) models are computationally efficient and have been widely used for modeling fluid migration in the subsurface. However, they rely on the assumption of instant gravity segregation of the two fluid phases which may not be valid especially for systems that have very slow drainage at low wetting phase saturations. In these cases, the time scale for the wetting phase to reach vertical equilibrium can be several orders of magnitude larger than the time scale of interest, rendering conventional VE models unsuitable. Here we present a pseudo-VE model that relaxes the assumption of instant segregation of the two fluid phases by applying a pseudo-residual saturation inside the plume of the injected fluid that declines over time due to slow vertical drainage. This pseudo-VE model is cast in a multiscale framework for vertically integrated models with the vertical drainage solved as a fine-scale problem. Two types of fine-scale models are developed for the vertical drainage, which lead to two pseudo-VE models. Comparisons with a conventional VE model and a full multidimensional model show that the pseudo-VE models have much wider applicability than the conventional VE model while maintaining the computational benefit of the conventional VE model.
• Guo, B., Bandilla, K., Nordbotten, J., Celia, M., Keilegavlen, E., & Doster, F. (2016). A multiscale multilayer vertically integrated model with vertical dynamics for CO2 sequestration in layered geological formations. Water Resources Research, 52(8). doi:10.1002/2016wr018714
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  Efficient computational models are desirable for simulation of large-scale geological CO2 sequestration. Vertically integrated models, which take advantage of dimension reduction, offer one type of computationally efficient model. The dimension reduction is usually achieved by vertical integration based on the vertical equilibrium (VE) assumption, which assumes that CO2 and brine segregate rapidly in the vertical due to strong buoyancy and quickly reach pressure equilibrium. However, the validity of the VE assumption requires small time scales of fluid segregation, which may not always be fulfilled, especially for heterogeneous geological formations with low vertical permeability. Recently, Guo et al. (2014a) developed a multiscale vertically integrated model, referred to as the dynamic reconstruction (DR) model, that relaxes the VE assumption by including the vertical two-phase flow dynamics of CO2 and brine as fine-scale one-dimensional problems in the vertical direction. Although the VE assumption can be relaxed, that model was limited to homogeneous geological formations. Here we extend the dynamic reconstruction model for layered heterogeneous formations, which is of much more practical interest for saline aquifers in sedimentary basins. We develop a new coarse-scale pressure equation to couple the different coarse-scale (vertically integrated) layers, and use the fine-scale dynamic reconstruction algorithm in Guo et al. (2014a) within each individual layer. Together, these form a multiscale multilayer dynamic reconstruction algorithm. Simulation results of the CO2 plume from the new model are in excellent agreement with full three-dimensional models, with the new algorithm being much more computationally efficient than conventional full three-dimensional models.

Proceedings Publications

• Chen, S., Jiang, J., & Guo, B. (2021). Effect of Pore Geometry and Heterogeneous Surface Wettability on the Nanoconfined Phase Behavior in Nanopore Networks of Shale Rocks. In SPE/AAPG/SEG Unconventional Resources Technology Conference.

Presentations

• Jianwen, D., Guo, B., Niu, G., Dontsova, K. M., Troch, P. A., & Chorover, J. D. (2024, December). H21B-01 A Two-Phase Flow and Reactive Transport Modeling Framework for Basalt Weathering at the Biosphere 2 Landscape Evolution Observatory. 2024 American Geophysical Union Fall Meeting. Washington, DC.
• Saleem, H., Guo, B., & Gupta, H. V. (2023). A Mathematical Model for Multi-component PFAS Solute Transport in Unsaturated Media. El Dia Del Agua y Atmosphera, Dept. of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Mar 28. Dept. of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson.
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  Saleem H, B Guo and H Gupta (2023), A Mathematical Model for Multi-component PFAS Solute Transport in Unsaturated Media, El Dia Del Agua y Atmosphera, Dept. of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Mar 28.
• Guo, B., Cao, Z., Du, J., Wang, Y., Niu, G., Dontsova, K. M., Hitzelberger, M., Chen, L., Troch, P. A., & Chorover, J. D. (2022, December 2022). Reactive transport modeling of basalt weathering and early soil formation within a highly-controlled, sloping lysimeter.
  . American Geophysical Union Fall Meeting. Chicago, IL: American Geophysical Union.
• Gupta, H. V., Guo, B., & Saleem, H. (2021). A Mathematical Model for the Transport of Multi-Component PFAS In Unsaturated Porous Media. El Dia Del Agua y Atmosphera, Department of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Mar 31/April 1. Online: Department of Hydrology and Atmospheric Sciences, The University of Arizona,.
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  Saleem H, B Guo and H Gupta (2021), A Mathematical Model for the Transport of Multi-Component PFAS In Unsaturated Porous Media, presented at El Dia Del Agua y Atmosphera, Department of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Mar 31/April 1
• Guo, B., Zeng, X., Stepanov, M., & Johnson, G. (2019, October 24). A numerical scheme for saturated flows based on convex optimization. Department of Hydrology and Atmospheric Sciences weekly Colloquium. University of Arizona.
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  A talk (by Misha Stepanov) at local (UofA) seminar, namely Colloquium at HAS. Presented the current state of the joint project on new numerical scheme for soil moisture evolution.

Poster Presentations

• Jianwen, D., Guo, B., Niu, G., Dontsova, K. M., Troch, P. A., & Chorover, J. D. (2023, December). H21M-1526: Quantifying the impact of CO2 transport and transient hydrological flow on basalt weathering at the Biosphere 2 Landscape Evolution Observatory.. 2023 American Geophysical Union Fall Meeting. San Francisco, CA.