Understanding the fate and transport of PFAS in the subsurface is essential for groundwater and contaminated site management. Most international studies focused on the transport in homogeneous sands, while other hydrogeological settings are less well studied. In this contribution, we investigate the impact of different glacial geological settings typically found in the Northern Hemisphere, possibly containing fractures and heterogeneities, on PFAS leaching through unsaturated glacial sediments.

We have implemented a vertical cross-section model that simulates transient groundwater flow and PFAS transport through the variably-saturated zone, accounting for sorption to the solid phase and to air-water interfaces. The model was tested on measured breakthrough curve data from saturated and unsaturated laboratory column experiments considering PFAS with different chain lengths. Model parameters were obtained from a comprehensive literature review, laboratory studies, and field investigations of contaminated sites.

The model was used to investigate the leaching of PFAS with different chain lengths through different setups with glacial sediment. We observed that the hydrogeological setting determines the magnitude of the air-water interfacial area and, thus, the retention of surface-active PFAS like PFOS and PFOA. Further, the model outcomes demonstrated a chromatographic separation of PFAS with different chain lengths due to different retention mechanisms. The longer-chained PFAS were retained more strongly in the unsaturated zone, while shorter-chained compounds were mobile.

Low-permeability clay-rich layers and inclusions generally provided less retention for surface-active PFAS due to a typically higher water saturation and, thus, smaller interfacial area compared to high-permeability media like sands. Fractures and heterogeneities may lead to the formation of preferential flow paths and thereby a potential bypassing of the unsaturated zone, where sorption to the air-water interface could occur. On the other hand, matrix diffusion may slow the rate of plume expansion by retaining PFAS in low-permeability layers. Over time, back diffusion from the matrix can result in long-term release to the groundwater.

The modeling investigations based on realistic data conducted in this study led to an improved understanding of the transport of short- and longer-chained PFAS in variably saturated glacial geological settings. Our findings allowed analyzing the influence of key parameters and processes on PFAS fate and transport.

How to cite: Mosthaf, K., Morsing, L., Bilic, N., Wienkenjohann, H., Fjordbøge, A. S., and Bjerg, P. L.: Modeling investigations of PFAS transport through variably-saturated glacial sediments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18846, https://doi.org/10.5194/egusphere-egu25-18846, 2025.

Microplastic pollution in terrestrial environments poses significant risks to soil health, groundwater quality, and ecosystem functionality. This study integrates findings from laboratory and field experiments to elucidate the dynamics of microplastic transport and retention in soils under various conditions. Laboratory experiments examined the fate of low-density polyethylene (LDPE), polybutylene adipate terephthalate (PBAT), and starch-based biodegradable microplastics in sandy loam and loamy sand soils under controlled rainfall intensities (22 mm/h and 35 mm/h). Effluent and soil analyses, coupled with microplastic balance assessments, revealed recovery rates between 64% and 104%, underscoring the reliability of the experimental approach. Transport varied with soil type, rainfall intensity, and polymer characteristics, with loamy sand exhibiting higher wash-off rates. LDPE consistently showed greater mobility than biodegradable polymers, particularly under higher rainfall intensities. Field studies complemented these findings, using loamy sand soil columns subjected to natural precipitation and fluctuating groundwater levels over 6- and 12-month periods. Retention profiles and particle size analyses highlighted distinct behaviors: LDPE persisted across soil depths, PBAT exhibited moderate redistribution due to partial biodegradation and starch-based microplastics underwent significant fragmentation and deeper transport. The field's natural precipitation and wet-dry cycles enhanced microplastic mobilization and degradation compared to laboratory conditions. HYDRUS-1D modeling was employed across both settings. Laboratory simulations showed depth-dependent deposition, particularly in upper soil layers, while field models reflected material-specific degradation and redistribution. Notably, LDPE exhibited stable retention parameters, whereas biodegradable polymers demonstrated declining attachment and detachment coefficients over time, indicating their biodegradability. These findings underscore the critical roles of soil type, rainfall intensity, polymer properties, and environmental conditions in shaping microplastic behavior in soils. Integrating controlled laboratory experiments and long-term field studies provides a comprehensive understanding of microplastic fate, offering essential insights for modeling and mitigating their impact on terrestrial ecosystems.
Keywords: microplastic transport, soil contamination, soil column experiment, HYDRUS-1D

How to cite: Soltani Tehrani, R., Yang, X., and van Dam, J.: Understanding microplastic transport and retention in soil: insights from laboratory and field studies, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6306, https://doi.org/10.5194/egusphere-egu25-6306, 2025.

Stable isotopes of nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) are powerful tools for tracing nitrogen sources and understanding transformation processes in soil-water systems. The isotopic composition of nitrogen and oxygen evolves due to isotope effects, which characterize processes such as nitrification and denitrification whilst offering insights into the environmental factors driving these reactions. Although isotope effects are often derived from laboratory experiments under controlled conditions, this study aims to derive them in situ within a dynamic natural system, where varying redox conditions, inflows, and substrate availability introduce complexities absent in controlled environments.

Combining high-resolution hydrochemical and stable isotopic monitoring of nitrate and water with numerical modeling and particle tracking using HYDRUS, we investigate the spatial variability of nitrogen transformations within an agricultural soil profile. Preliminary results indicate that nitrification, with nitrate concentrations exceeding 200 mg·l^-1, is prominent in the upper soil layers and exhibits isotopic signatures (δ¹⁵N = 4.2‰ ±0.9‰) characteristic of soil nitrogen, likely derived from the immobilization of applied fertilizer. Denitrification, reducing concentrations to as low as 0.2 mg·l^-1, occurs primarily within the capillary fringe, generating a linear Δδ¹⁸O:Δδ¹⁵N trajectory with a slope of 0.79 and a field based apparent isotopic enrichment factor for nitrogen of ε = -4.8‰. Below this zone, regions dominated by nitrification on denitrification exhibit curved Δδ¹⁸O:Δδ¹⁵N trajectories, highlighting the incorporation of oxygen from ambient water during re-nitrification.

How to cite: Richard-Cerda, J. C., Schulz, S., and Knöller, K.: Using in-situ monitoring and modeling to characterize isotope effects in nitrate cycling at an agricultural site, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18992, https://doi.org/10.5194/egusphere-egu25-18992, 2025.

In soils contaminated with petroleum hydrocarbons (PHCs), water table fluctuations (WTFs) affect the kinetics of PHC biodegradation and the generation and transport of carbon dioxide (CO₂) and methane (CH₄). Thus, understanding the impacts of WTFs on natural source zone depletion processes is critical for environmental risk assessment and the design of soil remediation strategies. In this study, a 300 day-long column experiment was conducted to simulate the effects of water table fluctuations on the aerobic and anaerobic PHC biodegradation pathways and rates. Eight columns were each filled with 45 cm of soil from undisturbed cores collected at a site formerly contaminated with PHCs. Four columns simulating fluctuating water table conditions were subjected to three successive 6-week cycles of drainage and imbibition. The remaining four columns remained fully saturated over the period of the experiment, simulating a static water table. Except for the controls, the columns received injections of ethanol or ethanol plus naphthalene after 111 days of pre-equilibration. Over the duration of the experiment, soil moisture, soil surface CO₂ and CH₄ effluxes, dissolved CO₂ and CH₄ concentrations, δ¹³C compositions of CO₂ and CH₄, dissolved naphthalene concentrations, and ancillary geochemical parameters were monitored. The remaining naphthalene depth distributions in the soil columns were also measured at the end of the experiment. A reactive transport model representing 13 biogeochemical reaction pathways was verified against the acquired data. The experimental and modeling results confirmed that the prevailing pathway generating CH₄ shifted from hydrogen-based to acetate-based methanogenesis in the ethanol and ethanol plus naphthalene spiked columns, while CH₄ oxidation played a key role in controlling the CH₄ efflux during the drainage periods. Compared to the static water table columns, the WTF columns exhibited significantly faster naphthalene attenuation while the cumulative CO₂ and CH₄ effluxes were about twice as high. These observations were attributed to the periodic incursion of air during the WTFs, which increased the porewater-air interface area for gas transfer while also accelerating the aerobic degradation of soil organic matter and naphthalene. Overall, our study advances the quantitative modeling of the biogeochemical reaction network in PHC contaminated soils under WTFs, including the role of methanogenic pathways.

How to cite: Rezanezhad, F., Ramezanzadeh, M., Slowinski, S., Ye, J., Vandergriendt, M., and Van Cappellen, P.: Effects of Water Table Fluctuations on Natural Source Zone Depletion of Petroleum Hydrocarbons in Contaminated Soils, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10142, https://doi.org/10.5194/egusphere-egu25-10142, 2025.

Evaporation from porous media is a key phenomenon in the terrestrial environment and is linked to soil salinization, degradation and weathering of building materials. Column-scale experiments could extend our understanding of the complex processes affecting saline water evaporation. In this context, the current study aims at investigating solute accumulation near evaporating surfaces and the resulting implications for time to salt crust formation. Previous numerical studies with REV-scale simulations predict the development of local instabilities due to density differences during saline water evaporation in case of saturated porous media with high permeability, eventually causing density-driven downward flow through fingering. To experimentally investigate this process on the column-scale, we performed evaporation experiments on two types of porous media: medium sand (F36) and fine sand/silt (W3) saturated with NaCl solution. The intrinsic permeability of the two packings differed by two orders of magnitude, i.e. 29×10^-12 m² for F36 and 0.56×10^-12 m² for W3. Using magnetic resonance imaging (²³Na-MRI), we monitored solute accumulation at the surface and subsequent downward redistribution of salt in time-lapse scans during evaporation with a continuous supply of water from below (wicking). Results showed key differences between the enrichment patterns of Na for the two types of porous media. Density-driven downward flow only occurred in F36, initially manifested by fingering, and resulted eventually in redistribution of Na throughout the sample. For W3, solute accumulated at the thin region at the surface with a thickness of a few mm. Despite similar average evaporation rates for both porous media, the concentration at the top reached the saturation limit (6.13 mol/L) for W3, whereas it remained relatively low (2.5 mol/L) for F36 due to the redistribution. This different behavior suggests that time-to-crust formation is longer for higher permeability porous media under similar evaporation conditions applied in our experiments. 
To investigate crust formation in more detail, additional column-scale evaporation experiments with wicking conditions were performed on three sands WS1, WS2 and WS3 with particle sizes ranging between 0.1 to 0.3 mm, 0.3 to 0.5 mm and 0.71 to 1.0 mm, respectively. To achieve well-controlled evaporation conditions, experiments were performed in a wind tunnel maintaining a constant wind speed of 5 ms^-1. Surface time-lapse imaging with a digital camera was performed to observe the onset time of crust formation as well as the resulting crust morphology after initiation. The results showed that for the relatively coarser WS2 sand, onset of crust took twice as long (40 hours) in comparison to the finer WS1 sand (20 hours). The significantly larger particle size of WS3 sand led to air entry, partially saturated conditions and an almost instantaneous crust formation. The crust formation affected also the evaporation rate of each sand, which is attributed to the formation of a new porous layer (crust) and its wetting-drying dynamics. These findings encourage further investigation into effects on crust development for heterogeneous porous media, redistribution and precipitation of different salt types, and the coupling of experimental results to numerical modelling.

How to cite: Chaudhry, M. A., Kiemle, S., Jannesarahmadi, S., Pohlmeier, A., Helmig, R., Shokri, N., and Huisman, J. A.: Solute redistribution during saline water evaporation in porous media and its effects on the onset of salt crust formation , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11280, https://doi.org/10.5194/egusphere-egu25-11280, 2025.

Infiltration and evaporation processes in porous media are crucial for understanding the hydrologic cycle and managing water resources, particularly in the context of climate change. Studying these processes requires models that accurately represent experimental and field measurements.

This work focuses on understanding flow and transport during water infiltration and evaporation cycles, considering factors such as soil heterogeneity, the non-linearity of its properties, and the formation of gravity fingers. We aim to improve the modeling of infiltration and evaporation processes in soils to accurately predict water flow and solute transport behavior, and to characterize the impact of soil heterogeneity, non-linear properties, and gravity fingers on these processes.

To address the effect of these factors, we simulate water infiltration and evaporation cycles, and solute transport in unsaturated soil. Two modeling approaches are compared: the traditional Richards’ equation and the fourth-order derivative in space model proposed by Cueto-Felgueroso and Juanes (2009), which is able to reproduce the formation of fingers and preferential flow during water infiltration. The flow and transport problems are solved using the finite element library FEniCS.

Soil heterogeneity is represented by Gaussian random permeability fields, with different correlation lengths and variance. To evaluate how heterogeneity affects dispersion and mixing during the solute transport, we analyze solute breakthrough curves at different depths, we calculate dispersion coefficients, concentration distribution and segregation index.

Keywords: infiltration and evaporation cycles, unsaturated flow, heterogeneity, gravity fingers, finite element method, solute transport, mixing.

References:

Cueto-Felgueroso, L., and R. Juanes (2009). A phase field model of unsaturated flow. Water Resources Research, 45, W10409. https://doi.org/10.1029/2009WR007945

How to cite: Castillo, Y., Hidalgo, J., and Dentz, M.: Unsaturated Flow and Solute Transport During Infiltration and Evaporation Cycles: Influence of Soil Heterogeneity and Gravity Fingers, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9139, https://doi.org/10.5194/egusphere-egu25-9139, 2025.

The source-responsive method (SRM), which accounts for film flow in macropores and matrix absorption phenomena, is an advanced dual-domain modeling framework and has been successfully applied in catchment scale. It also provides a parameter-predictive approach by introducing a parameter, M, to represent macropore area density. However, the capability of this parameter to accurately reflect macropore structure remains unclear. In this study, a 1-D infiltration model based on SRM was developed to simulate soil water dynamics across six experiments, and obtained calibrated M values. The results demonstrate that the SRM performs well (NSE>0.88) in most cases, except for two artificial-macropore experiments with low M values. Measured M values were extracted from horizontal image slices of dyeing experiments. In experiments with good infiltration simulation performance, the measured values align closely with calibrated values (RE<35%), though they are consistently slightly higher. Conversely, in poorly simulated experiments, significant deviations were observed, with RE exceeding one order of magnitude. Further analysis using HYDRUS-2D revealed that limited lateral water propagation from macropore walls contributed to poor simulation accuracy when M values were excessively low.

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How to cite: Shen, X., Liu, J., Vereecken, H., and Rahmati, M.: The potential of Source-responsive Method in representing macropore structural characteristics within soil profiles, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4980, https://doi.org/10.5194/egusphere-egu25-4980, 2025.

Frozen soils are typically considered impermeable due to their reduced infiltration capacity. During rainfall, water behavior depends on whether the soil thaws, allowing infiltration, or remains frozen, causing surface runoff that may lead to flooding or debris flows.
Open macropores, such as cracks, root channels, and wormholes, act as preferential flow paths, significantly altering these dynamics. Understanding these processes is crucial for improving hydrological models, evaluating slope stability, and assessing natural hazard risks in cold regions.

To investigate these mechanisms, a novel large-scale experimental setup has been developed, which is up to ten times larger than previous experiments and surpasses them in complexity.
The setup features a tiltable design, adjustable up to 20°, allowing the system to replicate natural slope conditions. An advanced irrigation system ensures automated and uniform rainfall distribution across the surface. Controlled climate conditions are maintained via a sophisticated climate chamber, enabling precise and realistic freezing processes. A macropore pattern plate facilitates the creation of a reproducible macropore network, ensuring consistency across experiments. Additionally, advanced sensors enable 3D visualization of soil temperature and moisture distribution, providing detailed insights into the internal processes during freezing and thawing.
This innovative approach reduces the gap between simplified small-scale experiments and the complexity of real-world scenarios.

Experimental results demonstrate that open macropores, despite their small volume fraction within the soil body, significantly facilitate infiltration and accelerate thawing of frozen slopes, directly influencing the hydrological cycle and slope stability.
The findings provide essential data for validating numerical models under climate-relevant freeze-thaw scenarios.

With the increasing frequency of freeze-thaw cycles driven by climate change, this research is essential for assessing infrastructure risks, managing groundwater resources, and mitigating natural hazards in cold and transitional regions.

How to cite: Bauer, J., Müller, S., and Baselt, I.: Macropore-Driven Infiltration in Frozen Slopes: Large-Scale Experimental Insights with Hydrological and Geotechnical Implications, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6622, https://doi.org/10.5194/egusphere-egu25-6622, 2025.

Non-uniform infiltration is a very common observation and one manifestation of non-diffusive physics in soil-water processes. It is posing a fundamental challenge to conventional Darcy-scale approaches in soil-water processes, revealing limitations in our current understanding of complex soil-water interactions. While infiltration is fundamental to many ecohydrologic and hydropedologic aspects, its manifestation through structured soil and dynamically connected flow paths demands a more sophisticated system description.

Through a series of plot-scale irrigation experiments, we characterized infiltration flow fields based on observed soil moisture, tracers and time-lapse GPR data. Based on these data we conceptualised a thermodynamic representation for characterizing soil-water dynamics and their interactions. We observe celerity distributions in flow fields shifting to higher values with higher antecedent soil moisture. We also see shifting soil reconfigurations in precipitation events. 

In the view of soil water dynamics as dissipative processes, we propose that these dynamic soil configurations systematically adapt to meet the system's dissipation demands. The thermodynamic perspective offers new insights into the physical constraints governing soil-water dynamics and provides a theoretical foundation for improved and scaleable prediction of non-uniform flow processes. Our results contribute to an advanced understanding of soil-water constitutive laws under non-equilibrium conditions and may help bridge the gap between observed soil-water dynamics and their representation in models.

How to cite: Jackisch, C., Hoffmeister, S., Stephan, S. M., and Zehe, E.: Thermodynamic Controls on Dynamic Soil Configuration and Non-uniform Infiltration Processes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18870, https://doi.org/10.5194/egusphere-egu25-18870, 2025.

Networks of subsurface pipes are widely used in humid regions to facilitate rapid removal of surface ponding and effectively decrease groundwater tables. Their application is also extended to arid regions as a strategic approach for mitigating soil salinization. Currently available subsurface pipe layout design methods require updating to properly incorporate salt discharge in such contexts. Numerical modeling is then a key tool for effective optimization of design parameters of such subsurface drainage systems. One of the challenges in subsurface drainage simulation is the inherent multiscale nature of the setting. A substantial scale disparity can be evidenced between millimeter-scale dimensions of subsurface drainage pipes and the meter-scale size of available field-scale soil profiles. Gradients of water potential and solute concentration near a subsurface pipe are typically high, thus challenging accuracy of numerical simulations targeting water and salt content across the soil-water system. To achieve accurate and computationally efficient simulations of water flow and solute transport in soil-water system within which subsurface drainage systems are in place, implementation of local grid refinement strategies is critical. In this context, we proposed (Qian et al., 2024) a soil water flow and solute transport model based on a vertex- centered finite volume method (VCFVM). The model has virtually no limitations on the cell shape as well as the number of neighbor cells, and strictly ensures local mass conservation. Here, we start by rigorously assessing the accuracy and efficiency of the algorithm upon considering test scenarios characterized by various soil textures and diverse boundary conditions. Our results show the that accurate solutions can be obtained upon relying on a grid whose number of nodes is only 5% of that of globally refined grid of the kind that are typically employed. The model is then further integrated with drainage equations to accurately simulate subsurface drainage process, so that the effect of placement of subsurface pipes can be effectively included. Our study suggests that the proposed model can accurately simulate soil water content, solute concentration, and subsurface drainage amount using a typical (globally refined) gridding procedure. Otherwise, it can save about 95% of CPU time by using nonmatching grids. Finally, a novel, user-friendly framework for the optimization of subsurface pipe layouts and corresponding leaching quota is proposed and demonstrated on a series of exemplary scenarios.

Reference:
Yingzhi Qian, Xiaoping Zhang, Yan Zhu, Lili Ju, Alberto Guadagnini, Jiesheng Huang, 2024, A novel vertex-centered finite volume method for solving Richards' equation and its adaptation to local mesh refinement, Journal of Computational Physics, 501, 112766,
https://doi.org/10.1016/j.jcp.2024.112766.

How to cite: Qian, Y., Zhu, Y., Zhang, X., Guadagnini, A., and Huang, J.: A multiscale algorithm for soil water flow and solute transport simulations and its application in subsurface drainage system optimization, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4935, https://doi.org/10.5194/egusphere-egu25-4935, 2025.