SSS6.1

Coupling of unsaturated flows and surface reactions, including sorption and desorption, takes place in environmental contexts such as soils and the unsaturated zone. Lower sorption coefficients than those observed from batch experiments are found in natural soils. However, a deep understanding of this coupling is missing. To achieve this objective, we designed microfluidics experiments and performed direct numerical simulations based on experimental images at the microscale. We explore the effects of fluid flow dynamics and reaction kinetics on the effective kinetics. We have observed that an increase in the air volume within the porous medium promotes more heterogeneous fluid flow velocities, controlling the mixing of the transported reactant and resulting in a non-monotonic effective kinetic of the surface reaction.

How to cite: Jimenez-Martinez, J., Markale, I., and Velásquez-Parra, A.: Effective kinetics and phases saturation impact on surface reactions in porous media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8994, https://doi.org/10.5194/egusphere-egu21-8994, 2021.

The interplay between chemical and transport processes can give rise to complex reaction fronts dynamics, whose understanding is crucial in a wide variety of environmental, hydrological and biological processes, among others. An important class of reactions is A+B->C processes, where A and B are two initially segregated miscible reactants that produce C upon contact. Depending on the nature of the reactants and on the transport processes that they undergo, this class of reaction describes a broad set of phenomena, including combustion, atmospheric reactions, calcium carbonate precipitation and more. Due to the complexity of the coupled chemical-hydrodynamic systems, theoretical studies generally deal with the particular case of reactants undergoing passive advection and molecular diffusion. A restricted number of different geometries have been studied, including uniform rectilinear [1], 2D radial [2] and 3D spherical [3] fronts. By symmetry considerations, these systems are effectively 1D.

Here, we consider a 3D axis-symmetric confined system in which a reactant A is injected radially into a sea of B and both species are transported by diffusion and passive non-uniform advection. The advective field v_r(r,z) describes a radial Poiseuille flow. We find that the front dynamics is defined by three distinct temporal regimes, which we characterize analytically and numerically. These are i) an early-time regime where the amount of mixing is small and the dynamics is transport-dominated, ii) a strongly non-linear transient regime and iii) a long-time regime that exhibits Taylor-like dispersion, for which the system dynamics is similar to the 2D radial case.

                                                   Fig. 1: Concentration profile of the product C in the transient (left) and asymptotic (right) regimes.

References:

[1] L. Gálfi, Z. Rácz, Phys. Rev. A 38, 3151 (1988);

[2] F. Brau, G. Schuszter, A. De Wit, Phys. Rev. Lett. 118, 134101 (2017);

[3] A. Comolli, A. De Wit, F. Brau, Phys. Rev. E, 100 (5), 052213 (2019).

How to cite: Comolli, A., De Wit, A., and Brau, F.: Effect of non-uniform passive advection on A+B->C radial reaction-diffusion fronts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5697, https://doi.org/10.5194/egusphere-egu21-5697, 2021.

We utilize effective dispersion coefficients to capture the evolution of the mixing interface between two initially segregated species due to the coupled effect of pore-scale heterogeneity and molecular diffusion. These effective dispersion coefficients are defined as the average spatial variance of the solute plume that evolves from a pointlike injection (the transport Green function). We numerically investigate the effective longitudinal dispersion coefficients in two porous media of different structure heterogeneity  and through different Péclet number regimes for each medium. We find that, as distance traveled increases (or time spent), the solute experiences the pore-scale velocity field heterogeneity due to advection and transverse diffusion, resulting in an evolution of the dispersion coefficients. They evolve from the value of molecular diffusion at early time, then undergo an advection dominated regime, to finally reach the value of hydrodynamic dispersion at late times. This means that, at times smaller than the characteristic diffusion time, the effective dispersion coefficients can be notably smaller than the hydrodynamic dispersion coefficient. Therefore, mismatches between pore-scale reaction data from experiment or simulations and Darcy scale predictions based on temporally constant hydrodynamic dispersion can be explained through these differences. We use the effective dispersion coefficients to approximate the transport Green function and to quantify the incomplete mixing occurring at the pore-scale. We evaluate the evolution of two initially segregated species via this methodology. The approach correctly predicts the amount of chemical reaction occuring in reactive bimolecular particle tracking simulations. These results shed light on the upscaling of pore-scale incomplete mixing and demonstrates that the effective dispersion is an accurate measure for the width of the mixing interface between two reactants. 

How to cite: Puyguiraud, A., Perez, L., Hidalgo, J. J., and Dentz, M.: Upscaling of pore-scale mixing and reaction through effective dispersion coefficients, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15090, https://doi.org/10.5194/egusphere-egu21-15090, 2021.

Remediation of groundwater contaminated by organic compounds in porous and fractured media is a persistent and not well understood challenge. In situ chemical oxidation (ISCO) is a remediation technology that delivers oxidants to the subsurface to transform contaminants into benign products. The reactions take place in the aqueous phase where the oxidant comes in contact with the dissolved phase of the contaminant. In this work, we report on the impact of by-product formation on the effectiveness of ISCO. We conducted a series of batch experiments to identify by-products and increase our understanding for time scales required for complete mineralization of petroleum hydrocarbons. This was coupled with micro-CT imaging of column experiments and imaging in a glass fractured rock replica to track the formation of gaseous and solid by-products and determine their effect on flow, transport, and mass transfer. The final aim of this study is to propose novel strategies for improved remediation efficiency.

How to cite: Kalogerakis, G. C., Boparai, H. K., and Sleep, B. E.: Underlying Mechanisms in Groundwater Remediation via In Situ Chemical Oxidation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3098, https://doi.org/10.5194/egusphere-egu21-3098, 2021.

Transport of dissolved substances through porous media is determined by the complexity of the pore space and diffusive mass transfer within and between pores. The interplay of diffusive pore-scale mixing and spatial flow variability are key for the understanding of transport and reaction phenomena in porous media. We study the interplay of pore-scale mixing and network-scale advection through heterogeneous porous media, and its role for the evolution and asymptotic behavior of hydrodynamic dispersion. In a Lagrangian framework, we identify three fundamental mechanisms of pore-scale mixing that determine large scale particle motion: (i) The smoothing of intra-pore velocity contrasts, (ii) the increase of the tortuosity of particle paths, and (iii) the setting of a maximum time for particle transitions. Based on these mechanisms, we derive an upscaled approach that predicts anomalous and normal hydrodynamic dispersion based on the characteristic pore length, Eulerian velocity distribution and Péclet number. The theoretical developments are supported and validated by direct numerical flow and transport simulations in a three-dimensional digitized Berea sandstone sample obtained using X-Ray microtomography. Solute breakthrough curves, are characterized by an intermediate power-law behavior and exponential cut-off, which reflect pore-scale velocity variability and intra-pore solute mixing. Similarly, dispersion evolves from molecular diffusion at early times to asymptotic hydrodynamics dispersion via an intermediate superdiffusive regime. The theory captures the full evolution form anomalous to normal transport behavior at different Péclet numbers as well as the Péclet-dependence of asymptotic dispersion. It sheds light on hydrodynamic dispersion behaviors as a consequence of the interaction between pore-scale mixing and Eulerian flow variability. 

How to cite: Dentz, M., Puyguiraud, A., and Gouze, P.: Pore-scale Mixing and the Evolution from Anomalous to Normal Hydrodynamic Dispersion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13655, https://doi.org/10.5194/egusphere-egu21-13655, 2021.

Electrokinetic (EK) remediation is one of the few in-situ remediation technologies that can effectively remove contaminants from low-permeability porous media. Process-based modeling, including the complex multiphysics and biogeochemical processes occurring during electrokinetic remediation, is instrumental to describe EK systems and to assist in their design. In this work we use NP-Phreeqc-EK [1], a multidimensional, multiphysics code which couples a flow and transport simulator (COMSOL Multiphysics) with a geochemical code (PhreeqcRM) through a MATLAB LiveLink interface. The model allows the simulation of coupled fluid flow, solute transport, charge interactions and biogeochemical reactions during electrokinetics in saturated porous media. The process-based code is applied for the modeling of electrokinetic delivery of amendments to enhance bioremediation (EK-Bio) of chlorinated compounds at a pilot test site [2]. We simulate both conservative and reactive transport scenarios and we compute and show the Nernst-Planck fluxes and the velocities of the different species (such as lactate, chlorinated ethenes and degrading microorganisms). To compare remediation performances and model outcomes we define different metrics quantifying the spatial distribution of the delivered reactants and the mass of the organic contaminants in the system. The process-based model allowed the simulation of the key processes occurring during EK-Bio, including 1) multidimensional electrokinetic transport such as electromigration of charged species and electroosmosis, 2) Coulombic interactions between ions in solution, 3) kinetics of contaminant biodegradation, 4) dynamics of microbial populations, 5) mass transfer limitations and 6) geochemical reactions.

[1] Sprocati, R., Masi, M., Muniruzzaman, M., & Rolle, M. (2019). Modeling electrokinetic transport and biogeochemical reactions in porous media: A multidimensional Nernst–Planck–Poisson approach with PHREEQC coupling. Advances in Water Resources, 127, 134-147.

[2] Sprocati, R., Flyvbjerg, J., Tuxen, N., & Rolle, M. (2020). Process-based modeling of electrokinetic-enhanced bioremediation of chlorinated ethenes. Journal of Hazardous Materials, 397, 122787.

How to cite: Sprocati, R. and Rolle, M.: Modeling of coupled flow, transport and biogeochemical reactions during electrokinetic bioremediation: model development and application, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14376, https://doi.org/10.5194/egusphere-egu21-14376, 2021.

We study the shear-induced migration of dilute suspensions of swimming bacteria (modelled as Active elongated Brownian Particles or ABPs) subject to plane Poiseuille flow in a confined channel. By incorporating very simple boundary conditions, we perform numerical simulations of the 3D equations of motion describing the change in position and orientation of the particles. We investigate the effects of confinement, of non-uniform shear and of aspect ratio of the particles on the overall dynamics of the ABPs population.

We particularly study the coupling between the local shear and the change in the orientation of the particles. We thus perform numerical simulations on both the case where the change in the orientation of the ABPs is purely diffusive (decoupled case) and the case where their orientation is coupled to the shear flow (coupled case). We observe that the decoupled case exhibits a Taylor dispersion i.e.  the effective dispersion coefficient of the ABPs along the direction of the flow is proportional to the square of the imposed shear at all shears. 

However, for all the coupled cases we observe a transition from a Taylor to an active-Taylor regime at a critical shear rate, indicating the effect of shear coupling on the orientation dynamics of the particles. This critical shear rate is directly correlated to the degree of confinement. The change in the dispersion coefficient along the direction of the flow as function of the shear rate is in qualitative agreement with previous studies[1]. 

To further understand these results, we also investigate the change in the dispersion coefficient in the other two directions along with the effect of the shape of the particles. We believe that this study should enhance our understanding of dispersion of bacteria through porous media, on surfaces etc. where shear flows are ubiquitous. 

[1] Sandeep Chilukuri, Cynthia H.Collins, and Patrick T. Underhill. Dispersionof flagellated swimming microorganisms in planar poiseuille flow.Physics offluids, 27, (031902):1 –17, 2015

How to cite: Ganesh, A., Rescanieres, R., Douarche, C., and Auradou, H.: Modelization of dispersion of swimming bacteria in Poiseuille flow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7183, https://doi.org/10.5194/egusphere-egu21-7183, 2021.

Capillary fringe plays an important role in the fate and transport of infiltrated solutes from agricultural lands. In this study, flow patterns and the vertical distribution of the velocity and hydraulic gradient inside the capillary fringe were investigated using FEFLOW calibrated by experimental data. An experimental box along with a real sample of capillary fringe from the study area (Sand and clay pit Brüggen, Germany) was used for the experiments. The dimension of the filled part of the box was 0.75 m long, 0.55 m high, and 0.150 m wide. To maintain a constant hydraulic gradient throughout the experiments the upstream and downstream groundwater levels were fixed to 7 cm and 3 cm, respectively. The horizontal velocity at different points inside the capillary fringe and the vadose zone was measured by injecting the fluorescent dye tracer (Uranin). At the end of the experiments, the soil samples are collected from different parts of the box for water content measurement. The results indicate that FEFLOW successfully estimates water content, overall flow pattern, and more importantly horizontal movement inside the capillary fringe. The streamlines are parallel to the groundwater table in the middle part.  Based on both experimental and numerical results, while there is a downward movement near the outflow, an upward movement was seen near the inflow. In previous studies, the velocity profile inside the capillary fringe was estimated using Darcy’s law, unsaturated hydraulic conductivity, and constant hydraulic gradient. The detailed comparison of measured water content and velocity with numerical modeling results showed that the constant hydraulic gradient assumption above the water table in previous studies is not valid. The vertical hydraulic gradient profile calculated by FEFLOW showed that the hydraulic gradient at the middle part of the box changes from 0.042 to 0.03. Moreover, the shape of the vertical hydraulic gradient profile is a function of the location in the box and soil type.

Keywords: Solute transport, Unsaturated zone, Streamline, Pore velocity, Hydraulic conductivity, FEFLOW

How to cite: Mahdavi Mazdeh, A. and Wohnlich, S.: Numerical and experimental study of the vertical distribution of velocity and hydraulic gradient in the capillary fringe, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9143, https://doi.org/10.5194/egusphere-egu21-9143, 2021.

Invasion-Percolation (IP) models are used to simulate multiphase flow in porous media across various scales (from pore-scale IP to Macro-IP). Numerous variations of IP models have emerged; here we are interested in simulating gas flow in a water-saturated porous medium. Gas flow in porous media occurs either as a continuous or as a discontinuous flow, depending on the rate of flow and the nature of the porous medium. A particular IP model version may be well suited for predictions in a specific gas flow regime, but not applicable to other regimes. Our research aims to compare various macro-scale versions of IP models existing in the literature and rank their performance in relevant gas flow regimes.

We test the performance of Macro-IP models on a range of gas-injection rates in water-saturated sand experiments, including both continuous and discontinuous flow regimes. The experimental data is obtained as a time series of images using the light transmission technique. To represent pore-scale heterogeneities of sand, we let each model version run on several random realizations of the initial entry pressure field. As a metric for ranking the models, we introduce a diffused version of the so-called Jaccard index (adapted from image analysis and object recognition). We average this metric over time and over all realizations per model version to evaluate each model’s overall performance. This metric may also be used to calibrate model parameters such as gas saturation. 

Our proposed approach evaluates the performance of competing IP model versions in different gas-flow regimes objectively and quantitatively, and thus provides guidance on their applicability under specific conditions. Moreover, our comparison method is not limited to gas-water phase systems in porous media but generalizes to any modelling situation accompanied by spatially and temporally highly resolved data.

How to cite: Banerjee, I., Guthke, A., Mumford, K., and Nowak, W.: Comparison of Competing Invasion-Percolation Models for Simulation of Multiphase Flow in Porous Media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8826, https://doi.org/10.5194/egusphere-egu21-8826, 2021.