Literature review on NAPL contamination and remediation

Remediation of polluted soils and groundwater is of major concern due to the increasing number of contaminated aquifers. Subsurface aquifers constitute one of the most important sources of drinkable water. In recent years, water needs have been increasing due to increases in development and human population. Several sorts of contaminants can be found in groundwater: metal ions, pesticides, aliphatic and aromatic hydrocarbons, polycyclic hydrocarbons, chlorinated hydrocarbons, etc. The toxicity of these compounds varies and so do guidelines that establish allowable concentration levels in drinking water. Among the aforementioned types of compounds, a particular importance is assumed by those which exist as a separate phase when their concentrations in water exceed a certain limit. The transport behavior and dynamics of multiphase contaminants are very different from their dissolved counterparts, and are very difficult both to describe and to model. Several phenomena can take place, such as organic phase trapping, formation of ganglia and pools of contaminant, sorption, hysteresis in both soil imbibition and drainage, capillarity, fingering, and mass-transfer. In such cases, our ability to describe and predict the fate of a contaminant plume in which a separate organic phase occurs is limited, and research within this field is quite open. Much effort has been devoted in trying to describe the characteristics of the phenomena occuring in multiphase systems, and several models and formulations have been proposed for predicting the fate of contaminants when present in such systems (see Miller et al. 1997) for a review on multiphase modeling in porous media). Work has also been done for modeling human intervention techniques for containing and/or reducing soil contaminantion (NRC, 1994), such as pumping, clean water-air-steam injection, soil heating, surfactants, biological methods, etc. Finally, much work has also been done on the numerical solution of mathematical models whose complexity does not allow for an analytical solution. Among the dozens of remediation methods which have been proposed and which are strongly dependent on site environmental conditions, biological methods are achieving increasing importance, due to their “naturalness" and their low costs (NRC, 1993) . It has been noticed that soil microorganisms are able to degrade several classes of compounds, in particular those which partition between an aqueous and an organic phase, or sometimes also gaseous phase, for e.g., hydrocarbons, chlorinated compounds, pesticides. These compounds, or better said, their fractions dissolved in water, are liable to be metabolized by subsurface microrganisms which have the capability to degrade the compounds and to transform them into carbon dioxide and/or other compounds, which are less toxic or unnoxious. Several laboratory and field studies have been conducted for assessing and evaluating the capability and the limits of soil microorganisms to degrade several classes of contaminants (Mayer et al., 1994, 1995, 1996, 1997) . Much work has also been devoted to modeling biodegration of groundwater contaminants. The outline of this report is as follows: section 2 gives a brief description of the characteristics and properties of NAPLs, including a review of the literature with regards to formulations and modeling; section 3 discusses biodegradation of contaminants and past efforts at modeling biodegradation; section 4 surveys specific remediation technologies and experiences; and section 5 discusses open issues for further research. In the final section possible lines of research for the second phase of the PhD program are indicated

Imaging spontaneous imbibition in full Darcy‐scale samples at pore‐scale resolution by fast X‐ray tomography

Spontaneous imbibition is a process occurring in a porous medium which describes wetting phase replacing nonwetting phase spontaneously due to capillary forces. This process is conventionally investigated by standardized, well-established spontaneous imbibition tests. In these tests, for instance, a rock sample is surrounded by wetting fluid. The following cumulative production of nonwetting phase versus time is used as a qualitative measure for wettability. However, these test results are difficult to interpret, because many rocks do not show a homogeneous but a mixed wettability in which the wetting preference of a rock varies from location to location. Moreover, during the test the flow regime typically changes from countercurrent to cocurrent flow and no phase pressure or pressure drop can be recorded. To help interpretation, we complement Darcy-scale production curves with X-ray imaging to describe the differences in imbibition processes between water-wet and mixed-wet systems. We found that the formation of a spontaneous imbibition front occurs only for water-wet systems; mixed-wet systems show localized imbibition events only. The asymmetry of the front depends on the occurrence of preferred production sites, which influences interpretation. Fluid layers on the outside of mixed-wet samples increase connectivity of the drained phase and the effect of buoyancy on spontaneous imbibition. The wider implication of our study is the demonstration of the capability of benchtop laboratory equipment to image a full Darcy-scale experiment while at the same time obtaining pore-scale information, resolving the natural length and time scale of the underlying processes

Advances in porous media science and engineering from InterPore2020 perspective

    Natural, artificial, and biological porous media can be seen everywhere in our daily lives. Transport phenomena in porous media, such as flow, diffusion, reaction, adsorption and deformation, are encountered in a wide variety of practical applications and scientific interests over widely disparate length scales, from molecular, to pore, core, and field scales. However, determination of transport properties in porous media remains a challenging issue. During the 12th Annual Meeting of the International Society for Porous Media (InterPore), held online from August 31-September 4, 2020, advances on porous media science and engineering in very broad areas were presented. The meeting was attended by more than 750 participants from across the globe, and a significant milestone was achieved in the history of InterPore conferences due to its online interactive platform. Participants could access the pre-recorded talks, leave comments and questions, chat with each other, one week before the conference. Then, all the feedback related to a talk was discussed in the presence of the author during several Q&A sessions. Invited and Keynote talks were live, and were also recorded. Each Q&A session was moderated by two experts, who first reviewed the 8 contributions of their session and then summarized the questions for each talk. The author could further elaborate their work and answer the questions.Cited as: Cai, J., Hajibeygi, H., Yao, J., Hassanizadeh, S.M. Advances in porous media science and engineering from InterPore2020 perspective. Advances in Geo-Energy Research, 2020, 4(4): 352-355, doi: 10.46690/ager.2020.04.0

Two sides of a coin: a critical review, and mathematical and phenomenological study of what we call hydromechanical coupling

In this paper a brief and critical review of the current literature on hydro-mechanical coupling is presented. Furthermore, anenhanced discrete element model is used to investigate the mutual relationship of soil water retention curve and suction stress curves and how the two are affected as a result of change in the initial porosity of the soil sample. The model revealed the suction stress values in wetting were less affected as in drying branch as a result of the change in the initial porosity of the soil sample

Modeling the dynamics of partial wetting

The behavior of interfaces and contact lines arises from intermolecular interactions like Van der Waals forces. To consider this multi–phase behavior on the continuum scale, appropriate physical descriptions must be formulated. While the Continuum Surface Force model is well–engineered for the description of interfaces, there is still a lack of treatment of contact lines, which are represented by the intersection of a fluid–fluid interface and a solid boundary surface. In our approach we use the “non compensated Young force” to model contact line dynamics and therefore use an extension to the Navier–Stokes equations in analogy to the extension of a two–phase interface in the CSF model. Because particle–based descriptions are well–suited for changing and moving interfaces we use Smoothed Particle Hydrodynamics. In this way we are not only able to calculate the equilibrium state of a two–phase interface with a static contact angle, but also for instance able to simulate droplet shapes and their dynamical evolution with corresponding contact angles towards the equilibrium state, as well as different pore wetting behavior. Together with the capability to model density differences, this approach has a high potential to model recent challenges of two–phase transport in porous media. Especially with respect to moving contact lines this is a novelty and indispensable for problems, where the dynamic contact angle dominates the system behavior

Modeling the Transport and Retention of Nanoparticles in a Single Partially Saturated Pore in Soil

Pore-network models are powerful tools for studying particle transport in complex porous media, and investigating the role of interfaces in their fate. The first step in simulating particle transport using pore-network models is to quantitatively describe particle transport in a single pore, and obtain relationships between pore-averaged deposition rate coefficients and various pore-scale parameters. So, in this study, a three-dimensional (3D) mathematical model is developed to simulate the transport and retention of nanoparticles within a single partially saturated pore with an angular cross-section. The model accounts for particle deposition at solid-water interfaces (SWIs), air-water interfaces (AWIs), and air-water-solid (AWS) contact regions. We provide a novel formulation for particle diffusive transport from AWI to AWS, where particles are assumed to be retained irreversibly by capillary forces. The model involves 12 dimensionless parameters representing various physicochemical conditions. The 3D model results are averaged over the pore cross-section and then fitted to breakthrough curves from one-dimensional (1D) advection-dispersion-sorption equations with three-site kinetics to estimate 1D-averaged deposition rate coefficients at interfaces. We find that half-corner angle, particle size, radius of curvature of AWI, and mean flow velocity have a significant effect on those coefficients. In contrast, chemical parameters such as ionic strength and surface potentials of particles and interfaces have negligible effects. AWS is found to be the major retention site for particles, especially for hydrophobic particles. We develop algebraic relationships between 1D-averaged deposition rate coefficients at interfaces vis-à-vis various pore-scale parameters. These relationships are needed for pore-network models to upscale nanoparticle transport to continuum scale

Experimental determination of in-plane permeability of nonwoven thin fibrous materials

Knowledge of hydraulic properties is crucial for understanding and modeling fluid flow in thin porous media. In this work, we have developed a new simple custom-built apparatus to measure the intrinsic permeability of a single thin fibrous sheet in the in-plane direction. The permeability was measured for two types of nonwoven thin fibrous porous materials using either the water or gas phase. For each layer, the measurements have been done for different combinations of flow direction and fiber orientation. The permeability values measured using gas and water were approximately close to each other. The permeability of the two samples was found to be anisotropic and the principal permeabilities were determined based on the measurements

Experimental Evaluation of Fluid Connectivity in Two-Phase Flow in Porous Media During Drainage

This study aims to experimentally investigate the possibility of combining two extended continuum theories for two-phase flow. One of these theories considers interfacial area as a separate state variable, and the other explicitly discriminates between connected and disconnected phases. This combination enhances our potential to effectively model the apparent hysteresis, which generally dominates two-phase flow. Using optical microscopy, we perform microfluidic experiments in quasi-2D artificial porous media for various cyclic displacement processes and boundary conditions. Specifically for a number of sequential drainage processes, with detailed image (post-)processing, pore-scale parameters such as the interfacial area between the phases (wetting, non-wetting, and solid), and local capillary pressure, as well as macroscopic parameters like saturation, are estimated. We show that discriminating between connected and disconnected clusters and the concept of the interfacial area as a separate state variable can be an appropriate way of modeling hysteresis in a two-phase flow scheme. The drainage datasets of capillary pressure, saturation, and specific interfacial area, are plotted as a surface, given by f (Pc, sw, awn) = 0. These surfaces accommodate all data points within a reasonable experimental error, irrespective of the boundary conditions, as long as the corresponding liquid is connected to its inlet. However, this concept also shows signs of reduced efficiency as a modeling approach in datasets gathered through combining experiments with higher volumetric fluxes. We attribute this observation to the effect of the porous medium geometry on the phase distribution. This yields further elaboration, in which this speculation is thoroughly studied and analyzed