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Preferential fluid flow and chemical transport occur on scales ranging from pores to aquifers and catchments, in both fully and partially water-saturated geological formations. Preferential flows can be considered, in a general sense, manifestations of self-organization that hinders perfect mixing within a system, and leads to faster throughput of water and chemicals. However, unified concepts for the onset, spatiotemporal patterning, and magnitude of such preferential flows are generally difficult to define; and this is compounded by the difficulty – or practical impossibility – of obtaining detailed measurements of the structure and hydraulic functioning of vadose zones, catchments, and aquifers. We propose that conceptualizations and quantitative characterizations of preferential fluid flow and chemical transport in all of these systems can be unified in terms of tools that connect them in a dynamic framework. Here, we discuss key, shared features of fluid flow and chemical transport dynamics in each of these two systems, based on both laboratory and field measurements, and numerical simulations. We show how even well-connected fracture networks can display highly non-uniform preferential paths for fluid and chemicals. We then recognize that this behavior is similar to that of rapid infiltration in soils and the vadose zone, which exhibits strongly localized preferential pathways in root channels, cracks, worm burrows or connected inter-aggregate pore networks. Moreover, both types of domains can display “memory effects”, in terms of the location and functioning of preferential paths even during perturbations in the velocity gradient and/or rates of infiltration. We argue that the ubiquity of unresolved (or uncharacterized) heterogeneity at all spatial and temporal scales necessitates the use of effective medium models that enable an accounting of a wide range of flow and transport behaviors. For chemical transport, we focus on a probabilistic modelling framework that can capture the dynamics in heterogeneous vadose zones and fractured (or otherwise heterogeneous) geological formations. We then demonstrate application of this model to interpret field-scale tracer breakthrough curves (concentration vs. time) in a highly fractured karst formation over length scales of up to more than 7 km.

We quantify the poorly-understood interactions between resident and infiltrating water during drainage-imbibition cycles in heterogeneous porous media, via a 2-D (and 3-D) lattice Boltzmann simulation method. These interactions are critical in the (partially saturated) soil layer, which is the interface from land surface to groundwater that controls the fate and transport of infiltrating water and chemicals to aquifers. The results demonstrate the sensitivity of these dynamic interactions to the contact angle, boundary flux condition, body force, and porosity. We show that the body force and contact angle have relatively small effects on interactions in the capillary flow regime, while the boundary flux and porosity alter the amount and distribution of resident and infiltrating water significantly. Moreover, air-water interfaces are shown to be dynamically concave/convex, determined by both contact angle and pore geometry. Our analysis demonstrates the coexistence of distinct pockets of resident and infiltrating water, and provides pore-scale, mechanistic evidence that supports field observations of soil water zones with different chemical signatures.

In parallel to technological advances and ever-increasing use of nanoparticles in industry, agriculture and consumer products, the potential ecotoxicity of nanoparticles and their potential accumulation in ecosystems is of increasing concern. Because scientific reports raise a concern regarding nanoparticle toxicity to plants, understanding of their bioaccumulation has become critical and demands more research. Here, the synthesis of isotopically-labeled nanoparticles of silver, copper and zinc oxide is reported; it is demonstrated that while maintaining the basic properties of the same unlabeled ("regular") nanoparticles, labeled nanoparticles enable more sensitive tracing of nanoparticles within plants that have background elemental levels. This technique is particularly useful for working with elements that are present in high abundance in natural environments. As a benchmark, labeled and unlabeled metal nanoparticles (Ag-NP, Cu-NP, ZnO-NP) were synthesized and compared, and then exposed in a series of growth experiments to Arabidopsis thaliana; the NPs were traced in different parts of the plant. All of the synthesized nanoparticles were characterized by TEM, EDS, DLS, ζ-potential and single particle ICP-MS, which provided essential information regarding size, composition, morphology and surface charge of nanoparticles, as well as their stability in suspensions. Tracing studies with A. thaliana showed uptake/retention of nanoparticles that is more significant in roots than in shoots. Single particle ICP-MS, and scanning electron micrographs and EDS of plant roots showed presence of Ag-NPs in particular, localized areas, whereas copper and zinc were found to be distributed over the root tissues, but not as nanoparticles. Thus, nanoparticles in any natural matrix can be replaced easily by their labeled counterparts to trace the accumulation or retention of NPs. Isotopically-labeled nanoparticles enable acquisition of specific results, even if there are some concentrations of the same elements that originate from other (natural or anthropogenic) sources.

The need for better understanding of the environmental fate and transport of engineered nanoparticles (ENPs) is now a scientific consensus. However, the partially saturated zone, a critical region that links the earth's surface to aquifers, has to date received only minor attention in the context of ENP mobility. We investigate the transport and fate of a representative ENP, silver nanoparticles (Ag-NPs), in partially saturated soil. Here we present a set of column experiments and modelling simulations to examine breakthrough curves (BTCs), retention profiles, and mass balances that characterize Ag-NP transport, and gain insights into retardation mechanisms. Unlike Ag-NP transport in sand columns, where the BTC pattern often resembles that of a conservative tracer, Ag-NP transport in soil columns shows moderate mobility and more complex BTC patterns; these results also emphasize the importance of employing realistic porous media in environmental studies. Overall, Ag-NP mobility decreases in the presence of Ca(NO3)2, and increases when the solution contains humic acid, at higher water saturation levels, or at higher input concentrations of Ag-NPs. In addition, a different pattern was observed for Ag-NP aggregates, indicating nanospecific behaviour. Modelling analysis of Ag-NP transport in partially saturated soil suggests that a two-site kinetic model with a time-dependent retention function quantifies the transport behaviour of Ag-NPs.

Temporal variations in the subsurface velocity field are often (if not always) present in the real world to at least some degree. However, an accounting of their effects on chemical transport has been largely neglected. Here we demonstrate experimentally the effects of a time-varying velocity field on conservative chemical tracer transport in porous media, as compared to constant velocity conditions. We find that velocity-field fluctuations increase chemical tracer spreading and residence time, which intensify the anomalous nature of the transport. This behavior is modeled by a continuous time random walk particle tracking method formulated to account for time-dependent velocity fields. The model matches the experimental results with a parsimonious and consistent set of parameters. The model is then applied to study the effects of different magnitudes in velocity-field fluctuations, as well as different degrees of porous media heterogeneity, on 1-D and 2-D spatiotemporal propagation of an injected, point-source, chemical plume. Increased intensity of velocity-field fluctuations, and increased porous medium heterogeneity, each serve to increase the extent of chemical spreading and anomalous behavior.

We present the synthesis of new composite materials based on copper nanoparticles (Cu NPs) deposited onto montmorillonite (MK10) and quartz sand, for degradation of atrazine, in the context of an advanced oxidation process (AOP). The synthesis involves a first step in which polyethylenimine (PEI) capped Cu NPs (PEI_Cu NPs) are prepared, and then deposited onto, separately, MK10 and sand, through a solvent impregnation method. The resulting products are characterized in detail; the copper is found to exist as a mixture of copper (I, II) oxide. The degradation of atrazine follows a second-order kinetic model with constant values of K2 = 1.7957 g mg−1 min−1 for MK10_PEI_Cu NPs and K2 = 0.8133 g mg−1 min−1 for sand_PEI_Cu NPs. The reaction rate is linked to Cu2O and CuO redox-active species within the layers, pores and surface of the host materials. A degradation mechanism is found with application of these composite materials in the presence of H2O2; adsorption occurs in the absence of H2O2. In contrast, the unmodified MK10 and sand exhibit adsorption in both of the above reaction conditions. Finally, the stability of the Cu NPs following degradation is evaluated, and no significant amount of copper leaching is found.

Copper oxide nanoparticles (CuO-NPs) have been suggested as effective catalysts to degrade many persistent organic contaminants. In parallel, CuO-NPs are considered toxic to soil microorganisms, plants and human cells, possibly because they induce oxidative stress and generation of reactive oxygen species (ROS). However, the mechanism of the catalytic process and the generated ROS are poorly understood. Here we discuss the reaction mechanism of CuO-NPs during the catalytic degradation of enrofloxacin – an antibiotic pharmaceutical used in this study as a representative persistent organic compound. The degradation of an aqueous solution of the enrofloxacin exposed to CuO-NPs and hydrogen peroxide was studied showing fast removal of the enrofloxacin at ambient conditionsns. ROS production was identified by electron spin resonance and a spin trapping technique. The distribution of the free radical species indicated production of a high percentage of superoxide (O2−.) radicals as well as hydroxyl radicals; this production is similar to the “radical production” activity of the superoxide dismutase (SOD) enzyme in the presence of hydrogen peroxide. This activity was also tested in the opposite direction, to examine if CuO-NPs show reactivity that potentially mimics the classical SOD enzymatic activity. The CuO-NPs were found to catalyze the dismutation of superoxide to hydrogen peroxide and oxygen in a set of laboratory experiments.

Path reversibility and radial symmetry are often assumed in push-pull tracer test analysis. In reality, heterogeneous flow fields mean that both assumptions are idealizations. To understand their impact, we perform a parametric study which quantifies the scattering effects of ambient flow, local-scale dispersion and velocity field heterogeneity on push-pull breakthrough curves and compares them to the effects of mobile-immobile mass transfer (MIMT) processes including sorption and diffusion into secondary porosity. We identify specific circumstances in which MIMT overwhelmingly determines the breakthrough curve, which may then be considered uninformative about drift and local-scale dispersion. Assuming path reversibility, we develop a continuous time random walk-based interpretation framework which is flow-field agnostic and well suited to quantifying MIMT. Adopting this perspective, we show that the radial flow assumption is often harmless: to the extent that solute paths are reversible, the breakthrough curve is uninformative about velocity field heterogeneity. Our interpretation method determines a mapping function (i.e. subordinator) from travel time in the absence of MIMT to travel time in its presence. A mathematical theory allowing this function to be directly “plugged into” an existing Laplace-domain transport model to incorporate MIMT is presented and demonstrated. Algorithms implementing the calibration are presented and applied to interpretation of data from a push-pull test performed in a heterogeneous environment. A successful four-parameter fit is obtained, of comparable fidelity to one obtained using a million-node 3D numerical model. Finally, we demonstrate analytically and numerically how push-pull tests quantifying MIMT are sensitive to remobilization, but not immobilization, kinetics. This article is protected by copyright. All rights reserved.

Reactive chemical transport plays a key role in geological media across scales, from pore scale to aquifer scale. Systems can be altered by changes in solution chemistry and a wide variety of chemical transformations, including precipitation/dissolution reactions that cause feedbacks that directly affect the flow and transport regime. The combination of these processes with advective-dispersive-diffusive transport in heterogeneous media leads to a rich spectrum of complex dynamics. The principal challenge in modeling reactive transport is to account for the subtle effects of fluctuations in the flow field and species concentrations; spatial or temporal averaging generally suppresses these effects. Moreover, it is critical to ground model conceptualizations and test model outputs against laboratory experiments and field measurements. This review emphasizes the integration of these aspects, considering carefully designed and controlled experiments at both laboratory and field scales, in the context of development and solution of reactive transport models based on continuum-scale and particle tracking approaches. We first discuss laboratory experiments and field measurements that define the scope of the phenomena and provide data for model comparison. We continue by surveying models involving advection-dispersion-reaction equation and continuous time random walk formulations. The integration of measurements and models is then examined, considering a series of case studies in different frameworks. We delineate the underlying assumptions, and strengths and weaknesses, of these analyses, and the role of probabilistic effects. We also show the key importance of quantifying the spreading and mixing of reactive species, recognizing the role of small-scale physical and chemical fluctuations that control the initiation of reactions.

Chemical action as an anthropogenic factor in soil formation was suggested 50 years ago as a part of the metapedogenetic process. Initially, agricultural practices such as irrigation with saline water and soil liming by natural carbonated earth materials (marling) were considered as examples of chemical contamination that alter soil properties. Over the years, however, new anthropogenic substances have emerged, and their disposal on the land surface, whether intentionally or by accident, is acting as another metapedogenetic factor, leading to formation of contemporary soils with altered properties. In this context, potential transformation of natural soils may occur as a result of exposure to engineered nanomaterials (ENMs). Due to the relatively small amounts used to date, and their size and associated chemical properties, the impact of ENMs on soil may be questioned. We argue here, however, that ENMs disposed on the land surface are irreversibly retained in the soil matrix, accumulating over time and becoming a metapedogenetic factor. In support of this hypothesis, we present examples from the literature which demonstrate that deposition of carbon-based and metal-based ENMs may cause, under specific conditions, changes to the matrix and properties of soil constituents and formation of contemporary anthropogenic soils. These examples reconfirm the hypothesis that chemicals are a factor in anthropogenically-induced metapedogenesis.

A continuous time random walk particle tracking (CTRW-PT) method was employed to model flow cell experiments that measured transport of engineered nanoparticles (ENPs) in a reactive porous medium. The experiments involved a water-saturated medium containing negatively charged, polyacrylamide beads, resembling many natural soils and aquifer materials, and having the same refraction index as water. Negatively and positively charged ENPs were injected into a uniform flow field in a 3-D horizontal flow cell, and the spatial and temporal concentrations of the evolving ENP plumes were obtained via image analysis. As a benchmark, and to calibrate the model, Congo red tracer was employed in 1-D column and 3-D flow cell experiments, containing the same beads. Negatively charged Au and Ag ENPs demonstrated migration patterns resembling those of the tracer but were slightly more dispersive; the transport was well represented by the CTRW-PT model. In contrast, positively charged AgNPs displayed an unusual behavior: establishment of an initial plume of essentially immobilized ENPs, followed by development of a secondary, freely migrating plume. The mobile plume was found to contain ENPs that, with aging, exhibited aggregation and charge inversion, becoming negatively charged and mobile. In this case, the CTRW-PT model was modified to include a probabilistic law for particle immobilization, to account for the decreasing tendency (over distance and time) of the positively charged AgNPs to attach to the porous medium. The agreement between experimental results and modeling suggests that the CTRW-PT framework can account for the non-Fickian and surface-charge-dependent transport and aging exhibited by ENPs in porous media.

Anomalous transport is ubiquitous in a wide range of disordered systems, notably in fractured porous formations. We quantitatively identify the structural controls on anomalous tracer transport in a model of a real fractured geological formation that was mapped in an outcrop. The transport, determined by a continuum scale mathematical model, is characterized by breakthrough curves (BTCs) that document anomalous (or “non-Fickian”) transport, which is accounted for by a power-law distribution of local transition times ψ(t) within the framework of a continuous time random walk (CTRW). We show that the determination of ψ(t) is related to fractures aligned approximately with the macroscopic direction of flow. We establish the dominant role of fracture alignment, and assess the statistics of these fractures by determining a concentration-visitation weighted residence time histogram. We then convert the histogram to a probability density function (pdf) that coincides with the CTRW ψ(t) and hence anomalous transport. We show that the permeability of the geological formation hosting the fracture network has a limited effect on the anomalous nature of the transport; rather, it is the fractures transverse to the flow direction that play the major role in forming the long BTC tail associated with anomalous transport. This is a remarkable result, given the complexity of the flow field statistics as captured by concentration transitions. This article is protected by copyright. All rights reserved.

Within historically accepted, major soil-forming major processes, the role of chemicals as a human-induced factor was neglected until the middle of the last century. Over the years, however, anthropogenic chemicals have emerged and are being released on the land surface in large amounts. Irreversible changes in the matrix of soil and soil constituents may occur as a result of both intentional and accidental release of anthropogenic chemicals, as well as a byproduct of human activity. After presenting an historical evolution of the discussion on soil-forming factors, we focus here on human impacts and examine the abiotic role of anthropogenic microchemical contaminant (AMCC) interactions with soils at the molecular level. Selected examples of microchemical contaminants, including heavy metals, pesticides, hydrocarbons, and engineered nanomaterials, are presented to demonstrate that AMCCs-even at low concentration-may irreversibly alter the matrix of the soil and soil constituents and lead to the formation of anthropogenic soils with different properties than those of the pristine soils.

We study an integro-differential equation that has important applications to problems of anomalous transport in highly disordered media. In one application, the equation is the continuum limit of a continuous time random walk used to quantify non-Fickian (anomalous) contaminant transport. The finite element method is used for the spatial discretization of this equation, with an implicit scheme for its time discretization. To avoid storage of the entire history, an efficient sum-of-exponential approximation of the kernel function is constructed that allows a simple recurrence relation. A 1D formulation with a linear element is implemented to demonstrate this approach, by comparison with available experiments and with an exact solution in the Laplace domain, transformed numerically to the time domain. The proposed scheme convergence assessment is briefly addressed. Future extensions of this implementation are then outlined.

Plastic nanoparticles (PNPs) are considered contaminants of emerging concern, but little information is available on their transport behavior in the soil-water environment, as well as their behavior relative to metal and other carbon-based nanoparticles. Here we show that size and surface functional groups affect the transport of polystyrene nanoparticles (PS-NPs) through saturated soil. Unmodified 110 nm and 50 nm PS-NPs demonstrated similar transport patterns in soil. However, a maximum elution value of 90% from the soil was found for the 50 nm PS-NPs, compared to a maximum value of ∼45% for 110 nm PS-NPs. The breakthrough curve for 190 nm PS-NPs demonstrated a maximum elution value of 60% from the soil. PS-NPs with surface functional groups display different mobility profiles: carboxylated PS-NPs demonstrated a plateau of 40% elution from the soil, while aminated PS-NPs were eluted only in small amounts and showed a spike pattern of elution from the column. These findings are attributed to the effects of common soil constituents such as calcium cations and humic acids on the size and charge of the PS-NPs with surface functional groups. Overall, PS-NP mobility in soil can vary widely, depending on PNP properties such as size and surface chemistry, and on matrix properties, such as the type of porous medium and its composition. These findings suggest that knowledge of inherent characteristics (size, surface charge, surface functional groups) of PNPs are required to elucidate the behavior of such particles in soil-water environments, and predict the extent of contaminant spreading.

Karst aquifers are important drinking water resources, but highly vulnerable to contamination. Contaminants can be transported rapidly through a network of fractures and conduits, with only limited sorption or degradation, which usually leads to a fast and strong response at karst springs. During migration, contaminants can also enter less mobile zones, such as pools or water in intra-karstic sediments, or advance from conduits into the adjacent fractured rock matrix. As contaminant concentrations in the main flow path(s) decrease, contaminants may migrate back into the main flow path and reach the karst springs at low (but significant) concentrations over a long time span. This is the conventional interpretation for the oft-observed steep rising limb and the long-tailed falling limb of tracer breakthrough curves in karst systems. Here, field measurements are examined from an alpine karst system in Austria where a series of distinctive, long-tailed breakthrough curves (BTCs) of conservative tracers were observed over distances up to 7400 m. Recognizing that the conventional advection-dispersion equation (ADE) cannot usually quantify such behavior, two other modeling approaches are considered, namely the two-region non-equilibrium (2RNE) model, which explicitly includes mobile and immobile zones, and a continuous time random walk (CTRW) model, which is based on a physically-based, probabilistic approach that describes anomalous (or non-Fickian) transport behavior characteristic of heterogeneous systems such as karst. In most cases, the ADE and 2RNE models do not quantify the low concentrations at longer travel times. The CTRW, in contrast, accounts for the long-tailed breakthrough behavior found in this karst system.

Indium and gallium are used widely in modern industry, mostly for the production of semiconductors. They are considered as Technology-Critical Elements and have therefore received growing attention in the past few years. We investigated the influence of different types of humic substances on the transport of indium and gallium in laboratory-scale, saturated column experiments, to gain understanding of their mobility in natural environments. We evaluated the effect of different humic substances on the transport of indium and gallium in quartz sand: a commercial humic acid (Aldrich Humic Acid, AHA), a fulvic acid (Suwannee River Fulvic Acid, SRFA) and an aquatic natural organic matter (Suwannee River Natural Organic Matter, SRNOM). The impact of the flow rate and the influence of different concentrations of organic matter were also investigated. Indium was shown to be more mobile than gallium in the presence of humic substances. The mobility of indium in sand was highest for SRNOM, followed by SRFA and then AHA, while for gallium the order was SRFA > SRNOM > AHA. These results can be significant in understanding the mobility of indium and gallium in soils with various compositions of organic matter.

This study reports the transport characteristics of the pharmaceutical compounds carboplatin and cisplatin, and their respective derivatives, in saturated sand and soil columns. Pharmaceuticals are recognized as emerging pollutants of soil and water resources, but studies of the transport characteristics of organometallic pharmaceuticals in soil-water environments are rare. A recent study of oxaliplatin transport in natural soil raises the question of whether or not its behavior is representative of all Pt-based pharmaceuticals behavior in soil-water systems. To address this question, transport behaviors of carboplatin and cisplatin species were studied individually in packed sand columns under unamended conditions, and in packed soil columns under unamended and acetate-amended conditions. In contrast to oxaliplatin, carboplatin species exhibited very low affinity to both sand and soil surfaces: the retention of injected carboplatin was 3% and6% for sand and soil, respectively. The affinity to soil was practically the same under the different redox conditions. The affinity of carboplatin to sand and soil surfaces was much smaller than the reported oxaliplatin affinity and the values reported in the literature. Cisplatin exhibited transport behavior similar to that of oxaliplatin in soil, including mild sensitivity to redox conditions (e.g., higher retention under acetate-amended conditions), overall exhibiting retention of 64-70% of the injected species. However, cisplatin also exhibited a similar retention in sand (retention of 45-53%), unlike the cases of carboplatin and oxaliplatin. The results indicate that similarly structured pharmaceuticals can exhibit very different transport characteristic in natural soil-water environments, and should therefore be studied and assessed individually.

An exposition is given of a finite-element method (FEM) software package to calculate solutions for the continuous time random walk (CTRW) integro-differential equation for non-Fickian (and Fickian) conservative or reactive transport in disordered media. The solutions encompass one-dimensional/two-dimensional (1D/2D) breakthrough curves and spatial concentration profiles for general geometry and grid. The velocity field, used as input to the 2D solutions, may also be calculated by applying a compatible Darcy flow 2D solver. The software enables both "forward" modeling (1D/2D) and "inverse" (best-fit) modeling (1D) of experimental data. Various inlet and outlet boundary conditions are implemented. The CTRW-FEM Package is freely downloadable and contains a User's Guide, source and executable files, and accompanying files with easy-to-use format, including descriptions of input/output files and pre- and post-processing. Several examples are provided to clearly demonstrate how to work with the CTRW-FEM Package in many typical analyses. The CTRW-FEM Package also allows users to adapt the software to specific needs, such as, for example, first- and second-order chemical reactions.

The release of pharmaceuticals and personal care products (PPCPs) to the soil-water environment necessitates understanding of PPCP transport behavior under conditions that account for dynamic flow and varying redox states. This study investigates the transport of two organometallic PPCPs, Gd-DTPA and roxarsone (arsenic compound) and their metal salts (Gd(NO3)3, AsNaO2); Gd-DTPA is used widely as a contrasting agent for MRI, while roxarsone is applied extensively as a food additive in the broiler poultry industry. Here, we present column experiments using sand and Mediterranean red sandy clay soil, performed under several redox conditions. The metal salts were almost completely immobile. In contrast, transport of Gd-DTPA and roxarsone was affected by the soil type. Roxarsone was also affected by the different redox conditions, showing delayed breakthrough curves as the redox potential became more negative due to biological activity (chemically-strong reducing conditions did not affect the transport). Mechanisms that include adsorptive retardation for aerobic and nitrate-reducing conditions, and non-adsorptive retardation for iron-reducing, sulfate-reducing and biologically-strong reducing conditions, are suggested to explain the roxarsone behavior. Gd-DTPA is found to be a stable complex, with potential for high mobility in groundwater systems, whereas roxarsone transport through groundwater systems is affected by redox environments, demonstrating high mobility under aerobic and nitrate-reducing conditions and delayed transport under iron-reducing, sulfate-reducing and biologically-strong reducing conditions.

We present quasi-3D visualization and analysis of engineered nanoparticle (ENP) transport behavior in an experimental setup that uses a transmitted light imaging technique. A flow cell was packed with specially adapted, water-transparent, spherical polyacrylamide beads, which carry a negative surface charge representative of many natural environments. Ubiquitous, oppositely-charged ENPs - Au and Ag NPs - were synthesized and introduced into a flow cell subjected to a macroscopically uniform flow field via point source pulse injection, at three different flow rates. The negatively-charged ENPs behaved like a conservative tracer, in terms of spatio-temporal plume evolution. The positive AgNPs, however, displayed a decrease in their initially strong tendency to attach to the oppositely-charged porous medium. As a result, immobilization of the positive AgNPs was spatially and temporally limited to the vicinity of the point of injection; beyond this region, the AgNPs were mobile and effluent contained AgNPs with hydrodynamic diameters significantly larger than those of the injected AgNPs. This behavior is understood by dynamic light scattering and ζ potential measurements, which showed aggregation processes and inversion in particle surface charge to occur during transport of the positive ENPs. These findings have broad implications for ENP mobility and reactivity in the environment.

A large number of research papers on the fate of engineered nanomaterials (ENMs) in the soil-water system have appeared in recent years, focusing on ENM transport, persistence and toxicological impact. It is clear from these publications that soil is a major sink for ENMs, and that only a small portion degrades or is mobilized further into groundwater. However, to date, very few studies have examined the impact of ENMs on the natural soil-subsurface matrix and its properties. Moreover, it is now well accepted that chemical contaminants are capable of changing soil properties either by inducing direct chemical or physical changes, or through indirect changes by, e.g., influencing biological activity that in turn modifies soil properties. Here, we review studies on the deposition, retention, and accumulation of ENMs in soil, indicative of the extent to which soil acts as a major sink of ENMs. We then examine evidence of how these retained particles lead to modification of surface properties, which are manifested by changes in the sorption capacity of soil for other (organic and inorganic) solutes, and by surface charges and composition different than the natural surfaces. Finally, we demonstrate how this results in physical and hydrological changes to soil properties, including hydraulic conductivity, swelling capacity and wettability. The overall picture revealed in this critical review sheds light on a perspective that has received little attention thus far. These aspects of soil change, due to exposure and subsequent accumulation of ENMs, may ultimately prove to be one of the most important impacts of ENM releases to the environment.

Nickel migration measured in laboratory-scale, natural soil column experiments is shown to display anomalous (non-Fickian) transport, nonequilibrium adsorption and desorption patterns, and precipitation/dissolution. Similar experiments using a conservative tracer also exhibit anomalous behavior. The occurrence of ion exchange of nickel, mainly with calcium (but also with other soil components), is measured in both batch and flow-through column experiments; adsorption and desorption isotherms demonstrate hysteresis. Strong retention of nickel during transport in soil columns leads to delayed initial breakthrough (∼40 pore volumes), slow increase in concentration, and extended concentration tailing at long times. We describe the mechanisms of transport and retention in terms of a continuous time random walk (CTRW) model, and use a particle tracking formulation to simulate nickel migration in the column. This approach allows us to capture the non-Fickian transport and the subtle local effects of adsorption/desorption and precipitation/dissolution. Consideration also of preferential pathways accounts for the evolution of the measured breakthrough curve and measured spatial concentration profiles. The model uses non-Fickian transport parameters estimated from the conservative tracer and, as a starting point, adsorption/desorption parameters based on batch experiments and a precipitation parameter based on Ksp values. The batch parameters are found to underestimate the actual amount of adsorption. We suggest that the sorption and precipitation/dissolution dynamics, and resulting breakthrough curves, are influenced strongly by preferential pathways; such pathways significantly alter the availability of sorption sites and ion availability for precipitation. Analysis of these results provides further understanding of the interaction and dynamics among transport, precipitation, and sorption mechanisms in natural soil.

We consider modeling approaches to characterize solute transport in porous media, integrating them into a unique theoretical and experimental framework for model evaluation and data interpretation. To date, development of (conservative and reactive chemical) transport models and formulation of model calibration methods grounded on sensitivity-based collection of measurements have been pursued in parallel. Key questions that remain include: For a given set of measurements, which conceptual picture of the transport processes, as embodied in a mathematical model or models, is most appropriate? What are the most valuable space-time locations for solute concentration measurements, depending on the model selected? How is model parameter uncertainty propagated to model output, and how does this propagation affect model calibration? We address these questions by merging parallel streams of research—model formulation, reduction, calibration, sensitivity analysis, and discrimination—offering our view on an emerging framework that guides (i) selection of an appropriate number and location of time-dependent concentration measurements given a transport model and (ii) assessment (through discrimination criteria) of the relative benefit of applying any particular model from a set of several models. Our strategy is to employ metrics to quantify the relative contribution of each uncertain model parameter to the variability of the model output. We evaluate these metrics through construction of a surrogate (or “meta”) transport model that has the additional benefit of enabling sensitivity analysis and model calibration at a highly reduced computational cost. We demonstrate the applicability of this framework, focusing on transport of reactive chemicals in laboratory-scale porous media.

This study reports the transport characteristics of the organometallic anticancer compound oxaliplatin and its derivatives in natural soil-water environments. Although pharmaceuticals and their derivatives have for many years been detected in water resources, and linked to toxicological impacts on ecological systems, their transport in soil and groundwater is not fully understood. Specifically, studies that describe transport of organometallic pharmaceuticals in porous media are rare, and the transport characteristics of platinum complexes have received little attention. Oxaliplatin transport was studied in sand, as a function of two added natural chelators (citrate and humic acid), and in soil, under four continuously monitored, environmentally-relevant redox conditions: oxic, nitrate reducing, iron reducing and methanogenic. In sand, oxaliplatin species retention was about 7%, and affected only mildly by added citrate, and by humic acid under buffered pH. Transport with unbuffered humic acid was affected significantly by pH variations, and exhibited strong retention at pH

We investigate the effects of high fluid velocities on flow and tracer transport in heterogeneous porous media. We simulate fluid flow and advective transport through two-dimensional pore-scale matrices with varying structural complexity. As the Reynolds number increases, the flow regime transitions from linear to nonlinear; this behavior is controlled by the medium structure, where higher complexity amplifies inertial effects. The result is, nonintuitively, increased homogenization of the flow field, which leads in the context of conservative chemical transport to less anomalous behavior. We quantify the transport patterns via a continuous time random walk, using the spatial distribution of the kinetic energy within the fluid as a characteristic measure.

International audience; We present a combined experimental and numerical modeling study that addresses two principal questions: (i) is any particular Eulerian-based method used to solve the classical advection-dispersion equation (ADE) clearly superior (relative to the others), in terms of yielding solutions that reproduce BTCs of the kind that are typically sampled at the outlet of a laboratory cell? and (ii) in the presence of matches of comparable quality against such BTCs, do any of these methods render different (or similar) numerical BTCs at locations within the domain? To address these questions, we obtained measurements from carefully controlled laboratory experiments, and employ them as a reference against which numerical results are benchmarked and compared. The experiments measure solute transport breakthrough curves (BTCs) through a square domain containing various configurations of coarse, medium, and fine quartz sand. The approaches to solve the ADE involve Eulerian-Lagrangian and Eulerian (finite volume, finite elements, mixed and discontinuous finite elements) numerical methods. Model calibration is not examined; permeability and porosity of each sand were determined previously through separate, standard laboratory tests, while dispersivities are assigned values proportional to mean grain size. We find that the spatial discretization of the flow field is of critical importance, due to the non-uniformity of the domain. Although simulated BTCs at the system outlet are observed to be very similar for these various numerical methods, computed local (point-wise, inside the domain) BTCs can be very different. We find that none of the numerical methods is able to fully reproduce the measured BTCs. The impact of model parameter uncertainty on the calculated BTCs is characterized through a set of numerical Monte Carlo simulations; in cases where the impact is significant, assessment of simulation matches to the experimental data can be ambiguous.

A nonlocal-in-time integro-differential equation is introduced that accounts for close coupling between transport and chemical reaction terms. The structure of the equation contains these terms in a single convolution with a memory function M (t), which includes the source of non-Fickian (anomalous) behavior, within the framework of a continuous time random walk (CTRW). The interaction is non-linear and second-order, relevant for a bimolecular reaction A + B → C. The interaction term ΓP A (s, t) P B (s, t) is symmetric in the concentrations of A and B (i.e. P A and P B ); thus the source terms in the equations for A, B and C are similar, but with a change in sign for that of C. Here, the chemical rate coefficient, Γ, is constant. The fully coupled equations are solved numerically using a finite element method (FEM) with a judicious representation of M (t) that eschews the need for the entire time history, instead using only values at the former time step. To begin to validate the equations, the FEM solution is compared, in lieu of experimental data, to a particle tracking method (CTRW-PT); the results from the two approaches, particularly for the C profiles, are in agreement. The FEM solution, for a range of initial and boundary conditions, can provide a good model for reactive transport in disordered media.

As a consequence of their growing use in electronic and industrial products, increasing amounts of technology critical elements (TCEs) are being released to the environment. Currently little is known about the fate of many of these elements. Initial research on their potential environmental impact identifies TCEs as emerging contaminants. TCE movement in the environment is often governed by water systems. Research on "natural" waters so far demonstrates that TCEs tend to be associated with suspended particulate matter (SPM), which influences TCE aqueous concentrations (here: concentration of TCEs in dissolved form and attached to SPM) and transport. However, the relative potential of different types of SPM to interact with TCEs is unknown. Here we examine the potential of various types of particulate matter, namely different nanoparticles (NPs; Al2O3, SiO2, CeO2, ZnO, montmorillonite, Ag, Au and carbon dots) and humic acid (HA), to impact TCE aqueous concentrations in aqueous solutions with soil and sand, and thus influence TCE transport in soil-water environments. We show that a combination of NPs and HA, and not NPs or HA individually, increases the aqueous concentrations of TCEs in soil solutions, for all tested NPs regardless of their type. TCEs retained on SPM, however, settle with time. In solutions with sand, HA alone is as influential as NPs+HA in keeping TCEs in the aqueous phase. Among NPs, Ag-NPs and Au-NPs demonstrate the highest potential for TCE transport. These results suggest that in natural soil-water environments, once TCEs are retained by soil, their partitioning to the aqueous phase by through-flowing water is unlikely. However, if TCEs are introduced to soil-water environments as part of solutions rich in NPs and HA, it is likely that NP and HA combinations can increase TCE stability in the aqueous phase and prevent their retention on soil and sand, thus facilitating TCE transport.

Environmental contextTechnology-critical elements, widely used in modern industry, are found in the environment as a result of both anthropogenic usage and natural sources. This review describes current knowledge on the transport of technology-critical elements in sand, soils and aquifer environments. The chemical compositions of the soils and groundwaters influence the transport of technology-critical elements, and natural colloids increase their mobility. AbstractTechnology-critical elements (TCEs) are now present in soil and aquifer environments, as a result not only of the geogenic origin but also of the recent anthropogenic activities and release. TCEs can interact with all components of the soil and water, which include inorganic and organic ligands (natural organic matter), clays, mineral surfaces and microorganisms. The literature regarding the transport and fate of TCEs in subsurface porous media (e.g. soil and aquifers) is limited and highly diverse. This review offers a detailed analysis of the existing literature on the transport and fate of TCEs in porous media, and emphasises what is still needed to fully understand their behaviour in the environment. Different modes of TCE transport are presented. First, the mobility of TCEs following interaction with colloids (e.g. natural organic matter, clays) is described. For these cases, an increase in the ionic strength and pH of aqueous solutions shows stronger retention or sorption of TCEs on porous matrices. The transport of nanoparticles (NPs) that contain TCEs is presented as a second mode of mobility. The ionic strength of the solution is the key parameter that controls the transport of cerium nanoparticles in porous media; natural organic matter also increases the mobility of nanoparticles. The third part of this review describes sorption and dissolution processes during transport. Finally, results from the field experiments are reported, which show that rare earth elements and indium are transported in the presence of natural organic matter. We conclude this review with suggested directions for future research.

The continuous time random walk (CTRW) has both an elegant mathematical theory and a successful record at modeling solute transport in the subsurface. However, there are some interpretation ambiguities relating to the relationship between the discrete CTRW transition distributions and the underlying continuous movement of solute that have not been addressed in existing literature. These include the exact definition of “transition”, and the extent to which transition probability distributions are unique/quantifiable from data. Here, we present some theoretical results which address these uncertainties in systems with an advective bias. Simultaneously, we present an alternative, reduced parameter CTRW formulation for general advective transport in heterogeneous porous media, which models early- and late-time transport by use of random transition times between sparse, imaginary planes normal to flow. We show that even in the context of this reduced-parameter formulation there is nonuniqueness in the definitions of both transition lengths and waiting time distributions, and that neither may be uniquely determined from experimental data. For practical use of this formulation, we suggest Pareto transition time distributions, leading to a two-degree-of-freedom modeling approach. We then demonstrate the power of this approach in fitting two sets of existing experimental data. While the primary focus is the presentation of new results, the discussion is designed to be pedagogical and to provide a good entry point into practical modeling of solute transport with the CTRW.

Both Eulerian and Lagrangian reactive transport simulations in natural media require selection of a parameter that controls the “promiscuity” of the reacting particles. In Eulerian models, measurement of this parameter may be difficult because its value will generally differ between natural (diffusion-limited) systems and batch experiments, even though both are modeled by reaction terms of the same form. And in Lagrangian models, there previously has been no a priori way to compute this parameter. In both cases, then, selection is typically done by calibration, or ad hoc. This paper addresses the parameter selection problem for Fickian transport by deriving, from first principles and D (the diffusion constant) the reaction-rate-controlling parameters for particle tracking (PT) codes and for the diffusion–reaction equation (DRE). Using continuous time random walk analysis, exact reaction probabilities are derived for pairs of potentially reactive particles based on D and their probability of reaction provided that they collocate. Simultaneously, a second PT scheme directly employing collocation probabilities is derived. One-to-one correspondence between each of D, the reaction radius specified for a PT scheme, and the DRE decay constant are then developed. These results serve to ground reactive transport simulations in their underlying thermodynamics, and are confirmed by simulations.

The catalytic degradation of two brominated flame retardants (BFRs), tribromoneopentyl alcohol (TBNPA) and 2,4 dibromophenol (2,4-DBP) by copper oxide nanoparticles (nCuO) was investigated. The degradation kinetics, the debromination, and the formation of intermediates by nCuO catalysis were also compared to Fenton oxidation and nano zero-valent iron (nZVI) reduction methods. BFRs have been added to various products like plastic, textile, electronics and synthetic polymers at growing rates. In spite of the clear advantages of reducing fire damages, many of these BFRs may be released to the environment after their beneficial use and become contaminants. The two studied BFRs were fully degraded with sufficient time (hours to days) and oxidation agent (H2O2). Shorter reaction times showed differences in reaction pathway and kinetics. The 2,4-DBP showed faster degradation than TBNPA, by nCuO catalysis. Relatively high resistance to degradation was recorded for 2,4-DBP with nZVI, yielding 20% degradation after 24h, while the TBNPA was degraded by 85% within 12h. Electron Spin Resonance (ESR) measurements show generation of both hydroxyl and superoxide radicals. In addition, inhibition of 2,4-DBP degradation in the presence of spin traps implies a radical degradation mechanism. A catalytic mechanism for radical generation and BFR degradation by nCuO is proposed. It is further suggested that H2O2 plays an essential role in the activation of the catalyst.

[1] We apply a general strategy based on Global Sensitivity Analysis (GSA) and model discrimination criteria to (a) calibrate the parameters embedded in competing models employed to interpret laboratory-scale tracer experiments, (b) rank these models, and (c) estimate the relative degree of likelihood of each model through a posterior probability weight. We consider a conservative transport experiment in a uniform porous medium. We apply GSA to three transport models, based on: the classical advection-dispersion equation (ADE), a dual-porosity (DP) formulation with mass transfer between mobile and immobile regions, and the Continuous Time Random Walk (CTRW) approach. GSA is performed through Polynomial Chaos Expansion of the governing equations, treating key model parameters as independent random variables. We show how this approach allows identification of (a) the relative importance of model-dependent parameters, and (b) the space-time locations, where the models are most sensitive to these parameters. GSA is then employed to assist parameter estimates within a Maximum Likelihood framework. Finally, formal model identification criteria are employed to (a) rank the alternative models, and (b) associate each model with a posterior probability weight for the specific case study. The GSA-based calibration of each model returns an acceptable approximation (remarkably accurate in the case of the CTRW model) of all available concentration data, with calibration being performed using minimum sets of observations corresponding to the most sensitive (space-time) locations.

In recent years the behavior and properties of nanoparticles released to the environment have been studied extensively to better assess the potential consequences of their broad use in commercial products. The fate, transport and mobility of nanoparticles in soil were shown to be strongly dependent on environmental conditions. However, little is known about the possible effects of nanoparticles on soil chemical, physical and biological properties. In this study, two types of metal oxide nanoparticles, CuO and Fe3O4 were mixed into two types of soil and the effects of the nanoparticles on various soil properties were assessed. Metal oxide nanoparticles were shown previously to catalyze the oxidation of organic pollutants in aqueous suspensions, and they were therefore expected to induce changes in the organic material in the soil, especially upon addition of an oxidant. It was found that the nanoparticles did not change the total amount of organic materials in the soil or the total organic carbon in the soil extract; however, three-dimensional fluorescence spectroscopy demonstrated changes in humic substances. The nanoparticles also affected the soil bacterial community composition, based on denaturing gradient gel electrophoresis (DGGE) fingerprinting, but had little impact on the macroscopic properties of the soil.

We review the analysis of the dynamics of reactive transport in disordered media, emphasizing the nature of the chemical reactions and the role of small-scale fluctuations induced by the structure of the porous medium. We are motivated by results and interpretations of laboratory-scale experiments, for which detailed characterization of the system is possible. Modeling approaches based on continuum and particle tracking (PT) schemes are examined critically, highlighting how fluctuations are incorporated. The continuum approach spans a large literature. Traditional formats of reactive transport equations, such as the advection–dispersion–reaction equation (ADRE), are based on a series of assumptions related mainly to scale separation and relative magnitude of time scales involved in the reactive transport setting. These assumptions as well as further developments are assessed in depth. PT methods offer an alternative means of accounting for pore-scale dynamics, wherein space–time transitions are drawn from appropriate probability distributions that have been tested to account for anomalous transport. While PT methods have been employed for many years to describe conservative transport, their application to laboratory-scale reactive transport problems in the context of both Fickian and non-Fickian regimes is relatively recent. We concentrate on experimental observations of different types of reactions in disordered media: (1) the dynamics of a bimolecular reactive transport (A + B → C) in passive (non-reactive) media, and (2) a multi-step chemical reaction, as exemplified in the process of dedolomitization involving both dissolution and precipitation. The fluctuations in a number of the key variables controlling the processes prove to have a dominant role; elucidation of this role forms the basis of the present study and the comparison of methods.

Author Brian Berkowitz answers questions about his recently published article and the scientific and societal implications of his findings.

Endocrine disrupting chemicals (EDCs) are detected in environmental matrices such as surface water, wastewater, groundwater, and drinking water in the ng/L range. For environmental sediments, values in the range of ng/g and even lower are reported. These almost omnipresent low concentrations of estrogens are of major concern, in particular as sorption parameters derived from laboratory investigations tend to imply the absence of estrogens in the natural environment. Very few studies consider the fate and transport of estrogens in column tests at environmentally relevant concentrations, including metabolite formation and sorption. Also, the effect of input concentration on hormone degradation is unclear. To close this gap, the estrogens 17s-estradiol (E2), estrone (E1), and the conjugated estrogen estrone-sulfate (E1-3S) were each injected into saturated, packed soil columns as pulses with subsequent analysis of soil and aqueous samples. The transformation pathway E2 to E1 and subsequently to E1-3S was verified in sandy clay loam soil from Bet Dagan, Israel. Furthermore, it was shown that (i) E2 is rapidly degraded in soil and transformed in soil to E1; (ii) E1 is subject to strong retardation by sorption; and (iii) E1-3S behaves almost conservatively and shows the lowest sorption and transformation potential.