Conservative Solute Transport Research Articles

Can Geometric Parameters Enable Direct Prediction of Non‐Fickian Transport in Rock Fractures Across Diverse Flow Regimes?

AbstractAnomalous solute migrations in fractured rocks are governed by geometric characteristics and flow regimes. Although existing inverse models can describe this behavior, the underlying physics for quantifying key transport coefficients remains largely unexplored. Here, we investigate the quantitative impacts of geometric heterogeneity and flow regimes on solute transport in rock fractures. We conduct numerical experiments to simulate water flow and conservative solute transport in 3D fractures with varying geometric features and Reynolds numbers. Our results show that the non‐Fickian transport is prevalent across the entire flow regime, with Darcy flows attributed to geometric heterogeneity and non‐Darcian flows influenced by additional eddy zones. We employ the mobile‐immobile (MIM) domain model and continuous time random walk (CTRW) model to inversely model simulated breakthrough curves. Inverse analyses demonstrate that both models effectively characterize anomalous transport behaviors. The fitted transport coefficients of the MIM model exhibit stronger quantitative relationships with aperture and roughness parameters, as well as Reynolds number, compared to the CTRW model. By incorporating parameterized transport coefficients, we propose physics‐ and statistics‐based models to directly predict anomalous transport behaviors under different flow regimes. These prediction models accurately reproduce solute transport processes of all simulated cases with acceptable errors. The feasibility of directly predicting solute transport under varying flow regimes using geometric information is thus validated. Our study not only supports the study of substance migration based on geometric structure features, but also serves as a foundation for investigating geological activities based on substance migration information.

Assessing transport timescales in Lake Huron's Hammond Bay: The crucial role of the Straits of Mackinac's exchange flows

The oscillating bidirectional exchange flows between Lakes Michigan and Huron in the Straits of Mackinac generate complex hydrodynamics and the exchange flows are known to alter hydrodynamics in regions as far down as 50–60 km from the Straits modulating physical, chemical, and biological processes in the region. Although previous research examined the effects of exchange flows on hydrodynamics, their impacts on transport time scales, including residence and flushing times, have not been quantified. We used observations and a three-dimensional hydrodynamic model to simulate bidirectional exchange flows in the Straits and their effects on hydrodynamics, temperature, and transport timescales in the Hammond Bay area, Lake Huron for the summers of 2018 and 2019. Comparisons with field observations showed that hydrodynamics can only be accurately described when the bidirectional flows are included in the modeling of the bays close to the Straits. Spectral analysis showed that the exchange flows play an important role in controlling conservative solute transport in bays close to the Straits. The residence time in the Hammond Bay area was calculated using a dye release approach with (without) the effects of bidirectional exchange flows producing estimates of 9.87 (16.00) and 13.75 (23.62) days for years 2018 and 2019 respectively based on a combined model of the two lakes and a model of Lake Huron only. Similarly, flushing times in the Hammond Bay area were estimated as 12.14 (14.38) and 8.96 (10.80) days for 2018 and 2019, respectively with (without) the exchange flows. Ignoring the exchange flows in the Straits was found to overestimate the residence time and flushing time in the Hammond Bay area by roughly 74 and 20 %, respectively. These results highlight the importance of including the bidirectional exchange flows in biophysical models of bays in Lake Huron closer to the Straits and in similar systems elsewhere.

MuFlowReacT: A Library to Solve Multiphase Multicomponent Reactive Transport on Unstructured Meshes.

In this paper we present a new reactive transport code for the efficient simulation of groundwater quality problems. The new code couples the two previously existing tools OpenFoam and PhreeqcRM. The major objective of the development was to transfer and expand the capabilities of the MODFLOW/MT3DMS-family of codes, especially their outstanding ability to suppress numerical dispersion, to a versatile and computationally efficient code for unstructured grids. Owing to the numerous, previously existing transport solvers contained in OpenFoam, the newly developed code achieves this objective and provides a solid basis for future expansions of the code capabilities. The flexibility of the OpenFoam framework is illustrated by the addition of diffusional processes for gaseous compounds in the unsaturated zone and the advection of gases (multiphase transport). The code capabilities and accuracy are illustrated through several examples: (1) a simple 2D case for conservative solute transport under saturated conditions, (2) a gas diffusion case with reactions in the unsaturated zone, (3) a hydrogeologically complex 3D reactive transport problem, and finally (4) the injection of CO2 into a deep aquifer with acidification being buffered by carbonate minerals.

Quantifying Mixing in Sewer Networks for Source Localization

There has been a recent increase of interest in sewer network water quality, both for pollutants and wastewater epidemiology. Of particular interest is the ability to perform cost-effective small-scale monitoring to understand the sewer network and perform source localization (the process of identifying the sources of materials of interest within the network), enabling prioritization of combined sewer overflow (CSO) interventions and targeted response to the detection of infectious diseases. Rhodamine WT fluorescent dye tracing was carried out in the combined sewer networks of four UK cities, for which network geometries were available. Over 100 dye concentration profiles were recorded, from which discharge, travel time (velocity), and dispersion were quantified. A simplified hydraulic and water quality (conservative solute transport) modeling approach was used to investigate dispersion further. A theoretical method for calculating dispersion over a reach with nonuniform properties was derived and used with the models and recorded data to develop a method for estimating the dispersion coefficient in sewers. Novel simultaneous injections into multiple manholes within one sewer network were conducted. Modeling of these injections validated the modeling approach and explained the measured concentration profiles, demonstrating both the potential of hydraulic and solute transport modeling and the new dispersion coefficient predictor for source localization. Such modeling can be used to develop sewer network “fingerprints” and source location probability plots based on residence time distribution (RTD) theory to maximize information from limited water quality monitoring. This will aid managers and operators in identifying potential intermittent sources of material within the network.

Upscaling dispersivity for conservative solute transport in naturally fractured media

Physical heterogeneities are prevalent features of fracture systems and significantly impact transport processes in aquifers across different spatiotemporal scales. Upscaling solute transport parameter is an effective way of quantifying parameter variability in heterogeneous aquifers including fractured media. This paper develops conceptual models for upscaling conservative transport parameters in fracture media. The focus is on upscaling dispersivity. Lagrangian-based transport model (LBTM) for dispersivity upscaling are derived for the solute transport in two-dimensional fractures surrounded by an impermeable matrix. The LBTM is validated against the random walk particle tracking (RWPT) model, which enables highly efficient and accurate predictions of conservative solute transport. The results show that the derived scale-dependent analytical expressions are in excellent agreement with RWPT model results. In addition, LBTM results are also compared to experimental results from the observed breakthrough curve of a conservative solute transport through a single natural fracture within a granite core. Comparing results from the LBTM and transport experiment shows that LBTM based estimated dispersivity is 10.55% higher than the measured value. Errors introduced by the experiments, the conceptual assumptions in deriving models, and the heterogeneities of fracture apertures not fully sampled by measuring instruments are main factor for such discrepancy. The sensitivity analysis indicates that the longitudinal and transverse dispersivities are positively related to the integral scale and the variance of the log-fracture aperture. The longitudinal dispersivity is strongly contolled by the variance of the log-fracture aperture. The LBTM may be useful for directly predicting solute transports, requiring only the acquisition of fractured geostatistical data. This work provides a better understanding of transport processes in fractured media which ultimately control water quality across scales.

Effect of Pore‐Wall Roughness and Péclet Number on Conservative Solute Transport in Saturated Porous Media

AbstractWhile modeling solute transport has been an active subject of research in the past few decades, the influence of pore‐wall roughness on contaminant migration has not yet been addressed. We therefore conduct particle tracking simulations in three porous domains that have different pore‐wall roughness characteristics. Specifically, we consider five surface fractal dimensions ds = 1.0, 1.1, 1.2, 1.4, and 1.6, and four different Péclet numbers Pe = 10, 102, 103, and 105. Overall, arrival time distributions are simulated for 60 scenarios (3 domains 5 surface fractal dimensions 4 Péclet numbers) some of which show heavy‐tailed patterns indicating non‐Fickian transport. To interpret the simulations and quantify the transport behavior, we analyze the resulting arrival time distributions by the continuous time random walk (CTRW) approach. Results show that, on average, as the surface fractal dimension increases from 1.0 to 1.6, the CTRW model parameters , an exponent showing the degree of anomalous transport, v, the average solute velocity, and t2, the cut‐off time to Fickian transport, remain nearly constant. However, the dispersion coefficient, D, increases and the characteristic transition time, t1, decreases. We found t1 and D are more sensitive to pore‐wall roughness compared to the other CTRW parameters. We also found that as the Péclet number increases from 10 to 105, on average, v and D increase, t1 and decrease, and t2 remains nearly constant. The simulations demonstrate that the exponent and the dispersion coefficient are correlated to the average solute velocity.

Macrodispersion in generalized sub-Gaussian randomly heterogeneous porous media

• Features of log-conductivity, Y , fields are captured by a non-Gaussian (GSG) model. • Expressions of flow and transport moments in three-dimensional GSG fields are derived. • Head and velocity fields are more correlated in GSG than in Gaussian Y structures. • Non-Gaussianity of Y heavily influences pre-asymptotic macrodispersion values. • Late time transport exhibits a Fickian behavior also in non-Gaussian Y fields. In this work, we explore the implications of modeling the logarithm of hydraulic conductivity, Y , as a Generalized Sub-Gaussian (GSG) field on the features of conservative solute transport in randomly heterogeneous, three-dimensional porous media. Hydro-geological properties are often viewed as Gaussian random fields. Nevertheless, the GSG model enables us to capture documented non-Gaussian traits that are not explained through classical Gaussian models. Our formulation yields lead- (or first-) order analytical solutions for key statistical moments of flow and transport variables. These include flow velocities, hydraulic head, and macrodispersion coefficients, as obtained across GSG log-conductivity fields. The analytical model is based on a first-order spectral theory, which constrains the rigorous validity of our results to small values of log-conductivity variance ( σ Y 2 < < 1 ). Analytical results are then compared against detailed numerical estimates obtained through a Monte Carlo scheme encompassing various levels of domain heterogeneity. An asymptotic Fickian transport regime is attained at late times in both Gaussian and GSG Y fields. Convergence to such regime is slower for GSG as compared to Gaussian fields. This suggests a strong impact of the heterogeneity structure on non-Fickian pre-asymptotic behaviors of the kind documented in the literature. The quality of the comparison between analytical and numerical results deteriorates with increasing σ Y 2 . Otherwise, our lead-order solutions frame macrodispersion coefficients in appropriate orders of magnitude also for values of σ Y 2 up to approximately 1.7, which are consistent with the spatial variability of Y across a single geological unit. In this sense, our analytical approach enables one to obtain prior information on solute plume evolution and to grasp the effects of non-Gaussian medium heterogeneity while favoring simplicity. Our findings also enhance the current level of understanding of the nature of mass transfer across heterogeneous media characterized by complex variability structures which cannot be reconciled with classical Gaussian scenarios.

Storage and release of conservative solute between karst conduit and fissures using a laboratory analog

The contaminant transport processes in karst water systems have a direct impact on the quality and utilization of karst water resources. The storage and release of contaminants or conservative solutes during the solute transport process is a common phenomenon in karst aquifers. The impact of the storage and release is more prevalent after focused recharge events. In this study, laboratory experimental and numerical studies were conducted to quantify the storage and release processes of conservative solute. The results showed that, conduit water recharges into fissures under high water head conditions, and the fissure water drains back into the conduit while the hydraulic gradient reverse. The conservative solute storage-release process controlled by hydrodynamic conditions produces strong asymmetry, long tailing, or bi-peak in the breakthrough curves (BTCs). The BTCs change from single peak to bi-peak with enhanced hydrodynamics under focused recharge conditions. The dual heterogeneous domain model was calibrated to simulate the long-tailing of the BTCs and their noticeably bimodal characteristics. The flow velocity and dispersion coefficient are the major variables that regulate the bimodal structure of the BTCs, which also control the solute storage-release differences between the conduit and fissures. The bimodal structure of the BTCs becomes more pronounced for large discrepancies in flow velocities. The total BTCs are a superposition of solute transport in the conduit path and storage-release path. A method to evaluate the mass of conservative solute transport in storage-release paths was proposed by segmenting the transport curve in the conduit from the total BTCs, thus quantifying the effects of the groundwater storage-release mechanism on the solute transport process in the karst water system.

Conservative solute transport processes and associated transient storage mechanisms: Comparing streams with contrasting channel morphologies, land use and land cover

AbstractLand use within a watershed impacts stream channel morphology and hydrology and, therefore, in‐stream solute transport processes and associated transient storage mechanisms. This study evaluated transport processes in two contrasting stream sites where channel morphology was influenced by the surrounding land use, land cover, climate and geologic controls: Como Creek, CO, a relatively undisturbed, high gradient, forested stream with a gravel bed and complex channel morphology, and Clear Creek, IA, an incised, low‐gradient stream with low‐permeability substrate draining an agricultural landscape. We performed conservative stream tracer injections at these sites to address the following questions: (1) How does solute transport vary between streams with differing morphologies? and (2) How does solute transport at each stream site change as a function of discharge? We analysed in‐stream tracer time series data and compared results quantifying solute attenuation in surface and subsurface transient storage zones. Significant trends were observed in these metrics with varying discharge conditions at the forested site but not at the agricultural site. There was a broad range of transport mechanisms and evidence of substantial exchange with both surface and hyporheic transient storage in the relatively undisturbed, forested stream. Changing discharge conditions activated or deactivated different solute transport mechanisms in the forested site and greatly impacted advective travel time. Conversely in the simplified agricultural stream, there was a narrow range of solute transport behaviour across flows and predominantly surface transient storage at all measured discharge conditions. These results demonstrate how channel simplification inhibits available solute transport mechanisms across varying discharge conditions.

Contributions of biofilm-induced flow heterogeneities to solute retention and anomalous transport features in porous media

Microbial biofilms are ubiquitous within porous media and the dynamics of their growth influence surface and subsurface flow patterns which impacts the physical properties of porous media and large-scale transport of solutes. A two-dimensional pore-scale numerical model was used to evaluate the impact of biofilm-induced flow heterogeneities on conservative transport. Our study integrates experimental biofilm images of Paenibacillus 300A strain in a microfluidic device packed with cylindrical grains in a hexagonal distribution, with mathematical modeling. Biofilm is represented as a synthetic porous structure with locally varying physical properties that honors the impact of biofilm on the porous medium. We find that biofilm plays a major role in shaping the observed conservative transport dynamics by enhancing anomalous characteristics. More specifically, when biofilm is present, the pore structure in our geometry becomes more spatially correlated. We observe intermittent behavior in the Lagrangian velocities that switches between fast transport periods and long trapping events. Our results suggest that intermittency enhances solute spreading in breakthrough curves which exhibit extreme anomalous slope at intermediate times and very marked late solute arrival due to solute retention. The efficiency of solute retention by the biofilm is controlled by a transport regime which can extend the tailing in the concentration breakthrough curves. These results indicate that solute retention by the biofilm exerts a strong control on conservative solute transport at pore-scale, a role that to date has not received enough attention.

A finite particle method (FPM) for Lagrangian simulation of conservative solute transport in heterogeneous porous media

When a smoothed particle hydrodynamics (SPH) method, a Lagrangian and meshfree numerical scheme, is used to solve the advection-dispersion equation (ADE), SPH solutions may not be accurate when particles are irregularly distributed, i.e., the distance between two neighbor particles varies irregularly in a simulation domain. Particle irregularity may be caused by nonuniform groundwater flow in a heterogeneous field of hydraulic conductivity. This study explores for the first time whether the Finite Particle Method (FPM) can provide more accurate ADE solutions than SPH does for irregularly distributed particles. FPM is similar to SPH in theory, but uses a modified kernel gradient to construct a SPH approximation of solute concentration gradients. Performance of SPH and FPM with irregularly distributed particles is evaluated by using two numerical cases. The first case considers only diffusive transport, and has an analytical solution for the evaluation. The second case considers both advection and dispersion, and uses a numerical solution as a reference for the evaluation. For each of the two cases, several numerical experiments are conducted using multiple sets of irregularly distributed particles with different levels of particle irregularity due to different levels of heterogeneity of hydraulic conductivity. Numerical results indicate that, for the numerical experiments of this study, FPM outperforms SPH to yield more accurate ADE solutions. However, FPM solutions are still subject to numerical errors, and the errors increase when the level of heterogeneity of hydraulic conductivity increases. Further improvement of FPM is warranted in a future study.

Modeling Reactive Solute Transport in Permafrost‐Affected Groundwater Systems

AbstractUnderstanding the interactions between ground freeze‐thaw, groundwater flow, and solute transport is imperative for evaluating the fate of contaminants in permafrost regions. However, predicting solute migration in permafrost‐affected groundwater systems is challenging due to the inherent interactions and coupling between subsurface mass and energy transport processes. To this end, we developed a numerical model that considers coupled groundwater flow, subsurface heat transfer, and solute transport, including water‐ice phase change, solute‐dependent porewater freezing, and temperature‐dependent solute reaction rates. As an illustrative example, we present simulations to investigate the potential for contamination from a municipal wastewater lagoon in the Canadian sub‐arctic. Two‐dimensional groundwater models assuming varying permafrost conditions were developed to evaluate possible contaminant migration scenarios associated with groundwater flow from the lagoon to a river, including the transport of conservative, degradable, and sorbing solutes. Model results reveal important transport mechanisms controlling the behavior of aqueous contaminants in permafrost landscapes as well as the hydrogeologic factors affecting reactive transport in cold regions. Seasonal freeze‐thaw episodically restricts connectivity of transport pathways, which attenuates both transport and reaction rates. However, elevated solute concentrations can depress the freezing temperature of porewater and produce thaw‐induced solute transport. Both thermally driven and solute‐enhanced thaw can decrease ice content in permafrost, which can have significant implications for solute migration. Therefore, thermo‐hydrogeologic process controlling reactive transport in cold‐region groundwater systems must be considered when quantitatively assessing the impact of changing environmental conditions on contaminant hydrogeology.

Analytical solution of two-dimensional conservative solute transport in a heterogeneous porous medium for varying input point source

In the present study, an analytical solution is obtained for two-dimensional advection–dispersion equation with variable coefficients in a semi-infinite heterogeneous porous medium. Dispersion coefficient and groundwater velocity are assumed to vary temporally as well as spatially while the retardation factor varies spatially only. The first-order decay and zero-order production terms are also considered into account. The variations in parameters along both longitudinal and transverse directions are of exponentially decreasing nature. Initially the geological formation is considered not solute free. The nature of pollutant is considered of conservative and is assumed to be originating from time-dependent varying point source. New time and space variables are introduced to convert the variable coefficients of the advection–dispersion equation into constant coefficients. Analytical solution of the proposed model is obtained by employing Laplace integral transformation technique (LITT). The proposed analytical solution is illustrated graphically to study the effects of various parameters on the solute transport for temporally exponentially decreasing and sinusoidal nature velocities.

Influence of the grain structure on solute transport in constructed inhomogeneous media from pore-scale simulations

Grain structures widely exist in porous media and play a significant role in solute transport in media. One homogeneous and three inhomogeneous grain structures were constructed, and the finite element method (FEM) was adopted to simulate conservative solute transport in the porous media. The velocity field and plume evolution were analysed. The breakthrough curves (BTCs) and residence time distributions (RTDs) were calculated, and the continuous time random walk (CTRW) inverse model was applied to fit the BTCs to reveal the influence of the grain structure on solute transport quantitatively. The flow velocity and plume evolution are homogeneously distributed in the monolayer homogeneous grain structure, but they are heterogeneous in multilayer inhomogeneous grain structures with non-Fickian characteristics. These results occur because the heterogeneity of the grain structure forms advantageous channels and the plume is divided into dominant branches with an “early arrival” character, which is consistent with the analysis of the BTCs. Conversely, the “long tail” characteristics occur because the main driving force is diffusion, which is consistent with the analysis of the RTDs. After further confirmation by the CTRW inverse model, it can be concluded that both the particle and layer thickness can influence solute transport. This paper proposed a new method for the construction of the grain structure and researched its influence on solute transport, which may improve our understanding of the mechanism of solute transport through inhomogeneous media at the pore scale.

Three-dimensional hydraulic tomography analysis of long-term municipal wellfield operations: Validation with synthetic flow and solute transport data

This study proposes the utilization of municipal well records as an alternative dataset for large-scale heterogeneity characterization of hydraulic conductivity (K) and specific storage (Ss) using hydraulic tomography (HT). To investigate the performance of HT and the feasibility of utilizing municipal well records, a three-dimensional aquifer/aquitard system is constructed and synthetic groundwater flow and solute transport experiments are conducted to generate data for inverse modeling and validation of results. In particular, we simultaneously calibrate four groundwater models with varying parameterization complexity using five datasets consisting of different time durations and periods. Calibration and validation results are qualitatively and quantitatively assessed to evaluate the performance of investigated models. The estimated K and Ss tomograms from different model cases are also validated through the simulation of independently conducted pumping tests and conservative solute transport. Our study reveals that: 1) the HT analysis of municipal well records is feasible and yields reliable heterogeneous K and Ss distributions where drawdown records are available; 2) accurate geological information is of critical importance when data density is low and should be incorporated for geostatistical inversions; 3) the estimated K and Ss tomograms from the geostatistical model with geological information are capable in providing robust predictions of both groundwater flow and solute transport. Overall, this synthetic study provides a general framework for large-scale heterogeneity characterization using HT through the interpretation of municipal well records, and provides guidance for applying this concept to field problems.

Liquid phase nonpoint source pollution dispersion through conveyance structures to sustainable urban drainage system within different land covers

Liquid-phase nonpoint source pollution dispersion and removal on sustainable urban drainage systems (SUDS) is an important issue for urban pollution mitigation which remains a challenge, as current researches mostly focus on pollutants removal by settling. Nevertheless, most of liquid-phase pollutants behave as dissolved substances on overland flow and, therefore, they cannot be trapped, but uptake by biological mechanisms and adsorbed by green infrastructure media. Hence, dispersion of dissolved pollutant is of great importance for liquid-phase pollution removal, as it also increases contact with underlying media in the SUDS. This paper addresses the liquid-phase pollutant dispersion on conveyance structures within different materials, using experimental and modelling analysis. Hydrodynamic dispersion and flow velocity were analysed separately, or conjoint, using dispersivity, as it is a key factor for porous solute transport and removal. Therefore, the effect of different covers on pavements draining to, or as part of, SUDS with very shallow runoff was investigated. Four scenarios were performed in triplicate to measure the flow velocity and the conservative solute transport across longitudinal section of flume (experimental indoors self-contained setup) using electrolyte tracer under different flow discharges (32–1813 ml s−1) with 0.8, 4.4 and 13.2% slopes. For one scenario, free water flow on a smooth surface was performed and results were used as control. For the three remaining scenarios: sand roughness, stone and synthetic grass covers were investigated. The ratio of the dispersion coefficient and flow velocity (i.e. dispersivity factor) was also determined and compared with control. Finally, data were analysed considering flow regimes, using the dimensionless Reynolds and Froude numbers. Results showed that surface covers caused reduction in the flow velocity, from 1.2 to 7.7 fold. However, dispersivity factor can be increased from 3 to nearly 10 orders of magnitude for the three scenarios, compared to control, due to the dual effect on hydrodynamic dispersion coefficient and flow velocity. Results here presented should be helpful to better understand dissolved non-point source pollution dispersion and how different land covers can effect pollutant removal.

Soil infiltration and conservative solute transport characteristics with different viscosity of hydroxypropyl methyl cellulose

AbstractHydroxypropyl methyl cellulose (HPMC), a potential soil improver with the ability to hold water, has practical applicability in alleviating soil and water nutrient loss. By using Cl− as tracer ions, we evaluated the effects of different HPMC viscosities on soil water infiltration characteristics and conservative solute migration characteristics. Hydroxypropyl methyl cellulose with viscosities of 2 × 104, 6 × 104, 10 × 104, and 20 × 104 mPa s−1 was selected. Experimental results revealed that the cumulative infiltration amount, wetting front migration distance, and infiltration rate decreased significantly with increased HPMC viscosity. The addition of HPMC delayed the soil solute transport process, and HPMC with a higher viscosity caused greater soil solute transport delay. Both the convection‐dispersion equation (CDE) and the two‐region model (TRM) accurately simulated solute transport, but the simulation accuracy of the TRM was higher. Results of parametric fitting based on the TRM indicated that the average pore water velocity, hydrodynamic dispersion coefficient, and dispersion degree decrease with increased HPMC viscosity. The average pore water velocity decreased by 13.6–28.2% relative to the control group (CG). Compared with the CG, the water content ratio in movable water of HPMC groups changed irregularly with an increase in viscosity. The mass exchange coefficient increased significantly compared with the CG, with an irregular trend. These data will be useful in the application and promotion of HPMC as a soil improver.

Influence of Streambed Heterogeneity on Hyporheic Flow and Sorptive Solute Transport

The subsurface region where river water and groundwater actively mix (the hyporheic zone) plays an important role in conservative and reactive solute transport along rivers. Deposits of high-conductivity (K) sediments along rivers can strongly control hyporheic processes by channeling flow along preferential flow paths wherever they intersect the channel boundary. Our goal is to understand how sediment heterogeneity influences conservative and sorptive solute transport within hyporheic zones containing high- and low-K sediment facies types. The sedimentary architecture of high-K facies is modeled using commonly observed characteristics (e.g., volume proportion and mean length), and their spatial connectivity is quantified to evaluate its effect on hyporheic mixing dynamics. Numerical simulations incorporate physical and chemical heterogeneity by representing spatial variability in both K and in the sediment sorption distribution coefficient ( K d ). Sediment heterogeneity significantly enhances hyporheic exchange and skews solute breakthrough behavior, while in homogeneous sediments, interfacial flux and solute transport are instead controlled by geomorphology and local-scale riverbed topographies. The hyporheic zone is compressed in sediments with high sorptive capacity, which limits solute interactions to only a small portion of the sedimentary architecture and thus increases retention. Our results have practical implications for groundwater quality, including remediation strategies for contaminants of emerging concern.

Upscaling transport of a sorbing solute in disordered non periodic porous domains

Spatial Markov random walk models (SMM) have been demonstrated to accurately predict conservative solute transport across a wide range of hydro-geological systems, with recent advances enabling the SMM to model systems with linear kinetic reactive processes. However, the proposed reactive SMM’s applicability is limited to systems that can be partitioned into a series of identical periodic cells where travel times across cells are highly correlated to the solute’s entrance position at the cell inlet. In real geologic settings, the spatial layout and size of grains varies through space, decorrelating the relationship between travel time and transverse position. Here, we generalize previous SMM implementations and implement a Bernoulli CTRW, where transport behavior can be captured in disordered and non-periodic porous media. We validate our upscaled model predictions with results from direct numerical simulation of transport in a 2D porous column that cannot be partitioned into identical periodic elements. We parameterize our model based on a subset of simulation statistics and explore how model accuracy changes due to our sampling method. This finding yields important insights for optimizing efficiency of the upscaled transport model parameterization and can guide field sampling of geological structures as well as multiscale investigation of laboratory observations.

Study on non-Fickian behavior for solute transport through porous media

ABSTRACT In this study, fractional advection-dispersion equation is used to model the conservative solute transport through porous media. Explicit finite difference method has been used to solve the spatial fractional advection dispersion equation. Observed concentration profiles of chloride tracer have been obtained through soil column experiments in the lab. Both advection-dispersion and spatial fractional advection dispersion models have been used to simulate observed concentration profile at various distances in the flow direction. Results indicate that the variation in the value of root mean square error is observed in the case of both homogeneous and heterogeneous porous media. Non-Fickian behavior is more in case of heterogeneous media as compared to homogeneous media. Finally, results indicate that the better simulation has been obtained from spatial fractional advection-dispersion equation model as compared to advection-dispersion equation model in case of heterogeneous porous media.