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Coastal wetlands in California are critically positioned at the interface between increasingly developed watersheds and the coastal ocean. These wetlands provide habitat for fish and wildlife, provide nutrients to surrounding coastal waters, and create recreational opportunities (Mitsch and Gosselink 1986). This report describes a circulation and transport model that is designed for tidal wetland circulation and mixing studies. Given the importance of wetland restoration projects to offset the impact of coastal development, wetland circulation and mixing models are of great utility for evaluating the advantages and disadvantages of various wetland restoration alternatives relative to issues such as bathing water quality, eutrophication, and sedimentation. An application of the model to a restored wetland and guidelines for its use are presented.

The effect of soil stratification was studied through numerical investigation based on the coupled model of solute transport in deformable unsaturated soil. The theoretical model implied two-way coupled excess pore pressure and soil deformation based on Biot's consolidation theory as well as a one-way coupled volatile pollutant concentration field developed from the advection-diffusion theory. Embedded in the model, the degree of saturation, fluid compressibility, self-weight of the soil matrix, porosity variance, longitudinal dispersion, and linear sorption were computed. Based on simulation results of a proposed three-layer landfill model using the finite element method, the multi-layer effects are discussed with regard to the hydraulic conductivity, shear modulus, degree of saturation, molecular diffusion coefficient, and thickness of each layer. Generally speaking, contaminants spread faster in a stratified field with a soft and highly permeable top layer; soil parameters of the top layer are more critical than the lower layers but controlling soil thicknesses will alter the results. This numerical investigation showed noticeable impacts of stratified soil properties on solute migration results, demonstrating the importance of correctly modeling layered soil instead of simply assuming the averaged properties across the soil profile.

Sea-level rise coupled with land subsidence from wetland drainage exposes increasingly large areas of coastal peatlands to seawater intrusion. Seawater contains high concentrations of sulfate (SO42−), which can alter the decomposition of organic matter thereby releasing organic and inorganic solutes from peat. In this study, a flow-through reactor system was used in order to examine the transport of SO42− through peat as well as its effect on solute release. Moderately-decomposed fen peat samples received input solutions with SO42− concentrations of 0, 100, 700, and 2,700 mg L−1; sample effluent was analyzed for a variety of geochemical parameters including dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and total dissolved nitrogen (TDN) as well as the concentrations of major cations and anions. The input solution remained anoxic throughout the experiment; however, no signs of a pronounced SO42− reduction were detected in the effluent. SO42− transport in the fen peat resembled non-reactive bromide (Br−) transport, indicating that in the absence of SO42− reduction the anion may be considered a conservative tracer. However, slightly elevated concentrations of DOC and TDN, associated with raised SO42− levels, suggest the minor desorption of organic acids through anion exchange. An increased solute release due to stimulated decomposition processes, including SO42− reduction, was observed for samples with acetate as an additional marine carbon source included in their input solution. The solute release of peats with different degrees of decomposition differed greatly under SO42−-enriched conditions where strongly-decomposed fen peat samples released the highest concentrations of DOC, DIC and TDN.

Transport of solutes in porous media at the laboratory scale is governed by an Advection Dispersion Equation (ADE). The advection is by the fluid velocity U and dispersion by DdL = UαdL, where the longitudinal dispersivity αdL is of the order of the pore size. Numerous data revealed that the longitudinal spreading of plumes at field scale is characterized by macrodispersivity αL, larger than αdL by orders of magnitude. This effect is attributed to heterogeneity of aquifers manifesting in the spatial variability of the logconductivity Y. Modeling Y as a stationary random field and for mean uniform flow (natural gradient), αL could be determined in an analytical form by a first order approximation in σY2 (variance of Y) of the flow and transport equations. Recently, models and numerical simulations for solving transport in highly heterogeneous aquifers (σY2>1), primarily in terms of the mass arrival (the breakthrough curve BTC), were advanced. In all cases ergodicity, which allows to exchange the unknown BTC with the ensemble mean, was assumed to prevail for large plumes, compared to the logconductivity integral scale. Besides, the various statistical parameters characterizing the logconductivity structure as well as the mean flow were assumed to be known deterministically. The present paper investigates the uncertainty of the non-ergodic BTC due to the finiteness of the plume size as well as due to the uncertainty of the various parameters on which the BTC depends. By the use of a simplified transport model we developed in the past (which led to accurate results for ergodic plumes), we were able to get simple results for the variance of the BTC. It depends in an analytical manner on the flow parameters as well as on the dimension of the initial plume relative to the integral scale of logconductivity covariance. The results were applied to the analysis of the uncertainty of the plume spatial distribution of the MADE transport experiment. This was achieved by using the latest, recent, analysis of the MADE aquifer conductivity data.

Microfluidic models are proving to be powerful systems to study fundamental processes in porous media, due to their ability to replicate topologically complex environments while allowing detailed, quantitative observations at the pore scale. Yet, while porous media such as living tissues, geological substrates, or industrial systems typically display a porosity that spans multiple scales, most microfluidic models to date are limited to a single porosity or a small range of pore sizes. Here, a novel microfluidic system with multiscale porosity is presented. By embedding polyacrylamide (PAAm) hydrogel structures through in-situ photopolymerization in a landscape of microfabricated polydimethylsiloxane (PDMS) pillars with varying spacing, micromodels with porosity spanning several orders of magnitude, from nanometers to millimeters are created. Experiments conducted at different porosity patterns demonstrate the potential of this approach to characterize fundamental and ubiquitous biological and geochemical transport processes in porous media. Accounting for multiscale porosity allows studies of the resulting heterogeneous fluid flow and concentration fields of transported chemicals, as well as the biological behaviors associated with this heterogeneity, such as bacterial chemotaxis. This approach brings laboratory studies of transport in porous media a step closer to their natural counterparts in the environment, industry, and medicine., (© 2024 The Authors. Advanced Science published by Wiley‐VCH GmbH.)

Purpose: The intervertebral disc being avascular depends on diffusion and load-based convection for essential nutrient supply and waste removal. There are no reliable methods to simultaneously investigate them in humans under natural loads. For the first time, present study aims to investigate this by strategically employing positional MRI and post-contrast studies in three physiological positions: supine, standing and post-standing recovery., Methods: A total of 100 healthy intervertebral discs from 20 volunteers were subjected to a serial post-contrast MR study after injecting 0.3 mmol/kg gadodiamide and T1-weighted MR images were obtained at 0, 2, 6, 12 and 24 h. At each time interval, images were obtained in three positions, i.e. supine, standing and post-standing recovery supine. The signal intensity values at endplate zone and nucleus pulposus were measured. Enhancement percentages were calculated and analysed comparing three positions., Results: During unloaded supine position, there was slow gradual increase in enhancement reaching peak at 6 h. When the subjects assumed standing position, there was immediate loss of enhancement at nucleus pulposus which resulted in reciprocal increase in enhancement at endplate zone (washout phenomenon). Interestingly, when subjects assumed the post-standing recovery position, the nucleus pulposus regained the enhancement and endplate zone showed reciprocal loss (pumping-in phenomenon)., Conclusions: For the first time, present study documented acute effects of physiological loading and unloading on nutrition of human discs in vivo. While during rest, solutes diffused gradually into disc, the diurnal short loading and unloading redistribute small solutes by convection. Standing caused rapid solute depletion but promptly regained by assuming resting supine position., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Contaminants that entered the streambed during previous surface water pollution events can be released to the stream, causing secondary pollution of the stream and impacting its eco‐environmental condition. By means of laboratory experiments and numerical simulations, we investigated density effects on the release of solute from periodic bedforms. The results show that solute release from the upper streambed is driven by bedform‐induced convection, and that density effects generally inhibit the solute release from the lower streambed. Density gradients modify the pore water flow patterns and form circulating flows in the area of lower streambed. The formation of circulating flows is affected by density gradients associated with the solute concentration and horizontal pressure gradients induced by stream slope. The circulating flows near the bottom of the streambed enhance mixing of the hyporheic zone and the ambient flow zone. [ABSTRACT FROM AUTHOR]

Self‐assembly of asymmetric block copolymers (BCPs) around active pore edges has emerged as an important strategy to produce smart membranes with tunable pathways for solute transport. However, thus far, it is still challenging to manipulate pore shape and functionality for directional transformation under external stimuli. Here, a versatile strategy by mesoscale simulations to design stimuli‐responsive pores with various edge decorations in hybrid membranes is reported. Dopant BCPs are used as decorators to stabilize pore edges and extend their function in reconfiguring pores in response to repeated membrane stretching/shrinking caused by external stimuli. The decoration morphologies are predictable since the assemblies of dopant BCPs around pore edges are closely related to their self‐assemblies in solution. The coassembly between different BCPs in the hybrid membrane for the control of pore morphology is featured, and the parameter settings, including block incompatibility and molecular architecture for the construction of a specific pore, are determined. Results show that harnessed dopant BCPs in the hybrid membrane can enhance pore formation and induce directional pore shape and functionality transformation. Diversified pore decorations exhibit potential that can be further explored in selective solute transport and the design of stimuli‐responsive smart nanodevices. [ABSTRACT FROM AUTHOR]

The present study is undertaken in the eastern coast of India, along the coastal tract of Bay of Bengal (BoB), to delineate the submarine groundwater discharge (SGD)-borne nutrient flux at temporal scale and their impact to coastal ecology and biogeochemical processes. Solutes chemistry, seepage meter study, stable-isotopic signature, and geophysical techniques were used to identify the surface water–groundwater interaction zone, SGD rate and nutrient flux. The estimated rate of major annual discharge of nutrient fluxes were 240 and 224 mM m−2 day−1 for NO3− and Fetot. The variation of solute and nutrient fluxes was depending on the load of terrestrial water masses, which is triggered by the local monsoonal meteoric recharge. The ecohydrological response to this solute flux results in spatio-temporal patterns of N and P-sensitive algal blooms in the intertidal zones. Most algae were identified as dinoflagellates and some haptophytes, with greenish and brownish hue that provides a distinct look to the coastal landscape. The algal blooms were found to be substantially influenced by the seasonal-nutrients flux and discharge location. Our study is expected to increase the understanding of a rarely reported eco-hydrological response to terrestrial–marine water interactions and their implications in the tropical ocean adjoining the Indian Subcontinent. [ABSTRACT FROM AUTHOR]

The code d3f++ developed under the leadership of GRS for simulating 3D flow and solute transport includes an advanced formulation for fracture flow and transport in lower-dimensional elements that has still been in need of qualification. A variety of in-situ experiments in the granitic rock at the Swedish Hard Rock Laboratory Äspö offers an excellent basis for this purpose. Particularly well suited is the work performed in the TRUE Block Scale Project where a hydrostructural model comprising 30 large-scale fractures in a cube-shaped domain with a side length of 200 m has been set up for simulating groundwater flow which in turn forms the basis for reproducing the results of the injection–extraction tracer test "C2". The present paper deals with modelling 3D flow and solute transport with the code d3f++ as a qualification exercise. The modelling results reveal an excellent representation of the contrasts in flow velocity across the fractures. Also the jumps in tracer concentration along intersecting fractures where waters with different tracer concentrations mix are handled well by the code. Calibration has nevertheless been required in order to achieve a reasonable fit of measured and calculated breakthrough curves. [ABSTRACT FROM AUTHOR]

Resazurin (Raz) is a phenoxazine dye that can be reduced irreversibly to the daughter compound resorufin (Rru) by aerobic respiration. Previous hydrologic studies using the Raz‐Rru reactive tracer system to quantify water‐sediment interactions and metabolic activity have reported that dilution‐corrected masses of Raz and Rru recovered are smaller than the mass of Raz injected. This lack of mass balance closure has been reported as a nonideality of this tracer system and, to date, it is still unclear what drives incomplete recovery. We used controlled laboratory experiments varying the initial concentrations of Raz, the duration of the experiments, and the type of microbial communities present to quantify mass balances of Raz and Rru under conditions that removed other suspected causes of incomplete recovery in field experiments, i.e., sorption to sediments and photodecay. We used the summation of Raz and Rru concentrations over time to assess mass recovery and variability and found mass recoveries in the range of 85.6–110.4%, with a maximum standard deviation of 7.5%. In three of the four experiments, no strong temporal trend in mass recovery is present. In an experiment with Bacillus subtilis bacteria, lower recovery and evidence of a temporal trend in recovery only occurred after 13 hr past the complete transformation of Raz (i.e., beyond the duration of most field experiments). These results suggest that the lack of mass recovery in field studies is likely associated with physical or chemical mechanisms rather than biological interactions with the Raz‐Rru tracer system. Plain Language Summary: Resazurin (Raz) is a fluorescent dye that can be transformed into resorufin (Rru) by microorganisms. It has been used for a decade to study how water interacts with sediments and microbial communities. However, most of these studies recover less Raz and Rru than the Raz that was added, prompting the question: Does the mass balance of Raz and Rru close at the microbial scale? We conducted multiple experiments with different microbes to answer this question. Our results generally show full mass recovery, meaning that the incomplete recovery reported in field studies is governed by physical or chemical factors. Key Points: Mass balances were used to assess if microbial transformation explains incomplete recovery of resazurin‐resorufin in field studiesExperiments conducted with different microorganisms indicate complete mass recovery during all conversion stagesComplete mass recovery at the microbial scale suggests sorption is responsible for incomplete recovery in mesocosm and field experiments [ABSTRACT FROM AUTHOR]

This paper reviews for the occurrence of groundwater contaminants including natural and manmade. Contaminants found in groundwater cover a broad range of physical, inorganic, and organic chemicals and radioactive parameters. The increasing trends of industrial effluents including municipal sewage are the main sources for contaminants in groundwater. In order to assess the level of groundwater contamination, appropriate mathematical model can be used and accordingly plans can be developed to combat the menace. Present study discusses the sources of contaminants, their movement, mathematical modeling, and remedial measures to stop further groundwater contamination in India and Australia. It is shown that in general, mobile immobile model gives better fit of observed data as compared to convection dispersion model. There is also a need to establish low-cost remediation methods to enable more reliable remediation to solve groundwater contamination problems. [ABSTRACT FROM AUTHOR]

Solute transport characteristics and groundwater connection structures are mainly studied in this paper by adopting the numerical simulation and field tracer test. Firstly, the simulation method of solute transport is proposed. Then two kinds of representative geological models of tracer test are built, and the process of solute transport is simulated. The variation characteristics of solute transport under the conditions of straight pipeline and branch pipeline are summarized by analysis of the simulation results. The effects of the pipeline width, flow velocity and path difference of branch pipelines on the tracer curve are discussed. General laws of groundwater connection tracer curve are obtained. Finally, based on the field test results, and combined with engineering geological conditions, characteristics of karst groundwater systems, the groundwater connection structures are analysed and speculated in detail by the flow velocity and local monitoring curve. The results show that: (i) As the flow velocity increases, the peak of the tracer curve decreases gradually, as well as the time to peak; (ii) The peak of the tracer curve gradually decreases with the increasing pipeline width; (iii) Under the different path difference of branch pipeline, the tracer curves all present obvious 'trailing' phenomenon; (iv) The analysis process of groundwater connection structures can provide references for the same type of hydrological geological problems. [ABSTRACT FROM AUTHOR]

Nowadays, with the increasing population in Iran, especially in arid and semi-arid areas, as a result of the growing importance of the quality of water resources, including groundwater, field experiments and many simulations have been conducted for the development of groundwater contamination through powerful and up- to- date software. However, in most cases, there is a tangible difference between the measured data in laboratories and the data produced with software; this is why the scientific validation and verification of the research results could be declined. In this study, in order to justify and correct these data, the calibration principle was used to minimize the error of testing and modeling. The purpose of this study was to validate and verify the SUTRA model for different scenarios of the solute transport in a sand tank with heterogeneous hydraulic conductivity to evaluate transverse dispersivity. In this study, coding was initially performed for the configuration and calibration of the SUTRA numerical model to simulate different scenarios of the solute transport in a heterogeneous sand Tank in the Hydraulic Laboratory of the University of Kassel, Germany, until acceptable values were obtained. Then the results were compared with the experimental model. In order to validate and verify the data obtained from the simulation with the SUTRA model, the relevant concentration profiles were compared with the results of the experimental model. The results of the numerical and laboratory models revealed the density effects by sinking the geometric center of the mixing zone for the low concentrations of salt, C0 = 250 ppm. The results also showed that the width of the mixing zone between salt and fresh water depended on the amount of longitudinal dispersivity, especially the transverse dispersivity. By analyzing the results of simulation and experiment, it was observed that with increasing the velocity, reducing the amount of sinking and raising the input concentration, the time needed to achieve the steady dispersion was decreased. [ABSTRACT FROM AUTHOR]

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Three landfills are located in the eastern part of the Chengdu City, and two traffic tunnels pass beneath the south portion of landfill-1. The landfills have a simple leachate collection system without any bottom liner and impermeable wall, which causes a severe corrosion of the traffic tunnels from 175 to 250 m in the test section. In order to explore how the traffic tunnels impact the transport path of contaminants infiltrating from landfills, a groundwater flow model and a solute transport model were established in the present study. It was found that, after 16 years of operation, the traffic tunnels will accelerate the vertical migration of sulfate ions in the area between the tunnels. Furthermore, 64 water samples along the traffic tunnels were collected to test the concentration of the contaminants. According to the distribution mechanism of the measured concentrations in the traffic tunnels, a preliminary treatment plan was proposed to control the further corrosion of the tunnels and the spread of the contaminants. [ABSTRACT FROM AUTHOR]

We determine the time-dependent behavior of the dispersion coefficient for transport in formations with isotropic log-conductivity fields showing fractal behavior. We consider two different dispersion coefficients for point-like injection: (1) the ensemble dispersion coefficients, defined as half the rate of change of the second central moments of the ensemble-averaged concentration distribution and (2) the effective dispersion, which is half the rate of change of the expected second central moments. Our results show, that the two longitudinal macrodispersion coefficients steadily grow with time and remain different at all times in a fully fractal regime, indicating that no Fickian transport regime is ever reached. The resulting effective longitudinal transport model is consequently a fractional advection–dispersion equation. In the semi-fractal regime, a Gaussian transport regime is reached eventually. However, compared to the case of a classic non-fractal regime, the transient non-Gaussian regime lasts much longer. In the transverse direction, the two dispersion coefficients approach the same large-time limit also in fractal media highlighting the fundamental difference between longitudinal and transverse dispersion. [ABSTRACT FROM AUTHOR]

Although 1,4-dioxane is generally thought to be recalcitrant, recent studies suggest it may degrade in the subsurface under ideal conditions. A fuller understanding of natural attenuation processes affecting 1,4-dioxane is therefore needed to assess its potential for in situ bioremediation. This investigation employed multiple lines of evidence to evaluate attenuation of 1,4-dioxane at the Gelman Site beneath the city of Ann Arbor, Michigan, USA. Data from a network of groundwater monitoring wells were used to determine attenuation metrics at individual wells and at the scale of a prominent 1,4-dioxane plume. A series of plume maps and historical remediation data were used to calculate changes in aqueous phase mass storage, mass influx rates, and mass removal rates over a 12-year period (2005–2017). Individual point and plume-scale metrics indicate that attenuation may be occurring at rates too small to meaningfully contribute to remediation results at the site. Conversely, plume-scale mass balance calculations reveal a 1,4-dioxane storage surplus for the first 6 years, followed by a storage deficit during the remaining 6 years that cannot be explained by mass influx or removal estimates, respectively. Mass balance deficits observed in this aquifer system are attributable to biodegradation and/or unrecognized discharge to surface water and storm drain systems at rates similar to remedial pump-and-treat mass removal during 2011–2017. [ABSTRACT FROM AUTHOR]

Many deterministic and stochastic approaches have been applied for groundwater contaminant source identification (GCSI) in recent decades. Usually, these implementations are based on a single groundwater model or fixed model structure and ignore the uncertainty of model structure. However, model structure uncertainty is inevitable for groundwater modeling, especially for complex geological environments and limited observations. This study evaluated the impact of model structure uncertainty on GCSI, and proposes an approach for GCSI based on Bayesian model selection. In the framework of multiple model analysis, a set of alternative model structures are used to represent the unknown groundwater system. Then, a novel nested sampling algorithm, POLYCHORD, is used for model selection and source identification. This algorithm is capable of estimating the model's marginal likelihood and inferring the posterior distribution of the contaminant source's characteristics simultaneously. Finally, this proposed approach is verified through two GCSI case studies, which include a synthetic groundwater contamination problem and a groundwater transport column experiment. The results demonstrated that GCSI could be inconsistent when using different model structures. Models with higher marginal likelihoods tend to have better performance on the predictions of the contaminant source's characteristics. It was concluded that POLYCHORD is efficient in marginal likelihood estimation and GCSI. [ABSTRACT FROM AUTHOR]

This paper presents a time-space Hausdorff derivative model for depicting solute transport in aquifers or water flow in heterogeneous porous media. In this model, the time and space Hausdorff derivatives are defined on non-Euclidean fractal metrics with power law scaling transform which, respectively, connect the temporal and spatial complexity during transport. The Hausdorff derivative model can be transformed to an advection-dispersion equation with time- and space-dependent dispersion and convection coefficients. This model is a fractal partial differential equation (PDE) defined on a fractal space and differs from the fractional PDE which is derived for non-local transport of particles on a non-fractal Euclidean space. As an example of applications of this model, an explicit solution with a constant diffusion coefficient and flow velocity subject to an instantaneous source is derived and fitted to the breakthrough curves of tritium as a tracer in porous media. These results are compared with those of a scale-dependent dispersion model and a time-scale dependent dispersion model. Overall, it is found that the fractal PDE based on the Hausdorff derivatives better captures the early arrival and heavy tail in the scaled breakthrough curves for variable transport distances. The estimated parameters in the fractal Hausrdorff model represent clear mechanisms such as linear relationships between the orders of Hausdorff derivatives and the transport distance. The mathematical formulation is applicable to both solute transport and water flow in porous media. [ABSTRACT FROM AUTHOR]

A single-well push–pull test is one of the most effective ways to estimate aquifer transport parameters. However, a patchy aquifer of finite thickness may surround the test well due to a gravel pack, mud invasion and stress redistribution during the well construction, which is usually neglected for single-well push–pull tests. In such a case, the aquifer should be regarded as a radial two-zone system, i.e., patchy zone and aquifer formation zone. In this study, a numerical model of a single-well push–pull test for a radial two-zone confined aquifer was developed using finite-element COMSOL Multiphysics. Two special cases, i.e., uniform patchy aquifer and non-uniform patchy aquifer, were considered. For the uniform patchy aquifer, results indicate that larger values of effective porosity and dispersivity in the patchy zone results in larger values of the breakthrough curves in the pumping phase. Patchy-aquifer thickness was also noted to have considerable impact on the breakthrough curves. As for the nonuniform patchy aquifer, the variation of hydraulic conductivity, dispersivity and effective porosity in the patchy zone leads to a change of concentration along the z-direction around the well screen, and as such, a smaller dispersivity of patchy regions results in more fluctuation in the concentration curve, while little impact can be found for a larger dispersivity. In addition, results showed that the average concentration over the entire screened section with an equivalent uniform patchy aquifer can be applied to interpret the data of a single-well push–pull test for the case of a nonuniform patchy aquifer. [ABSTRACT FROM AUTHOR]

Groundwater vulnerability maps can be combined with pollution hazards to assess risks of groundwater pollution. However, groundwater vulnerability maps are generally difficult to interpret because they differ according to the factors considered and the way they are combined. Here, starting from process-based concepts and criteria, a robust definition for groundwater vulnerability to pollution is discussed. A methodology is developed based on processes governing the fate of pollutants at the land surface (i.e. runoff and infiltration) and below ground (i.e. pollutant transport in the subsurface). Groundwater vulnerability is evaluated based on combination of the land surface hazard and the subsurface attenuation capacity. Land surface hazard is defined to consider direct and lateral infiltration capacity of pollutants, regardless of any subsurface attenuation capacity, which refers to any process that leads to pollutant mass reduction from the infiltration location to the water table. The concept of subsurface attenuation capacity is adapted to the case of groundwater intrinsic vulnerability assessment, considering three process-based vulnerability coefficients, which are the pollutant minimum travel time from the hazard location to the water table, the pollution duration at the water table, and the maximum concentration of pollutant discharging into the groundwater. The concepts are illustrated by applying the developed method (named APSÛ) for intrinsic groundwater vulnerability assessment in the Néblon catchment, a karstified limestone/sandstone aquifer system in Belgium. The APSÛ method results are discussed and the perspectives for generalizing the method to groundwater-specific vulnerability and risk mapping are presented. [ABSTRACT FROM AUTHOR]

Solute transport is important in a variety of applications regarding flow in porous media, such as contaminant groundwater remediation. Most recent experimental studies on this process focus on field‐scale or centimeter‐scale data. However, solute spreading and mixing are strongly influenced by pore‐scale heterogeneity. To study this, we developed a novel methodology to quantify transient solute concentration fields at the pore scale using fast laboratory‐based microcomputed tomography. Tracer injection experiments in samples with different degrees of pore‐scale heterogeneity (porous sintered glass and Bentheimer sandstone) were imaged in 3D by continuous scanning at a time resolution of 15 s and a spatial resolution of 13.4 μm. While our calibration experiments indicated a high uncertainty (1σ) on the concentration in single voxels due to imaging noise (± 27% of the total concentration range), we show that coarse gridding these values per individual pore significantly lowers the uncertainty (± 1.2%). The resulting pore‐based tracer concentrations were used to characterize the transport by calculating the solute's arrival time and transient (filling) time in each pore. The average velocities estimated from the arrival times correspond well to the interstitial velocities calculated from the flow rate. This suggests that the temporal resolution of the experiment was sufficient. Finally, the pore‐based transient filling times, the global concentration moment and the global scalar dissipation rate calculated from our experiments, indicated more dispersion in the sandstone sample than in the more homogeneous sintered glass. The developed method can thus provide more insight in the influence of pore‐scale heterogeneity on solute transport. Plain Language Summary: In groundwater reservoirs, contaminants are often dissolved in the water that flows through the porous sediment or rock layers. Different transport processes like advection, diffusion, and mechanical dispersion influence the concentration distribution of these contaminants. A thorough understanding of the transport processes is thus key for, for example, groundwater remediation and waste management. This can be studied by simulations and experimental work. Most of the recent experimental studies gather field‐scale or centimeter‐scale data. However, transport processes are significantly impacted by the microscopic structure of the pores in the sediment or rock. In this methodological study, we thus focus on direct observations of pore‐scale solute transport in porous materials with a different amount of pore‐scale heterogeneity (sintered glass and sandstone samples) by fast laboratory‐based microcomputed tomography. Tracer concentrations within individual pores are quantified with a standard deviation of 1.2% during tracer injection experiments. Based on these concentration fields, an arrival time and transient time for every pore are defined. These parameters provide insight in pore‐scale mechanical dispersion processes. This methodology can contribute to understand the influence of pore‐scale heterogeneity on solute transport. Key Points: Fast micro‐CT (15‐s temporal resolution) was used to quantify transient pore‐scale concentration fields in tracer injection experimentsEnhanced image analysis reduced the average uncertainty of the micro‐CT based concentration in each individual pore to 1.2%Solute spreading and mixing were analyzed by defining pore‐scale transport properties like arrival and transient (filling duration) time [ABSTRACT FROM AUTHOR]

Hydroelectric dams often create highly dynamic downstream flows that promote surface water‐groundwater (SW‐GW) interactions including bank storage, the temporary storage of river water in the riverbank. Previous research on SW‐GW exchanges in dammed rivers has primarily been at single study sites, which has limited the understanding of how these exchanges evolve as dam releases travel downstream. This study evaluates how dam releases affect SW‐GW exchange continuously over a 100‐km distance. This is accomplished by longitudinally routing water releases through a synthetic river and modeling bed and bank fluid and solute exchange across transverse transects spaced along the reach. Peak and square dam release hydrograph shapes with three magnitudes (0.5, 1.0, and 1.5 m) were considered. The effect of four ambient groundwater flow conditions (very slightly losing, neutral, and two gaining from the perspective of the river) was evaluated for each dam release scenario. Both types of dam release shapes cause SW‐GW interaction over the entire 100‐km distance, and our results show that square type releases cause bank storage exchange well beyond this distance. Strongly gaining conditions reduce the amount of exchange and allow flushing of river‐sourced solute out of the bank after the dam pulse has passed. Both neutral and losing conditions have larger fluid and solute flux into the bank and limit the amount of solute that returns to the river. Our results support that river corridors downstream of dams have increased river‐aquifer connectivity and that this enhanced connectivity can extend at least 100 km downstream. Plain Language Summary: Rivers downstream from dams that generate hydroelectric power can experience frequent water level changes as discharge from the dam is increased or decreased to meet electricity demands. Dam releases can be of similar size to storm runoff events caused by precipitation, but they often occur with more regularity. A large body of research indicates that the exchange of surface water and groundwater (SW‐GW) can play an important role in the ecology, nutrient cycling, and overall ecosystem health of river environments. This study uses computer simulations to evaluate how dam releases affect SW‐GW interactions. Instead of collecting observations at a handful of field locations as typically done, our approach enabled us to test how a wide range of dam release properties affect SW‐GW interactions continuously over a long distance downstream. Our results show that depending on the type and size of hydropower release and the groundwater conditions, the volume of exchange between the river and groundwater can range from 1 to almost 200 Olympic swimming pools over a 100 km length of river. Our findings can help predict where hydropower operations are more likely to cause SW‐GW exchange and thus help management of river corridors and guide future studies in dammed rivers. Key Points: Dam‐induced bank storage fluid and solute exchange was simulated for a 100‐km synthetic river subjected to realistic dam release signalsA large fraction of initial exchange near a dam can occur well beyond 100 km downstreamPositive, neutral, or negative groundwater gradients along the reach substantially change the quantity of bank storage exchange [ABSTRACT FROM AUTHOR]

This paper presents a demonstration of an integrated risk assessment and site investigation for groundwater contamination through a case study, in which the geologic and hydrogeological feature of the site and the blueprint of the fossil power plant (FPP) were closely analyzed. Predictions for groundwater contamination in case of accidents were performed by groundwater modeling system (GMS) and modular three-dimensional multispecies transport model (MT3DMS). Results indicate that the studied site area presents a semi-isolated hydrogeological unit with multiplicity in stratum lithology, the main aquifers at the site are consisted of the filled karst development layer with a thickness between 6.0 and 40.0 m. The poor permeability of the vadose zone at the FPP significantly restricted the infiltration of contaminants through the vadose zone to the subsurface. The limited influence of rarely isotropic porous karstified carbonate rocks on the groundwater flow system premised the simulate scenarios of plume migration. Analysis of the present groundwater chemistry manifested that that the groundwater at the site and the local area are of the HCO3–Ca, HCO3, and SO4–Ca types. A few of the water samples were contaminated by coliform bacteria and ammonia nitrogen as a result of the local cultivation. Prediction results indicate that the impact of normal construction and operation processes on the groundwater environment is negligible. However, groundwater may be partly contaminated within a certain period in the area of leakage from the diesel tanks, the industrial wastewater pool, and the cooling tower water tank in case of accidents. On a positive note, none of the plumes would reach the local sensitive areas for groundwater using. Finally, an anti-seepage scheme and a monitoring program are proposed to safeguard the groundwater protection. The integrated method of the site investigation and risk assessment used in this case study can facilitate the protection of groundwater for the construction of large-scale industrial project. [ABSTRACT FROM AUTHOR]

The transport and distribution of charged molecules in polyelectrolyte solutions are of both fundamental and practical importance. A practical example, which is the specific subject addressed in the present paper, is the transport and distribution of charged species into cartilage. The charged species could be a contrast agent or a drug molecule involved in diagnosis or treatment of the widespread degenerative disease osteoarthritis, which leads to degradation of articular cartilage. Associated scientific issues include the rate of transport and the equilibrium concentrations of the charged species in the cartilage and the synovial fluid. To address these questions, we present results from magnetic resonance micro-imaging experiments on a model system of articular cartilage. The experiments yield temporally and spatially resolved data on the transport of a negatively charged contrast agent (charge = -2), used in medical examinations of cartilage, into a polyelectrolyte solution, which is designed to capture the electrostatic interactions in cartilage. Also presented is a theoretical analysis of the transport where the relevant differential equations are solved using finite element techniques as well as treated with approximate analytical expressions. In the analysis, non-ideal effects are included in the treatment of the mobile species in the system. This is made possible by using results from previous Monte Carlo simulations. The results demonstrate the importance of taking non-idealities into account when data from measurements of transport of charged solutes in a system with fixed charges from biological polyelectrolytes are analyzed. [ABSTRACT FROM AUTHOR]

The time domain-random walk method was developed further for simulating mass transfer in fracture flows together with matrix diffusion in surrounding porous media. Specifically, a time domain-random walk scheme was developed for numerically approximating solutions of the advection-diffusion equation when the diffusion coefficient exhibits significant spatial variation or even discontinuities. The proposed scheme relies on second-order accurate, central-difference approximations of the advective and diffusive fluxes. The scheme was verified by comparing simulated results against analytical solutions in flow configurations involving a rectangular channel connected on one side with a porous matrix. Simulations with several flow rates, diffusion coefficients, and matrix porosities indicate good agreement between the numerical approximations and analytical solutions. [ABSTRACT FROM AUTHOR]

Deterioration of groundwater resources in coastal regions due to the progression of saline water in aquifers in these regions is currently one of the important issues in providing water needs in these areas. In coastal regions, saline water enters the aquifer from below in shape of wedge. Due to the difference in the density between fresh and salty water, an interface zone forms between two fluids. In order to better understanding the importance of this issue, experiments and numerical investigations of density-depended flow and transport through a tank filled with a variety of sand, are great help in achieving this. In this research, the real sand tank was simulated using SUTRA model. This simulation includes configuration, discretization, property assignment and boundary conditions determination. Finally, the transverse macro-dispersivity coefficient was estimated for different scenarios of the solute transport in this tank. The purpose of this research is to analyze of the solute dispersion, in mixing salt and fresh water, and the effect of seepage velocity, concentration of pollutant source and heterogeneity of porous media on the flow dispersivity property. In this research, after studying the effect of different boundary conditions in SUTRA model on the development of the salt water plume, simulation of the model of heterogeneous sand tank and comparing its results with laboratory model and homogeneous model were performed. As a practical result of this research, the diagram of changes in the coefficient of transverse dispersivity against the source concentration and seepage flow velocity was plotted. In numerical simulation of heterogeneous Porous media, for all concentrations, with the exception of the concentration C0= 35000, with increasing flow velocity, the values of the transverse dispersivity coefficient AT calculated by SUTRA decreased. Also AT for all seepage velocities, with the exception of seepage velocity u=4 m/day, increased with increasing source concentration. Also, the values obtained AT from the SUTRA model were more than the values of AT obtained from experiments. In numerical simulation of the homogeneous porous media, for all velocities, as the concentration source C0 increases, the transvers macro dispersivity coefficient AT increases. According to the applied results, suitable solutions can be found to improve the quality of groundwater and prevent the mixing of fresh and saltwater resources. [ABSTRACT FROM AUTHOR]

The groundwater contamination risk in future climates was investigated at three locations in Sweden. Solute transport penetration depths were simulated using the HYDRUS-1D model using historical data and an ensemble of climate projections including two global climate models (GCMs), three emission scenarios and one regional climate model. Most projections indicated increasing precipitation and evapotranspiration until mid-century with a further increase at end-century. Results showed both increasing and decreasing groundwater contamination risks depending on emission scenario and GCM. Generally, the groundwater contamination risk is likely to be unchanged until mid-century, but higher at the end of the century. Soil and site specific relationships between Δ(P – PET) (i.e. change in the difference between precipitation, P, and potential evapotranspiration, PET) and changes in solute transport depths were determined. Using this, changes in solute transport depths for other climate projections can be assessed. [ABSTRACT FROM AUTHOR]

The 2D solute transport equation can be incorporated into the 2D shallow water equations in order to solve both flow and solute interactions in a coupled system of equations. In order to solve this system, an explicit finite volume scheme based on Roe's linearization is proposed. Moreover, it is feasible to decouple the solute transport equation from the hydrodynamic system in a conservative way. In this case, the advection part is solved in essence defining a numerical flux, allowing the use of higher order numerical schemes. However, the discretization of the diffusion–dispersion terms have to be carefully analysed. In particular, time-step restrictions linked to the nature of the solute equation itself as well as the numerical diffusion associated to the numerical scheme used are question of interest in this work. These improvements are tested in an analytical case as well as in a laboratory test case with a passive solute (fluorescein) released from a reservoir. Experimental measurements are compared against the numerical results obtained with the proposed model and a sensitivity analysis is carried out, confirming an agreement with the longitudinal coefficients and an underestimation of the transversal ones, respectively. [ABSTRACT FROM AUTHOR]

• An innovative framework for PDE discovery is developed by integrating data-driven and data-assimilation methods. • Physical processes and empirical models can be concurrently identified. • The developed method is applicable to complex physical problems with nonlinear models. • Discovery of the PDE of reactive solute transport in subsurface formation is investigated. With the advent of modern data collection and storage technologies, data-driven approaches have been developed for discovering the governing partial differential equations (PDE) of physical problems. However, in the extant works the model parameters in the equations are either assumed to be known or have a linear dependency. Therefore, most of the realistic physical processes cannot be identified with the current data-driven PDE discovery approaches. In this study, an innovative framework is developed that combines data-driven and data-assimilation methods for simultaneously identifying physical processes and inferring model parameters. Spatiotemporal measurement data are first divided into a training data set and a testing data set. Using the training data set, a data-driven method is developed to learn the governing equation of the considered physical problem by identifying the occurred (or dominated) processes and selecting the proper empirical model. Through introducing a prediction error of the learned governing equation for the testing data set, a data-assimilation method is devised to estimate the uncertain model parameters of the selected empirical model. For the contaminant solute transport problem investigated, the results demonstrate that the proposed method can adequately identify the considered physical processes via concurrently discovering the corresponding governing equations and inferring uncertain parameters of nonlinear models, even in the presence of measurement errors. This work helps to broaden the applicable area of the research of data driven discovery of governing equations of physical problems. [ABSTRACT FROM AUTHOR]

Purpose: Systemic and intraperitoneal inflammation are characteristic features of patients with end-stage renal disease undergoing chronic peritoneal dialysis (PD). Arginine vasopressin (AVP) and its surrogate marker copeptin play important roles in many pathophysiological processes in chronic kidney disease. The aim of this study was to assess if copeptin concentrations in plasma and dialysate were related to peritoneal transport parameters and residual renal function (RRF) in incident PD patients. Methods: In 37 clinically stable incident PD patients (mean age 50 years, 68% women, 32% diabetes), a 4 h peritoneal equilibration test (PET) was performed 4–6 weeks after the onset of PD. Plasma (at 2 h of PET) and dialysate (at 4 h) concentrations of copeptin, high-sensitivity C-reactive protein and interleukin-6 (IL-6) were determined. Results: Plasma (80.7 ± 37.3 pg/mL) and dialysate (33.2 ± 18.0 pg/mL) concentrations of copeptin were correlated (Rs = 0.52, p = 0.001). Plasma and dialysate copeptin concentrations were negatively correlated with renal function as assessed by renal Kt/V (Rs = − 0.38; p = 0.021 and Rs = − 0.33; p = 0.047, respectively). At PET, dialysate copeptin negatively correlated with D/P creatinine (Rs = − 0.35, p = 0.033), and positively with D/D0 glucose (Rs = 0.33, p = 0.045) and ultrafiltration (Rs = 0.37, p = 0.024). Multivariate analysis showed that low dialysate copeptin (β = –0.30, p = 0.049) and high dialysate IL-6 (β = + 0.40, p = 0.012) were independent determinants of higher D/P creatinine. Conclusions: Dialysate copeptin was negatively associated with D/P creatinine in incident PD patients suggesting a potential influence of copeptin or AVP on peritoneal solute transport rate that might involve vasoactive mechanisms. [ABSTRACT FROM AUTHOR]

Multispecies solute transport through soils is characterized by the dynamic processes of convection, diffusion, decay and a series of chemical reactions, posing difficulties when seeking to derive analytical solutions related to subsurface hydrology. At present, most analytical solutions for multiple-species contaminant transport problems are based on the Laplace transform or Fourier transform technique, a decomposition strategy such as ingenious transform format and matrix diagonalization procedure, classical integral methods, the generalized integral transform technique, etc. These methods have been limited to some cases with simple initial conditions, identical retardation coefficients for all species, and complicated mathematical procedures, which restrict their use for cases under complex initial conditions. To cope with these limitations, the homotopy analysis method (HAM) is implemented to solve multispecies reactive transport models with more general, smooth initial conditions. Applying an auxiliary linear and nonlinear operator, a zero-order deformation equation is derived to obtain an approximate solution with high accuracy (different cases with various initial and boundary conditions). The semi-analytical solutions are compared with exact analytical solutions from the literature; good agreement between the resulting simulation and analytical solutions has been achieved. This illustrates that HAM is one of the most effective methods for solving the multispecies reactive transport models with more general and smooth initial conditions. [ABSTRACT FROM AUTHOR]