Concentration Breakthrough Curves Research Articles

Unraveling the complex dynamics of temporal moments for multispecies contaminant transport in saturated porous media: A global sensitivity analysis approach

In this study, we focus on the application of global sensitivity analysis (GSA) to gain insight into the influence of model parameters on the outputs of multispecies contaminant transport model. In this context, employing GSA methods (Morris and FAST) allows us to comprehend the relative significance of each input parameter in the model. We specifically analyzed the effects of model parameters on the concentration breakthrough curves and associated temporal moments of concentration of contaminant species. The spatial variability of sensitivity indexes from Morris and FAST methods for parent and daughter contaminant species is also investigated. We represent uncertainties of longitudinal dispersivity, porosity, first-order degradation constants of parent and daughter contaminant species, and sorption distribution coefficient of parent species into the coupled multispecies contaminant transport model and GSA framework. Our results show a complex interplay between different model parameters and their varying influences on different output metrics. The ranking of parameters based on GSA shows that (i) the contaminant mass recovery of final daughter species of three species chain reaction is governed by the first-order degradation coefficient of parent and first daughter species; (ii) dispersivity and porosity are critical parameters that govern the mean residence time and variance of breakthrough curve of final daughter species; (iii) the coefficient of skewness is more sensitive to dispersivity, sorption distribution coefficient, and first-order degradation coefficient of parent species as compared to other model parameters. The variation in the sensitivity indices among contaminant species and with travel distance is observed. This study highlights the significance of global sensitivity analysis when simulating the multiple species contaminants in saturated porous media.

The bimolecular reactive transport in heterogeneous porous media: Sub-diffusion in interpretation of laboratory experiment

This present work consists of investigating the effects of particle size heterogeneity and flow rates on transport-reaction kinetics of CuSO4 and Na2EDTA2− in porous media, via the combination of a bimolecular reaction experiment and model simulations. In the early stages of transport, a peak is observed in the concentration breakthrough curve of the reactant CuSO4, related to the delayed mixing and reaction of the reactants. The numerical results show that an increase in flow rate promotes the mixing processes between the reactants, resulting in a larger peak concentration and a slighter tail of breakthrough curves, while an increase in medium heterogeneity leads to a more significant heavy tail. The apparent anomalous diffusion and heavy-tailing behavior can be effectively quantified by a novel truncated fractional derivative bimolecular reaction model. The truncated fractional-order model, taking into account the incomplete mixing, offers a satisfactory reproduction of the experimental data.

Study on Numerical Simulation of Reactive-Transport of Groundwater Pollutants Caused by Acid Leaching of Uranium: A Case Study in Bayan-Uul Area, Northern China

Acid in situ leaching (ISL) is a common approach to the recovery of uranium in the subsurface. In acid ISL, there are numerous of chemical reactions among the injected sulfuric acid, groundwater, and porous media containing ore layers. A substantial amount of radioactive elements including U, Ra, Rn, as well as conventional elements like K, Na, and Ca, and trace elements such as As, Cd, and Pb, are released into the groundwater. Thus, in acid ISL, understanding the transport and reactions of these substances and managing pollution control is crucial. In this study, a three-dimensional reactive transport modeling (RTM) using TOUGHREACT was built to investigate the dynamic reactive migration process of UO22+, H+, and SO42− at a typical uranium mine of Bayan-Uul. The model considering the partial penetration through wellbore in confined aquifer and complex chemical reactions among main minerals like uranium, K-feldspar, calcite, dolomite, anhydrite, gypsum, iron minerals, clay minerals, and other secondary minerals. The results show that after mining for one year, from the injection well to the extraction well, the spatial distribution of uranium volume fraction does not consistently increase or decrease, but it decreases initially and then increases. After mining for one year, the concentration front of UO22+ is about 20 m outside the mining area, the high concentration zone is mainly inside the mining area. The concentration front of H+ is no more than 50 m. SO42− is the index with the highest concentration among the three indexes, the concentration front of SO42− is no more than 100 m. The concentration breakthrough curve of the observation well 10 m from the mining area indicates that the concentrations of the three indicators began to significantly rise approximately after mining 0.05 years, reached the maximum value after mining 0.08 to 0.1 years, and then stabilized. The parameter sensitivity of absolute permeability and specific surface area of minerals shows that the concentration of H+ and SO42− is positively correlated with absolute permeability. The concentration of H+ is negatively correlated with the specific surface area of calcite, anhydrite, K-feldspar, gypsum, hematite, and dolomite. The concentration of SO42− is positively correlated with the specific surface area of K-feldspar and Hematite, and negatively correlated with the specific surface area of calcite, anhydrite, gypsum, and dolomite. The influence analysis of pumping ratio and non-uniform injection ratio shows that the non-uniform injection scheme has a more significant impact on pollution control. The water table, streamline, capture envelope, and the concentration breakthrough curve of five schemes with different pumping ratios and non-uniform injection ratio were obtained. The water table characteristics of five schemes shown that increase in the pumping ratio and the non-uniform injection ratio, the water table convex near the outer injection well is weakened and the groundwater depression cone near the pumping well is strengthened. This characteristic of water table exerts a notable retarding influence on the migration of pollutants from the mining area to the outside. For the scheme with a pumping ratio is 0 (the total pumping flow rate is equal to the total injection flow rate) and a non-uniform injection ratio is 0 (the flow rate of inner injection well Q1,Q2,Q3 is equal to the flow rate of outer injection well Q4,Q5,Q6), the streamline characteristics shown that a segment of the streamline of is diverging from inner region to the outer region. For other schemes, the streamline exhibits a convergent feature. It is indicated that by increasing the pumping ratio and non-uniform injection ratio, a closure flow field can be established, confining the groundwater pollutants resulting from mining within the capture envelope. Hence, the best scheme for preventing pollution migration is the scheme with a pumping ratio is 0 (the total pumping flow rate is equal to the total injection flow rate) and a non-uniform injection ratio is 0.1 (the flow rate of inner injection well Q1,Q2,Q3 is 10% more than the flow rate of outer injection well Q4,Q5,Q6). In this scheme, the optimal stable concentration of UO22+, H+, and SO42− at the observation well obtained by RTM is lower than other schemes, and the values are 0.00316 mol/kg, 2.792 (pH), and 0.0952 mol/kg. The inner well injection rate is 194.09 m3/d, the outer well injection rate is 158.89 m3/d, and the pumping rate is 264.00 m3/d. Numerical simulation analysis suggests that a scheme with a larger non-uniform injection ratio is more conducive to the formation of a strong hydraulic capture zone, thereby controlling the migration of pollutants in the acid ISL. A reasonable suggestion is to adopt non-uniform injection mining mode in acid ISL.

Effective removal of arsenic from contaminated groundwater using an iron-based metal-organic framework

This study investigates the potential of an iron-based metal–organic framework (MOF) material, Fe-benzene-1,3,5-tricarboxylate (Fe-BTC), for arsenic removal from water. We conducted batch and column experiments to evaluate the effects of varying mass dosages of MOF and pH ranges on arsenic adsorption. Our batch experiments revealed that increasing the mass of Fe-BTC MOF led to higher adsorption capacity. Furthermore, within the range of pH values analyzed, Fe-BTC demonstrated stable arsenic adsorption capacity, suggesting that pH conditions did not significantly affect its performance. In the column experiments, we used granitic material amended with compost and compared arsenic concentration breakthrough curves with and without the presence of MOF to assess its efficiency in arsenic removal. Without MOF, we observed rapid arsenic arrival followed by a slow increase in concentration, indicating anomalous transport dynamics induced by the compost. The addition of MOF resulted in prolonged arsenic arrival and a stabilized lower concentration, indicating effective arsenic adsorption. Fe-BTC MOF exhibited a six-fold increase in arsenic adsorption compared to its absence when added to the soil material. We employed a fractional order advection–dispersion equation model to characterize and predict the physical and chemical dynamics in the column experiments. The transport model accurately matched the arsenic breakthrough curves in the column experiments by capturing the non-Fickian transport and sorption chemical dynamics. The model indicated that compost had an insignificant impact on arsenic adsorption due to flow heterogeneity, while higher dosages of MOF resulted in increased arsenic adsorption and non-Fickian dynamics.

Effects of macro-scale heterogeneity on the kinetic interface-sensitive tracer test for measuring the fluid–fluid interfacial area in dynamic two-phase flow in porous media

A novel reactive smart tracer method, termed the kinetic interface-sensitive (KIS) tracer test, has been demonstrated in laboratory column experiments to enable measurement of the specific capillary-associated fluid–fluid interfacial area in dynamic two-phase flow displacement processes in porous media. Development of the tracer method towards effective application in real field conditions requires investigation of the influence of the porous media heterogeneity on the front size and the specific interfacial area, and, consequently, in how far a kinetic interface-sensitive tracer experiment, and the corresponding breakthrough curves, are affected. This study employs a two-dimensional Darcy-scale two-phase flow reactive transport model to investigate numerically the KIS tracer transport in heterogeneous porous media. Simulations were carried out for the primary drainage process in a domain formed of fine and coarse porous media. Various heterogeneity patterns, having different numbers of inclusions and different geometrical distributions, were studied. It is shown that the shape of the breakthrough curves can be used as an indicator for quantifying the displacement front roughness, the specific interfacial area in the domain, and the domain heterogeneity, e.g., the existence of preferential flow pathways inside the porous media. The results indicate that when the displacement front roughness is small, the concentration breakthrough curves exhibit a linear increase. The slopes of the breakthrough curves linearly depend on the fraction of the bulk volume occupied by the low-permeability sand inclusions. The volume-averaged specific interfacial area and the size of the transition zone can be determined from the slopes of the breakthrough curves.

Contaminated water flow modelling through the porous media by using fractional advection-dispersion equation (FADE)

ABSTRACT This study aims to evaluate the validity of Fractional Advection-Dispersion equation (FADE) in breakthrough curves in saturated homogenous soil media, i.e. clay, and to examine the three primary restraints, pore water velocity, dispersion coefficient, and order of fractional differentiation which are impacting solute transport behavior (Fickian or Non-Fickian). In this study, the FADE framework used to characterize the transport process at depth 50 cm soil (clay). FADE Main based on FORTRAN, to estimate the parameters of FADE including fractional differentiation (λ), the dispersive coefficient (D), and the average pore-water velocity (v). If the value of is equal to or greater than 2, the transport is said to be Fickian otherwise is non-Fickian. In this study, the fractional differentiation of non-Fickian behavior was found 1.85 which is less than 2. On the other hand, early long-time tailing to the soil media showed an increase in the dispersion coefficient (D) for FADE and higher values in differentiation coefficient (λ = 1.85–1.99). The results of breakthrough curves (BTCs) of relative concentration (C/C0) show that it is best fitted by using FADE. For the assessment of fitting, Root Mean Square Error (RMSE) and determination Coefficient was used to find the quality of fit. .

Reservoir characterization by push-pull tests employing kinetic interface sensitive tracers - a pore-scale study for understanding large-scale processes

The kinetic interface-sensitive (KIS) tracer experiment is a newly developed method to quantify the fluid-fluid interfacial area (FIFA) during drainage processes. The concentration breakthrough curves, obtained by measuring reaction product concentration in brine/water samples after breakthrough, are interpreted with a specialized numerical model to determine the FIFA. However, this method has its own limitations since the volume of the usable water sample (after breakthrough), based on which the analysis takes place, is often insufficient. This happens because this volume strongly depends on the aquifer material, i.e. sand type and system parameters. An alternative is to employ KIS tracers in a “push-pull” experimental set-up, i.e. primary drainage is followed by a consequent main imbibition process, with the flow direction being reversed. This study applies a pore-scale numerical model to study the KIS tracer reactive transport during a push-pull experiment in a 2D porous medium. The advective-dispersive-reactive transport during both primary drainage and main imbibition processes is discussed. The concentration breakthrough curves show a non-linear descending trend with time, until reaching a constant concentration at the steady state. By analyzing the spatial concentration distribution and the concentration breakthrough curve, the macro-scale KIS tracer reactive transport model can be interpreted for the push-pull process. Finally, the new method is applied in a column experiment, where the determined specific interfacial area is found to be close to literature data.

Behavior characteristics of bimolecular reactive transport in heterogeneous porous media

In order to study the mechanism of bimolecular reactive solute transport in heterogeneous porous media, the chemical reaction (CuSO4 + Na2EDTA2−→CuEDTA2) was carried out by laboratory experiments and numerical simulation in heterogeneous porous media. Three different kinds of heterogeneous porous media (Sd2 = 1.72, 1.67 and 0.80 mm2) and flow rates (1.5, 2.5 and 5.0 mL/s) were considered. The increase of flow rate would promote the mixing between reactants, resulting in a greater peak value and a slighter “tailing” of product concentration, while the increase of medium heterogeneity would result in a more significant “tailing”. It was found that the concentration breakthrough curves of reactant CuSO4 had a peak in the early stage of the transport, and the peak value increased with the increase of flow rate and medium heterogeneity. The concentration peak of CuSO4 was caused by the delayed mixing and reaction of reactants. The IM-ADRE (The advection–dispersion–reaction equation considering incomplete mixing) model could well simulate the experimental results. The simulation error of IM-ADRE model for the concentration peak of product was less than 6.15%, and the fitting accuracy for “tailing” increased with the increase of flow. The dispersion coefficient increased logarithmically with the increase of flow, and was negatively correlated to the heterogeneity of the medium. In addition, the dispersion coefficient of CuSO4 simulated by IM-ADRE model was one order of magnitude larger than that simulated by ADE model, indicating that the reaction promoted dispersion.

Simple approximate closed-form expressions for tracer injection in aquifer with a radially converging or diverging flow field

The exact solution for the concentration breakthrough curves resulting from a solute injection in radially converging flow in an aquifer obtained using Laplace transforms has a complex form difficult to compute, and cannot be given in closed-form expression. This makes it difficult to use for tracer tests analysis in radial flow. To overcome this difficulty new simple approximate but accurate closed-form expressions are derived in the present paper for slug and continuous injections. The improvement of accuracy of these new closed-form expressions is demonstrated. The paper establishes also that the breakthrough curves resulting from slug injections into a radially diverging or radially converging flow are exactly identical at the sampling well in domains having the same spatial extension. Solutions were also derived for tracers or solutes subject to degradation.The new closed-form expressions were used for the analysis of 12 field tracer tests to highlight the difference in transport parameters determination compared to previous approximate expressions.

Surface complexation reactions in sandy porous media: Effects of incomplete mixing and mass-transfer limitations in flow-through systems

Although mixing and surface complexation reactions are key processes for solute transport in porous media, their coupling has not been extensively investigated. In this work, we study the impact of mass-transfer limitations on heterogeneous reactions taking place at the solid-solution interface of a natural sandy porous medium under advection-dominated flow-through conditions. A comprehensive set of 36 column experiments with different grain sizes (0.64, 1.3 and 2.3 mm), seepage velocities (1, 30 and 90 m/day), and hydrochemical conditions were performed. The injection of NaBr solutions of different concentrations (1–100 mM) led to the release of protons via deprotonation reactions of the quartz surface. pH and solute concentration breakthrough curves were measured at the outlet of the columns and the propagation of pH fronts in the column setups was tracked inside the porous medium with non-invasive optode sensors. The experimental results show that the deprotonation of the reactive surfaces, resulting from their interactions with the injected ionic species, strongly depends on the hydrodynamic conditions and differs among the tested porous media despite their apparent similar surface properties. Reactive transport modeling was used to quantitatively interpret the experimental results and to analyze the effects of mass-transfer limited physical processes on surface complexation reactions, propagation of pH fronts, transport of major ions and spatio-temporal evolution of surface composition. A dual domain mass transfer formulation (DDMT) combined with a surface complexation model (SCM) allowed capturing the effects of incomplete mixing on the surface reactions and to reproduce the experimental observations collected in the experiments with high flow velocities. The SCM was parameterized with a single set of surface complexation parameters, accounting for the similar surface properties of the porous media, and was capable of describing the surface complexation mechanisms and their impact on the hydrochemistry over the large range of tested ionic strengths.

Surface Transient Storage Under Low‐Flow Conditions in Streams With Rough Bathymetry

AbstractIn natural and restored rivers, protruding river‐bed elements can cause complex flow patterns. We hypothesize that, particularly at very low flows, solute spreading in such streams is controlled by different mechanisms than at high flows, questioning existing parameterizations. We investigate two‐dimensional flow and transport in a synthetic channel with rough bathymetry under low‐flow conditions with a numerical model solving the full shallow‐water equations. Recirculation cells form behind protruding bed elements and trap water, macroscopically acting as transient‐storage domain. We fit the one‐dimensional mobile‐immobile transport model to the concentration breakthrough curves of the two‐dimensional model and relate the fitted parameters to easy‐to‐measure properties of the stream. The velocity in the mobile domain undergoes a power‐law relation with the discharge and the channel slope, differing from the relationship expected for normal flow in streams with large water depths. Both the fitted dispersion coefficient and the mass‐transfer coefficient between the mobile and immobile domains scale linearly with the velocity in the mobile domain. The ratio of immobile to mobile storage volumes β first increases then decreases with increasing discharge. We suggest measuring the surface area of bed elements protruding the water surface and that of the open water. The product of the corresponding area ratio with the velocity exerts a power‐law control on β. Altogether, we can relate the coefficients of the mobile‐immobile transport model to a few easy‐to‐measure properties of the stream. The findings of the present work contribute to the understanding of surface transient storage in natural and restored streams.

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.

Fractional-derivative model simulations of reach-scale uptake and transport dynamics of natural fluorescent dissolved organic matter in a temperate forested stream in southeastern U.S.

Dissolved organic matter (DOM) entering the river networks affects aquatic ecosystems, yet it remains difficult to quantify the strength of various environmental factors in controlling reach-scale uptake and transport of DOM. This study combined reach-scale field experiments and stochastic modeling to investigate the uptake and transport dynamics of natural fluorescent DOM (FDOM) in a forested coastal plain stream located in the southeastern U.S. FDOM from litter leachate and sodium chloride (NaCl) were injected into the stream under base flow conditions (with different flow rates) on three separate occasions over a 3-month period. Field measurements showed that FDOM and NaCl exhibited different anomalous dynamics characterized by the peak and tailing behaviors of concentration breakthrough curves (BTC), indicating that the transport of FDOM was regulated more by physical retention and/or biogeochemical recycling than that of NaCl. The preferential removal of humic fluorescence than protein fluorescence before the BTC peak suggests that physical sorption was more important than biological degradation in early FDOM removal at the field site. A tempered fractional advection–dispersion-reaction model (TFADRM) was proposed to describe the FDOM migration process and estimate the FDOM spiraling metrics, and the resultant metrics were compared to those from conventionally used BTC-integrated approach. Results revealed that solute retention can lower the peak concentration and enhance the late-time tail of the FDOM BTC, whereas irreversible biogeochemical reaction processes can consume FDOM and decrease the BTC peak. Relative to the commonly used OTIS model, TFADRM can better capture the observed BTC late-time tail by considering multiple rates for the mass exchange/uptake of FDOM in streams, and therefore, this study provided a more reasonable estimate of FDOM spiraling metrics and transport dynamics at the reach scale.

2D model of the transfer processes in S-traps with wash-coated monolith channels and fixed bed for the application of H2S chemisorption in a fuel processor/fuel cell system

A two-dimensional model was developed for H2S chemisorption in different S-trap designs, including (i) ZnO coated monolith channels, (ii) ZnO fixed bed, and (iii) a new design. The new design was based on a modification of the monolithic reactor with adsorbent (ZnO) particles for filling the microchannels. The predicted concentration breakthrough curves were consistent with experimental literature data dealing with fixed bed and monolith S-traps.

Spatiotemporal geo-electrical sensing of a Pluronic-coated cobalt ferrite nanoparticle slug in natural sand flow-through columns

Rising industrial interest in the application of nanomaterials for the remediation of contaminated sites has led to concern over the environmental fate of the nanoremediation agents used. A critical requirement in evaluating and understanding nanoparticle (NP) behaviour in porous media is the development of analytical methods capable of in situ monitoring of complex NP transport dynamics. Spectral induced polarization (SIP), a non-invasive geo-electrical technique, offers a promising tool for detecting and quantifying NPs in soil and aquifer media. However, its application for monitoring the spatial migration and attachment behaviour of NPs remains uninvestigated. Here, we present results from flow-through experiments where we monitored the transport of cobalt ferrite nanoparticles (CoFe-NPs) coated with Pluronic, an amphiphilic polymer, in natural aquifer sand columns. We coupled concentration breakthrough curve analysis with SIP monitoring and reactive transport modeling to relate spatiotemporal NP concentration distributions to geo-electrical signals. Changes in the real (σ′) conductivity at three different locations along the columns closely correlated with model-computed total (solid plus aqueous phase) NP concentrations during the propagation of a NP slug. The imaginary conductivity (σ″) correlated closely with the arrival of the NP-slug. However, during the receding front, bimodal σ″-signal peak behaviour was observed propagating through the columns, indicating the existence of complex in situ NP transport dynamics, potentially revealing the rupture of nanoclusters upon straining and their effect on bulk charge storage that may not be obvious from breakthrough curve data alone. Fitting of a double Cole-Cole relaxation model yielded distinct shifts in relaxation time (τ) associated with the polarization of smaller length-scale particles. Post-NP pulse τ and σ″ did not return to pre-injection values; these lingering signals were caused by retained NP concentrations as low as 8.8 mg kg−1. Our results support the applicability of SIP for spatial and temporal monitoring of NP distributions, with implications for the investigation of NP transport and nanoremediation strategies.

Simultaneous identification of groundwater pollution source location and release concentration using Artificial Neural Network

Groundwater pollution source identification problem plays an important role in designing the groundwater remediation measures. This study aims at identifying the groundwater pollutant source location and its strength given a set of concentration breakthrough curves at different locations. Feed forward three layered artificial neural network (ANN) has been used to identify these two source parameters: location, i.e., the distance of source from the observation point and strength, i.e., the release concentration. A simplified ANN architecture is achieved by presenting the available concentration breakthrough curves in a specific manner as input to the model. The database of 1750 patterns for training and testing of the model is generated employing analytical solution of one dimensional steady flow and transient contaminant transport in a homogeneous aquifer. The performance of the ANN model was evaluated using standard statistical methods. A network with architecture 22-16-2 was found to be optimum and resulted in a normalized root mean square error (NRMSE) of 0.014 for identifying the source location and NRMSE of 0.044 for identifying the source release concentration. The results show that ANN can be a very efficient tool for locating pollution sources and estimating the release concentration at source. A good ANN model performance was obtained even with a simple architecture and with a small number of input variables.

Snowmelt as a determinant factor in the hydrogeological behaviour of high mountain karst aquifers: The Garcés karst system, Central Pyrenees (Spain)

Time series of environmental tracers (groundwater stable isotope composition, electrical conductivity and temperature) and concentration breakthrough curves of artificial tracers (uranine, eosine, amino-G and naphtionate) have been analyzed to characterize fast preferential and slow matrix in-transit recharge flows in the Paleocene-Eocene limestone aquifer of the Ordesa and Monte Perdido National Park, an alpine karst system drained by a water table cave, a rare hydrological feature in high mountain karst systems with similar characteristics. Snowmelt favors the areal recharge of the system. This process is reflected in the large proportion of groundwater flowing through the connected porosity structure of the karst aquifer, which amounts the 75% of the total system water discharge. From the perspective of water resources recovery, the water capacity of the fissured-porous zone (matrix) represents 99% of the total karst system storage. The volume associated to the karst conduits is very small. The estimated mean travel times are 9 days for conduits and 475 days for connected porosity. These short travel times reveal high vulnerability of the karst system to pollutants in broad sense and a great impact of climate change on the associated water resources.

MFIT 1.0.0: Multi-Flow Inversion of Tracer breakthrough curves in fractured and karst aquifers

Abstract. More than half of the Earth's population depends largely or entirely on fractured or karst aquifers for their drinking water supply. Both the characterization and modeling of these groundwater reservoirs are therefore of worldwide concern. Artificial tracer testing is a widely used method for the characterization of solute (including contaminant) transport in groundwater. Tracer experiments consist of a two-step procedure: (1) introducing a conservative tracer-labeled solution into an aquifer, usually through a sinkhole or a well, and (2) measuring the concentration breakthrough curve (BTC) response(s) at one or several downstream monitoring locations, usually spring(s) or pumping well(s). However, the modeling and interpretation of tracer test responses can be a challenging task in some cases, notably when the BTCs exhibit multiple local peaks and/or extensive backward tailing. MFIT (Multi-Flow Inversion of Tracer breakthrough curves) is a new open-source Windows-based computer package for the analytical modeling of tracer BTCs. This software integrates four transport models that are all capable of simulating single- or multiple-peak and/or heavy-tailed BTCs. The four transport models are encapsulated in a general multiflow modeling framework, which assumes that the spatial heterogeneity of an aquifer can be approximated by a combination of independent one-dimensional channels. Two of the MFIT transport models are believed to be new, as they combine the multiflow approach and the double-porosity concept, which is applied at the scale of the individual channels. Another salient feature of MFIT is its compatibility and interface with the advanced optimization tools of the PEST suite of programs. Hence, MFIT is the first BTC fitting tool that allows for regularized inversion and nonlinear analysis of the postcalibration uncertainty of model parameters.

The role of particle size and molar ratio of Ca2+/Na+ in the retention and remobilization of colloids in saturated porous media

The paper examined the effects of colloid size and molar ratio of Ca2+/Na+ on the retention and release of colloids under transients in ionic strength (IS). Column experiments were conducted to elucidate the transport behavior of three-sized polystyrene latex microspheres (300 nm, 500 nm, and 1000 nm) by varied molar ratios of Ca2+/Na+. Colloid concentration breakthrough curves showed the retention of colloids enhanced with the increase of molar ratio and the decrease of colloid size, which can be explained by Derjaguin–Landau–Verwey–Overbeek theory and the existence of nanoscale surface roughness. When the IS of the eluting solution was reduced from 10 to 1 mM, and when the eluting solution changed to ultrapure water, peak concentrations of colloid release were observed. Colloid recovery rates during the injection of ultrapure water were low, and substantial proportion of colloid still retained in the column even after the injection of ultrapure water, which suggested a stronger influence caused by the primary minimum compared with the secondary minimum on the retention of colloid. A modified numerical model considering the processes of Langmuirian dynamics blocking associated with a conservative tracer equation was successfully developed to simulate the transport process of all the experiments.

Comparative analyses of alternative breakthrough curves from cumulative mass column testing of soil–bentonite backfills

The results of eight cumulative mass column tests were analyzed via several different methods to evaluate the dispersion coefficient, D, and the retardation factor, Rd, governing the migration of chloride (Cl−), potassium (K), and zinc (Zn) through soil–bentonite backfills for vertical cutoff walls. Regression of the measured relative (effluent) concentration (RC) breakthrough curves (BTCs) resulted in relatively accurate determinations of Rd, but relatively inaccurate determinations of D for all three solutes. Values of Rd based on dimensionless time, T, corresponding to an RC of 0.5 were underestimated for all three solutes due to the significance of diffusion on solute transport. With a few exceptions, Rd for K and Zn based on analyses of the steady-state portions of measured cumulative mass ratio (CMR) BTCs and T – CMR BTCs were relatively accurate, whereas analysis of measured T – CMR BTCs was more accurate for determining Rd of Cl−. Overall, there is no advantage to analyzing the results of cumulative mass column tests in the form of RC BTCs, whereas the CMR and T – CMR BTCs offer the advantage of determining Rd based on simple linear regressions of the steady-state portions of the BTCs, i.e., provided steady-state solute transport has been established.