Convection-dispersion Equation Research Articles

Elemental and particle size fractionation during the transport of Eu(III)-silicate colloids in water-saturated porous media

Actinides (An)-bearing colloids could facilitate An migration in the environment. However, little is known about the transport behavior of An(III)-silicate colloids, which are readily formed by the reaction of An3+ with silicic acid under environmental conditions. Column experiments were conducted to investigate the transport of Eu(III)-silicate colloids (chemical analog of An(III)-silicate colloids) in water-saturated porous media as a function of pH, ionic strength (IS) and the presence of fulvic acid (FA). The results showed that colloid transport was more favorable at relatively low IS (≤ 50 mM) and high pH levels (pH ≥ 7). The presence of FA (5−10 mg/L) significantly enhanced the colloid transport. Under high IS (≥ 100 mM), the transport feature of colloids was turned from blocking to ripening due to the on-going aggregation of colloids. Additionally, an interesting elemental fractionation, i.e., a discrepancy in the breakthrough curves (BTCs) with respect to the C/C0 values of Si and Eu, was observed in the IS of 100−500 mM. A detailed investigation indicated that the elemental fractionation could be attributed to the partial Si dissolution of the colloids, the heterogeneity of the colloid size and element composition, and particle size fractionation during colloid transport. Extended Derjaguin-Landau-Verwey-Overbeek interaction energy calculations and convective-dispersive equation modeling were performed to illustrate variations in the colloid transport profiles. These findings illustrate the importance of Si dissolution in the migration of metal-silicate colloids and highlight the significant influence of the heterogeneity of colloid size and composition on the transport/migration behavior of colloids in the environment.

Dynamic changes in dissolved organic matter during transport of landfill leachate in porous medium.

The dynamic changes in dissolved organic matter (DOM) during the transport of landfill leachate (LL) in porous medium should be explored, considering the high levels of DOM in the LL of municipal solid waste. Column experiments were carried out at 25°C at a Darcy's flux of 0.29cm/h for 2722h to compare the transport of Cl-, ultraviolet absorbance at 254nm (UV254), chemical oxygen demand (COD), and dissolved organic carbon (DOC) in the simulated porous medium by using the CXTFIT2.1 code. Results showed that the convection-dispersion equation (CDE) could describe Cl- transport well. The high levels of λ and D could be highly correlated with the physicochemical properties of the porous medium. The transport of the studied DOM with evident aromatic character could be described appropriately by the CDE model with the first-order reaction assumption, considering the similar variation trends of UV254, COD, and DOC in the effluent during experiments. Specifically, the values of retardation factor (R) were in the following order: DOC > UV254 > COD, whereas the low values of the first-order decay coefficient (k1) for DOC and COD were still higher than that for UV254. High contents of humic-like substances in the DOM with complex toxic components resulted in the natural low removal efficiencies of COD, DOC, and UV254 (≤ 23%), which could be confirmed by the variations of fluorescence index (FI) and humification index (HIX) in the effluent. The results should be helpful in evaluating the environmental risk induced by the LL leakage in a landfill site.

Effect of Biochar on NO3--N Transport in Loessial Soil and Its Simulation

The objective of this study was to explore the effects of different amounts of biochar on the migration process and characteristics of NO3--N in loessial soil. In this study, six groups of mixed soil samples with biochar and loessial soil mass ratios of 0% (T0), 1% (T1), 2% (T2), 3% (T3), 4% (T4), and 5% (T5) were used as research objects. NO3--N was used as the tracer. Through the indoor soil column solute transport simulation tests, the effects of different biochar application amounts on the NO3--N transport process in loessial soil were simulated and studied. The results showed that the breakthrough curve of NO3--N in loessial soil shifted to the right with the increasing of biochar application, and the peak value gradually decreased. The initial penetration time, complete penetration time, and total penetration time increased with the increasing of biochar application amount. The total penetration time of NO3- in the T1, T2, T3, T4, and T5 treatments was 1.26, 2.31, 2.72, 3.22, and 3.57 times that of T0, respectively. The R2 was > 0.997 and RMSE was < 2.083 of the two-zone model (TRM). Compared with the convection-dispersion equation (CDE), the TRM model had higher fitting accuracy and could better simulate the NO3--N migration process in loessial soil after the application of different contents of biochar. The analysis of the fitting parameters of the TRM model showed that the average pore velocity, hydrodynamic dispersion coefficient, and water content ratio in the movable zone gradually decreased with the increasing of biochar application, whereas the dispersion and mass exchange coefficient showed an increasing trend. The results showed that biochar application could effectively enhance the ability of loessial soil to fix NO3--N, reduce the leakage of NO3--N to groundwater, and play an important role in maintaining soil fertility and preventing groundwater pollution.

Annual atrazine residue estimation in Chinese agricultural soils by integrated modeling of machine learning and mechanism-based models

This study presents a comprehensive approach to estimating annual atrazine residues in China's agricultural soils, integrating machine learning algorithms and mechanism-based models. First, machine learning was used to predict essential parameters influencing atrazine's adsorption, degradation, and dispersivity of solute transport. The results demonstrated that soil organic matter was the most important input variable for predicting adsorption and degradation; clay content was the primary variable for predicting dispersivity. The SHapley Additive exPlanations (SHAP) contribution of various soil properties on target variables were also analyzed to reveal whether each input variable has a positive, negative, or complex effect. Subsequently, these parameters inform the construction of a detailed model across 23,692 subregions of China, with a 20 km × 20 km resolution. The model considered regional variations and soil layer heterogeneity, including rainfall, soil depth-specific properties, and parameters for adsorption, degradation, and dispersivity. Utilizing the convection-dispersion equations and the Phydrus, the model simulated atrazine's transport and degradation patterns across diverse soil environments after applying 250 mL of atrazine (40%) per Chinese mu. The outcomes provided a spatially explicit distribution of atrazine residues, specifying that the arid areas have the highest residual risk, followed by the Northeast, Southwest, and Southeast. Atrazine levels may exceed national drinking water standards at 50 cm depth in Inner Mongolia, the Qinghai-Tibet Plateau, and the Jungar Basin. This study's integrative approach may also offer valuable insights and tools for evaluating residues of various pesticides and herbicides in agricultural soils.

Recovery of biochar particles laden with lead in saturated porous media by DC electric field

The co-transport behavior of environmental pollutants with biochar particles has aroused great interests from researchers due to the concerns about pollutant diffusion and environmental exposure after biochar is applied to soil. In this work, the recovery and co-transport behavior of biochar micron-/nano-particles (BCMP and BCNP) and lead (Pb2+) in saturated porous media were investigated under different ionic strength conditions (IS = 1, 5 and 10 mM) under a direct current electric field. The results showed that the electric field could significantly enhance the mobility of Pb adsorbed biochar particles, particularly BCNP. The recovery of Pb laden biochar particles was improved by 1.8 folds, reaching 78.8% at maximum under favorable condition at +0.5 V cm−1. According to the CDE (Convection-Dispersion-Equation) model and DLVO (Derjaguin-Landau-Verwey-Overbeek) theory analysis, the electric field facilitated the transport of Pb carried biochar mainly by increasing the negative charges on biochar surface and improving the repulsive force between biochar and porous media. High IS was favorable for biochar transport under the electric field, but inhibited desorbing Pb2+ from biochar (18% by maximum at IS = 10 mM). By switching the electric field power, a two-stage strategy was established to maximize the recovery of both biochar particles and Pb, where BCNP and Pb recovery were higher than electric field free case by 90% and 35%, respectively. The findings of this study can help build a biochar recovery approach to prevent potential risks from biochar application in heavy metal contaminated soil remediation.

Vertical migration behavior simulation and prediction of Pb and Cd in co-contaminated soil around Pb-Zn smelting slag site

Heavy metal migration in soil poses a serious threat to the soil and groundwater. Understanding the migration pattern of heavy metals (HMs) under different factors could provide a more reasonable position for pollution evaluation and targetoriented treatment of soil heavy metal. In this study, the migration behavior of Pb and Cd in co-contaminated soil under different pH and ionic strength (NaCl concentration) was simulated using convective dispersion equation (CDE). We predicted the migration trends of Pb and Cd in soils after 5, 10, and 20 years via PHREEQC. The results showed that the migration time of Cd in the soil column experiment was about 60 days faster than that of Pb, and the migration trend was much steeper. The CDE was proved to describe the migration behavior of Pb and Cd (R2 > 0.75) in soil. The predicted results showed that Cd migrated to 15–20 cm of soil within 7 years and Pb stayed mainly in the top 0–6 cm of soil within 5 years as the duration of irrigation increased. Overall, our study is expected to provide new insight into the migration of heavy metal in soil ecosystems and guidance for reducing risk of heavy metal in the environment.

Effects of Coal Gasification Slag on the Migration of Cd2+ and Pb2+ in Soil

In recent years, coal gasification slag has been widely used as an adsorbent in many countries worldwide. To better understand remediation of soil heavy metal pollution by coal gasification slag, it is necessary to examine and analyze the influence mechanism of the adsorption characteristics of coal gasification slag on the migration of heavy metal ions in the soil. In this study, we used a soil water movement model and convection-dispersion equation and constructed a model of heavy metal migration after soil improvement by coal gasification slag; then, we evaluated and predicted the influence of heavy metal infiltration with liquid on groundwater after soil improvement. Combined with the soil column experiment, HYDRUS-1D software was used to predict the migration behavior of heavy metals in the soil. The results showed that the dispersion of the sample with coal gasification slag was smaller than that of the original soil sample, and the permeability coefficient of the soil was reduced by 59.34%; after adding coal gasification slag, the adsorption rates of Cd2+ and Pb2+ on the soil samples were higher than 95% and 99% respectively. The constructed heavy metal migration model was fitted well and predicted the migration behavior of heavy metals in the soil satisfactorily. The addition of coal gasification slag can effectively prevent the infiltration of Cd2+ and Pb2+ in the soil and can mitigate groundwater pollution. Therefore, using the adsorption characteristics of coal gasification slag to study the migration behavior of Cd2+ and Pb2+ in soil has important practical significance for soil adsorption and provides a new idea for the comprehensive utilization of coal gasification slag.

Spatial distribution and modelling of 239+240Pu in the sediments and seawater columns of the South China Sea and Indian Ocean

In order to investigate the 239+240Pu potential influence in the ocean, and develop a new method for rapidly monitoring radioactive pollution, the 239+240Pu spatial distribution in the South China Sea (SCS) and the Indian Ocean (IND) sediments is analyzed by SF-ICP-MS (ELEMENT 2). The inventory-weighted mean activities of 239+240Pu were 0.413 ± 0.333 mBq/g, 0.128 ± 0.044 mBq/g, and 0.483 ± 0.606 mBq/g in the sediments of the SCS, eastern IND, and Arabian Sea, respectively. The 239+240Pu activity spatial distribution in the SCS sediments was influenced by the current, the vertical distribution of Pu in seawater, and the transport of particulate matter. The 239+240Pu activity spatial distribution in the IND sediments could be impacted by Antarctic Intermediate Water. The average of 240Pu/239Pu atomic ratios were 0.258 ± 0.034, 0.219 ± 0.031, and 0.212 ± 0.028 in the sediments of the SCS, eastern IND, and Arabian Sea, respectively. The 240Pu/239Pu atomic ratios in the SCS and IND indicate that Pu from the Pacific Proving Ground (PPG) is transported to the IND via the SCS internal current and transverse ocean currents within Indonesia. In addition, a seawater advection-dispersion equation (S-ADE) model is established based on the actual physical processes of radionuclides in the seawater column and well fitting results were obtained (R2 = 0.49 to 0.99). The 239+240Pu data and the geographic information from the sample site were used to correct the Pu distribution in the seawater. The calculated 239+240Pu mean concentrations in the surface seawater were 2.465 mBq/m3 and 2.205 mBq/m3 for the SCS and the eastern IND seawater, respectively, and the result is consistent with the previous measurements. Then, the 239+240Pu stored in the study area of SCS and eastern IND was estimated to be approximately 1.0–1.4% of the global ocean based on the model. This study provides a useful model for guiding and designing future monitoring of pollution by anthropogenic Pu and other isotopes.

Performance of a pesticide fate model for predicting multi-year surface runoff contamination in a Mediterranean vineyard

Characterization of the risk of water contamination by pesticide runoff is an essential issue in the assessment of the environmental impact of pesticide use in agriculture. Models are valuable tools for this purpose. However, the use of pesticide runoff models implies that they can simulate the intra- and interannual variations in runoff contamination with a sufficient accuracy. In this study, we aimed to assess the performance of a modelling approach coupling Richards and convection-dispersion equations with the uniform mixing cell concept and an overland flow routine to predict the between- and within-year variability of the pesticide concentration in runoff water at the outlet of an agricultural field within a Mediterranean context. The model performance was evaluated by calibrating and validating model predictions to observed concentrations of two herbicides, diuron and glyphosate. The temporal structure of the model performance was analysed over 10 and 9 years for diuron and glyphosate, respectively. The results showed a favourable average model performance for runoff pesticide concentration prediction over approximately ten years. However, the annual performance largely varied, suggesting that at least 3 years of data are required to obtain reasonably precise runoff pesticides estimates in terms of the Nash–Sutcliffe efficiency in a Mediterranean climate. Although limited differences in the prediction performance were found between the two compounds, the within-year fluctuation of the predictions was more accurate for glyphosate than for diuron. This is interpreted by ageing effects that affect diuron sorption but not glyphosate sorption. Furthermore, the study results indicated that the parameters critical to precise estimation depend on the simulation objectives. The main parameter values to be determined include the pesticide half-life time when estimating chronic exposure to pesticide runoff and the sorption coefficient and mixing depth when predicting the maximum observed concentration during a given season.

Cadmium and copper transport in alluvial soils in the Brazilian semiarid region: column percolation and modeling

ABSTRACT Regarding the Brazilian textile industry, part of the northeast region stands out as the second-largest textile manufacturing hub in the country. Despite its importance, this industrial activity has been bringing relevant environmental concerns regarding the disposal of textile effluents, especially from industrial laundries. This waste contains many chemicals and among them are various types of heavy metals. To assess environmental risks associated with heavy metals, pollutant transfer needs to be investigated. This study evaluated the retention and mobility of heavy metals Cd and Cu in alluvial soil, through soil column tests. The up-flow column percolation tests were performed using a nonreactive tracer (KBr) at a concentration of 0.3 mol L -1 and injecting a metallic solution containing Cu and Cd at 100 and 60 mg L -1 , respectively. The injection flow rate was 0.75 mL min -1 . The hydro-dispersive parameters were obtained by modeling the observed breakthrough curves with the convection-dispersion equation (CDE) and the two-region model, also referred to as the MIM (Mobile-IMmobile waters) model. The transport parameters were obtained from the two-site model (TSS). All elution curves were fitted to the models with the CXTFIT 2.0 program. The Two-Site Sorption Model was the best for the case studied, with R 2 of 0.985 and 0.995 for Cu and Cd, respectively. The values of R were considerably higher than the unit, presenting an average of 2.138 for Cu and 1.907 for Cd. This indicates a delay of these contaminants when leaving the column, which is caused by the interaction of these chemical compounds with the soil. The values obtained for parameter D were 3.469 for Cu and 5.205 for Cd. Thus, the metals in this study present a risk of groundwater contamination for the local alluvial aquifers. The main reason for that is the physicochemical features of the soil, such as high sand content (85 %) and low OM content (2.1 %). The results also indicated greater retention and less mobility for Cu than for Cd, pointing to a greater risk for Cd.

The effect of slow adsorption of phosphate on its transport during the infiltration process in saturated agricultural soils

ABSTRACT Assessment of phosphorus (P)-infiltrating croplands is essential for the preservation of the water environment. It has been pointed out that a huge discrepancy lies in the different evaluation methods of P adsorption, such as batch experiments and column experiments, which makes it difficult to demonstrate P mobility under flow conditions. The objective of this study was to evaluate the applicability of the convective-dispersion equation using the parameters of the Langmuir-type isotherm obtained from batch experiments with different reaction times: the adsorption capacity of soil (q max) = 0.112 (g kg−1) for a Gray lowland soil with 24 h reaction time, q max = 0.484 (g kg−1) for an Andosol (volcanic ash soil) with 24 h reaction time, and q max = 1.17 (g kg−1) for an Andosol with 32 d reaction time, for describing P mobility in typical Japanese agricultural soils under fast flow conditions. The breakthrough curves of P infiltrating the soil columns demonstrate nonequilibrium P adsorption by the soil. The chemical nonequilibrium model, with a kinetic adsorption rate of α = 0.40 (Gray lowland soil) and 0.098 (Andosol), succeeded in describing the observations in the column experiments. Compared with Gray lowland soil, which is relatively rich in iron oxide, P mobility was largely affected by kinetic sorption in Andosol, which is relatively rich in allophane. It is suggested that the P adsorption capacity of soils should be evaluated reflecting the soil composition in order to simulate the P mobility under flow conditions. In particular, the slow adsorption (long-lasting adsorption) of P by the soil should be considered in the estimation of the P transport.

Numerical solution of non-linear advection-dispersion equation in a finite porous domain

This study presents a numerical simulation of the one-dimensional concentration-dependent convection-dispersion equation in a finite heterogeneous porous formation. The solution of the convection-diffusion equation with variable coefficients is obtained with the help of MATLAB pdepe solver. The groundwater flow velocity depends on the pollutant concentration, and the dispersion coefficient is proportional to the groundwater flow velocity. The effects of the zero-order production term and the first-order decay are also considered. The aquifer is assumed to be heterogeneous and finite, with sources concentrated in the flow direction. It is assumed that the porous media and the pollutant are chemically non-reactive. Initially porous domain is considered not solute free. The model assumes a uniform continuous input point source and a variable input point source released from the left end of the aquifer domain. The obtained results graphically describe the importance of the dispersion coefficient and other relevant parameters for solute transport in porous media. The developed numerical solution is verified with an analytical solution, and it is found that they are in good agreement.

Study on Migration Monitoring Technology of Chromium-Contaminated Site Based on Dual-Frequency Induced Polarization Method

In order to reveal the migration process of chromium-contaminated sites, COMSOL5.6 software was used to build the initial model of the chromium pollution source and analyze the migration change characteristics over time based on the rule of groundwater movement and Convection–Dispersion equation. COMSOL provides fully coupled multiphysics modeling for most engineering fields. The results show that: the permeable layer with a high permeability coefficient during the migration of chromium pollutants has a strong adsorption effect on the heavily polluted part, and it will enhance the lateral diffusion of the polluted area within a short period. Based on the migration model, the intermediate gradient and the symmetrical quadrupole sounding model are built. The variation law of apparent amplitude frequency and apparent resistivity under flat and undulating terrain is analyzed based on the dual frequency IP (Induced Polarization) method. The results show that: The intermediate gradient detection is better than the symmetrical quadrupole sounding in the correspondence between the peak value of apparent amplitude frequency and the valley value of apparent resistivity. The arrangement of the bathymetric measurement point in the center of the projected edge of the pollution field can be a better way to monitor the pollution. Monitoring with the intermediate gradient detection device and the symmetrical quadrupole sounding device creates “multiple peaks” in the curve as it passes through the valley. Arranging the power supply electrodes on the mountain frontiers on both sides of the raised peaks and synthesizes the apparent amplitude frequency and apparent resistivity curves of the pollution field, which can effectively determine the specific orientation of the heavily polluted area of the pollution field. This research makes theoretical additions to the migration characteristics of the Cr pollution field. It provides technical guidance for the real-time monitoring of the pollution field, which is of positive significance for promoting ecological and environmental protection.

Stability of Spectral Collocation Schemes with Explicit-Implicit-Null Time-Marching for ConvectionDiffusion and Convection-Dispersion Equations

Stability of Spectral Collocation Schemes with Explicit-Implicit-Null Time-Marching for ConvectionDiffusion and Convection-Dispersion Equations

Laboratory Studies on the Influence of Ionic Strength on Particle Transport Behavior in a Saturated Porous Medium

An experimental study has been undertaken to investigate the effect of flow velocity and ionic strength on the transport of suspended particles (SP) and their deposition in a saturated porous medium. The SP injections were carried out using a laboratory column filled with sand and a pulse injection method. Ionic strengths varying between 0 and 600 mM (NaCl) have prospected. Two velocities were tested: 0.15 and 0.30 cm/s. Selected polydisperse particles diameters ranging from 0.27 to 5 μm and a median diameter (dp50) equal to 2.25 μm were used. An analytical solution of the convection–dispersion equation with first-order deposition kinetics was used to describe the experimental breakthrough curves and to identify the transport parameters. The results show that the increase of ionic strength promotes the retention of the SP in the porous medium. In addition, retention is more important when the flow velocity is low. The deposition kinetics coefficient increases with increasing ionic strength and flow velocity.

Laws Governing Nitrogen Loss and Its Numerical Simulation in the Sloping Farmland of the Miyun Reservoir.

Surface flow (SF) and subsurface flow (SSF) are important hydrological processes occurring on slopes, and are driven by two main factors: rainfall intensity and slope gradient. To explore nitrogen (N) migration and loss from sloping farmland in the Miyun Reservoir, the characteristics of total nitrogen (TN) migration and loss via SF and SSF under different rainfall intensities (30, 40, 50, 60, 70, and 80 mm/h) and slope gradients (5°, 10°, and 15°) were studied using indoor stimulated rainfall tests and mathematical models. Nitrogen loss via SF and SSF was found to increase exponentially and linearly with time, respectively, with SSF showing 14-78 times higher loss than SF. Under different rainfall intensities, SSF generally had larger TN loss loading than SF, thereby indicating that SSF was the main route for TN loss. However, the TN loss loading proportion via SF increasing from 14.03% to 35.82% with increasing rainfall intensity is noteworthy. Furthermore, compared with the measurement data, the precision evaluation index Nash-Suttcliffe efficient (NSE) and the determination coefficient (R2) of the effective mixing depth model in the numerical simulation of TN loss through SF in the sloping farmland in the Miyun Reservoir were 0.74 and 0.831, respectively, whereas those of the convection-dispersion equation for SSF were 0.81 and 0.811, respectively, thus indicating good simulation results. Therefore, this paper provides a reference for studying the mechanism of N migration and loss in sloping farmland in the Miyun Reservoir.

Analytical Solution of the One-Dimensional Transport of Ionic Contaminants in Porous Media with Time-Varying Velocity

The one-dimensional convection–dispersion equation has been widely used to describe the migration process of contaminant leachate through barriers. However, most of the existing solutions are limited to simple conditions. In this study, a one-dimensional convection–dispersion model with time-dependent velocity was established while considering the change in the permeability coefficient. The analytical solution of the model was obtained by using the integral transformation method. Based on the analytical model, three special conditions were assumed for comparison. The results showed that the concentration levels of pollutants inside the barrier would significantly increase with the increase in the flow velocity, and the pollutant concentrations inside the barrier would be increased by four times compared with the normal flow velocity when the flow rate increased by two times. The transports of heavy metal ions with variable velocities through soil–bentonite and soil–attapulgite barriers were predicted under field conditions. The predicted results showed that the breakthrough time would be reduced by as much as two times. In engineering practice, a barrier’s service performance can be improved by controlling the temperature of the seepage field and improving the chemical compatibility of the barrier materials.

Wolf prize awarded to Rien van Genuchten

Martinus Th. (“Rien”) van Genuchten has been awarded the 2023 Wolf Prize in Agriculture. The Wolf Prize is awarded to “outstanding scientists and artists from around the world for achievements in the interest of humanity and friendly relations among people.” The prize is awarded in the disciplines of medicine, agriculture, mathematics, chemistry, and physics. For agricultural sciences, the Wolf Prize is widely considered one of the world's most prestigious recognitions. Rien van Genuchten was born in 1945 in Vught, The Netherlands. He received his early education at the Agricultural University of Wageningen, The Netherlands, and his PhD at New Mexico State University in the United States. He then spent most of his career at the US Salinity Laboratory in Riverside, CA, before moving to the University of Rio de Janeiro, Brazil, and to Utrecht University, The Netherlands. One of his most influential publications describes a closed-form equation for predicting the hydraulic conductivity of unsaturated soils, known as the van Genuchten equation (van Genuchten, 1980). He was, together with Jirka Simunek, the developer of the widely used HYDRUS software package for simulating the movement of water, heat, and solutes in variably saturated media (Šimůnek et al., 2016). Early in his career, he also published a compendium on analytical solutions of the one-dimensional convection–dispersion equation (van Genuchten & Alves, 1982), a resource widely used to analyse solute transport at the column and field scale. Rien van Genuchten was the founding editor of Vadose Zone Journal, when he, 21 years ago, had the insight to initiate a new, multi-disciplinary journal to promote research related to the vadose zone, the zone between the soil surface and the groundwater table. This was at a time when environmental research was still in its infancy, and the new journal provided a common outlet for agronomy, soil science, hydrology, hydrogeophysics, biogeochemistry, civil engineering, and the emerging environmental sciences. Rien van Genuchten's foresight paved the way for innovative research and helped elevate vadose zone science to an established and respected field of research. The Wolf Prize was awarded to Rien van Genuchten “for his ground-breaking work in understanding water flow and predicting contaminant transport in soils.” On behalf of the editorial board of Vadose Zone Journal, we congratulate and celebrate Rien van Genuchten and his receipt of the Wolf Prize. Venkataraman Lakshmi: Conceptualization; writing—review and editing. Markus Flury: Conceptualization; writing—original draft; writing—review and editing.

Mechanistic insights into the migration behavior of cadmium (Cd) in the soil and groundwater systems at a construction site: Experimental and numerical analysis

The environmental fates of cadmium (Cd) in the soil and groundwater systems largely depend on its adsorption isotherms and kinetic with the soil matrix. Combined with HYDRUS–1D software, batch and column studies were conducted in this study to investigate the adsorption and movement process of Cd in soil and groundwater systems. The results from static adsorption experiment indicated that the second order model and Langmuir model could provide the better description for adsorption behavior of Cd. The results from SEM –EDS and XPS analysis revealed that the main adsorption mechanism of Cd were ion exchange, complexation and precipitation. The results from Tessier's sequential extraction suggested that more Cd would be diffused to the deeper soils, since the higher percentage of Cd were found in the exchangeable fractions. The results from soil column experiments and numerical analyses demonstrated that standard convection–dispersion equation (CDE) could better describe Cd breakthrough curve (BTCs) of column adsorption than the other three solute transport models within HYDRUS–1D software. Noteworthy, the horizontal and vertical migration distance of Cd (II) reached about 8 m after 50,000 days and 10 m after 15,000 days, respectively, indicating the easily mobile Cd (II) might be eventually released from contaminated soils to groundwater. Based on the above results, this study not only provided a new insight into the migration mechanism of Cd in the soil and groundwater environment, but was also beneficial to formulating remediation strategies of contaminated sites.

Study on non-Darcian flow and solute transport characteristics through a filled single fracture

ABSTRACT In this study, well-controlled laboratory experiments on flow and solute migration through a Filled-Single Fracture (FSF) of 3 mm aperture size were performed for different flow rates. Non-Darcian flow conditions prevailed throughout the experiment, which can be effectively explained using Forchheimer or Izbash’s equations. The constant mode was utilized to inject a conservative solute (NaCl) evenly through the FSF, following which the resulting Breakthrough curves (BTCs) were developed for three different average flow velocities. The computed BTCs were simulated by the conventional Convection-Dispersion Equation (CDE); and the single rate Mobile-Immobile (MIM) Model. A non-Fickian trend was observed for the attained BTCs, which cannot be interpreted by the Fickian CDE; however, the CDE model serves as the comparing base. MIM model was found much better than the CDE in fitting BTCs for three different average values of pore velocities. Statistical analysis on the goodness-of-fit further confirmed the suitability of MIM as much greater correlation coefficients were observed compared to the CDE model. Further, the BTCs were found to fit better at lower velocities. The greater computed dispersivity values are compatible with Taylor who suggested that much stronger dispersion occurs under non-Darcian flow conditions.