Miscible Displacement Experiments Research Articles

Effects of water content on reactive transport of 85Sr in Chernobyl sand columns

It is known that under unsaturated conditions, the transport of solutes can deviate from ideal advective–dispersive behaviour even for macroscopically homogeneous porous materials. Causes may include physical non-equilibrium, sorption kinetics, non-linear sorption, and the irregular distribution of sorption sites. We have performed laboratory experiments designed to identify the processes responsible for the non-ideality of radioactive Sr transport observed under unsaturated flow conditions in an Aeolian sandy deposit from the Chernobyl exclusion zone. Miscible displacement experiments were carried out at various water contents and corresponding flow rates in a laboratory model system. Results of our experiments have shown that breakthrough curves of a conservative tracer exhibit a higher degree of asymmetry when the water content decreases than at saturated water content and same Darcy velocity. It is possible that velocity variations caused by heterogeneities at the macroscopic scale are responsible for this situation. Another explanation is that molecular diffusion drives the solute mass transfer between mobile and immobile water regions, but the surface of contact between these water regions is small. At very low concentrations, representative of a radioactive Sr contamination of the pore water, sorption and physical disequilibrium dominate the radioactive Sr transport under unsaturated flow conditions. A sorption reaction is described by a cation exchange mechanism calibrated under fully saturated conditions. The sorption capacity, as well as the exchange coefficients are not affected by desaturation. The number of accessible exchange sites was calculated on the basis that the solid remained in contact with water and that the fraction of solid phase in contact with mobile water is numerically equal to the proportion of mobile water to total water content. That means that for this type of sandy soil, the nature of mineral phases is the same in advective and non-advective domains. So sorption reaction parameters can be estimated from more easily conducted saturated experiments, but hydrodynamic behaviour must be characterized by conservative tracer experiments under unsaturated flow conditions.

Displacement of oil by different interfacial tension fluids under ultrasonic waves

Insight into the physical principles governing the mobilization of oil by ultrasonic waves is vital for developing novel techniques of oil extraction. Various mechanisms such as peristaltic transport due to mechanical deformation of the pore walls, reduction of capillary forces due to the destruction of surface films, coalescence of oil drops due to the Bjerknes forces, oscillation and excitation of capillary trapped oil drops, forces generated by cavitating bubbles, and sonocapillary effects may be responsible for enhancing the flow of oil through porous media in the presence of an acoustic field. This paper aims at analyzing the influence of high-frequency, high-intensity ultrasonic radiation on the immiscible and miscible displacement. To analyze the viscous displacement under ultrasonic waves, experiments were performed on Hele–Shaw models. Immiscible (for mineral oil–brine and mineral oil–surfactant pairs) and miscible (for mineral oil–pentane, mineral oil–kerosene and mineral oil–2-propanol pairs) displacement experiments were performed. Changes on the displacement fronts and patterns were observed under ultrasound. Fractal techniques were used to analyze the changes and correlate them to the intensity of ultrasonic waves.

The Longitudinal Dispersion Coefficient of Soils as Related to the Variability of Local Permeability

Solute (NaCl) miscible displacement experiments are performed on long disturbed soil columns to determine the hydrodynamic longitudinal dispersion coefficient and correlate it with the variability of the local permeability. The solute concentration, averaged over several cross-sections along the soil column, is monitored by measuring the electrical resistance between rod electrodes. The measured solute concentration breakthrough curves are fitted simultaneously with the one-region and two-region analytical models of the 1-D advection–dispersion equation to estimate the longitudinal dispersion coefficient, D L, as a function of Peclet number, Pe, for common groundwater flow velocities (2 < Pe < 50). Macroscopic simulations of miscible displacement in 2-D porous media described by a periodic permeability field with low, moderate and high variability are employed to evaluate the predictability of the one-region and two-region models, and the sensitivity of the dispersion coefficients and flow velocities estimated from soil column displacement tests to the variance of local permeability. When the variability of the local permeability becomes high, the one-region model fails, while the two-region model is capable of reproducing satisfactorily the breakthrough curves, and providing reliable values of dispersion coefficients. The two mean pore velocities estimated by the two-region model represent, on average, a fast and a slow mean velocity of the dispersion front, whereas their difference is a measure of the transient evolution of the width of the equi-concentration dispersion front.

Transport of simazine in unsaturated sandy soil and predictions of its leaching under hypothetical field conditions

The potential contamination of groundwater by herbicides is often controlled by processes in the vadose zone, through which herbicides travel before entering groundwater. In the vadose zone, both physical and chemical processes affect the fate and transport of herbicides, therefore it is important to represent these processes by mathematical models to predict contaminant movement. To simulate the movement of simazine, a herbicide commonly used in Chilean vineyards, batch and miscible displacement column experiments were performed on a disturbed sandy soil to quantify the primary parameters and processes of simazine transport. Chloride (Cl −) was used as a non-reactive tracer, and simazine as the reactive tracer. The Hydrus-1D model was used to estimate the parameters by inversion from the breakthrough curves of the columns and to evaluate the potential groundwater contamination in a sandy soil from the Casablanca Valley, Chile. The two-site, chemical non-equilibrium model was observed to best represent the experimental results of the miscible displacement experiments in laboratory soil columns. Predictions of transport under hypothetical field conditions using the same soil from the column experiments were made for 40 years by applying herbicide during the first 20 years, and then halting the application and considering different rates of groundwater recharge. For recharge rates smaller than 84 mm year − 1 , the predicted concentration of simazine at a depth of 1 m is below the U.S. EPA's maximum contaminant levels (4 μg L − 1 ). After eight years of application at a groundwater recharge rate of 180 mm year −1 (approximately 50% of the annual rainfall), simazine was found to reach the groundwater (located at 1 m depth) at a higher concentration (more than 40 μg L − 1 ) than the existing guidelines in the USA and Europe.

Modeling Cadmium Transport in Soils Using Sequential Extraction, Batch, and Miscible Displacement Experiments

Asymmetric transport behavior of Cd in soils can result from sorption nonlinearity or rate-limited sorption, of which the relative significance is difficult to be discerned by transport modeling alone. Multiple approaches incorporating batch, sequential extraction, and miscible displacement experiments were therefore used in this study. Batch isotherm was used to indicate the range of Cd concentration where sorption became nonlinear. It is noteworthy that batch kinetics illustrated a shorter apparent equilibrium time of sorption at a higher Cd concentration, which presumably correlates with the significance of rate-limited sorption. In addition, sequential extraction results indicated that the relative contribution of rate-limited specific sorption to total sorption became negligible with increasing Cd concentration. Since the rate-limited specific sorption appears to be of low capacity, it is only important for sorption at low concentration. In miscible displacement experiments, Cd transport at 10 -5 M could only be simulated using a nonequilibrium model, whereas a nonlinear model was necessary for describing Cd transport at 10 -3 M. Therefore, this study demonstrated that transport behavior is predominantly influenced by rate-limited sorption at sufficiently low concentration, while by sorption nonlinearity at sufficiently high concentration. Additionally, sequential extraction can be a useful tool to illustrate the significance of rate-limited specific sorption and the corresponding chemical nonequilibrium transport behavior.

Simultaneous Adsorption of Calcium and Sulfate and Its Effect on Their Movement

Ion retention in variable-charge soils can be enhanced by the presence of certain ions with opposite charge, thereby influencing the movement of these ions through the soil profile. Studies examining these interactions are still incipient, however, especially regarding its modeling. We present results from batch and miscible displacement experiments describing Ca2+ and SO42−movement in a variable-charge soil from New Zealand. Evidence was found for ion-pair adsorption (IPA) of both Ca2+and SO42− The results were modeled using the convection–dispersion equation (CDE), coupled with two different mathematical approaches proposed to account for IPA. The first approach related IPA to the single soil adsorption capacity, which is governed by particle-surface phenomena. For the second approach, IPA was related solely to the soil solution concentration. Both these approaches described the adsorption data from the batch experiment reasonably well, as well as the breakthrough curves from the miscible displacement experiments. The first approach showed better overall agreement. Significant differences were found, however, when the adsorption parameters were identified by fitting models to data from either batch or miscible displacement experiments. Although more studies are needed to better understand IPA, our results showed that the extent of IPA can be large and it should not be ignored when predicting SO42− and Ca2+movement in variable-charge soils.

Discerning and Modeling the Fate and Transport of Testosterone in Undisturbed Soil

Testosterone is an endocrine disruptor that is released into the environment from natural and anthropogenic sources. The objective of this study was to achieve a better understanding of the complex fate and transport of this labile compound in an undisturbed agricultural soil (a Hamar Sandy, mixed, frigid typic Endoaquolls). This was done by using batch and miscible-displacement experiments, and by using a chemical nonequilibrium transport model. Sorption and transformations of testosterone were discerned using various batch experiments. The batch experiments indicated that the aqueous phase concentrations of testosterone rapidly decreased from 12 to 15% of the initial aqueous concentration within 5 h, but then gradually increased through time and reached 28 to 29% of the initial aqueous concentration at 168 h. The increase in the aqueous concentration was explained by mineralization and biodegradation. Multiple first-order models were used to describe batch experiments where simultaneous degradation and sorption processes occurred. An evolutionary global optimization strategy was used to estimate the process parameters from these batch experiments and there was high confidence in these parameter estimates. The result of column experiments also showed that 23.4% of testosterone was mineralized to CO2 as it transported through the column. Combustion analyses of extracted soil from inside the columns showed that most of the 14C retained in the column (69-74%) was sorbed in the top 5 cm. The independently determined batch parameters were incorporated into a chemical nonequilibrium transport model, which provided an excellent description of the hormone in the effluent, and vertical redistribution in the soil column.

Cation influence on sulfate leaching in allophanic soils

We have examined the influence of the ionic composition of the soil solution on the movement of sulfate and calcium in 2 New Zealand soils with differing allophane content. For this study, we have carried out a series of miscible displacement experiments using repacked and intact soil columns, in which sulfate was applied in the presence of either calcium or potassium as the accompanying cation. Our results showed that sulfate leaching was significantly retarded in the soil with higher allophane content when applied with calcium. On the other hand, no effects were observed for all studied soils when potassium was used as the accompanying cation. In addition, in soils with high allophane content, calcium also had its retention increased when sulfate was present. The increase in sulfate adsorption was accompanied by corresponding increase in calcium adsorption. These findings evidenced the presence of co-adsorption, or ion-pair adsorption (IPA), in allophane-containing soils. The extent of this adsorption is dependent on the soil pH, the accompanying cation, and the allophane content of the soil.

Dispersion of Dissolved Contaminants in Groundwater: from Visualization Experiments to Macroscopic Simulation

In order to estimate the longitudinal and transverse dispersivities of pore networks, an explicit method, based on the calculation of the transient variation of the spatial moments of the solute concentration over a disordered medium, is used to analyze visualization miscible displacement experiments and calculate the components of velocity vector and dispersion tensor. The longitudinal dispersion coefficient commonly estimated by using the 1-D advection dispersion coefficient is smaller than that calculated by the present method which accounts for the transverse dispersivity. The numerical predictions of the contaminant spreading in a macroscopically homogeneous porous medium are reliable presuming that correct values of longitudinal and transverse dispersivity are used.

Fate and transport of 1278-TCDD, 1378-TCDD, and 1478-TCDD in soil–water systems

The most toxic dioxin is 2,3,7,8-tetrachlorodibenzo-p-dioxin (2378-TCDD), and obtaining comprehensive experimental data for this compound is challenging. However, several nontoxic isomers of 2378-TCDD exist, and can provide significant experimental evidence about this highly toxic dioxin. The goal of this study was to obtain experimental evidence for the fate and transport of 2378-TCDD in natural soils using its nontoxic isomers, 1,2,7,8-tetrachlorodibenzo-p-dioxin (1278-TCDD), 1,3,7,8-tetrachlorodibenzo-p-dioxin (1378-TCDD), and 1,4,7,8-tetrachlorodibenzo-p-dioxin (1478-TCDD). Batch sorption and miscible-displacement experiments, in various soils, were done using [4-(14)C]-radiolabeled TCDDs, while metabolism of these compounds was monitored. The results from the batch experiments indicated a high sorption affinity of all the TCDD isomers to soils and a strong correlation to organic matter (OM) content. 1278-TCDD, 1378-TCDD and 1478-TCDD (TCDDs) were more tightly bound to the soil with high OM than to the soil with low OM; however, it took a longer contact time to approach sorption equilibrium of TCDDs in the soil with high OM. Miscible-displacement breakthrough curves indicated chemical nonequilibrium transport, where there was a rate-limited or kinetic sorption that was likely caused by OM. Combustion analyses of extracted soil from the soil columns showed that most TCDDs were adsorbed in the top 1-5 cm of the column. These column combustion results also showed that sorption was correlated to specific surface and soil depth, which suggested the possibility of colloidal transport.

Influence of DOC on MS-2 Bacteriophage Transport in a Sandy Soil

The purpose of this study was to investigate the influence of dissolved organic carbon (DOC) on MS-2 bacteriophage transport in a sandy soil. Miscible-displacement experiments were conducted to examine the retention and transport of MS-2, at two influent concentrations, in the absence and presence of DOC. Effluent recoveries of MS-2 were similar for both sets of experiments. The results of the experiments were analyzed with a mathematical model that incorporated inactivation and rate-limited attachment/detachment. The optimized attachment rate coefficients were similar for all experiments. These results indicate that DOC had no significant influence on the transport of MS-2 in this soil. A mass-balance analysis based on the mathematical-modeling results revealed that attachment was significant during the early stages of the experiments, and that the majority of attached MS-2 became inactivated by the time the experiments were completed.

Simulação do transporte e da sorção de imazaquin em colunas de solo

Dentre os métodos disponíveis para avaliar o comportamento de agrotóxicos no solo, destacam-se o método "batch" (análise em batelada) e o deslocamento miscível. O objetivo deste estudo foi avaliar a sorção e a mobilidade do imazaquin em solos com diferentes características químicas, físicas e mineralógicas, utilizando as técnicas de deslocamento miscível e "batch". Os ensaios foram realizados no laboratório de Ecotoxicologia do Centro de Energia Nuclear na Agricultura da Universidade de São Paulo (CENA/USP), Piracicaba, São Paulo. Os solos utilizados, classificados como Neossolo Quartzarênico (RQ), Latossolo Vermelho-Amarelo (LVA) e Latossolo Vermelho distroférrico (LVdf), foram secos ao ar e peneirados em malha 2 mm, sendo então procedida a caracterização química, física e mineralógica das amostras. Foi selecionada para o estudo a molécula do Imazaquin. No ensaio do deslocamento miscível, foi aplicada uma solução de CaCl2, obtendo-se uma curva de eluição do Cl-, utilizado como traçador. As curvas de eluição do Cl- foram ajustadas a um modelo baseado na equação de convecção-dispersão para transporte ideal. Após a eluição do Cl-, aplicou-se a solução que continha 14C-imazaquin que gerou as curvas de eluição para esta molécula. A curva foi ajustada a um modelo bicontínuo que considera o não-equilíbrio durante o transporte. No ensaio de sorção com o método "batch", foram utilizadas soluções de imazaquin radiomarcado variando de 0,67 a 10,72 mmol L-1 para obtenção das isotermas ajustadas pela equação de Freundlich. A taxa de lixiviação foi alta para todos os solos, sendo maior para o RQ. As curvas de eluição do Cl- foram simétricas para todos os solos e apresentaram um bom ajuste ao modelo que considera o transporte ideal, indicando que não houve não-equilíbrio relacionado com o transporte. Para o imazaquin, as curvas de eluição foram assimétricas, com prolongamento da parte posterior da curva, e apresentaram bom ajuste ao modelo bicontínuo, evidenciando que ocorreu não-equilíbrio no processo de sorção. A comparação dos dois métodos empregados mostrou que o coeficiente de sorção determinado no método "batch" foi maior. Tal resultado evidencia que a sorção, durante o processo de lixiviação, pode ser superestimada por métodos que consideram a sorção sob condições de equilíbrio químico.

Comparison of Escherichia coli and Campylobacter jejuni Transport in Saturated Porous Media

Due to the difficulties in testing for specific pathogens, water samples are tested for the presence of nonpathogenic indicator organisms to determine whether a water supply has been contaminated by fecal material. An implicit assumption in this approach is that where pathogenic microorganisms are present fecal indicator organisms are present as well; yet surprisingly few studies have been conducted that directly compare the transport of indicator organisms with pathogenic organisms in ground water environments. In this study we compared the cell properties and transport of Escherichia coli, a commonly used indicator organism, and Campylobacter jejuni, an important enteropathogen commonly found in agricultural wastes, through saturated porous media. Differences in cell properties were determined by measuring cell geometry, hydrophobicity, and electrophoretic mobility. Transport differences were determined by conducting miscible displacement experiments in laboratory columns. Under the experimental conditions tested, C. jejuni was much more negatively charged and more hydrophobic than E. coli. In addition, C. jejuni cells were slightly longer, narrower, and less spherical than E. coli. The variations in cell properties, primarily surface charge, resulted in significant differences in transport between these two microorganisms, with the transport of C. jejuni exceeding that of E. coli when conditions favored low attachment rates, thus calling into question the usefulness of using E. coli as an indicator organism for this important pathogen.

Transport of molybdenum in a biosolid-amended alkaline soil

The transport of molybdenum (Mo) in a biosolid-amended, alkaline, agricultural soil was examined in the research reported herein. Batch-equilibrium and miscible-displacement experiments were conducted to examine the transport of Mo in soil with and without 10% by weight biosolid amendment. The results of geochemical modeling, conducted using PHREEQC, indicated that no mineral dissolution or precipitation reactions were expected for the system under the prevailing conditions. Sorption equilibrium coefficients ( K d ) obtained from moment analysis of the Mo breakthrough curves were similar to those calculated from the results of the batch-equilibrium experiments. Mo sorption was greater for the biosolid-amended soil ( K d of 1.3 versus 0.35). Sorption of Mo was shown to be linear, rate limited, and reversible for both unamended and amended soil. The results suggest that Mo associated with biosolid-amended soils is relatively bioavailable and mobile.

Biodegradation during contaminant transport in porous media: 8. The influence of microbial system variability on transport behavior and parameter determination

The impact of microbial system variability on the biodegradation and transport behavior of a model solute, salicylate, was investigated with a series of miscible displacement experiments. Four systems of increasing complexity were employed: a sterilized, well‐sorted sand inoculated with a single bacterial isolate, a sterilized soil inoculated with the same isolate, and two soils, each of which contained an indigenous multiple‐population community of bacteria. The experiments were conducted in replicate (three or four experiments per set) and with paired controls. The biodegradation and transport behavior of salicylate exhibited a small degree of variability among the replicates for the two inoculated systems and a relatively large degree of variability for the two indigenous systems. The greater variability observed for the two indigenous systems is attributed primarily to greater variability of microbial system properties, such as initial cell density, metabolic status, and community composition. Values for maximum specific growth rate coefficient, mean lag time, and lag time variance were determined by model calibration to the measured breakthrough curves and compared to values obtained from batch experiments. Reasonable correspondence was observed between the two sets of values for both the inoculated and indigenous systems. The maximum specific growth rate coefficient exhibited a relatively small degree of uncertainty for all four systems, whereas greater uncertainty was associated with the lag time mean and variance. The variability in calibrated parameters among each set of replicate experiments was significantly greater than the uncertainty associated with the individual experiment calibrations and the measured input parameters. These results illustrate that variability inherent to natural microbial systems can cause variability in transport behavior even under controlled laboratory conditions and concomitantly enhance the uncertainty of biokinetic parameters obtained from laboratory studies.

Measurement of effect of chemical reactions on the hydrologic properties of fractured glass media using a tri-axial flow and transport apparatus

Summary Understanding the effect of chemical reactions on the hydrologic properties of sub-surface media is critical to many natural and engineered sub-surface systems. Methods and information for such characterization of fractured media are severely lacking. Influence of glass corrosion (precipitation and dissolution) reactions on fractured glass blocks HAN28 and LAWBP1, two candidate waste glass forms for a proposed immobilized low-activity waste (ILAW) disposal facility at the Hanford, WA site, was investigated. Flow and tracer transport experiments were conducted in such randomly and multiply fractured ILAW glass blocks, before and after subjecting them to corrosion using vapor hydration testing (VHT) at 200 °C temperature and 200 psig (1379 KPa) pressure, causing the precipitation of alteration products. A tri-axial fractured media flow and transport experimental apparatus, which allows the simultaneous measurement of flow and transport properties and their anisotropy, has been designed and built for this purpose. Such apparatus for fractured media characterization are being reported in the literature only recently. Hydraulic properties of fractured blocks were measured in different orientations and along different cardinal directions, before and after glass corrosion reactions. Miscible displacement experiments using a non-reactive dye were also conducted, before and after glass corrosion reactions, to study the tracer transport behavior through such media. Initial efforts to analyze breakthrough curve (BTC) data using a 1D advection dispersion equation (ADE) solution revealed that a different fractured media transport model, which accurately accounts for the heterogeneous transport behavior in 3D, may be necessary for such interpretation. It was found that glass reactions could have a significant influence on the hydrologic properties of fractured ILAW glass media. The methods and results are useful to better understand the effect of chemical reactions on the hydrologic properties of fractured geomedia in general and glass media in particular.

Mobility of pharmaceuticals carbamazepine, diclofenac, ibuprofen, and propyphenazone in miscible-displacement experiments

Many pharmaceuticals pass the unsaturated zone before reaching an aquifer. Therefore, laboratory sand column transport experiments were conducted to study the transport behavior of carbamazepine, diclofenac, ibuprofen, and propyphenazone under unsaturated conditions. The test water was artificial sewage effluent to simulate the infiltration of reused wastewater. The test water was spiked with the pharmaceutically active compounds and the tracer LiCl. Afterwards it was passed through laboratory sand columns, one experiment for each pharmaceutical. The physical and chemical parameters were recorded and general ions measured. Pharmaceuticals were measured using solid phase extraction, derivatization, and detection with GC-MS. The column experiments indicate a significant elimination of ibuprofen (54%), propyphenazone (55%), and diclofenac (35%), whereas carbamazepine was not eliminated. Retardation factors varied between 1.84 for carbamazepine, 2.51 for propyphenazone, 3.00 for ibuprofen, and 4.80 for diclofenac. These results show that mobility and elimination of diclofenac, ibuprofen, and propyphenazone is about in the same range as for experiments under saturated conditions whereas carbamazepine had a significantly lower sorption and elimination under unsaturated conditions.

Biodegradation during contaminant transport in porous media: 7. Impact of multiple‐degrader community dynamics

The biodegradation and transport of phenanthrene in porous media containing multiple populations of phenanthrene degraders is examined with a series of miscible-displacement experiments. A long-term experiment was conducted with a soil containing an indigenous microbial community comprised of 25 identified phenanthrene-degrading isolates. The rate and magnitude of phenanthrene biodegradation oscillated throughout the six-month experiment. This behavior, at least in part, is attributed to multiple-population dynamics associated with the indigenous community of phenanthrene degraders, the composition of which changed during the experiment. This hypothesis is supported by the results of experiments conducted using sterilized porous media that were inoculated with selected isolates obtained from the indigenous soil community. The results of experiments conducted with sterilized soil inoculated with isolate A exhibited an initial extended period of steady phenanthrene effluent concentrations, followed by a uniform decline. The results of experiments conducted using sterilized sand for single-isolate systems with one of three selected isolates and for systems of two-isolate combinations, indicate the existence of apparent synergistic and antagonistic interactions among the isolates. For example, phenanthrene biodegradation was relatively extensive and occurred without a lag phase for isolate A alone. However, biodegradation was constrained when isolate A and B were combined, indicating an antagonistic interaction. Conversely, whereas extensive lag phases were exhibited by both isolates B and C for the single-isolate experiments, there was minimal lag when isolates B and C were combined, indicating a synergistic interaction.

Transport of cadmium, nickel, and zinc in Taoyuan red soil using one-dimensional convective–dispersive model

Pollution of agricultural lands by heavy metals has become more serious with increasing pollutants emitted from electroplate plants of northern Taiwan. This study aimed to assess the retardation factor ( R) and the dispersion coefficient ( D) using one-dimensional convective–dispersive model by comparing the breakthrough curve (BTC) and least squares methods. An acrylic column, with an inner diameter of 9 cm and a length of 15 cm, was uniformly packed with soil for metal leaching experiments. An unpolluted surface (0–20 cm) of the Taoyuan red soil was collected for this study. In miscible displacement experiments, a mixture of 4 mg L − 1 CdCl 2, 20 mg L − 1 NiCl 2, and 60 mg L − 1 ZnCl 2 was leached from top of the column and the effluent was collected in increments with the aid of a fraction collector. The retardation factor ( R) calculated from the least squares method was close to the R value observed from breakthrough curves. The R value showed the trend: Cd>Zn>Ni, indicating the rate of metals transport in red soils: Ni>Zn>Cd. Their reactivity was assessed within the framework of the hard–soft acid–base principle (HSAB). From sequential extractions, most of the Cd remained in exchangeable fractions, and Ni and Zn were found to exist in residual Fe- or Mn-oxides, and organic matter fractions.

Using TDR and Inverse Modeling to Characterize Solute Transport in a Layered Agricultural Volcanic Soil

Volcanic soils exhibit particular physical‐chemical properties (i.e., strong and stable natural aggregation and high content of variable‐charge minerals) that may influence solute transport. To determine if such techniques like TDR and inverse modeling are useful for analyzing solute transport in volcanic soils, we studied the governing transport processes by means of a miscible displacement experiment of Br− in a large undisturbed soil monolith. Bromide resident concentrations at several depths were monitored successfully with TDR technology, while parameters for the convective–dispersive (CDE) and mobile–immobile (MIM) transport models were estimated by inverse modeling. For the relatively high soil moisture conditions, typical of high frequency‐irrigation systems that we considered, Br− was found to move slowly by convection–dispersion. Simulations with the CDE and MIM transport models yielded very similar results. Although Br− is generally assumed to behave as a tracer, we found that this anion in our experiment was subject to adsorption at the bottom part of the monolith. This may be explained by the variable‐charge nature of the minerals (Fe and Al oxihydroxides) present in this volcanic soil, which exhibited anion exchange when the pH of the soil solution decreased below the zero point of charge.