Mobile Water Fraction Research Articles

Simulation of chloride transport in an aggregated soil using three conceptual models

Breakthrough curves (BTCs) of chloride displaced through columns of loessial soil aggregates of different sizes were measured under saturated steady flow conditions. The data were simulated using three conceptual models. Model I (CDE) assumed that all soil water was mobile and physical equilibrium existed in the system. Model II (two-region model) partitioned the soil water into mobile and immobile regions, and convective diffusive solute transport was limited to the mobile water region. Model III (two-flow region model) also divided the soil water into two regions based on their flow velocities, but both of the regions had a non-zero flow rate. Transfer of the chloride solute between the two soil water regions was assumed to occur at a rate proportional to the difference in solute concentration. The two unknown parameters in model I, three in model II, and four in model III were estimated by fitting the experimental data. The three models could well describe all the BTCs measured for columns packed with all the aggregate sizes at the low pore water velocity (0.68 cm/h); however, the values of the fitted parameters varied greatly. The Peclet numbers derived from both the two-region (model II) and two-flow region (model III) models behaved similarly and increased with increases in aggregate size. But the Peclet numbers derived from the convection dispersion equation (model I) were about two orders of magnitude greater than those derived from the other two models. The mobile water fraction obtained for the two-flow region model decreased with increases of aggregate size. The mass transfer coefficient decreased with an increase in pore water velocity due to the shorter residence time of the chloride solute in the soil columns.

On Evaluating Characteristics of the Solute Transport in the Arid Vadose Zone

The transport of bromide (Br) under matric heads of 0, -2, -5, and -10 cm using undisturbed soil columns was investigated for understanding the solute transport in arid soils. Undisturbed soil cores were collected at ground surface, directly below where tension infiltrometer measurements were made in the Amargosa Desert, Nevada, United States. Laboratory experiments were conducted by introducing water containing Br tracer into a soil column maintained at steady-state conditions. The observed data of breakthrough curves (BTC) were well fitted to an one-region model, except for the cores at saturation, and a core at the matric head of -5 cm, from which the observed data were better fitted to a two-region model. Fitted pore water velocities with the one-region model ranged from 1.2 to 56.6 cm/h, and fitted dispersion coefficients (D) ranged from 2.2 to 100 cm² /h. Results for the core analyzed with the two-region model indicated that D ranged from 27.6 to 70.9 cm² /h at saturation, and 25.7 cm² /h at the matric head of -5 cm; fraction of mobile water (β) ranged from 0.18 to 0.65, and mass transfer coefficient (ω) ranged from 0.006 to 0.03. In summary, the water fluxes and Br dispersion coefficients at investigated matric heads were very high due to the coarseness of the soils and possibly due to preferential flow pathways. These high water fluxes and Br dispersion coefficients would lead to a higher risk of deeper leaching accumulating nitrate nitrogen to the groundwater, and have significant effects on the desert ecosystem.

Caracterização hidrodinâmica e hidrodispersiva de um latossolo amarelo na microrregião do brejo paraibano

Para modelar o fluxo de água e o transporte de soluto em solos são necessários parâmetros hidrodinâmicos e hidrodispersivos como dados de entrada nos modelos numéricos. O experimento de infiltração axissimétrica utilizando um infiltrômetro de anel simples com um traçador conservativo (KCl) é um método simples e eficiente para determinar esses parâmetros no campo. O objetivo deste trabalho consistiu na caracterização hidrodinâmica e hidrodispersiva de um Latossolo Amarelo situado na microrregião do brejo paraibano, no município de Areia, PB, em nível de campo, por meio da infiltrometria de anel simples. Os experimentos foram realizados em uma área cultivada com feijão em uma malha de 50 x 50 m. A condutividade hidráulica saturada, Ks, e a sorvidade, S, foram estimadas pelas análises de infiltração tridimensional axissimétrica para tempos curto e longo. O soluto inerte (Cl-) foi utilizado para o cálculo da fração de água móvel, Φ, por meio da medição da concentração de soluto na camada do solo (0-15 cm) ao final do experimento de infiltração. Um modelo numérico de transferência de soluto com base no conceito de convecção-dispersão (CD) foi utilizado para definir os parâmetros (D e R) de transporte. A infiltrometria de anel simples demonstrou ser um método simples e eficiente para obtenção dos parâmetros hidrodinâmicos e hidrodispersivos. Observou-se boa concordância entre os valores medidos e calculados (v, D e R) com o modelo CD observado pelos coeficientes de determinação (r²).

Characterization of Cross-Linked Porous Gelatin Carriers and Their Interaction with Corneal Endothelium: Biopolymer Concentration Effect

Cell sheet-mediated tissue regeneration is a promising approach for corneal reconstruction. However, the fragility of bioengineered corneal endothelial cell (CEC) monolayers allows us to take advantage of cross-linked porous gelatin hydrogels as cell sheet carriers for intraocular delivery. The aim of this study was to further investigate the effects of biopolymer concentrations (5–15 wt%) on the characteristic and safety of hydrogel discs fabricated by a simple stirring process combined with freeze-drying method. Results of scanning electron microscopy, porosity measurements, and ninhydrin assays showed that, with increasing solid content, the pore size, porosity, and cross-linking index of carbodiimide treated samples significantly decreased from 508±30 to 292±42 µm, 59.8±1.1 to 33.2±1.9%, and 56.2±1.6 to 34.3±1.8%, respectively. The variation in biopolymer concentrations and degrees of cross-linking greatly affects the Young’s modulus and swelling ratio of the gelatin carriers. Differential scanning calorimetry measurements and glucose permeation studies indicated that for the samples with a highest solid content, the highest pore wall thickness and the lowest fraction of mobile water may inhibit solute transport. When the biopolymer concentration is in the range of 5–10 wt%, the hydrogels have high freezable water content (0.89–0.93) and concentration of permeated glucose (591.3–615.5 µg/ml). These features are beneficial to the in vitro cultivation of CECs without limiting proliferation and changing expression of ion channel and pump genes such as ATP1A1, VDAC2, and AQP1. In vivo studies by analyzing the rabbit CEC morphology and count also demonstrate that the implanted gelatin discs with the highest solid content may cause unfavorable tissue-material interactions. It is concluded that the characteristics of cross-linked porous gelatin hydrogel carriers and their triggered biological responses are in relation to biopolymer concentration effects.

Transport and retention of strontium in surface-modified quartz sand with different wettability

Instead of radioactive 90Sr, common strontium chloride was used to simulate the migration of radioactive strontium chloride in surface hydroxylated, silanized, and common quartz sand. The sorption and retardation characteristics of strontium (Sr2+) in these surface modified quartz sands were studied by batch tests and column experiments. The equilibrium sorption data for Sr2+ on different wettability sands were described by the Langmuir and Freundlich isotherm models, and the Langmuir model has been found to provide better correlation for hydrophilic sand. The breakthrough curves (BTCs) of Sr2+ in these media were analyzed with the equilibrium convection–dispersion equation (CDE) and a non-equilibrium two-region mobile–immobile model (TRM) using a nonlinear least square curve-fitting program CXTFIT. The TRM model showed better fit to the measured BTCs of Sr2+, and the parameters of the fraction of mobile water indicated that significant preferential flow effected the non-equilibrium transport of Sr2+. Although TRM model could not fit the Sr2+ BTCs very well, the parameter estimated by TRM model may be more reliable than those obtained from batch experiments because the transport of Sr2+ in these kind of sand is non-equilibrium processes.

Nutrients transport through variably structured soils

Soil structure development has a direct and indirect relation with hydraulic properties that generally control the transport of surface applied nutrients in unsaturated zones causing environmental implications. Nitrate () and phosphate () transport was assessed using undisturbed columns of variably structured calcareous soils, i.e. Shahdara (massive to weak coarse sub-angular blocky silt loam), Sultanpur (weak coarse, sub-angular blocky very fine sandy loam), Lyallpur (weak, coarse and medium sub-angular blocky silt loam), and Pacca (moderate, medium and fine sub-angular blocky silty clay). After conditioning the columns with 0.01 M CaCl2 (≈2 pore volumes), a pulse of KNO3 (50 mM) and KH2PO4 (32 mM) solution was applied and afterwards leached with 0.01 M CaCl2. The experimental breakthrough curves (BTCs) of and were described with one-domain and two-domain (mobile–immobile) convective dispersive equations (CDEs). Results showed that both and , applied in the same pulse, appeared in the effluent almost immediately after pulse application and peaked after 0.4 pore volume cumulative drainage in Lyallpur and Pacca columns. In contrast, in the Sultanpur and Shahdara columns, appearance of in the effluent was delayed till 0.6 pore volume and 0.4 pore volume cumulative drainage, respectively, and peaked at ≈1.0 pore volume cumulative drainage. The appearance of in the effluent was further delayed until 0.8 pore volume and 0.6 pore volume cumulative drainage, respectively. The despersivity, λ, that occurred in the Lyallpur and Pacca columns was significantly greater than in Shahdara and Sultanpur columns. While the fitted mobile water fraction, β, was significantly greater in Shahdara (0.81) and Sultanpur (0.90) than that of Lyallpur (0.27) and Pacca (0.28). It was concluded that variable structured soils had significant effect on solute transport parameters and should be considered in nutrient management studies.

Experimental study of solute transport under non-Darcian flow in a single fracture

Summary We have experimentally studied solute transport in a single fracture (SF) under non-Darcian flow condition which was found to closely follow the Forchheimer equation at Reynolds numbers around 12.2–86.0 when fracture apertures were between 4 mm and 9 mm. The measured breakthrough curves (BTCs) under the non-Darcian flow condition had some features that are difficult to explain using the Fickian type advection–dispersion equation (ADE). All the measured BTCs showed long tails, which might be caused by mass transfer between the boundary layer near the fracture wall and the mobile domain near the plane of symmetry, as supported by the boundary layer dispersion theories of Koch and Brady, 1985 , Koch and Brady, 1987 . A mobile–immobile (MIM) model was used to simulate the measured BTCs. To show that the MIM model was doing a better job than the ADE model in describing the observed BTCs, we conducted statistical analysis on the goodness of fitting with these two models. The results showed that the correlation coefficients for the MIM model were greater than those for the ADE model and were close to unity, indicating a nearly perfect fit with the MIM. The mass transfer rate between the mobile domain and the boundary layer increased when the mobile water fraction became larger. The best fit dispersivity values using the MIM model varied between 1.05 mm and 9.29 mm whereas their counterparts using the ADE model varied between 245 mm and 462 mm for the experimental condition of this study. Several issues such as the possible bimodal concentration distribution and the scale-dependent transport in a SF were discussed and would be investigated in the future.

Effects of Coal Gangue Content on Water Movement and Solute Transport in a China Loess Plateau Soil

AbstractThe mining industry has grown strongly in China in recent decades, resulting in large amounts of coal gangues, which cause water and soil pollution, soil erosion, and various other environmental problems. They are often used in reclamation projects in attempts to restore land damaged by mining, hence they are frequently present (in widely varying proportions) in the topsoil in areas around mines. Their presence can strongly affect key soil variables, including its bulk density, structure, water retention, water movement, and solute transport rates. In the study presented here, the effects of gangue contents on infiltration, saturated hydraulic conductivity, and solute transport parameters of a Chinese Loess plateau soil were examined. The results show that infiltration rates and saturated hydraulic conductivity decreased with increasing gangue content. The Peck–Watson equation modeled these relationships well, but Bouwer–Rice equations provided poorer matches with the acquired data. Cumulative infiltration over time was described well by both the Philip equation and Kostiakov equation. Both the simplified convection–dispersion equation and a two‐region model described the solute transport processes well. In addition, the dispersion increased, while both the Peclet number and mobile water fraction decreased, with increases in gangue contents.

Functional Assessment of Cross-Linked Porous Gelatin Hydrogels for Bioengineered Cell Sheet Carriers

An efficient carrier for corneal endothelial cell therapy should deliver and retain the cell sheet transplants at the site of injury without causing adverse effects. Here we introduced a simple stirring process combined with freeze-drying (SFD1) method for the development of gelatin hydrogels with enlarged pore structure that can improve the aqueous humor circulation. Samples fabricated by air-drying (AD) or freeze-drying method were used for comparison. After cross-linking with 1 mM 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC), the discs were investigated to assess their functionality. The simultaneous presence of ice crystals and gas bubbles resulted in large pore size (461 +/- 85 mum) and high porosity (48.0 +/- 1.9%) of SFD1 carriers. Among all of the samples studied, the SFD1 hydrogels showed the most appropriate swelling characteristics without squeezing effect on the anterior segment tissues of the eye. The enlarged pore structure also allowed carriers to contain the highest fraction of mobile water and exhibit the lowest resistance to the glucose permeation. In comparison with AD samples, the SFD1 materials had better cytocompatibility and biocompatibility and more effectively prevented a drastic change of intraocular pressure. Rheological measurements showed that the SFD1 hydrogels behaved like an elastic solid and had a tough (rigid and deformable) texture. As a temporary supporter, the biodegradable gelatin hydrogel could facilitate cell sheet transfer and avoid long-term residence of foreign carriers in the body. Our findings suggest that the gelatin discs with enlarged pore structure have potential as cell sheet carriers for intraocular delivery and corneal tissue engineering.

Comparison of the Mechanical Properties of Cellulose and Starch Films

Some effects of water at levels up to 25% (dry solids basis db) on regenerated cellulose films with a thickness of 100 mum were investigated by dynamic vapor sorption (DVS), X-ray diffraction, tensile testing, and proton-nuclear magnetic resonance (NMR). The sorption isotherm fitted by the D'Arcy and Watt model and the increase in NMR T(2) with water content suggest that a mobile water fraction appears at water contents above 10%db. Water absorption increased the crystallinity of cellulose films from 31% (dry) to 38% (25%db) and altered the dimensions of crystallites. Mechanical measurements on planar and notched samples at all the water contents used here showed ductile fracture behavior. Although the properties of water in these cellulose films are comparable to previously reported data on starch, cellulose films at low water content are much less brittle than starch. The reasons for this difference are explored.

Improving parameter estimation for column experiments by multi-model evaluation and comparison

The equilibrium convection dispersion equation model is often unable to accurately simulate breakthrough curves from column experiments. While the non-equilibrium convection dispersion equation model may match the data well, uncertainty in parameter estimates is often large. In this work we investigate approaches to improve match for the equilibrium model and reduce parameter estimate uncertainty for the non-equilibrium model. Four column experiment data sets are selected from the literature for the illustration. For the equilibrium convection dispersion equation model, we show that measurement error, presence of immobile water, and other mechanisms can cause mismatch between model predictions and observations because the model is sensitive to water content. The mismatch may be overcome by calibrating the effective water content. For the non-equilibrium convection dispersion equation model, simultaneous fitting of multiple tracers with reduced number of calibration parameters (e.g., assuming the dispersivity and mobile water fraction to be identical for different tracers, the mass transfer coefficient to be proportional to tracer molecular diffusion coefficient) can reduce the uncertainty in parameter estimate and better identify/quantify the non-equilibrium processes. By evaluating and comparing the multiple estimates obtained with different choices of calibration parameters (e.g., fixing or estimating water content), parameterizations and models (e.g., equilibrium or non-equilibrium), the reliability of the data interpretation can be improved by quantifying uncertainty in the experiment, considering alternative transport processes, and following the principle of parsimony.

Evaluation of Anomalous Solute Transport in a Large Heterogeneous Soil Column with Mobile-Immobile Model

This study uses the mobile-immobile model (MIM) and the traditional convection-dispersion equation (CDE) to analyze the observed breakthrough curves (BTCs) at different distances in a 1,250-cm-long saturated and highly heterogeneous soil column. It provides a simple method to determine the mobile water fraction independently as the ratio of effective porosity over total porosity of the packed soil materials. The effective porosity is calculated a priori as the ratio of measured flow rate and estimated pore-water velocity. It is found that there is a significant amount of immobile water in the soil column, resulting in the anomalous early breakthrough and tailing behaviors of the measured BTCs. Comparing to the CDE, the measured asymmetric BTCs at various scales can be better described by the MIM, especially their early arrival and long tailing parts. The degree of anomalous transport behavior in this large heterogeneous soil column is reduced with transport scale due to the increased mobile water fraction...

Effect of Particle‐Scale Heterogeneity on Uranium(VI) Transport in Unsaturated Porous Media

Uranium(VI) sorption and transport was evaluated in mixtures of silt loam and coarse sand sediments using traditional static batch sorption, saturated column, and unsaturated centrifugation experiments to evaluate the association of mobile and immobile water domains with particles of different size and surface reactivity. Exclusion of conservative tracers and a decrease in U sorption compared with what was predicted by the mass‐averaged equilibrium distribution coefficient (Kd‐mass‐avg) was observed in sediment mixtures where the mass fraction of silt loam was 10%. This is consistent with behavior that was previously reported for coarse and fine sand separates. No exclusion of the conservative tracer, as predicted for the moderate water content range, was measured during unsaturated transport in sediment mixtures that contained 30% or more silt loam by mass. Sorption under unsaturated conditions was greater than predicted based on the batch sorption measurement of Kd‐mass‐avg value, however, which suggests that the fine‐textured silt was in contact with the mobile water domain. This is the first evidence linking sorption to transport in a particular water domain. Results of this investigation demonstrate that the interaction between the geochemical and hydrodynamic processes has a profound effect on transport in unsaturated sediments. Definition of the fraction of mobile water was especially important for defining the front of the breakthrough curve, which is integral to predicting the arrival time of solutes at a particular depth and location in the sediment.

DSC approach for the investigation of mobile water fractions in aqueous solutions of NaCl and Tris buffer

The fraction of mobile (unfrozen) water during phase transitions of NaCl–H 2O solution and tris(hydroxymethyl)aminomethane (Tris) buffer solution has been determined by differential scanning calorimetry (DSC). Measurements were carried out in the temperature range between −50 and +30 °C with a heating rate of 2 K min −1. The fraction of mobile water was estimated from the enthalpy of melting of the different frozen phases present in aqueous solution samples. Results are supplemented by proton NMR intensity measurements in the same temperature range. A small endothermic peak was detected in the DSC curves at the same temperature where the change in the NMR intensity occurs. We assume that activation/deactivation of rotational molecular motion of hydration water molecules occurs at the steps of the NMR intensity at higher and lower temperatures during heating and cooling, respectively. The rotational motion is probably the initial stage of the eutectic phase separation. Tris additive to NaCl solution causes thermal shifts of the small endothermic peak and in the related NMR intensity. These qualitative differences indicate interactions of Na + and Cl − ions with the small organic molecule constituents.

Conceptual Model for Landfill Hydrologic Transport Developed Using Chloride Tracer Data and Dual-Domain Modeling

Waste-derived chloride present in a landfill cell was used as a hydrologic tracer in conjunction with a mathematical model to better elucidate landfill hydrologic dynamics for a municipal solid-waste landfill in which the cover material is significantly more permeable than the waste material. Long-term temporal chloride data from a lined landfill cell in Florida were used to calibrate a variably saturated dual-domain model (originally developed for soil-science research) to landfill hydrology and solute transport. The model successfully simulated the chloride temporal trend. The dual-domain processes were needed to simulate the long-term decline in conservative chloride concentrations. However, the temporal variations about the general trend could only be simulated accurately by considering the variations in landfill recharge. The fitted value of Darcy flux (0.274 cm/day) was less than literature values, and the dispersion coefficient (900 cm2/day) was higher than simple estimates based on heterogeneous aquifers. The other fitted parameters, first-order mass transfer coefficient (0.0018/day) and fraction of mobile water (0.22), were within the range of values reported in the literature. Model sensitivity studies were conducted to assess the relative importance of each parameter. The most sensitive parameter was recharge rate to the landfill, a factor that can be measured with the proper instrumentation. Although the model performance was not as sensitive to the values of the dual-domain parameters, inclusion of these parameters was required to accurately simulate the long-term temporal trend.

Tillage effects on soil strength and solute transport

To study the effect of different soil tillage practices and the consequences of soil deformation on the functioning of the pore system, we performed unsaturated leaching experiments (by applying a suction of −10 kPa) on undisturbed soil columns from the Ap-horizon of a luvisol derived from glacial till (agricultural site at Hohenschulen, North Germany). We compared two different tillage practices (conventionally tilled to 30 cm depth, and conservational chiselled to a depth of 8 cm-Horsch system) with respect to soil strength, pore connectivity and their effect on the fate of surface-applied fertilisers. The soil strength was measured by determining the precompression stress value (PCV). The conventionally tilled topsoil had a PCV of 21 kPa at a pore water potential of −6 kPa, while the conservation treatment resulted in a slightly higher PCV of 28 kPa, suggesting a slowly increasing soil strength induced by the formation of aggregates under reduced tillage practice. The leaching experiments were modelled using the convection dispersion equation (CDE) and a modified version of the mobile–immobile approach (MIM), which included three water fractions: mobile, immobile and totally immobile water. From the CDE mobile water fractions ( θ m) ranging from 47 to 67% were found, and θ m was slightly higher in the ploughed seedbed compared to the conservation-tilled one. This could be due to higher aggregation in the latter one. Dispersivities were relatively large, ranging from 44 to 360 mm, but no difference was found for the treatments. The MIM could simulate the drop in concentration when leaching was interrupted, but overall did not improve the simulation, despite the larger number of fitting parameters. Compacting the soil with loads of 70 kPa prior to the leaching experiment did not affect solute transport in the conservational tilled soil. In the conventional-tilled soil, however, the dispersivity decreased and the mobile water content increased compared to the non-compacted soil, suggesting that the former one is less prone to deformation by mechanical loads.

TDR estimation of electrical conductivity and saline solute concentration in a volcanic soil

Relative to montmorillonitic or kaolinitic soils, volcanic soils have atypical dielectric characteristics that interfere with the applicability of the Time Domain Reflectometry (TDR) technique for soil moisture ( θ) determination when common, empirical calibration equations are used. This particular dielectric response affects estimation of salinity in volcanic soils. Six TDR-based methods to estimate bulk electrical conductivity ( σ a) on a range of KCl saline reference solutions were compared, with Nadler's method giving the best results ( R 1:1 2=0.988). Three models (linear, non-linear and empirical) for predicting soil solution electrical conductivity ( σ w) based on σ a and θ, were experimentally tested on 24 hand-packed soil columns varying in salinity (Br −) from 0.2 to 4.0 dS m −1, each in four θ levels (36–58%). Rhoades' linear model performed better, especially for large water contents, than the other two ( R 1:1 2=0.986 vs. 0.976 and 0.983, respectively). An interpretation in terms of mobile vs. immobile volumetric fractions of water present in volcanic soils is suggested as a possible explanation for these results. The empirical model resulted over-parameterized and an alternative equation with fewer non-correlated parameters, σ a=( aθ 2+ bθ) σ w+ cθ 2, is proposed and tested with good results in volcanic soils from the Canary Islands and New Zealand. The equation encompasses both the relative dielectric dominance of the mobile water fraction at high water content typical of volcanic soils, and of the immobile fraction at low water contents. Simultaneous measurements made with a standard four-electrode probe and TDR gave good correlation ( R 2=0.964). A good linear correlation was also found between tracer concentration in the soil solution and σ w ( R 2=0.960). Nadler's and the new empirical model also tested with good results under dynamic (flow) conditions during a miscible displacement experiment in a large monolith using bromide as a tracer. The method reveals itself as a robust tool for solute transport studies under controlled salinity conditions in a volcanic soil.

Hydrodynamic Dispersion in an Unsaturated Dune Sand

Solutes spread relative to the mean displacement position during water flow in soils as a result of meandering through the (partially) saturated pore complex. Spreading is characterized by the hydrodynamic dispersion coefficient in the convection-dispersion equation (CDE). This coefficient has been extensively studied for saturated soils. In this study hydrodynamic dispersion coefficients for nonaggregated dune sand were determined as a function of volumetric water contents, θ, ranging from saturation to 0.08 cm3cm−3 in columns of 5-cm diam. and 25- to 40-cm length. Unit-gradient flow experiments were conducted to measure solute breakthrough curves (BTCs) using four-electrode salinity probes at several column depths. Transport parameters for the CDE and the mobile-immobile model (MIM) were determined by optimizing analytical solutions to observed BTCs. A maximum dispersivity, λ, of 0.97 cm was found at θ = 0.13, whereas for saturated flow λ ≈ 0.1 cm irrespective of pore-water velocity ranging from 208 to 5878 cm d−1 For the MIM, the mobile water fraction, θm/θ, gradually deceased from almost unity at saturation to a minimum of 0.85 at θ = 0.15 followed by a slight increase with further desaturation. The exchange time between the mobile and immobile phases, 1/α, was 0.1 to 0.2 d for θ > 0.15 presumably because of the relatively homogeneous flow with convective solute mixing. For lower θ, the exchange became much slower since flow predominantly occurs in water films enveloping sand particles. The Peclet number for molecular diffusion, Pe, decreases as the role of transverse diffusion increases at lower θ because of smaller v and thinner water films while the resistance increases for solute exchange between mobile and immobile phases. These combined effects lead to a maximum dispersivity value at intermediate water contents in the case of the nonaggregated dune sand.

Semianalytical solutions of radioactive or reactive solute transport in variably fractured layered media

In this paper, semianalytical solutions are developed for the problem of transport of radioactive or reactive solute tracers through a layered system of heterogeneous fractured media with misaligned fractures. The tracer transport equations in the nonflowing matrix account for (1) diffusion, (2) surface diffusion, (3) mass transfer between mobile and immobile water fractions, (4) linear kinetic or equilibrium physical, chemical, or combined sorption or colloid filtration, and (5) radioactive decay or first‐order chemical reactions. The tracer transport equations in the fractures account for the same processes, in addition to advection and hydrodynamic dispersion. Any number of radioactive decay daughter products (or products of a linear, first‐order reaction chain) can be tracked. The solutions, which are analytical in the Laplace space, are numerically inverted to provide the solution in time and can accommodate any number of fractured and/or porous layers. The solutions are verified using analytical and numerical solutions for limiting cases of solute and colloid transport through fractured and porous media. The effect of important parameters on the transport of 3H, 99Tc, 237Np, and 239Pu (and its daughters) is investigated in several test problems involving layered geological systems of varying complexity.

WATER TRANSFER AND MOBILE WATER CONTENT MEASUREMENT IN A CULTIVATED CRUSTED SOIL

In crusted soils, runoff and erosion at the surface are strongly controlled by soil infiltrability. An in situ hydrodynamic characterization of a cultivated crusted soil was conducted to define the factors that reduce infiltrability. Experiments were carried out on three layers of two different profiles in the topsoil: (i) on the surface crust, which was either sedimentary or structural, (ii) within the underlying soil, and (iii) at the plow pan surface. A structural crust is the result of gradual coalescing of aggregates by raindrop compaction, whereas a sedimentary crust is formed by deposition of the particles suspended in overland flow. The purpose here was to characterize water transfer as a function of vertical heterogeneity. A tension disc infiltrometer, along with an 18O solution, was used to create a near-saturated flow. Hydrodynamic properties and mobile water fraction of the soil surface were inferred from the cumulative infiltration and the soil solute concentration at the end of the experiments. Visual observations of X-ray images obtained from thin sections were used to emphasize some of the conclusions about the hydrodynamic characterization. Results of infiltration in soil covered with either more or less developed crusts were compared. Then, comparisons were made between the infiltrability of the underlying soil, which was covered by sedimentary or structural crusts. Finally, estimated values of hydraulic conductivity and the mobile water fraction for each layer of the two profiles provided information on water transfer. Results showed that the fraction of the soil surface covered by sedimentary crusts and structural crusts was an important factor for infiltrability (the cumulative infiltration at t = 5000 s varied between 11.5 mm and 14.8 mm in soil covered by a sedimentary crust, whereas the variation was between 18 mm and 22 mm in soil covered by a structural crust). On the other hand, infiltrability did not depend on the developmental stage of the surface crusts as the differences between the cumulative infiltration in more or less developed crusts were not significantly different at P = 0.05 (according to the Student’s t test). The sedimentary crust seemed to protect the underlying soil from aggregate coalescence. Thus, collapsing was less important in the underlying plowed material covered by a sedimentary crust. As a consequence, the mobile water fraction and effective mean pore size estimations showed that in the case of strongly collapsed material, coalescing increased the pore connection: water transferred through small but well connected pores (the effective mean pore size was λm = 0.105 mm, and the mobile water fraction was f = 0.93). When there was less collapsed underlying soil, the pores participating in transfer were bigger but less connected (λm = 1.2 mm and f = 0.5). The plow pan did not show strong impermeable behavior because the macropores made by roots were not sealed by plowing.