Unsaturated Soil Columns Research Articles

Estimating Soil Hydraulic Parameters By Using Green and Ampt Infiltration Equation

Infiltration is the process of water movement from the ground surface into the soil and is an important process in the hydrological cycle. In this study, a mathematical equation of the upward-advancing wet front is derived from the Green and Ampt infiltration model, and a new method is proposed to estimate the values of soil hydraulic parameters such as unsaturated hydraulic conductivity (K) and the capillary pressure head (Ψf). In the proposed method, these parameters are estimated from the advancing wet front data or infiltration test data under constant head. Experiments in which water flow is entered into the unsaturated soil column from beneath under constant head and the advancing wet front is recorded with the time are conducted on the five uniform sandy soils for different values of the saturation ratio (Sr). Results indicated that hydraulic parameters of unsaturated soils are not constant values. With increasing soil water content, K is decreased from 1.69Ks at Sr=0% to Ks at Sr=100%. But the val...

Leaching of Total Phosphorus in Greenhouse Soil in Relation to Soil Fertility and Fertilizer Application

Inappropriate applications of phosphorus (P) in agricultural production lead to the leaching loss of P, which subsequently contributes to the eutrophication of water bodies. A leaching experiment using unsaturated intact soil columns was conducted to study the influence of fertilizer application on leaching of phosphorus in a gley meadow soil at different fertility levels (low-, medium and high fertility levels). The soil column at each fertility level received three fertilization treatments (control [CK], manure [M] and chemical fertilizer [F]). The results indicated that the leaching loss of total P (TP) from the soil column was induced by the P input from either manure or chemical fertilizer application, and the extent of leaching loss of P was also positively related to the soil fertility level. In addition, the TP concentrations in the leachates from all fertilization treatments exceeded the critical value for water eutrophication (0.02 mg P/L). This suggests that applications of manure and chemical fertilizer at proper rates with close consideration of the soil fertility level are essential to reduce the leaching loss of TP to the environment.

Instability analysis and simulation of water infiltration into an unsaturated elasto-viscoplastic material

A linear instability analysis was performed in order to investigate which variables have a significant effect on the onset of the instability of an unsaturated viscoplastic material subjected to water infiltration. It was found that the onset of the growing instability of the material system mainly depends on the specific moisture capacity, the suction, and the hardening parameter. Then, in order to simulate the water infiltration process of a one-dimensional unsaturated soil column, a multiphase coupled elasto-viscoplastic finite element analysis was performed based on the theory of porous media. The results of the numerical simulations are discussed with respect to the effect of the specific moisture capacity and the initial suction on the development of volumetric strain. We found that rapid transitions from unsaturated to saturated states and higher levels of initial suction lead to the contractive behavior of the material and instability. The instability detected by the numerical results is consistent with the theoretical results obtained through the linear instability analysis.

EVALUATION OF SOLUTE TRANSPORT IN UNSATURATED SOIL COLUMN BY MULTI-COMPONENT FLOW MODEL

EVALUATION OF SOLUTE TRANSPORT IN UNSATURATED SOIL COLUMN BY MULTI-COMPONENT FLOW MODEL

Quantifying the Transport of Energetic Materials in Unsaturated Sediments from Cracked Unexploded Ordnance

Dissolved explosive species have been found in the groundwater under military training areas. These explosives are thought to originate from munitions although the mechanism of transport to the groundwater is poorly understood. This study was conducted to determine whether ruptured unexploded ordnance may be a viable source term for these explosives. The rupturing effect of one 81 mm-mortar exploding in close proximity to another 81-mm mortar was observed and the resulting contaminants were collected. These contaminants were then subjected to leaching experiments on repacked, jack drill compacted unsaturated sediment columns in a climate controlled laboratory. The mortars which were exposed to nearby explosions were shown to be susceptible to rupturing rather than sympathetically detonating under certain conditions. The ruptured mortars released up to 166+/-2 g of pulverized explosive residues (largely Composition B) and the results from the subsequent leaching tests showed that this explosive residue is highly mobile in unsaturated sandy soil. Up to 4.45+/-1.00 g of dissolved explosive contamination was transported through the unsaturated soil columns during the first year of infiltration. The results indicate the mass of transported explosive residue dissolved in the leachate was primarily caused by the preferential dissolution of explosive contaminants having a grain size under 0.125 mm. Surface or near-surface unexploded ordnance (UXO) on live fire ranges may therefore be significant sources of explosive environmental contamination after they have been exposed to other rounds which explode nearby.

Readily dispersible clay and particle transport in five Swedish soils under long-term shallow tillage and mouldboard ploughing

Abstract Soil disturbance by tillage and traffic often adversely affects aggregate stability, leading to colloid and particle mobilisation, which may result in crusting, low water infiltration rate and sediment transport. Hence, minimising soil tillage is an important step in conserving soil quality. This study investigated readily dispersible clay (RDC) and particle transport as affected by shallow tillage or mouldboard ploughing in five Swedish long-term reduced tillage experiments that had been underway for 15–31 years at the time of soil sampling. The soils of the experimental sites are Eutric and Dystric Cambisols. RDC of soil samples, collected in 0–10 cm and 12–17 cm depth, from mouldboard ploughed (MP) and shallow tillage (ST) plots was compared by measuring turbidity. Particle transport was also determined in undisturbed soil columns (20 cm in diameter and 20 cm high) from these treatments. RDC and particle transport were significantly lower for ST than for MP at three sites where clay content was above 30%. Particle transport in two soils (clay content above 40%) increased with irrigation events, indicating that preferential transport dominated in unsaturated soil columns, but matrix flow and, consequently, particle dispersion increased as the soil approached saturation. Despite more organic C accumulation in the upper 0–10 cm of ST than in the 12–17 cm soil layer, clay dispersion was lower in the latter. Particulate P (Total P − dissolved P) was well-correlated to turbidity, suggesting that analysis costs often can be cut by only measuring turbidity. Shallow tillage generally produced positive environmental effects without negative effects on crop yield, especially on soils with high clay content.

Leaching and transformability of transgenic DNA in unsaturated soil columns

Unsaturated soil columns were used to examine the transport of the plasmid pLEPO1 and plant DNA (transplastomic tobacco DNA), both carrying an antibiotic resistance gene ( aadA gene), and the capacity of bacteria to incorporate the gene in their genome after its passage through the soil. Soil columns containing a top leaf layer had sterile water percolated through them at a rate of 0.5 mL h −1. DNA from column leachate water was extracted and analyzed. Quantitative measurements included total DNA concentrations in the water and the transformation frequencies of Acinetobacter sp. BD413 by DNA in the column effluent. Qualitative measurements included the relative degradation of DNA after passage in the columns by agarose gel electrophoresis and the potential of effluent DNA to transform bacteria, leading to the production of antibiotic-resistant bacteria. The presence of aadA gene in the leachate water of soil columns suggests the mobility of DNA in unsaturated soil medium. The extent of DNA degradation was found to be proportional to its residence time in the soil column while a fraction of DNA was always able to incorporate into the Acinetobacter genome under all conditions studied. These results suggest that biologically active transgenic DNA might be transported downward by rain in unsaturated soils.

Noninvasive 3‐D Transport Characterization in a Sandy Soil Using ERT: 1. Investigating the Validity of ERT‐derived Transport Parameters

We used time‐lapse electrical resistivity tomography (ERT) and time‐domain reflectometry (TDR) probes to noninvasively capture three‐dimensional solute transport during four tracer experiments under different steady‐state irrigation rates in a large unsaturated undisturbed soil column (140 cm length and 116 cm inner diameter). The transport was characterized by means of apparent convection–dispersion parameters that were derived from breakthrough curves (BTCs) at different lateral scales: the ERT voxel scale, the sampling volume of TDR, and the cross‐section of the column. We validated the ERT‐derived data by means of mass balance, TDR probes, and the effluent BTC. We observed an excellent mass recovery by ERT. The ERT‐derived transport velocities exhibited minimal bias and high precision at the scale of the TDR measurements. On average the ERT‐derived column‐scale transport velocities were also not biased; however, the spatial variability of the voxel‐scale velocities within the column's cross‐sections underestimated the true velocity variability. In contrast to the transport velocities, the ERT‐derived dispersivities exhibited a large bias and low precision and were sensitive to temporal smoothing. Unlike previous studies, we did not find evidence that the ERT‐derived voxel‐scale dispersivities increase with decreasing ERT sensitivity. Although ERT provided unprecedented information about transport processes, resolution and uncertainty analyses remain important issues requiring further investigation.

Reflectometria no domínio do tempo em estudos de lixiviação de potássio em colunas de solo não-saturado

Este trabalho tem como objetivos projetar e avaliar o potencial de colunas de solo não-saturado equipadas com sondas de TDR (Time domain reflectometry) e acopladas a um sistema automatizado de aquisição de dados no monitoramento da dinâmica da água e de potássio no solo. O ensaio experimental foi conduzido em uma coluna de acrílico de 35cm de diâmetro interno e 105cm de comprimento preenchida com terra fina seca ao ar. A técnica da TDR possibilitou a obtenção de uma malha espaço-temporal em curtos intervalos de tempo e da umidade e da condutividade elétrica (Ce) nas diferentes profundidades consideradas. Tal resultado pode trazer uma importante contribuição para o entendimento da dinâmica da água e do potássio na terra, auxiliando os trabalhos para proposição e validação de modelos matemáticos que descrevem numericamente esses processos.

Effect of the Lower Boundary Condition and Flotation on Colloid Mobilization in Unsaturated Sandy Sediments

In unsaturated soil columns, the boundary condition imposed at the column outlet may cause experimental artifacts. Our objective was to study in situ colloid mobilization during transient, unsaturated flow as affected by the boundary condition at the column outflow. We conducted colloid mobilization experiments by infiltrating unsaturated sandy sediment columns under different bottom boundary conditions: a seepage and a suction control. The mechanisms of colloid mobilization were investigated using force calculations (adhesive and interfacial forces), complemented with flotation experiments, where colloids in the bulk fluid and at the liquid–gas interface were measured separately. More colloids were mobilized under seepage than under suction‐controlled boundary conditions. The shape of the colloid breakthrough curves also differed: for the seepage boundary, the maximum of the colloid concentration occurred at the beginning of the column outflow, but for the suction‐controlled boundary, colloid concentrations in the outflow increased gradually before reaching a maximum. Colloid mobilization increased with flow rate and decreased with ionic strength for both boundary conditions; however, colloids were mobilized even at ionic strength exceeding the critical coagulation concentration (CCC). Flotation experiments showed that colloids were located both in the bulk fluid and at the liquid–gas interface at electrolyte concentrations less than the CCC, but only at the liquid–gas interface when the CCC was exceeded. Theoretical considerations confirm that interfacial forces at the liquid–gas interface exceeded adhesive forces at all ionic strengths. Both experiments and theory show that the liquid–gas interface had a dominant effect on colloid mobilization.

Influence of organic waste type and soil structure on the bacterial filtration rates in unsaturated intact soil columns

Organic wastes are considered to be a source for the potentially pathogenic microorganisms found in surface and sub-surface water resources. Following their release from the organic waste matrix, bacteria often infiltrate into soil and may be transported to significant depths contaminating aquifers. We investigated the influence of soil texture and structure and most importantly the organic waste properties on the transport and filtration coefficients of Escherichia coli and total bacteria in undisturbed soil columns. Intact soil columns (diameter 16 cm and height 25 cm) were collected from two soils: sandy clay loam (SCL) and loamy sand (LS) in Hamadan, western Iran. The cores were amended with cow manure, poultry manure and sewage sludge at a rate of 10 Mg ha −1 (dry basis). The amended soil cores were leached at a steady-state flux of 4.8 cm h −1 (i.e. 0.12 of saturated hydraulic conductivity of the SCL) to a total volume of up to 4 times the pore volume of the columns. The influent ( C 0) and effluent ( C) were sampled at similar time intervals during the experiments and bacterial concentrations were measured by the plate count method. Cumulative numbers of the leached bacteria, filtration coefficient ( λ f), and relative adsorption index ( S R) were calculated. The preferential pathways and stable structure of the SCL facilitated the rapid transport and early appearance of the bacteria in the effluent. The LS filtered more bacteria when compared with the SCL. The effluent contamination of poultry manure-treated columns was greater than the cow manure- and sewage sludge-treated ones. The difference between cow manure and sewage sludge was negligible. The λ f and S R values for E. coli and total bacteria were greater in the LS than in the SCL. This indicates a predominant role for the physical pore-obstruction filtration mechanisms as present in the poorly structured LS vs. the retention at adsorptive sites (chemical filtration) more likely in the better structured SCL. While the results confirmed the significant role of soil structure and preferential (macroporous) pathways, manure type was proven to have a major role in determining the maximum penetration risk of bacteria by governing filtration of bacteria. Thus while the numbers of bacteria in waste may be of significance for shallow aquifers, the type of waste may determine the risk for microbial contamination of deep aquifers.

Impact of different land uses on the migration of two herbicides in a silt loam soil: unsaturated soil column displacement studies

SummaryDisplacement experiments of bromide and two herbicides, isoproturon and metribuzin, were performed in unsaturated undisturbed soil cores sampled on a unique site under (i) conventional wheat/maize rotation, (ii) 10‐year‐old grassed strip and (iii) 80‐year‐old oak/chestnut forest. Steady state conditions at constant rainfall intensity were used to compare the effect of the different land uses on soil herbicide transport and dissipation parameters. For each herbicide, mobility was similar between grassed strip and cultivated soils while it was reduced in the forest soils. The descriptive and moment analyses of the experimental breakthrough curves of bromide and herbicides demonstrated a similar degree of physical non‐equilibrium transport for the cropped and grassland soil columns, with transport ranging from a less intense physical non‐equilibrium status to the complete physical equilibrium state for the forest soil columns. Chemical non‐equilibrium transport was shown for the three situations. Degradation occurred in some columns as demonstrated by the elution of a metabolite of isoproturon. Previous data on sorption and degradation of isoproturon on the same soils were used to explain the differences observed in the analysis of the elution curves between grassed strip, forest and cultivated soil. Averaging Kd values from individual soil layers of each profile gave a satisfying understanding of isoproturon retardation factors. Sorption on soil organic matter was the main factor in herbicide mobility. However, other phenomena such as degradation and formation of non‐extractable residues were involved in the differences observed in leached masses of isoproturon between the three situations.

Release and leaching of plant DNA in unsaturated soil column

Little information is available on the process of DNA release from plants and neither is there much information to be found regarding DNA transport in the vadose zone. Unsaturated soil columns were used to examine the release and transport of DNA content in the leaf of tomato variety Palmiro, which was introduced into the soil columns after being dried at 35 °C for 3 days. Soil columns were leached with sterile water at a rate of 0.5 mL h −1 for 104 days. DNA from column leachate water was extracted and analysed. Both quantitative and qualitative measurements of extracted DNA were taken. Tomato gene sequences were detected using PCR amplification based on tomato specific 18S primers. The concentration of total DNA in soil column leachate water ranged from 6.7 to 50.4 μg L −1. Genomic analysis using agarose gel electrophoresis showed degradation of DNA after its passage through unsaturated soil columns. The presence of tomato genes in the leachate water of soil columns suggests a potential release of tomato DNA from leaves and potential transport of DNA over considerable distances in water-unsaturated soil. Consequently, transport of plant DNA in vadose zone is indicated, and there is a risk that the DNA may reach the groundwater.

HYSTERETIC MODELING ON THE CONVECTIVE TRANSPORT OF ORGANIC SOLVENT IN AN UNSATURATED SOIL ZONE

A mathematical model is described for the prediction of convective upward transport of an organic solvent driven by evaporation at the surface, which is known as the major transport mechanism in the in-situ photolysis of a soil contaminated with 2,3,7,8-tetrachlorodibenzo-p-dioxin(TCDD). A finite-element model was proposed to incorporate the effects of multiphase flow on the distribution of each fluid, gravity as a driving force, and the use of hysteretic models for more accurate description of k-S-p relations. Extensive numerical calculations were performed to study fluid flow through three types of soils under different water table conditions. Predictions of relative permeability-saturation-pressure (k-S-p) relations and fluids distribution for an illustrative soil indicate that hysteresis effects may be quite substantial. This result emphasizes the need to use hysteretic models in performing flow simulations including reversals of flow paths. Results of additional calculations accounting for hysteresis on the one-dimensional unsaturated soil columns show that gravity affects significantly on the flow of each fluid during gravity drainage, solvent injection, and evaporation, especially for highly permeable soils. The rate and duration of solvent injection also have a profound influence on the fluid saturation profile and the amount of evaporated solvent. Key factors influencing water drainage and solvent evaporation in soils also include hydraulic conductivity and water table configuration.

Vertical advection of extracellular DNA by water capillarity in soil columns

The fate of extracellular DNA in the environment concerns both the fate of transgenes from genetically modified organisms and the evolution of active bacteria capable of incorporating this DNA into their genomes. This study addressed the possibility that DNA, like other organic molecules, could move vertically in the capillary fringe of groundwater aquifers. The targeted gene fragment used here was the 35S- nptII sequence, which was below detection levels in controls. Initial microcosm studies detected the DNA target molecule by PCR during the entire experiment. The vertical advection of water and DNA were monitored for a period of 3 days in soil columns. DNA was added as a water solution at the bottom of the unsaturated soil column, and then DNA-free water was added at the bottom after 12 and 24 h. After the addition of the DNA solution, capillary water rose 4 cm within the soil column and the target DNA was detected up to that height. After 60 min, the entire soil column (10 cm) was wetted and the target sequence was detected up to a height of 7.5 cm. After the second wetting (12 h later), the target sequence was detected up to the top of the soil column (10 cm). However, after the third wetting (24 h later), the marker sequence was only found at heights from 0.5 to 4 cm. Results clearly show the vertical movement of DNA due the capillary rise and suggest the possibility of DNA degradation within the soil column.

Behavior of Unsaturated Layered Soil Columns during Infiltration

This paper presents the laboratory test results of vertical infiltration on two soil columns of finer over coarser soils subject to simulated rainfalls under conditions of no-ponding at the surface and constant head at the bottom. The main objectives were to evaluate the effect of rainfall intensity and duration; and to provide experimental evidence for soil water redistribution and hysteresis. The results show that rainfall intensity had a major effect on infiltration in the finer layer but had limited effect in the coarser layer due to the large difference of saturated permeability between the two layers. A relatively short rainfall duration resulted in a delayed response of pore pressure and water content to the rainfall after its cessation, while a relatively long duration did not result in such a delayed response. The delayed response indicated the redistribution of soil water in infiltration. Different paths of water content versus matric suction were followed during the tests indicating the apparent hysteretic behavior of soil water. In addition, the coarser layer restricted the increase of pore pressure in the finer layer. A minor variation of saturated soil permeability had minimal effect on infiltration.

A Flexible Water Content Probe for Unsaturated Soil Column Experiments

A commercially available soil moisture probe was modified by replacing rigid electrode waveguides with flexible electrode traces that can be attached to the interior of soil column walls. This new design minimizes soil packing difficulties and potential bias in water and solute flow pathways commonly associated with rigid probe installations in column experiments. Tests demonstrated that the modified flexible electrode design maintains a voltage response that is proportional to the electrode trace length and is reduced by approximately one-half when only one side of the electrode trace is in contact with the soil media. Laboratory experiments confirmed that the longer electrode traces attached to the interior of a column wall have a comparable sensitivity of that of a commercially available rigid probe inserted horizontally across the diameter of a soil column. The replacement of the rigid electrode with a flexible electrode offers an improved method of measuring the water content in solute transport experiments while minimizing sensor intrusion into laboratory soil columns.

Nitrate Leaching Through Unsaturated Soil Columns: Comparison Between Numerical and Analytical Solutions

Nitrate Leaching Through Unsaturated Soil Columns: Comparison Between Numerical and Analytical Solutions

Simulação do deslocamento de potássio em colunas verticais de solo não-saturado

O estudo do transporte de água e potássio em solo não-saturado é importante, tanto do ponto de vista do ambiente quanto do econômico. Assim sendo, o uso da modelagem computacional é importante, pois permite de maneira precisa e rápida o monitoramento do deslocamento de solutos, importante na prevenção de impactos ao ambiente. No presente trabalho, teve-se o objetivo de avaliar a simulação do deslocamento do íon potássio em colunas de solo não-saturado, utilizando o modelo MIDI, bem como apresentar a determinação dos parâmetros de transporte do íon potássio em um Latossolo Vermelho-Amarelo, fase arenosa. Concluiu-se que o modelo foi capaz de simular de maneira satisfatória o perfil de umidade e o deslocamento do íon potássio.

Sorbent Wicking Device for Sampling Hydrophobic Organic Compounds in Unsaturated Soil Pore Water. II: Chemical Capture, Recovery, and Analysis

Experiments investigated the chemical capture and quantitative recovery of chlorinated hydrophobic organic compounds (HOCs) sampled from the pore-water of unsaturated soils with a new in situ sorbent wicking device. The sampling device is comprised of an annular porous stainless-steel interface encasing granular-activated carbon and a fiberglass wick. Laboratory column studies using Ottawa sand spiked with tri- and tetrachlorobenzenes evaluated the accuracy of the sampling device in assessing average pore-water concentrations. The findings from these experiments showed that the sorbent-wick sampler provided reliable and repeatable estimates of the pore-water concentrations of the chlorobenzenes in the unsaturated soil columns at pore-water concentrations as low as several parts per billion. Field trials were conducted in a land biotreatment unit containing industrial soil/sludge materials to assess the availability of trace levels of aqueous-phase polychlorinated biphenyls (PCBs). The land treatment unit had undergone 5 years of intrinsic remediation and the test results indicated that the samplers were able to replicate the aqueous-phase PCB homolog pattern measured in ex situ aqueous extraction tests. Orthosubstituted PCBs were dominant in sorbent-wick samples and aqueous extracts; these PCBs appear relatively recalcitrant in land biotreatment units. The sorbent-wicking device offers the ability to obtain time-averaged mass and volumetric flux rates of HOCs in land treatment units.