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  • 1
    In: Soil Science Society of America Journal, January 2012, Vol.76(1), pp.51-60
    Description: Solute diffusivity in soil plays a major role in many important processes with relation to plant growth and environmental issues. Soil solute diffusivity is affected by the volumetric water content as well as the morphological characteristics of water‐filled pores. The solute diffusivity in intact soil samples from two different tillage treatments (soil from below the depth of a harrow treatment and soil from within a moldboard plowed plow layer) was estimated based on concentration profiles using a newly developed method. The method makes use of multiple tracers (two sets of counterdiffusing tracers) for a better determination of the diffusivity. The diffusivity was higher in the below‐till soil than the plowed soil at the same soil water matric potential due to higher water content but also due to higher continuity and lower tortuosity of the soil pores. We measured identical solute diffusivities independent of the tracer set used. We analyzed the whole data set using Archie's law and found a linear relation between Archie's exponent and the logarithm of the soil water matric suction in centimeters of water (pF). An analysis of seven data sets from the literature showed that this was a general trend for soils with moderate to low clay contents.
    Keywords: Clay ; Data Collection ; Soil Water Content ; Soil Pore System ; Soil Treatment ; Solutes ; Soil Sampling ; Soil Water ; Plowing ; Tracer Techniques ; Plant Growth ; Water Content ; Harrows ; Diffusivity;
    ISSN: 0361-5995
    E-ISSN: 1435-0661
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  • 2
    Language: English
    In: Journal of Hydrology, February 2015, Vol.521, pp.498-507
    Description: The present study proposes a new two-step approach to prediction of the continuous soil water characteristic (SWC) from saturation to oven-dryness from a limited number of measured textural data, organic matter content and dry bulk density. The approach combines dry- and wet-region functions to obtain the entire SWC by means of parameterizing a previously developed continuous equation. The dry region function relates gravimetric soil fractions to adsorptive forces and the corresponding water adsorbed to soil particles. The wet region function converts the volumetric particle size fractions to pore size fractions and utilizes the capillary rise equation to predict water content and matric potential pairs. Twenty-one Arizona source soils with clay and organic carbon contents ranging from 0.01 to 0.52 kg kg and 0 to 0.07 kg kg , respectively, were used for the model development. The SWCs were measured with Tempe cells, a WP4-T Dewpoint Potentiameter, and a water vapor sorption analyzer (VSA). The model was subsequently tested for eight soils from various agricultural fields in Denmark with clay contents ranging from 0.05 to 0.41 kg kg . Test results clearly revealed that the proposed model can adequately predict the SWC based on limited soil data. The advantage of the new model is that it considers both capillary and adsorptive contributions to obtain the SWC from saturation to oven-dryness.
    Keywords: Capillarity ; Adsorption ; Unsaturated Soil ; Water Retention ; Soil Moisture ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 3
    Language: English
    In: Soil Science Society of America Journal, March-April, 2014, Vol.78(2), p.422(12)
    Description: Changes in soil water content are known to affect soil reflectance. Even though it was suggested some time ago that the phenomenon of increased forward scattering due to the presence of water in the soil is related to water film thickness and matric potential, there has been no detailed investigation of this in any studies. The effects of moisture conditions on visible near-infrared (vis-NIR) spectra of four representative soils in Denmark have been assessed as a function of both water film thickness (expressed as the number of molecular layers) and matric potential. Complete water retention curves, from wet (pF 0.3, pF = log(|j|), where ? is the matric potential in cm) to hyper dry end (oven-dried and freeze-dried soil), were obtained by initial wetting followed by successive draining and drying of soil samples, performing NIR measurements at each step. Soil reflectance was found to decrease systematically, yet not proportionally, with decreasing matric potential and increasing molecular layers. The changes in molecular layers were best captured by the soil reflectance of clay-rich soils. Here the largest increase in reflectance occurred between pF 3 and 4, caused by the shift from capillary to adsorptive surface forces. In support of this, the smallest changes in reflectance were seen in the sandiest soil. Freeze drying the soil highest in organic C increased reflectance, possibly due to an alteration in organic matter during freezing. The different reflectance behavior of soil with a higher organic C content may be linked to differences in the amount, but also the quality (higher hydrophobicity) of the organic matter. However, this needs to be confirmed in further studies.
    Keywords: Reflectance -- Research ; Molecules -- Research ; Soil Research
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 4
    Language: English
    In: Soil Science Society of America journal, 2011, Vol.75(4), pp.1315-1329
    Description: Accurate predictions of the soil-gas diffusivity (Dp/Do, where Dp is the soil-gas diffusion coefficient and Do is the diffusion coefficient in free air) from easily measureable parameters like air-filled porosity (ε) and soil total porosity (ϕ) are valuable when predicting soil aeration and the emission of greenhouse gases and gaseous-phase contaminants from soils. Soil type (texture) and soil density (compaction) are two key factors controlling gas diffusivity in soils. We extended a recently presented density-corrected Dp(ε)/Do model by letting both model parameters (α and β) be interdependent and also functions of ϕ. The extension was based on literature measurements on Dutch and Danish soils ranging from sand to peat. The parameter α showed a promising linear relation to total porosity, while β also varied with α following a weak linear relation. The thus generalized density-corrected (GDC) model gave improved predictions of diffusivity across a wide range of soil types and density levels when tested against two independent data sets (total of 280 undisturbed soils or soil layers) representing Danish soil profile data (0–8 m below the ground surface) and performed better than existing models. The GDC model was further extended to describe two-region (bimodal) soils and could describe and predict Dp/Do well for both different soil aggregate size fractions and variably compacted volcanic ash soils. A possible use of the new GDC model is engineering applications such as the design of highly compacted landfill site caps. ; p. 1315-1329.
    Keywords: Data Collection ; Sand ; Soil Profiles ; Texture ; Landfills ; Prediction ; Engineering ; Soil Density ; Porosity ; Models ; Peat ; Greenhouse Gas Emissions ; Aeration ; Air ; Volcanic Ash Soils ; Diffusivity
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 5
    Language: English
    In: Vadose Zone Journal, 2016, Vol.15(4), p.0
    Description: Detailed characterization of partially saturated porous media is important for understanding and predicting vadose zone transport processes. While basic properties (e.g., particle- and pore-size distributions and soil-water retention) are, in general, essential prerequisites for characterizing most porous media transport properties, key transport parameters such as thermal conductivity and gas diffusivity are particularly important to describe temperature-induced heat transport and diffusion-controlled gas transport processes, respectively. Despite many experimental and numerical studies focusing on a specific porous media characteristic, a single study presenting a wide range of important characteristics, together with the best-performing functional relationships, can seldom be found. This study characterized five differently textured sand grades (Accusand no. 12/20, 20/30, 30/40, 40/50, and 50/70) in relation to physical properties, water retention, hydraulic conductivity, thermal conductivity, and gas diffusivity. We used measured basic properties and transport data to accurately parameterize the characteristic functions (particle- and pore-size distributions and water retention) and descriptive transport models (thermal conductivity, saturated hydraulic conductivity, and gas diffusivity). An existing thermal conductivity model was improved to describe the distinct three-region behavior in observed thermal conductivity–water saturation relations. Applying widely used parametric models for saturated hydraulic conductivity and soil-gas diffusivity, we characterized porous media tortuosity in relation to grain size. Strong relations among average particle diameter, characteristic pore diameter from soil-water retention measurements, and saturated hydraulic conductivity were found. Thus, the results of this work are useful toward better describing, linking, and predicting mass transfer and pore network properties in variably saturated porous media.
    Keywords: Agriculture;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 6
    Language: English
    In: Vadose Zone Journal, 2012, Vol.11(1), p.0
    Description: Gas diffusion is a dominant transport mechanism for climate and regulated gases in the vadose zone. Soil-gas diffusion is governed by the gas diffusion coefficient (D (sub p) , m (super 2) s (super -1) ) which is highly dependent on soil texture, such as sand, silt, clay, and organic matter contents, as well as soil physical properties such as soil-air content (epsilon , m (super 3) m (super -3) ) or total porosity (Phi , m (super 3) m (super -3) ). Soil organic matter is a key contributor to the formation of the soil pore structure (i.e., total porosity and air-filled pore tortuosity), and it highly affects D (sub p) behavior under variably saturated conditions. In this study, based on numerous D (sub p) data sets across soil types including sands, loamy clay soils, volcanic ash soils, and organic soils, predictive D (sub p) models incorporating a percolation threshold (epsilon (sub th) , m (super 3) m (super -3) ) and pore tortuosity factor (X') are proposed. The observed relations between either epsilon (sub th) or X' and either Phi or volumetric organic matter fraction (OMF, m (super 3) m (super -3) ) were embedded in the proposed D (sub p) model. The proposed D (sub p) models, coupled with predictive epsilon (sub th) and OMF models, performed well against the measured D (sub p) data across soil types. Finally, a sensitivity analysis of the OMF in relation to the D (sub p) and pore-network tortuosity (T) showed a reduction in D (sub p) and increase in T with increasing OMF under the same epsilon conditions.
    Keywords: Soils ; Asia ; Diffusion ; Far East ; Gases ; Grain Size ; Hokkaido ; Honshu ; Hydrology ; Japan ; Loam ; Memuro Japan ; Moisture ; Nishi-Tokyo Japan ; Organic Compounds ; Percolation ; Permeability ; Physical Properties ; Porosity ; Power Law ; Saitama Japan ; Saturation ; Size Distribution ; Soil Gases ; Soils ; Statistical Analysis ; Tortuosity ; Transport ; Unsaturated Zone ; Volcanic Soils ; Water;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 7
    Language: English
    In: Soil Science Society of America journal, 2012, Vol.76(3), pp.845-852
    Description: Subsurface gaseous-phase transport is governed by three gas transport parameters: the air permeability coefficient (k(a)), gas diffusion coefficient (D(P)), and gas dispersion coefficient (D(H)). Among these, D(H) is the least understood due to hitherto limited research into the relationship between gas dispersion and soil physical characteristics. In this study, a series of advection-dispersion experiments was performed on granular porous media to identify the effects of soil column dimensions (length and diameter), particle size and shape, dry bulk density, and moisture content on the magnitude of gas dispersion. Glass beads and various sands of different shapes (angular and rounded) with mean particle diameters (d(50)) ranging from 0.19 to 1.51 mm at both air-dry and variable moisture contents were used as granular porous media. Gas dispersion coefficients and gas dispersivities (α = D(H)/v, where v is the pore-gas velocity) were determined by fitting the advection-dispersion equation to the measured breakthrough curves. For all test conditions, D(H) increased linearly with v. The test results showed that neither soil column length nor diameter had significant effect on gas dispersivity. Under air-dry conditions, higher gas dispersivities were observed for media with wider particle size distribution and higher dry bulk density. The minor effect of particle shape on gas dispersivity was found under both air-dry and wet conditions. Under wet conditions, the variations in gas dispersivity were mainly controlled by the air-filled porosity. An empirical model was also proposed for the prediction of gas dispersivity in granular, unsaturated porous media. ; p. 845-852.
    Keywords: Soil Physical Properties ; Particle Size ; Sand ; Bulk Density ; Porous Media ; Prediction ; Equations ; Porosity ; Models ; Glass ; Water Content ; Air ; Permeability ; Particle Size Distribution ; Diffusivity
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 8
    Language: English
    In: Vadose Zone Journal, 2012, Vol.11(1), p.0
    Description: Bench testing of petroleum biodegradation rates in vadose zone soils is typically associated with errors because transport conditions in laboratory systems are different from those found in the vadose zone. This work addressed the effect of soil structure and gas transport properties on hydrocarbon biodegradation in the unsaturated zone and we present a novel method for measuring biodegradation rates in intact and undisturbed soil columns of 100 cm (super 3) . To determine whether soil structure and gas diffusivity, defined as the ratio of the gas diffusion coefficient in soil to that in free air (D (sub p) /D (sub 0) ), influence the outcome of aerobic benzene biodegradation experiments, measurements using identical sandy soils were performed on (i) undisturbed 100-cm (super 3) core samples; (ii) sieved (2-mm) and repacked 100-cm (super 3) core samples; and (iii) soil samples (10 g) prepared as slurry microcosms. While slurry reactor experiments changed the first-order rate constant (k (sub w,1) ) significantly compared with undisturbed core samples, this was not the case for soil cores that had been sieved and repacked. This suggests that soil structure on a millimeter scale does not affect aerobic biodegradation in relatively unstructured sandy soils. Within differently textured soil cores, the biodegradation rate was found to increase with gas diffusivity when D (sub p) /D (sub 0) 〈0.02. This establishes gaseous O (sub 2) and petroleum vapor diffusion and distribution in soil profiles as a controlling factor for natural biodegradation of petroleum vapors.
    Keywords: Environmental Geology ; Aromatic Hydrocarbons ; Bacteria ; Benzene ; Biodegradation ; Biota ; Btex ; Cores ; Denmark ; Diffusivity ; Ethylbenzene ; Europe ; Experimental Studies ; Gas Transport ; Gases ; Hjorring Denmark ; Hydrocarbons ; Jutland ; Laboratory Studies ; Moisture ; Morphology ; Nyborg Denmark ; Organic Compounds ; Petroleum Products ; Pollution ; Sampling ; Scandinavia ; Slurries ; Soils ; Toluene ; Transport ; Unsaturated Zone ; Western Europe ; Xylene;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 9
    Language: English
    In: Vadose Zone Journal, 2012, Vol.11(1), p.0
    Description: Soil thermal conductivity (lambda ) and heat capacity (C) control heat transport and the thermal environment for biogeophysical processes in the vadose zone. Accurate lambda and C predictions for peaty soils with high organic contents are particularly important for assessing emissions of greenhouse gases formed during microbial activity in wetlands. In this study, we measured the lambda and C at different soil-water matric potentials on undisturbed samples for three peaty soil profiles at the Hokkaido Bibai marsh in Japan, representing a total of 10 different soil horizons. The thermal properties under air-dried conditions, lambda (sub dry) and C (sub dry) , were measured separately at changing volumetric solids contents (sigma ). For each sample, volume shrinkage was observed to varying degrees during the drying process. Measured lambda and C increased linearly with increasing volumetric water content (theta ). Applying the concept of a three-phase mixing model and incorporating the lambda -theta or C-theta and the lambda (sub dry) -sigma or C (sub dry) -sigma relations, predictive lambda and C models were developed as functions of sigma and theta . The new mixing model is represented by lambda = lambda (sub dry) +f (sub lambda ) theta lambda (sub w) and C = C (sub dry) +f (sub C) {theta}C (sub w) , where lambda (sub w) and C (sub w) are the thermal conductivity and heat capacity of water, respectively, and f is an impedance factor that takes into account the liquid-phase tortuosity. The new mixing model predicted literature lambda -theta data on peaty and highly organic soils under variable saturation well. The probable ranges of lambda and C under variable saturation were proposed based on the sensitivity analysis.
    Keywords: Soils ; Asia ; Bibai Marsh ; Biogenic Processes ; Bogs ; Carbon ; Equations ; Far East ; Ground Water ; Heat Capacity ; Heat Flow ; Heat Transport ; Hokkaido ; Horizons ; Hydrologic Cycle ; Hydrology ; Impedance ; Japan ; Liquid Phase ; Mires ; Organic Carbon ; Peat ; Physicochemical Properties ; Retention ; Sediments ; Sensitivity Analysis ; Soils ; Statistical Analysis ; Thermal Conductivity ; Thermal Properties ; Transport ; Unsaturated Zone ; Water Table ; Wetlands;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 10
    Language: English
    In: Vadose Zone Journal, 2015, Vol.14(7), p.0
    Description: A new parameter for representing the matrix bulk density, computed tomography (CT) number derived matrix density, was used in studying preferential flow and transport in an agricultural soil. Results illustrate that CT number derived matrix density provides useful information regarding the initiation of macropore flow and the magnitude of preferential flow. Preferential flow and transport in structured soils can be intimately linked to numerous environmental problems. Surface-applied chemicals are susceptible to rapid transport to deeper depths in structural soil pores, thereby potentially contaminating valuable environmental resources and posing risks to public health. This study focused on establishing links between the structural pore space and preferential transport using a combination of standard physical measurement methods for air and water permeabilities, breakthrough experiments, and X-ray computed tomography (CT) on large soil columns. Substantial structural heterogeneity that resulted in significant variations in flow and tracer transport was observed, despite the textural similarity of the investigated samples. Quantification of macropore characteristics with X-ray CT was useful but not sufficient to explain the variability in air permeability, saturated hydraulic conductivity, and solute transport. This was due to the limited CT scan resolution and large structural variability below this resolution. However, CTmatrix, a new parameter derived from the CT number of the matrix excluding stones and large mostly air-filled macropores, was found to be useful for determining the magnitude of preferential flow under boundary conditions of constant, near-saturated flow.
    Keywords: Tracers ; Solutes ; Permeability ; X-Rays ; Soils ; Public Health ; Flow ; Permeability ; Pores ; Variability ; Public Health ; Density ; Standards ; Preferential Flow ; Heterogeneity ; Coastal Zone Management ; General ; CT, Computed Tomography ; Ctmatrix, Computed Tomography Number of the Soil Matrix ; Hu, Hounsfield Units ; Roi, Region of Interest ; Btc, Breakthrough Curve;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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