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• 1
Article
Language: English
In: PLoS ONE, 01 January 2016, Vol.11(7), p.e0159948
Description: Matter turnover in soil is tightly linked to soil structure which governs the heterogeneous distribution of habitats, reaction sites and pathways in soil. Thereby, the temporal dynamics of soil structure alteration is deemed to be important for essential ecosystem functions of soil but very little is known about it. A major reason for this knowledge gap is the lack of methods to study soil structure turnover directly at microscopic scales. Here we devise a conceptual approach and an image processing workflow to study soil structure turnover by labeling some initial state of soil structure with small garnet particles and tracking their fate with X-ray microtomography. The particles adhere to aggregate boundaries at the beginning of the experiment but gradually change their position relative to the nearest pore as structure formation progresses and pores are destructed or newly formed. A new metric based on the contact distances between particles and pores is proposed that allows for a direct quantification of soil structure turnover rates. The methodology is tested for a case study about soil compaction of a silty loam soil during stepwise increase of bulk density (ρ = {1.1, 1.3, 1.5} g/cm3). We demonstrate that the analysis of mean contact distances provides genuinely new insights about changing diffusion pathways that cannot be inferred neither from conventional pore space attributes (porosity, mean pore size, pore connectivity) nor from deformation analysis with digital image correlation. This structure labeling approach to quantify soil structure turnover provides a direct analogy to stable isotope labeling for the analysis of matter turnover and can be readily combined with each other.
Keywords: Sciences (General)
E-ISSN: 1932-6203
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• 2
Article
Language: English
In: Science of the Total Environment, 01 December 2015, Vol.535, pp.113-121
Description: Chemical factors and physical constraints lead to coupled effects during particle transport in unsaturated porous media. Studies on unsaturated transport as typical for soils are currently scarce. In unsaturated porous media, particle mobility is determined by the existence of an air–water interface in addition to a solid–water interface. To this end, we measured breakthrough curves and retention profiles of citrate-coated Ag nanoparticles in unsaturated sand at two pH values (5 and 9) and three different flow rates corresponding to different water contents with 1 mM KNO as background electrolyte. The classical DLVO theory suggests unfavorable deposition conditions at the air–water and solid–water interfaces. The breakthrough curves indicate modification in curve shapes and retardation of nanoparticles compared to inert solute. Retention profiles show sensitivity to flow rate and pH and this ranged from almost no retention for the highest flow rate at pH = 9 to almost complete retention for the lowest flow rate at pH = 5. Modeling of the breakthrough curves, thus, required coupling two parallel processes: a kinetically controlled attachment process far from equilibrium, responsible for the shape modification, and an equilibrium sorption, responsible for particle retardation. The non-equilibrium process and equilibrium sorption are suggested to relate to the solid–water and air–water interfaces, respectively. This is supported by the DLVO model extended for hydrophobic interactions which suggests reversible attachment, characterized by a secondary minimum (depth 3–5 kT) and a repulsive barrier at the air–water interface. In contrast, the solid–water interface is characterized by a significant repulsive barrier and the absence of a secondary minimum suggesting kinetically controlled and non-equilibrium interaction. This study provides new insights into particle transport in unsaturated porous media and offers a model concept representing the relevant processes.
Keywords: Air–Water Interface ; Solid–Water Interface ; Engineered Nanoparticle ; Extended Dlvo Theory ; Unsaturated Flow ; Pore Structure ; Environmental Sciences ; Biology ; Public Health
ISSN: 0048-9697
E-ISSN: 1879-1026
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• 3
Article
Language: English
In: Transport in Porous Media, 2016, Vol.112(1), pp.207-227
Description: According to experimental observations, capillary trapping is strongly dependent on the roughness of the pore–solid interface. We performed imbibition experiments in the range of capillary numbers ( Ca ) from $$10^{-6}$$ 10 - 6 to $$5\times 10^{-5}$$ 5 × 10 - 5 using 2D-micromodels, which exhibit a rough surface. The microstructure comprises a double-porosity structure with pronounced macropores. The dynamics of precursor thin-film flow and its importance for capillary trapping are studied. The experimental data for thin-film flow advancement show a square-root time dependence. Based on the experimental data, we conducted inverse modeling to investigate the influence of surface roughness on the dynamic contact angle of precursor thin-film flow. Our experimental results show that trapped gas saturation decreases logarithmically with an increasing capillary number. Cluster analysis shows that the morphology and number of trapped clusters change with capillary number. We demonstrate that capillary trapping shows significant differences for vertical flow and horizontal flow. We found that our experimental results agree with theoretical results of percolation theory for $$Ca =10^{-6}$$ C a = 10 - 6 : (i) a universal power-like cluster size distribution, (ii) the linear surface–volume relationship of trapped clusters, and (iii) the existence of the cutoff correlation length for the maximal cluster height. The good agreement is a strong argument that the experimental cluster size distribution is caused by a percolation-like trapping process (ordinary percolation). For the first time, it is demonstrated experimentally that the transition zone model proposed by Wilkinson (Phys Rev A 30:520–531, 1984) can be applied to 2D-micromodels, if bicontinuity is generalized such that it holds for the thin-film water phase and the bulk gas phase.
Keywords: 2D-micromodel with rough surface ; Precursor thin-film flow ; Snap-off trapping ; Universal power law ; Ordinary bond percolation
ISSN: 0169-3913
E-ISSN: 1573-1634
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• 4
Article
In: Water Resources Research, May 2014, Vol.50(5), pp.4514-4529
Description: A major difficulty in modeling multiphase flow in porous media is the emergence of trapped phases. Our experiments demonstrate that gas can be trapped in either single‐pores, multipores, or in large connected networks. These large connected clusters can comprise up to eight grain volumes and can contain up to 50% of the whole trapped gas volume. About 85% of the gas volume is trapped by gas clusters. This variety of possible trapped gas clusters of different shape and volume will lead to a better process understanding of bubble‐mediated mass transfer. Since multipore gas bubbles are in contact with the solid surface through ultrathin adsorbed water films the interfacial area between trapped gas clusters and intergranular capillary water is only about 80% of the total gas surface. We could derive a significant (R = 0.98) linear relationship between the gas‐water‐interface and gas saturation. We found no systematic dependency of the front velocity of the invading water phase in the velocity range from 0.1 to 0.6 cm/min corresponding to capillary numbers from 2 × 10 to 10. Our experimental results indicate that the capillary trapping mechanism is controlled by the local pore structure and local connectivity and not by thermodynamics, i.e., by the minimum of the , at least in the considered velocity range. Consistent with this physical picture is our finding that the trapping frequency (= bubble‐size distribution) reflects the pore size distribution for the whole range of pore radii, i.e., the capillary trapping process is determined by statistics and not by thermodynamics. No systematic dependency of trapping efficiency on capillary number Majority of trapped gas bubbles (85%) are multipore trapped Trapping of gas clusters is determined by statistics and not by thermodynamics
Keywords: Gas Clusters ; Capillary Trapping ; Interfacial Area
ISSN: 0043-1397
E-ISSN: 1944-7973
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• 5
Article
In: Water Resources Research, November 2015, Vol.51(11), pp.9094-9111
Description: We study the impact of pore structure and surface roughness on capillary trapping of nonwetting gas phase during imbibition with water for capillary numbers between 10 and 5 × 10, within glass beads, natural sands, glass beads monolayers, and 2‐D micromodels. The materials exhibit different roughness of the pore‐solid interface. We found that glass beads and natural sands, which exhibit nearly the same grain size distribution, pore size distribution, and connectivity, showed a significant difference of the trapped gas phase of about 15%. This difference can be explained by the microstructure of the pore‐solid interface. Based on the visualization of the trapping dynamics within glass beads monolayers and 2‐D micromodels, we could show that bypass trapping controls the trapping process in glass beads monolayers, while snap‐off trapping controls the trapping process in 2‐D micromodels. We conclude that these different trapping processes are the reason for the different trapping efficiency, when comparing glass beads packs with natural sand packs. Moreover, for small capillary numbers of 10, we found that the cluster size distribution of trapped gas clusters of all 2‐D and 3‐D porous media can be described by a universal power law behavior predicted from percolation theory. This cannot be expected a priori for 2‐D porous media, because bicontinuity of the two bulk phases is violated. Obviously, bicontinuity holds for the thin‐film water phase and the bulk gas phase. The snap‐off trapping process leads to ordinary bond percolation in front of the advancing bulk water phase and is the reason for the observed universal power law behavior in 2‐D micromodels with rough surfaces. Surface roughness controls capillary trapping efficiency The transition‐zone model can be applied to 2‐D micromodels with rough surfaces The 2‐D and 3‐D porous media belong all to the same universality class
Keywords: Surface Roughness ; Precursor Thin‐Film Flow ; Snap‐Off Trapping ; Universal Power Law ; Ordinary Bond Percolation
ISSN: 0043-1397
E-ISSN: 1944-7973
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• 6
Article
Language: English
In: Vadose Zone Journal, 2011, Vol.10(3), p.1082
Description: Predicting solute transport through structured soil based on observable structural properties of the material has not been accomplished to date. We evaluated a new approach to predicting breakthrough curves (BTCs) of dissolved chemicals in intact structured soil columns based on attributes of the pore structure at hierarchical spatial scales. The methodology centers on x-ray computed microtomography of a hierarchic suite of undisturbed soil samples (diameters 1, 4.6, 7.5, and 16 cm) to identify the network of pores 〉10 mu m in diameter. The pore structure was quantified in terms of pore size distribution, interface area density, and connectivity. The pore size distribution and pore connectivity were used to set up an equivalent pore network model (PNM) for predicting the BTCs of Br (super -) and Brilliant Blue FCF (BB) at unsaturated, steady-state flux. For a structured silt loam soil column, the predictions of Br (super -) tracer breakthrough were within the variation observed in the column experiments. A similarly good prediction was obtained for Br (super -) breakthrough in a sandy soil column. The BB breakthrough observed in the silt loam was dominated by a large variation in sorption (retardation factors between R = 2.9 and 24.2). The BB sorption distribution coefficient, k (sub d) , was measured in batch tests. Using the average k (sub d) in the PNM resulted in an overestimated retardation (R = 28). By contrast, breakthrough of BB in the sandy soil (experimental R = 3.3) could be roughly predicted using the batch test k (sub d) (PNM simulation R = 5.3). The prediction improved when applying a sorption correction function accounting for the deviation between measured interface area density distribution and its realization in the network model (R = 4.1). Overall, the results support the hypothesis that solute transport can be estimated based on a limited number of characteristics describing pore structure: the pore size distribution, pore topology, and pore-solid interfacial density.
Keywords: Soils ; Bad Lauchstadt Germany ; Boundary Conditions ; Breakthrough Curves ; Bromine ; Central Europe ; Central Germany ; Chemical Dispersion ; Chernozems ; Computed Tomography ; Convection ; Density ; Dye Tracers ; Equations ; Europe ; Experimental Studies ; Fuhrberg Germany ; Germany ; Halogens ; Image Analysis ; Laboratory Studies ; Lower Saxony Germany ; Microtomography ; Minckowski Functions ; Morphology ; Networks ; Podzols ; Porosity ; Quantitative Analysis ; Saxony-Anhalt Germany ; Simulation ; Soils ; Solute Transport ; Spectra ; Tomography ; Topology ; Transport ; X-Ray Spectra;
E-ISSN: 1539-1663
Source: CrossRef
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• 7
Article
Language: English
In: Computers and Geosciences, 2010, Vol.36(10), pp.1246-1251
Description: For many analyses, grey scale images from X-ray tomography and other sources need to be segmented into objects and background which often is a difficult task and afflicted by an arbitrary and subjective choice of threshold values. This is especially true if the volume fraction of objects is small and the histogram becomes unimodal. Bi-level segmentation based on region growing is a promising approach to cope with the fuzzy transition zone between object and background due to the partial volume effect, but until now there is no method to properly determine the required thresholds in case of unimodality. We propose an automatic and robust technique for threshold selection based on edge detection. The method uses gradient masks which are defined as regions of interest for the determination of threshold values. Its robustness is analysed by a systematic performance test and finally demonstrated for the segmentation of pores in different soils using images from X-ray tomography.
Keywords: Segmentation ; Thresholding ; Edge Detection ; Region Growing ; Tomography ; Geology
ISSN: 0098-3004
E-ISSN: 1873-7803
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• 8
Article
In: Water Resources Research, November 2018, Vol.54(11), pp.9033-9044
Description: Structural hierarchy is a fundamental characteristic of natural porous media. Yet it provokes one of the grand challenges for the modeling of fluid flow and transport since pore‐scale structures and continuum‐scale domains often coincide independent of the observation scale. Common approaches to represent structural hierarchy build, for example, on a multidomain continuum for transport or on the coupling of the Stokes equations with Darcy's law for fluid flow. These approaches, however, are computationally expensive or introduce empirical parameters that are difficult to derive with independent observations. We present an efficient model for fluid flow based on Darcy's law and the law of Hagen‐Poiseuille that is parameterized based on the explicit pore space morphology obtained, for example, by X‐ray μ‐CT and inherently permits the coupling of pore‐scale and continuum‐scale domain. We used the resulting flow field to predict the transport of solutes via particle tracking across the different domains. Compared to experimental breakthrough data from laboratory‐scale columns with hierarchically structured porosity built from solid glass beads and microporous glass pellets, an excellent agreement was achieved without any calibration. Furthermore, we present different test scenarios to compare the flow fields resulting from the Stokes‐Brinkman equations and our approach to comprehensively illustrate its advantages and limitations. In this way, we could show a striking efficiency and accuracy of our approach that qualifies as general alternative for the modeling of fluid flow and transport in hierarchical porous media, for example, fractured rock or karstic aquifers. A model for the simulation of pore‐scale and continuum‐scale flow in hierarchically structured porous media is developed Explicit pore space morphology obtained by image analysis of X‐ray micro‐CT images is used for parameterization Predictions of solute breakthrough obtained by particle tracking perfectly match observations
Keywords: Darcy'S Law ; Particle Tracking ; Column Experiments ; X‐Ray Μ‐Ct ; Pore Space Morphology ; Image Analysis
ISSN: 0043-1397
E-ISSN: 1944-7973
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• 9
Article
In: Soil Science, 2012, Vol.177(1), pp.1-11
Description: ABSTRACT: It is important to understand the impact of texture and organic carbon (OC) on soil structure development. Only few studies investigated this for silt-dominated soils. In this study, soil physical properties were determined on samples from a controlled experiment (Static Fertilization Experiment, Bad Lauchstädt, Germany) on a loess soil that started more than 100 years ago with six different combinations of organic and mineral fertilizers. The parameters measured include soil texture, water retention curve, air-connected porosity, gas diffusion coefficient, air permeability, and saturated hydraulic conductivity. The management resulted in a distinct gradient in OC. A bulk density gradient developed from differences in amount of clay not complexed with OC. This gradient in bulk density mainly affected content of pores larger than 3 μm. The air-connected porosity measured by a pycnometer was highly similar to the total air-filled porosity calculated from gravimetric water content. For all six treatments, diffusivities and permeabilities were quite similar; both suggested that air-filled pore space was inactive for gas transport for air saturation below 0.1, but became highly connected around 0.2 to 0.25. Furthermore, diffusion data from intact cores compared well with data from repacked samples measured at low air-filled porosities and another high-silt soil (Yolo silt loam, USA) measured at higher air-filled porosities. A two-parameter fitting model was used to analyze gas diffusion coefficient data; the model pore-connectivity factor was fairly constant, whereas the water blockage factor was markedly different. Water and air parameters both implied that change in bulk density was the major driver for diffusive and convective parameters in the experiment.
Keywords: Soil Sciences ; Physical Properties ; Carbon ; Porosity ; Diffusion;
ISSN: 0038-075X
E-ISSN: 15389243
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• 10
Article
Language: English
In: Geoderma, April 2014, Vol.217-218, pp.181-189
Description: The study characterized soil structure development and evolution in six plots that were amended with varying amounts of animal manure (AM) and NPK fertilizer over a period of 106 years in a long-term fertilization experiment in Bad Lauchstädt, Germany. Two intact soil cores (10-cm diameter and 8-cm tall) and bulk soil samples were extracted from a depth between 5 and 15-cm from each plot. Soil properties including texture, organic carbon, soil–water characteristic, air permeability and diffusivity were measured and analyzed along with X-ray computed tomography (CT) data. Long-term applications of AM and NPK had a major impact on soil organic carbon content which increased from 0.015 kg kg (unfertilized plot) to 0.024 kg kg (well fertilized plot, 30 T ha 2y AM with NPK). Total porosity linearly followed the organic carbon gradient, increasing from 0.36 to 0.43 m m . The water holding capacity of the soil was considerably increased with the increase of AM and NPK applications. Gas diffusivity and air permeability measurements clearly indicated that the level of soil aeration improved with increasing AM and NPK fertilizer amount. The three-dimensional X-ray CT visualizations revealed higher macroporosity and biological (earthworm) activity in the well fertilized areas when compared to plots without or only a small amount of fertilizer applied. A combined evaluation of the soil water characteristic, gas transport and X-ray CT results suggested that pore size distributions widened, and pore connectivity was significantly improved with increasing fertilizer amount. Furthermore, the soils fertilized with both AM and NPK showed a more aggregated structure than soils amended with AM only.
Keywords: Animal Manure ; Npk Fertilizers ; Soil–Water Characteristic ; Gas Diffusivity ; Air Permeability ; X-Ray Computed Tomography ; Agriculture
ISSN: 0016-7061
E-ISSN: 1872-6259
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