Tracer experiments within composite soil column investigated by MRI

The detection of a nonequilibrium water flow and solute transport in structured soil at various scales is essential for better understanding of these phenomena. This study focused on the visualization of preferential flow in a Haplic Luvisol and Haplic Cambisol and their horizons by performing field ponding dye infiltration experiments. In addition, thin soil sections were made and micromorphological images were used to study soil aggregate structure and dye distribution at the microscale. The staining patterns within the vertical and horizontal field‐scale sections documented the different nature of preferential flow in different soil types and also within the soil profiles. While preferential flow in the Haplic Luvisol was caused by soil aggregation and biopores, preferential flow in the Haplic Cambisol was caused only by biopores, large soil fractures, and incorporated straw material. Micromorphological images showed that, in the case of the Haplic Luvisol, the dye was primarily distributed either in the interaggregate pores and then in the pores inside the aggregates or in the isolated large pores connected to the dye source and then into the matrix pores. The dye distribution in the soil matrix was uneven as well. Accumulated organic matter, clay coating, and isolated larger capillary pores, which initially did not contain the dye tracer, behaved as less‐permeable or impermeable barriers. Uneven distribution was caused by hierarchical pore size distribution of the soil matrix. Results indicated a multimodal character of preferential flow in this soil. In the case of the Haplic Cambisol, the dye pattern studied at the microscale was mostly affected by fractures and the size and shape of mineral grains.

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confidence: 99%

Using Dye Tracer for Visualization of Preferential Flow at Macro‐ and Microscales

Kodešová

Němeček

Kodeš

et al. 2012

“…Based on the fact that for some natural soils only water in large pores produces a significant magnetic resonance (MR) signal, MRI studies of preferential flow in undisturbed samples of heterogeneous soil were performed (Císlerová et al, 1999, 2002; Votrubová, 2002; Císlerová and Votrubová, 2002; Votrubová et al, 2003; Sněhota et al, 2003; Sněhota, 2003). Recently, MRI was used to investigate the fate and transport of nonaqueous‐phase liquids (Chu et al, 2004; Zhang et al, 2008; Nestle et al, 2008), to study finger flow in sand (Posadas et al, 2009), to trace the transport of D 2 O (Pohlmeier et al, 2009) and the transport of Ni(NO 3 ) 2 (Jelínková et al, 2010), and to monitor flow in a root zone (Pohlmeier et al, 2008). Magnetic resonance imaging was used for spatial measurement of local flow velocities (Seymour and Callaghan, 1997; Baumann et al, 2000).…”

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confidence: 99%

Tracing the Entrapped Air in Heterogeneous Soil by Means of Magnetic Resonance Imaging

Sněhota

Cı́slerová

Amin

et al. 2010

Magne c resonance imaging (MRI) was used to study the process of infi ltra on in an undisturbed core of coarse sandy loam during a repeated ponded infi ltra on experiment (RPI), in which the fi rst infi ltra on was conducted into a naturally dry sample and the second infi ltra on into a wet sample. The primary aim was to assess the change in distribu on of entrapped air in the undisturbed soil sample and its infl uence on steady-state fl ow rates. The RPI experiment was conducted on a sample 8.9 cm in diameter and 8.4 cm high. Concurrently with the MRI monitoring, pressure heads and fl uxes were measured. Mul pleslice maps of longitudinal relaxa on me (T1), which is a parameter related to the surface/ volume ra o, and proton density (M0), related to the water content, covered almost the en re volume of the sample. The T1 mapping was conducted during four stages of the experiment: steady-state fl ow of the fi rst (I1) and second (I2) infi ltra ons and the equilibrium stage a er drainage of the sample (Stages D1 and D2). During I1 and I2, the highest values of M0 and T1 were detected in the upper 3 cm of the sample, which is in good agreement with fi ndings of computer tomography showing lower sample density and higher porosity in this region. The drop in the quasi-steady-state fl ow rate that was observed between the I1 and I2 runs corresponded to the decline in T1 and M0 in the upper 4 cm of the sample. The decrease in T1 indicates replacement of water in large pores with trapped air, while a lower M0 signifi es a general decrease of water content in the aff ected areas.Abbrevia ons: a.u., arbitrary units; CT, computed tomography; D1 and D2, fi rst and second drainages, respec vely; I1 and I2, fi rst and second infi ltra ons, respec vely; MR, magne c resonance; MRI, magne c resonance imaging; RPI, repeated ponded infi ltra on; T1, longitudinal relaxa on me; T2, transverse relaxa on me; TE, me to echo; TR, repe on me.

“…A noninvasive visualization by means of MR imaging has proven to be useful in three‐dimensional investigation of both water flow and solute transport in porous media (e.g., Deurer et al, 2004; Pohlmeier et al, 2009; Jelinkova et al, 2010). Votrubova et al (2003) reported a decrease in the steady‐state infiltration rate between two successive ponded infiltrations and used MR imaging to quantify changes in the water distribution within a sample.…”

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confidence: 99%

Tree‐Dimensional Numerical Analysis of Water Flow Affected by Entrapped Air: Application of Noninvasive Imaging Techniques

Dohnal

Jelínková

Sněhota

et al. 2013

Recurrent ponded infi ltra on experiments on undisturbed samples of coarse sandy loam have revealed a signifi cant fl ow instability characterized by a decrease in the steady-state fl ow rate of the second infi ltra on run, conducted into wet soil, compared with the fi rst infi ltra on run, conducted into drier soil. It has been hypothesized that this decrease was caused by air entrapment during the second run, with subsequent blocking of the preferen al pathways. In this study, entrapped air distribu on and its impact on water fl ow was studied through a novel combina on of magne c resonance (MR) imaging and numerical modeling. An undisturbed sample of coarse sandy loam was subject to recurrent ponded infi ltra on while being monitored by MR. Internal structure of the soil sample was visualized by x-ray computed tomography (CT). A parallel version of the three-dimensional water fl ow model based on Richards' equa on was used to simulate water fl ow through the heterogeneous soil sample. Informa on from the CT was used to describe internal heterogeneity of the soil sample via scaling factors. Magne c resonance relaxometry imaging data were u lized to derive three-dimensional maps of entrapped air. These were implemented into a simula on of the second infi ltra on run as regions of no fl ow. The MR images were used to assess the water content distribu on within the sample. The threedimensional model was able to describe measured ou low rates and pressure heads and also to reproduce the heterogeneous distribu on of water content within the sample. The results obtained support the assump on that the observed decrease in the ou low rate could be caused by entrapped air in large pores of the soil sample. Current theory for mathema cal predic ons of water fl ow and solute transport in porous media assumes that the air phase plays a relatively minor role during unsaturated fl ow and transport (e.g., van Genuchten et al., 1999). Th ere is a long-term evidence, however, of air-phase discontinuities occurring in porous media and leading either to water entrapment (Papafotiou et al., 2008) or to air entrapment eff ects on water fl ow (Christiansen, 1944;Faybishenko, 1995) and solute transport in soils (Geistlinger et al., 2005). For instance, entrapped air was found to be responsible for an increase in infi ltration variability (Wang et al., 1998), temporal changes in hydraulic conductivity (Sakaguchi et al., 2005), and fl ow instabilities (Schultze et al., 1999;Wildenschild et al., 2001).To study infi ltration processes exhibiting air entrapment eff ects, Snehota et al. (2010) performed a recurrent ponded infi ltration experiment on an undisturbed heterogeneous soil column. With the use of MR imaging, they detected signifi cant air entrapment in large pores and associated this with the decrease in the quasi-steady-state outfl ow rate during infi ltration into a wet sample. Th ey presented only qualitative results because the MR pulse sequence used did not provide spatially resolved information about changes in water cont...