[Show abstract][Hide abstract] ABSTRACT: The soil in direct vicinity of the roots, the root-soil interface or so called rhizosphere, is heavily modified by the activity of roots, compared to bulk soil, e.g. in respect to microbiology and soil chemistry. It has turned out that the root-soil interface, though small in size, also plays a decisive role in the hydraulics controlling the water flow from bulk soil into the roots. A promising approach for the non-invasive investigation of water dynamics, water flow and solute transport is the combination of the two imaging techniques magnetic resonance imaging (MRI) and neutron imaging (NI). Both methods are complementary, because NI maps the total proton density, possibly amplified by NI tracers, which usually corresponds to total water content, and is able to detect changes and spatial patterns with high resolution. On the other side, nuclear magnetic resonance relaxation times reflect the interaction between fluid and matrix, while also a mapping of proton spin density and thus water content is possible. Therefore MRI is able to classify different water pools via their relaxation times additionally to the water distribution inside soil as a porous medium. We have started such combined measurements with the approach to use the same samples and perform tomography with each imaging method at different location and short-term sample transfer.
[Show abstract][Hide abstract] ABSTRACT: NMR relaxometry has developed into a method for rapid pore-size determination of natural porous media. Nevertheless, it is prone to uncertainties because of unknown surface relaxivities which depend mainly on the chemical composition of the pore walls as well as on the interfacial dynamics of the pore fluid. The classical approach for the determination of surface relaxivities is the scaling of NMR relaxation times by surface to volume ratios measured by gas adsorption or mercury intrusion. However, it is preferable that a method for the determination of average pore sizes uses the same substance, water, as probe molecule for both relaxometry and surface to volume measurements. One should also ensure that in both experiments the dynamics of the probe molecule takes place on similar length scales, which are in the order of some microns. Therefore, we employed NMR diffusion measurements with different observation times using bipolar pulsed field gradients and applied them to unconsolidated sediments (two purified sands, two natural sands, and one soil). The evaluation by Mitra's short-time model for diffusion in restricted environments yielded information about the surface to volume ratios which is independent of relaxation mechanisms. We point out that methods based on NMR diffusometry yield pore dimensions and surface relaxivities consistent with a pore space as sampled by native pore fluids via the diffusion process. This opens a way to calibrate NMR relaxation measurements with other NMR techniques, providing information about the pore-size distribution of natural porous media directly from relaxometry.
[Show abstract][Hide abstract] ABSTRACT: The stable isotope compositions of soil water (δ2H and δ18O) carry important information about the prevailing soil hydrological conditions and for constraining ecosystem water budgets. However, they are highly dynamic, especially during and after precipitation events. The classical method of determining soil water δ2H and δ18O at different depths, i.e., soil sampling and cryogenic extraction of the soil water, followed by isotope-ratio mass spectrometer analysis is destructive and laborious with limited temporal resolution. In this study, we present a new non-destructive method based on gas-permeable tubing and isotope-specific infrared laser absorption spectroscopy. We conducted a laboratory experiment with an acrylic glass column filled with medium sand equipped with gas-permeable tubing at eight different soil depths. The soil column was initially saturated from the bottom, exposed to evaporation for a period of 290 days, and finally rewatered. Soil water vapor δ2H and δ18O were measured daily, sequentially for each depth. Soil liquid water δ2H and δ18O were inferred from the isotopic values of the vapor assuming thermodynamic equilibrium between liquid and vapor phases in the soil. The experimental setup allowed following the evolution of typical exponential-shaped soil water δ2H and δ18O profiles with unprecedentedly high temporal resolution. As the soil dried out, we could also show for the first time the increasing influence of the isotopically depleted ambient water vapor on the isotopically enriched liquid water close to the soil surface (i.e., atmospheric invasion). Rewatering at the end of the experiment led to instantaneous resetting of the stable isotope profiles, which could be closely followed with the new method.
Hydrology and Earth System Sciences Discussions 04/2015; 12(4):3893-3918. DOI:10.5194/hessd-12-3893-2015 · 3.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Bare soils are natural porous media where moisture and transport properties may change considerably. Under very dry conditions, it is predicted that capillary continuity from deeper soil layers to the surface ceases and evaporation decreases drastically because it is only sustained by vapor transport through an increasing dry surface layer (stage II). In this study, we firstly confirm this effect by investigating the drying of a lab-scale sand column using various MRI sequences as well as a unilateral NMR sensor (NMR-MOUSE). Proofing the convenience of the unilateral sensor, we take a step forward by monitoring moisture development of a natural soil under controlled ambient conditions. Finally, the experimental results clearly validate the prediction of a coupled water, vapor and heat flow model regarding the onset of stage II evaporation and the subsequent receding secondary evaporation front.
[Show abstract][Hide abstract] ABSTRACT: The bioavailability of water for plant nutrition in natural soils is controlled by the pore system structure and the interaction of water with the pore walls at variable degrees of saturation. For the characterization of these processes T 1 relaxometry is particularly suitable because it is not influenced by internal gradients and the frequency dependence of T 1 includes detailed information about the local dynamics at the pore walls. Using Fast Field Cycling Relaxometry, we have determined T 1 relaxation dispersion curves of unsaturated soil materials which cover a broad range of textures between pure sand and silt-loam. The mean relaxation rates scale inversely with the water content, as expected according to the Brownstein–Tarr model, which means that the effective pore volume is the only water-contributing fraction. By further analysis of the relaxation dispersion curves we find a bi-logarithmic behavior which is describable by a model of two-dimensional diffusion at the liquid–solid interface in the neighborhood of paramagnetic impurities at the surface. The microscopic wettability, as expressed by the ratio of surface residence time and correlation time, is identical for the soil material but decreases by a factor of two for the sand. This relaxation mechanism is unique for all textures and water contents and proves that the water mobility at the surface does not decrease even at the lowest water contents.
Applied Magnetic Resonance 10/2014; 45(10):1099-1115. DOI:10.1007/s00723-014-0599-2 · 1.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Near surface soil moisture profiles contain important information about the evaporation process from a bare soil. In this study, we demonstrated that such profiles could be monitored non-invasively and with high spatial resolution using Nuclear Magnetic Resonance (NMR). Soil moisture profiles were measured in a column exposed to evaporation for a period of 67 days using a stationary Magnetic Resonance Imaging (MRI) high field scanner and a unilateral NMR sensor. The column was packed with medium sand and initially saturated. Two distinct shapes of soil moisture profiles that are characteristic for stage I (evaporation rate is controlled by atmospheric demand) and stage II (evaporation rate is controlled by the porous medium) of the evaporation process were followed by both, MRI and unilateral NMR. During stage I, an approximately uniform decrease of soil moisture over time was monitored, whereas during stage II, s-shaped moisture profiles developed which receded progressively into the soil column. These promising results and the specific design of the unilateral NMR system make it very well suited for determining soil moisture profiles in the field.
[Show abstract][Hide abstract] ABSTRACT: Water exchange between bare soil and atmosphere is controlled by
evaporation. In the topmost soil layer moisture content and hydraulic
conductivity may change strongly and capillary film flow (stage I) from
saturated regions to the surface discontinues. Evaporation is now mainly
driven by vapor diffusion through a dry layer (stage II). Water
vaporizes in the unsaturated zone inside the soil what strongly reduces
the evaporation rate and also soil surface temperature to a considerable
amount. The dynamics of the transition from stage I to stage II as well
as film flow and vapor diffusion at low water contents have received
little attention. In this study we investigated water content changes in
the uppermost soil layer with high spatial resolution using nuclear
magnetic resonance (NMR). NMR is a feasible noninvasive method where the
received signal of hydrogen protons allows conclusions on moisture and
pore size distribution. The overall aim is to apply a mobile nuclear
magnetic resonance surface sensor (NMR-MOUSE) directly for field
measurements. This sensor has a max. measurement depth of 25 mm and
operates at a Larmor frequency of 13.4 MHz. The general challenges of
NMR in soils are the inherent fast transversal relaxation times of the
soil matrix especially next to the residual moisture content.
Therefore, as a first step of validation we applied and compared
NMR-MOUSE measurements with magnetic resonance imaging (MRI) using an
initially saturated sand column. The column was evaporated over 67 days
and water content profiles were recorded by 1D-T2 relaxation
measurements using the NMR-MOUSE as well as different 3D-MRI sequences
during drying. Firstly, we report on the sensitivities and limits of
the different devices and measurement sequences. Considering these data,
we could monitor that over a period of 58 days the moisture decreased
rather uniform until the onset of stage II. Thereafter, a dry surface
layer developed and a retreating drying front was observed.
[Show abstract][Hide abstract] ABSTRACT: Nuclear Magnetic Resonance (NMR) relaxation and NMR diffusion
measurements are two of a series of fast and non-invasive NMR
applications widely used e.g. as well logging tools in petroleum
exploration . For experiments with water, NMR relaxation measures the
relaxation behaviour of former excited water molecules, and NMR
diffusion evaluates the self-diffusion of water. Applied in porous
media, both relaxation and diffusion measurements depend on intrinsic
properties of the media like pore size distribution, connectivity and
tortuosity of the pores, and water saturation [2, 3]. Thus, NMR can be
used to characterise the pore space of porous media not only in
consolidated sediments but also in soil. The physical principle behind
is the relaxation of water molecules in an external magnetic field after
excitation. In porous media water molecules in a surface layer of the
pores relax faster than the molecules in bulk water because of
interactions with the pore wall. Thus, the relaxation in smaller pores
is generally faster than in bigger pores resulting in a relaxation time
distribution for porous media with a range of pore sizes like soil .
In NMR diffusion experiments, there is an additional encoding of water
molecules by application of a magnetic field gradient. Subsequent
storage of the magnetization and decoding enables the determination of
the mean square displacement and therefore of the self-diffusion of the
water molecules . Employing various relaxation and diffusion
experiments, we get a measure of the surface to volume ratio of the
pores and the tortuosity of the media. In this work, we show the
characterisation of a set of sand and soil samples covering a wide range
of textural classes by NMR methods. Relaxation times were monitored by
the Carr-Purcell-Meiboom-Gill sequence and analysed using inverse
Laplace transformation. Apparent self-diffusion constants were detected
by a 13-intervall pulse sequence and variation of the storage time. We
correlated the results with various soil properties like texture, water
retention parameters, and hydraulic conductivity. This way we show that
we can predict soil properties by NMR measurements and that we are able
use results of NMR measurements as a proxy without the need of direct
measurements.  Song, Y.-Q., Vadose Zone Journal, 9 (2010) 
Stingaciu, L. R., et al., Water Resources Research, 46 (2010)  Vogt,
C., et al., Journal of Applied Geophysics, 50 (2002)  Barrie, P. J.,
Annual Reports on NMR Spectroscopy, 41 (2000)  Stallmach, F.,
Galvosas, P., Annual Reports on NMR Spectroscopy, 61 (2007)
[Show abstract][Hide abstract] ABSTRACT: The interface between roots and soil plays a key role in water transport
in the Soil-Plant-Atmosphere-Continuum (SPAC). The transport which
changes with the degree of dehydration is influenced by both the
hydraulic conductivity of roots and the soil. One important factor in
plant growth is the amount of available water in the soil, which
correlates directly with soil texture. Water uptake of plant roots and
water uptake patterns in soil can be monitored using non-invasive 1H
Nuclear Magnetic Resonance Imaging (MRI). In a preceding study the
effect of root water uptake and uniform desiccation patterns under
drought conditions were observed for Ricinus communis grown in a model
medium (Pohlmeier et al. 2008). Continuing these studies, the new aspect
is the determination of water uptake patterns and root system
architecture in a natural soil. The general challenge of MRI in soils
are the inherent fast relaxation times T2* and T2 of the soil matrix.
With the use of conventional sequences only water in macropores can be
determined. The loss of sensitivity can be overcome by MRI sequences
with sufficiently short detection times. In this work we employed and
assessed two methods: SPI (Single Point Imaging) detects the T2*
relaxation with a dead time of < 0.05 ms and SE3D (Spin Echo 3D)
probes T2,eff with an echo time of about 0.8 ms. Zea mays, planted in a
cylindrical container filled with a natural soil was completely sealed
after 4 weeks of growth to avoid evaporation, so water loss took place
via transpiration only. The water content of the soil was determined
gravimetrically and by means of MRI each 2nd day over a period of 14
days. Furthermore a SEMS (Spin Echo Multi Slice) sequence was used to
visualize the growth of root system architecture. This study shows that
SPI3D and SE3D are feasible for the determination of water content in a
natural soil up to a certain detection limit. We observed quite uniform
water uptake patterns during drying of the soil until water content was
less than 0.15 cm^3/cm^3, which is the detection limit of both sequences
for the used soil material. Accordingly, this indicates an always
sufficiently high hydraulic conductivity of the soil to sustain water
supply for the plant. The growth of the root system architecture could
reliably been visualized with SEMS sequence where the best
differentiation between soil and roots is obtained by the choice of long
echo time and small voxel size. During the whole drought period we
observed an increase in root growth what is an effect of the high water
supply. Pohlmeier, A., Oros-Peusquens, A., et al. (2008) " Changes
in Soil Water Content Resulting from Ricinus Root Uptake Monitored by
Magnetic Resonance Imaging" Vadose Zone J. 7: 1010-1017.
[Show abstract][Hide abstract] ABSTRACT: Water fluxes in soils control many processes in the environment like
plant nutrition, solute and pollutant transport. In the last two decades
non-invasive visualization methods have been adapted to monitor flux
processes on the small scale. Magnetic resonance imaging (MRI), also
well known from medical diagnostics, is one of the most versatile ones.
It mostly probes directly the substance of interest: water, and it
offers many opportunities to manipulate the observed signals for
creating different contrasts and thus probing different properties of
the porous medium and the embedded fluids. For example, one can make the
signal sensitive to the total proton density, i. e. water content, to
spatial distributions of relaxation times which reflect pore sizes, to
spatial distributions of transport coefficients, and to concentration of
contrast agents by using strongly T1 weighted MRI pulse sequences. In
this presentation we use GdDTPA2- for monitoring flux processes in soil
columns in an ultra-wide bore MRI scanner. It offers the opportunity for
monitoring slow water fluxes mainly occurring in soil systems which are
not monitorable with direct MRI flow imaging. This contrast agent is
most convenient since it behaves conservatively, i.e. it does not sorb
at different soil materials and it is chemically stable. Firstly, we
show that its mode of action in natural porous media is identical to
that known from medical applications as proved by the identical
relaxivity parameters . Secondly, the tracer is applied for the
visualization of flux processes during evaporation-driven flow.
Theoretical considerations by forward simulation predicted a lateral
redistribution of solutes during evaporative upward fluxes from highly
conductive fine material to neighbouring domains with low water content
and conductivity. Here we could prove that such near-surface
redistribution really takes place . Thirdly, this tracer is applied
for the investigation of water uptake by root systems. Depending on the
transpiration conditions slow uptake in the dark is present, where the
tracer moves directly into the xylem. When fully illuminated, the tracer
uptake is limited by the Caspari band, and it is enriched strongly in
the roots cortex. The results so far show that this tracer offers a new
window for monitoring slow water fluxes in bare and grown soil columns.
 Haber-Pohlmeier S, Bechtold M, Stapf, S, Pohlmeier, A. (2010) Vadose
Zone Journal 9, 835-845  Bechtold M, Haber-Pohlmeier S, Vanderborght
J, Pohlmeier A, Ferré T, Vereecken H. (2011) Geophysical Research
Letters 38, L17404
[Show abstract][Hide abstract] ABSTRACT: An automated method for root system architecture reconstruction from three-dimensional volume data sets obtained from magnetic resonance imaging (MRI) was developed and validated with a three-dimensional semimanual reconstruction using virtual reality and a two-dimensional reconstruction using SmartRoot. It was tested on the basis of an MRI image of a 25-d-old lupin (Lupinus albus L.) grown in natural sand with a resolution of 0.39 by 0.39 by 1.1 mm. The automated reconstruction algorithm was inspired by methods for blood vessel detection in MRI images. It describes the root system by a hierarchical network of nodes, which are connected by segments of defined length and thickness, and also allows the calculation of root parameter profiles such as root length, surface, and apex density The obtained root system architecture (RSA) varied in number of branches, segments, and connectivity of the segments but did not vary in the average diameter of the segments (0.137 cm for semimanual and 0.143 cm for automatic RSA), total root surface (127 cm(2) for semimanual and 124 cm(2) for automatic RSA), total root length (293 cm for semimanual and 282 cm for automatic RSA), and total root volume (4.7 cm(3) for semimanual and 4.7 cm(3) for automatic RSA). The difference in performance of the automated and semimanual reconstructions was checked by using the root system as input for water uptake modeling with the Doussan model. Both systems worked well and allowed for continuous water flow. Slight differences in the connectivity appeared to be leading to locally different water flow velocities, which were 30% smaller for the semimanual method.
Vadose Zone Journal 01/2012; 12(1). DOI:10.2136/vzj2012.0019 · 1.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present results from solute transport experiments in an evaporating composite porous medium consisting of a cylindrical inner core with coarse sand that was surrounded by a mantle with fine sand. Small volumes of dye and salt tracer were applied at the surface of the fine material of the evaporating column. The pressure head at the bottom boundary was kept constant using a hanging water table ensuring liquid phase continuity to top surface in both fine and coarse material, whereby the latter was hydraulically less conductive at these pressure conditions. Contrary to the expectation that solute accumulation at an evaporating surface is proportional to local cumulative evaporation, high concentration spots developed at the surface of the coarse material, for which IR surface temperature measurements did not indicate higher evaporation fluxes. 3D unsaturated flow and transport simulations and a second tracer experiment monitored with magnetic resonance imaging (MRI) demonstrated that preferential upward water flux in the fine sand deeper in the column and near-surface lateral water flow from the fine into the coarse sand in combination with a downward diffusive flux are responsible for the local solute accumulation. We propose that at the wet regions of a soil surface, solute accumulation is largely decoupled from local evaporation fluxes and strongly governed by relative differences of the hydraulic conductivities. The possible formation of high solute concentration spots at the surface of coarser regions usually representing preferential flow pathways during strong precipitation may have an accelerating effect on the leaching of solutes.
[Show abstract][Hide abstract] ABSTRACT: A low-field NMR sensor has been built to measure partial saturation of soils in a non-invasive way. The sensor was deployed in a set-up where a one-step outflow experiment was carried out. Partial saturation before, during and after the experiment was acquired for two model soils. Hydraulic characteristics of the soils were obtained through inverse analysis. NMR signal was analyzed to gain information about the microscopical conditions of the liquid phase.
Journal of Hydrology 08/2011; 406(1):30-38. DOI:10.1016/j.jhydrol.2011.05.045 · 3.05 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Characterization and quantification of root water uptake processes play a key role in understanding and managing the effects of global climate change on agricultural production and ecosystem dynamics. Part of this understanding is related to the flow of water towards plant roots in soils. In this study we demonstrate for the first time, to our knowledge, that fluid flow in the voids of the pore space of a model soil system (natural sand) can be detected and mapped to an NMR image for mean flows as low as 0.06 mm/s even under the influence of internal magnetic field gradients. To accomplish this we combined multi-slice imaging with a 13-interval pulse sequence to the NMR pulse sequence 13-interval stimulated echo multi-slice imaging (13-interval STEMSI). The result is a largely reduced influence of the internal magnetic field gradients, leading to an improved signal-to-noise ratio which in turn enables one to acquire velocity maps where conventional stimulated echo methods fail.
Journal of Magnetic Resonance 07/2011; 212(1):216-23. DOI:10.1016/j.jmr.2011.07.004 · 2.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Understanding root water uptake in soils is of high importance for securing nutrition in the context of climate change and linked phenomena like stronger varying weather conditions (draught, strong rain). One step to understand how root water uptake occurs is the knowledge of the water flow in soil towards plant roots. Magnetic Resonance Imaging (MRI) in combination with q‐space imaging is potentially the most powerful analytical tool for non‐invasive three dimensional visualization of flow and transport in porous media. Numerous attempts have been made to measure local velocity in porous media by combining velocity phase encoding with fast imaging methods, where flow velocities in the vascular bundles of plant stems were investigated. In contrast to water situated in the cellular structure of plants, NMR signal arising from water in the pore space in soil may be much more affected by the presence of internal magnetic field gradients. In this work we account for the existence of these gradients by employing bipolar pulsed field magnetic gradients for velocity encoding. This enables one to study flow through sand (as a model system for soil) at flow rates relevant for the water uptake of plant roots.
[Show abstract][Hide abstract] ABSTRACT: Magnetic resonance imaging (MRI) was used to study the process of infiltration and solute transport in an undisturbed soil sample of coarse sandy loam. The sample was subjected to the recurrent ponded infiltration (RPI) experiment, which was carried out in order to assess the changes in the entrapped air volume and its impact on steady state flow rates and solute breakthrough. The main stages of the first and second experimental RPI runs were monitored using an MRI sequence that follows both water density and magnetic relaxation. In a steady state stage of each experimental run a nickel nitrate pulse was injected in order to visualize the solute breakthrough. Effluent from the sample was collected for chemical analysis and a breakthrough curve of the nickel was constructed. To obtain information about the soil structure and to reveal potential preferential pathways, the soil sample was scanned using computed tomography. The local nickel ion transport breakthrough was evaluated from MR images in a series of local observation points distributed along the selected preferential pathways.The preferential flow instability phenomenon with the emphasis on air bubble formation was shown by detecting a 60% decrease of the steady state infiltration rate. The detailed analyses of MRI measurements at observation points revealed air bubble formation, producing a flow rate decrease accompanied by redirection of nickel ion transport trajectories. By analyzing M0 maps it was found that the volumetric water content decrease was 2.2%.
[Show abstract][Hide abstract] ABSTRACT: Water and solute movement as any other transport processes through soil are influenced by the hydraulic properties of the soils. The heterogeneities of the soils imply heterogeneous spatial distribution of the hydraulic properties leading to heterogeneous distribution of soil water content. This may affects the water availability for plant growth, the groundwater contamination and nutrients losses within the root zone. The measurement techniques available today for the estimation of soil hydraulic parameters do not account for the heterogeneity of the sample and treat each measurement sample as a homogeneous representative volume. On the other side natural soils contain large heterogeneities mostly in terms of inclusions of different materials. Therefore the purpose of this study is to estimate soil hydraulic properties of a heterogeneous sample by combining classical multi-step-outflow (MSO) with magnetic resonance imaging (MRI) experiments. MSO experiments were performed on a sample filled with sand and sand-clay mixture in a coaxial structure. During each pressure application MRI images at 4.7 T (200 MHz) were recorded using a pure phase-encoding MRI sequence in order to provide information about the soil water content at specific locations within the coaxial sample. The recorded cumulative outflow and water content data were used as input data in the inversion of the MSO experiment. For the simulation and inversion of the MSO experiment we used the hydrological model HYDRUS-2D3D in which the initial hydraulic parameters of the two materials were estimated based on CPMG-T2 relaxation measurements on homogeneous sub-samples. The results show conclusively that the combination of the two MRI and MSO methods leads to a unique estimation of the hydraulic properties of two materials simultaneously.
[Show abstract][Hide abstract] ABSTRACT: In this study, we evaluate the feasibility of using nuclear magnetic resonance (NMR) relaxometry measurements to characterize pore size distribution and hydraulic properties in four porous samples with different texture and composition. We compare NMR with two classical techniques based on water retention and mercury intrusion measurements. Both T2 and T1 NMR relaxation measurements at 6.47 MHz were carried out for three saturated model samples (medium sand, fine sand, and a homogenous sand/kaolin clay mixture) and one saturated natural silt loam soil. Cumulative pore size distribution functions and mean pore diameters were calculated assuming average surface relaxivity parameters and a cylindrical capillary model of the pores. The mean pore diameters derived from T2 and T1 distributions as well as the cumulative pore size distribution functions agree satisfactorily with those derived from mercury intrusion and retention curves. The observed deviations are due to limitations of each method, sample preparation, and sample composition. To evaluate the influence of the variations observed in the hydraulic properties of the samples, the pore size distribution functions were scaled back to water retention functions, and the van Genuchten hydraulic parameters were estimated by inversion using the RETC software. The comparison shows that both T2 and T1 NMR relaxation measurements can be used to estimate pore size distribution and mean pore diameter, as well as the retention function and corresponding hydraulic properties.
Water Resources Research 11/2010; 46(11). DOI:10.1029/2009WR008686 · 3.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Magnetic resonance imaging (MRI) was applied to the study of flow processes in model and natural soil cores. Flow velocities in soils are mostly too slow to be monitored directly by MRI flow velocity imaging. Therefore, we used for the first time diethylenetriaminepentaacetate in the form Gd-DTPA(2-) as a tracer in spin echo multislice imaging protocols with strong weighting of longitudinal relaxation time T(1) for probing slow flow velocities in soils. Apart from its chemical stability, the main advantage of Gd-DTPA(2-) is the anionic net charge in neutral aqueous solution. We showed that this property hinders adsorption at soil mineral surfaces and therefore retardation. We found that Gd-DTPA(2-) is a very convenient conservative tracer for the investigation of flow processes in model and natural soil cores. With respect to the flow processes in the coaxial model soil column and the natural soil column, we observed totally different flow patterns. In the first case, the tracer plume moved quite homogeneously in the inner highly conductive core only and the migration into the outer fine material was very limited. A numerical forward simulation based on independently obtained parameters showed good agreement between experiment and simulation and thus proves the convenience of Gd-DTPA as a tracer in MRI for soil physical investigations. The natural soil core, in contrast, showed a flow pattern characterized by preferential paths, avoiding dense regions and preferring loose structures. In the case of the simpler model column, the local flow velocities were also calculated by applying a peak tracking algorithm.
Vadose Zone Journal 11/2010; 9(4-4):835-845. DOI:10.2136/vzj2009.0177 · 1.78 Impact Factor