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DIVISION S-1-SOIL PHYSICS

Preferential Solute Flow in Intact Soil Columns Measured by SPECT Scanning

^a Dep. of Agric. and Biosyst. Eng., McGill Univ., 21111 Lakeshore Road, Ste-Anne-de-Bellevue, QC, Canada H9X-3V9
^b Dep. of Chem. and Petrol. Eng., Univ. of Calgary, 2500 University Dr. N.W., Calgary, AB, Canada T2N-1N4
^c TIPM Lab., Univ. of Calgary, 2500 University Dr. N.W., Calgary, AB, Canada T2N-1N4
^d Dep. of Chemistry, Univ. of Calgary, 2500 University Dr. N.W., Calgary, AB, Canada T2N-1N4

prasher{at}macdonald.mcgill.ca

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
Single photon emission computed tomography (SPECT) is an imaging technique that is widely used in medical diagnosis. This technique has never been applied to soils. The objective of this study was to investigate the capabilities of SPECT scanning for visualizing preferential flow in soil. This paper describes the principle of SPECT scanning and its application to tracer breakthroughs in four large undisturbed soil columns (800-mm length x 77-mm diam.). This new approach allows real-time analysis of flow patterns of radioactive tracers in 2-D using planar imaging, and in 3-D using the tomographic capabilities of the SPECT scanner. Not only does SPECT scanning provide qualitative data, but it also allows for the quantification of a tracer's spatial distribution. Our results characterized preferential flow very clearly in soil columns. SPECT scanning opens up new avenues for 2-D and 3-D tracer studies in porous media such as soils.

Abbreviations: CAT, computer assisted tomography • SPECT, single photon emission computed tomography

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
WATER AND SOLUTE TRANSPORT THROUGH SOIL MACROPORES can be quite significant and is now well documented (Perret et al., 1997; Perret et al., 2000; Beven and Germann, 1982; Bouma, 1990; Hamblin, 1985; Logsdon, 1995; McCoy et al., 1994; Steenhuis et al., 1990; Singh and Kanwar, 1991; White, 1985). Macropore flow results in rapid movement of water and solutes through the soil profile with important implications for ground water quality. Short travel time may not allow contaminants, such as pesticides, to be adsorbed on and into soil particles. Consequently, most solutes in the water will be quickly delivered to the drains or ground water aquifers without being degraded by chemical and biological actions. However, quantification and prediction of preferential flow has been difficult because of the complexity of soil structure.

Knowledge of soil structure, along with a suitable technique for measuring water and associated solute flow characteristics through soil, is essential to understanding the mechanisms of preferential flow. Unfortunately, progress in this area has been severely limited by difficulties in obtaining direct and nondestructive measurements of the preferential flow in a structured soil. Computer assisted tomography (CAT) scanning offers a powerful approach to the study of preferential flow (Anderson and Hopmans, 1994; Perret et al., 2000). However, whereas CAT scanning provides high-resolution cross-sectional images in usually more than a second, single photon emission computed tomography (SPECT) allows longitudinal views at a sub-second level.

To the best of our knowledge, SPECT has never been applied to studies in soil science. However, it may provide a new and exciting approach for visualizing and characterizing preferential flow phenomenon. The primary objective of this paper is to investigate the capabilities of SPECT scanning for visualizing preferential flow in soil during solute transport and distribution.

Single photon emission computed tomography is based on detecting nuclear radiation emitted from the body or from an object into which a very small amount of radioactive material, called a radiopharmaceutical, has been introduced. A special type of camera, known as a scintillation or gamma camera, is used to transform these radioactive emissions into images or data, which describe the location and intensity of these emissions.

Single photon emission computed tomography scanning is widely used as a diagnostic technique in nuclear medicine. Ten to twelve million nuclear medicine imaging and therapeutic procedures are performed each year in the USA alone (SNM, 1999). Today, nearly all cardiac patients in developed countries receive a SPECT scan to detect arterial diseases or a damaged heart. Investigation of the liver, kidneys, thyroid gland, and many other organs are, similarly, leading applications of SPECT (Coleman et al., 1986). It is used routinely to help diagnose restriction of blood flow to parts of the brain, as well as cancer, stroke, lung disease, and many other physiological abnormalities.

Scintillation camera technology has been used in industrial applications since the mid-1980s. For instance, photon emission imaging has been used to perform visual and radiometric scans of nuclear facilities. Sedaghat et al. (1988) developed a technique to characterize cross-flow in a two sub-channel nuclear fuel assembly using a gamma camera. Gamma camera technology has also been adapted to conduct contamination surveys inside buildings that are connected with nuclear production (Chesnokov et al., 1997; Mottershead and Orr, 1996).

Single photon emission computed tomography has also brought a new dimension to disciplines such as particle tracking and flow analysis in mixing reactor vessels. Castellana and Dudley (1984) were among the first researchers to visualize particle motion in fluid–solid systems using a gamma camera. With a similar approach, Lin et al. (1985) used a series of photomultiplier tubes to measure the solid motion of radioactive particles in gas fluidized beds. Today, radioactive-particle tracking continues to offer great potential for measuring recirculating phase velocities in gas-fluidized beds and bubble columns. Scintillation cameras have also been used to determine the hydrodynamics and radial distributions of velocity in vertical riser reactors and mixing reactor vessels (Castellana et al., 1984; Berker and Tulig, 1986; Legoupil et al., 1997). The potential of SPECT scanning for imaging a gas-flow fluidization test rig and fluid flow in a gasification unit is discussed by Jonkers et al. (1990). Other applications to the field of engineering include measuring liquid film thickness on the surface of a rotating disk (Castellana and Hsu, 1984) and analyzing the radioactive ball trajectories within the charge of a rotary grinding mill (Powel and Nurick, 1996).

Nuclear technology has also been utilized in oil-recovery fields to visualize dynamic oil displacement in porous media. For instance, Huang and Gryte (1988) used a gamma camera to observe immiscible displacement of oil in thin slabs of a porous medium saturated with water. In their study, technitium-99m was used as a tracer for the water phase. The authors showed that photon emission imaging provides a powerful approach for determining local fluid saturations in quantitative terms. Lien et al. (1988) studied the one-dimensional distribution of saturation in 0.75-m long sandstone cores, operating at reservoir pressure and temperature. Information on one-dimensional fluid saturation distributions was obtained by labeling fluid phases with nuclear tracers and detecting radiation with a gamma camera. They reported that the apparatus fulfilled its objective of imaging displacement processes at reservoir conditions. Charlier et al. (1995) applied gamma ray absorption techniques to determine the permeability of oil during a tertiary-gas gravity-drainage experiment. Using this technique, they were able to visualize fluid saturation distribution in the core as a function of the injected gas's volume.

Although the power of noninvasive and in situ SPECT scanning has been demonstrated for dynamic industrial processes and for oil recovery, this technique has never been applied to soil studies, nor to the visualization and characterization of preferential flow. This approach opens new avenues, both in 2-D and in 3-D, for tracer studies in porous media, such as soil.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
This study is the first of its kind, dealing with the application of SPECT scanning in soil hydrology. This section is divided into eight subsections. The first subsection presents the extraction and preparation of the column. The second subsection explains the choice of the column size while the third one deals with breakthrough measured by SPECT scanning. In the fourth subsection, the basic principles and operation of the SPECT scanner is presented. The following subsection presents the radioactive tracer. Finally, the last three subsections discuss the processing data generated by SPECT scanning, errors associated with SPECT scanning, and the use of CAT scanning as a complementary tool for soil structure characterization.

Soil Core Extractions and Column Preparation
Four undisturbed soil columns (800-mm length x 77-mm diam.) were taken from a field site at the Macdonald Campus of McGill University in Quebec, Canada. The soil cores were obtained by driving a polyvinyl chloride (PVC) pipe into the soil with a backhoe. Efforts were made to reduce soil compaction by inserting the PVC pipe very slowly and in increments of 0.1 m. As the pipes were inserted, the soil around them was removed to reduce friction. The lower end of the PVC pipe was sharpened to reduce compaction inside the column and to facilitate pipe insertion into the soil. The columns were extracted from an uncultivated field border that had been covered for many years with a combination of quack grass [Elytrigia repens (L.) Nevski], white clover (Trifolium repens L.), and wild oat (Avena fatua L.). Periodic mowing during the summer was the only cultural practice.

The soil belongs to the Chicot series. These soils are developed from sandy materials over a calcareous till and, as a result, they are generally well-drained. The soil was predominantly a sandy loam with an A horizon thickness of about 0.4m. The land slope was less than 1%. PVC caps were installed to create an empty space at the end of the column. This space allows water and tracers to penetrate uniformly through the column cross-section. A plastic screen was placed on both ends of the soil column to prevent the soil from collapsing. One of the soil columns was drilled from top to bottom to verify the gamma camera's ability to portray preferential flow in this pore. This artificial macropore consisted of a 1-mm i.d. polyethylene tube. Air inside the soil columns was removed by diffusing CO₂ through the soil columns for a period of 24 h. Carbon dioxide was made to infiltrate the soil by connecting the bottom ends of soil columns to a CO₂ pressurized tank. On the other ends, one-way valves were installed to prevent air from reentering the soil column. After saturating the soil cavities with CO₂, the undisturbed soil cores were slowly saturated by gradually raising the water level over a 3-day period. Since CO₂ dissolves in water, empty spaces were easily saturated. This technique was found to be effective in preventing entrapment of air bubbles in the soil. Before core saturation, water was deaerated in a 25-L vessel that was connected to a vacuum pump (at -4.5 kPa) for 12 h.

Selection of the Size of the Columns
The maximum diameter of the column that can be scanned by SPECT scanning depends on four factors: (i) the radioactive absorption characteristics of the soil, (ii) the energy of the gamma radiation of the tracer (i.e., 140 keV for technetium [Tc]), (iii) the radioactivity level of the tracer (i.e., 1.11 GBq for example), and (iv) the sensitivity of the gamma camera. In this study, the column size was selected not only based on the necessity of allowing the gamma radiation to emit from the core, but also based on the need to make it large enough to contain preferential flow paths, yet small enough to be handled easily when full of soil.

Given the size of the columns, the variability of macroporosity in the field, and the artifacts created during column extractions, the intact soil cores provide only an approximation of the preferential flow occurring under field conditions.

Breakthrough Measured by SPECT Scanning
The breakthrough experiment was performed on four saturated soil columns. A hydraulic head of 0.1 m was maintained at the upstream face of the soil columns in order to reach steady-state flow. A simple system was constructed for this purpose and is described in Perret et al. (1999). A thin layer of mineral oil was maintained on top of the water to minimize gas exchange between air and de-aerated water. A tracer pulse (i.e., 60 mL of radioactive solution at 0.43 GBq) was initiated at the upstream end of the soil columns. Twenty-milliliter samples were collected in the effluent every 30 s. The radioactivity level was evaluated with the gamma camera at the end of the breakthrough experiments. Sampling time was carefully taken in order to account for radioactive decay and to estimate flow rate. During the tracer breakthrough, the spatial distribution of radioactivity was monitored in the soil column with a SPECT scanner. A small cotton thread, saturated with a radioactive solution, was taped around the soil columns to delimit their boundaries.

Single Photon Emission Computed Tomography Scanning
Single photon emission computed tomography is based on detecting nuclear radiation emitted from a body or an object into which a radiopharmaceutical has been introduced. It is different from x-ray CAT scanning in that while x-ray CAT scanning generates transmission images, SPECT scanning produces emission images (Palmer et al., 1992). In other words, during x-ray CAT scanning, an external x-ray beam passes through the patient or object and the x-ray attenuation is recorded on the opposite side, whereas during SPECT scanning, the source of radiation is coming from inside the object and is recorded externally. Single photon emission computed tomography is unique in that it documents organ function or a dynamic process, in contrast to x-ray CAT scanning, which is based on anatomy of structure.

The spatial resolution of SPECT is usually worse than that of a CAT scanner. For instance, the pixel resolution of a fourth-generation CAT scanner is normally less than 200 by 200 µm, whereas the resolution of a SPECT scanner ranges generally from 2 by 2 mm to 9 by 9 mm depending on the size of the scintillation camera and the collimator (Palmer et al., 1992). However, typical SPECT scanners have a temporal resolution of about 1.5 to 3 ms (Palmer et al., 1992). Therefore, SPECT imaging can be used in dynamic studies in which changes in the distribution of radiation need to be monitored in a very small time scale. The time resolution of a CAT scanner is usually of the order of 1 to 2 s.

Single photon emission computed tomography scanning can be seen as a complementary approach to CAT imaging because it allows visualization of physiological functions or dynamic processes that are not usually seen by x-rays. X-ray CAT is primarily designed for visualizing the structure of an object or patient's anatomy, whereas SPECT imaging, with the use of radiopharmaceuticals, provides a powerful technique for inspecting specific dynamic processes.

A Siemens Orbiter (Siemens Medical Systems, Iselin, NJ) was used for this study (Fig. 1) . This equipment is located at the Tomographic Imaging and Porous Media (TIPM) Laboratory in Calgary, Alberta, Canada. It consists of three main components: the Orbiter detector stand assembly, the operator's console, and the NucLear MAC computer. The latter is a Power PC with NucLear software for displaying and processing SPECT data installed on it. This computer is used to acquire, display, store, and post-process data generated by the gamma camera. To understand the basic concepts of SPECT scanning, let us follow the process from emission of gamma radiation to images or matrices generated by the NucLear MAC computer. The radioactive source emits gamma rays in all directions. The first task is to detect radiation. This is achieved by the gamma camera of the Orbiter stand assembly. The main components of the gamma camera are the collimator, the NaI crystal and the photomultiplier tubes. Their functions are briefly discussed below.

View larger version (119K): [in this window] [in a new window]   Fig. 1 Siemens Orbiter gamma camera at the Tomographic Imaging and Porous Media Laboratory in Calgary, AB

Sodium Iodide Crystal and Photomultiplier Tubes
Gamma rays, travelling along a path that coincides with one of the collimator channels, pass through the collimator unabsorbed and interact with a large NaI crystal, creating light. These photons are then guided toward photocathodes on an array of photomultiplier tubes (light sensitive), where they are converted into electrons, multiplied, and finally converted into an electrical signal (Cho et al., 1993). In other words, behind the crystal, a grid of photomultiplier tubes collect light for processing. The Siemens Orbiter has a NaI crystal with a 0.375-m diam. and 37 photomultiplier tubes. The principal components of the gamma camera are presented in Fig. 2 .

View larger version (33K): [in this window] [in a new window]   Fig. 2 Basic principles and components of a gamma camera

Transformation of the Output Signal
The output from the gamma camera is analog and must therefore be converted to digital form by an analog-to-digital converter. After the x (horizontal) and y (vertical) position signals of the gamma camera are digitized, their position values are used to generate matrices. The numeric value of each pixel represents the counts recorded at that particular location. In this study, count rates were stored in a 160 by 160 matrix. Once the digital form is memorized, images can be displayed with various contrasts and brightness, or stored for further and more detailed analysis on the NucLear MAC computer.

Dynamic Scanning
The Siemens Orbiter is capable of generating matrices in a dynamic mode. Dynamic investigation of sequential matrices is similar to the analysis of time-lapse photography. As soon as acquisition of the first matrix is completed, it immediately begins to collect a second matrix, and so on. The time interval between acquisition of matrices is set at the beginning of the acquisition process. It is possible to acquire matrices at a sub-second level. This allows for real-time detection of the tracer's spatial distribution and provides a powerful approach to visualizing and quantifying tracer displacement in dynamic flow studies. However, an acquisition time of 1 s per matrix was sufficient to monitor breakthrough in the four soil columns. The breakthrough for certain soil columns was monitored for more than 3 h. Matrices produced during the acquisition process are two-dimensional and can be recorded either as planar projection images or in a tomographic fashion.

Planar Imaging
A planar single photon image is a pictorial representation of the radioactive decay that emanates from within the patient or the object of interest. More precisely, it is a 2-D representation of a 3-D object. It depicts the radiopharmaceutical's 3-D distribution onto a planar 2-D surface producing a projection matrix. A planar single photon image is very similar to a standard radiograph, however, the photons do not pass through the object, but are emitted from within it. Planar imaging was performed on Columns 1, 2, and 3.

Tomographic Imaging
Tomographic single photon images are acquired by detecting radioactive decay activities from different angles around the object. This is accomplished by rotating the camera head around the object and recording data from multiple projections. The stand of the Orbiter supports and counterbalances the gamma camera to accomplish this task. The Orbiter is designed to allow the camera head to orbit around an object as shown in Fig. 3 . During its rotation, the gamma camera stops at predefined angles to record spatial distribution of the radioactive decay within the object's volume. These multiple planar single photon images are then stored in a computer until the entire object has been viewed from multiple angles. A series of cross-sectional images or matrices of the object are then generated on the NucLear MAC computer using a filtered back-projection reconstruction algorithm. Tomographic imaging could be performed only on a 0.4-m length at a time, because of the size of the gamma camera head. More precisely, nine SPECT sequences were performed to monitor the three-dimensional breakthrough at different times. The time required for SPECT acquisition was 6 min. During that period, 64 planar matrices were recorded around the soil column. There should be no tracer displacement during SPECT acquisition in order to accurately reconstruct the radioactive tracer's spatial distribution. For this purpose, the tracer was allowed to move for 2 min before SPECT scanning. Then the flow was interrupted to freeze convection through macropores. At that time, the scanning sequence was initiated. This operation was repeated nine times. Each time, 64 planar views of the tracer distribution were recorded by the NucLear MAC computer.

View larger version (132K): [in this window] [in a new window]   Fig. 3 Rotation of the gamma camera around a column of soil

(1)

where RC_corrected is the corrected radioactive count, RC_measured is the measured radioactive count at time t (in minutes).

View larger version (15K): [in this window] [in a new window]   Fig. 4 Comparison of the radioactivity level of Tc in water and in soil at different concentrations

A 1.11-GBq dose was used in our experiments. This is an average dose for a number of nuclear medicine procedures that are performed on humans. It is considered a safe dose of radiation, which means that there are no harmful side effects to the patient or technologist performing the procedure. Normal medical radiation protection procedures were followed by (i) using protective gear (lead gloves, lead apron), and (ii) minimizing exposure to radiation by staying as little time as possible near the columns. As for disposal, the columns were stored in lead for 24 h. Since the half-life of Tc is short (i.e., 6 h), most of it was expected to have decayed in one day.

Preprocessing of Gamma Camera Output
The NucLear MAC computer is a high-performance system for acquisition, display, and post-processing data generated by the gamma camera. The NucLear MAC software follows standard user interface guidelines. After acquisition of the gamma camera output, the NucLear MAC was used to generate files containing 1000 matrices of 160 by 160 elements. Each matrix portrays 1 s of breakthrough in the soil columns. These files were saved in 8-bit tagged image file format (TIFF). A program called Readg.pro was written in the PV-WAVE language to read the TIFF files and to export them in ASCII format for further analysis on a Pentium II 300 MHz, equipped with 128 Mb of RAM.

The numerical value of each pixel of the cross-sectional matrices is equal to the radioactive count of a small-volume element (voxel) recorded at that particular location. The size of this voxel was equal to 2.5 by 2.5 by 2.4 mm. Therefore, each cross-sectional matrix had a thickness of 2.4 mm. The superposition of these 2-D matrices allows us to visualize the 3-D tracer distribution in the part of the soil column that has been scanned in tomographic mode. The NucLear MAC software has a special module that allows reconstruction of cross-sectional matrices acquired in tomographic mode. In this study, this module was used to acquire and superimpose 160 cross-sections in order to reconstruct and visualize the 3-D distribution of ^99mTc in a region of 400 mm in Column 4. For each SPECT acquisition, the module stored the cross-sections in a 3-D matrix of 160 by 160 by 160 pixels. These matrices were then converted from TIFF to ASCII format using Readg.pro. A second program (SPECTjo.pro) was developed for selecting a region of interest. Only part of the 160 by 160 cross-sectional matrices is actually used to investigate tracer distribution. The resulting matrix requires only 35 kB of disc space and is composed of an array of 40 by 40 elements. SPECTjo.pro also contains a filtering subroutine to remove noise, created by the radioactive string that was used to delimit the boundaries of the columns. Two additional programs were developed in PV-WAVE to generate 3-D reconstructions of the tracer distribution in the soil column. The first program (SPECT-3-D.pro) combines the 40 by 40 matrices into 3-D arrays and stores them in binary I/O format. The second program (SPECT-3-Dview.pro) produces a list of vertices and polygons that describes the 3-D distribution of Tc. The list of vertices and polygons is then used to generate 3-D images for visualization of the breakthrough.

Errors Associated with SPECT Scanning
During SPECT scanning, the radioactive count rates are stored in a computer's memory within a matrix of pixels. Once in digital form, images can be displayed with various contrasts and brightness levels, or stored for further analyses. The precision of the acquired information depends on the sensitivity of the SPECT camera. Low sensitivity is a source of error. The sensitivity of Siemens Orbiter used in this study was 0.131 cps GBq^-1 (counts per second per gigabecquerel of activity).

The spatial resolution can also be a source of imprecision. The radioactive counts per second are recorded over a specific area (i.e., pixel area), depending on the size of the scintillation camera and the collimator. The coarser the size of the pixels, the greater the imprecision.

During SPECT scanning, the scintillation camera rotates around the object of interest, stopping at selected intervals and generating images at each view. Before acquiring the SPECT data, it is important to calibrate the rotation of the camera and calculate its center of rotation. If this step is not done, the 3-D reconstructions will not be representative of the actual radioactive tracer distribution.

Computer Assisted Tomography Scanning
The soil columns were also scanned using a fourth-generation CAT scanner for the visualization and characterization of their interconnecting macropore structure. Computer programs were developed to generate 3-D models of the macropores, based entirely on the CAT scan data. These 3-D reconstructions provide us with an element of comparison between the actual macropore structure (measured by CAT scanning) and the preferential flow phenomenon monitored by SPECT scanning. For more details on macropore visualization and quantification using CAT scanning, the reader is referred to Perret et al. (1997), Perret et al. (2000), and Perret et al. (1999).

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 
Visualization of Macropore Flow
Most studies on the characterization of macropore flow consider the soil system to be a black box. Knowledge of flow behavior in macropores has been inferred by analyzing tracer variations in one or more detection sites of a soil column or a field plot. Visual inspection of the tracer distribution would be a new and exciting step towards analysis of the preferential flow phenomenon.

Figure 5 shows a planar view of the tracer breakthrough in the top 0.4 m of one of the soil columns. The three-dimensional rendered surface represents the tracer's spatial distribution. The vertical axis indicates the level of radioactivity.

View larger version (68K): [in this window] [in a new window]   Fig. 5 Schematic representation of the distribution of the radioactive tracer in the top region of one of the soil columns

View larger version (53K): [in this window] [in a new window]   Fig. 6 Three-dimensional representation of 99mTc distribution at different times after injection: (a) 25 s, (b) 50 s, (c) 75 s, (d) 100 s, (e) 125 s, (f) 150 s, (g) 200 s, (h) 300 s, (i) 400 s, (j) 500 s, (k) 750 s, and (l) 1000 s in Column 1

In Fig. 6f (150 s), radioactivity in the macropore has built up significantly. Moreover, it seems that flux through that macropore is branching out to the left, to another network. The tracer front in Fig. 6f has moved down substantially. This suggests that between 125 and 150 s, the tracer reached a region of high macroporosity, where the tracer moved rapidly. Radioactivity in the top 0.4 m of Column 1 reached a maximum at 200 s after injection (Fig. 6g).

The tracer is flushed from the column after that time and the radioactivity level decreases. It is interesting to note that after 300 s (Fig. 6h), the large plateau in the top 1/3 of the column does not seem to progress any more. In fact, it appears to be draining away from that region through the continuous macropore. From 400 (Fig. 6i) to 1000 s (Fig. 6l) after injection, the tracer moves slowly through the soil matrix.

Figure 7 shows the relationship between macropore structure and tracer distribution in Columns 1 (Fig. 7a and b) and 4 (Fig. 7c and d). Figures 7a and c show 3-D reconstructions of macropores, based on the CAT scan data (Perret et al., 1997, 1999). The colors used in the 3-D reconstruction generated by CAT scanning (Fig. 7a and c) depict the macropores (i.e., gray gradient) and the background (i.e., black). The macropores have been rendered and are displayed as gray descending structures. To give them a 3-D effect, their color varies from dark to light gray, depending on their position and geometry. Figures 7b and d show the tracer distribution in soil. These images were generated using a Gamma-II color palette in PV-WAVE, where dark gray represents low radioactivity and bright gray indicates tracer saturation. If we compare the macropore structure to tracer distribution in Column 1, it is quite obvious that the tracer moves preferentially through a continuous macropore. Moreover, it is interesting that not all macropores contribute to flow. The top region of Column 1 shows high macroporosity. Since the activities of arthropods, oligochaetes, and plant roots tend to be more important close to the soil surface, this observation was anticipated. Figure 7b indicates clearly that this region of high macroporosity, organized in a complex network of interconnected macropores, allows the tracer to move rapidly.

View larger version (96K): [in this window] [in a new window]   Fig. 7 Relation between 3-D macropore space and tracer distribution. Three-dimensional reconstructions of macropore networks are shown for (a) Column 1 and (c) Column 4. The spatial distributions of the tracer were evaluated at (b) 165 s in Column 1 and (d) 80 s in Column 4 after tracer injection

Three-dimensional reconstructions of the tracer distribution in the top 0.4 m of Column 4 are shown for different times in Fig. 8 . Two minutes separate each 3-D reconstruction. Figure 8a shows distribution 2 min after injection; Fig. 8b, 4 min after injection; Fig. 8c, 6 min after injection, and so on.

View larger version (78K): [in this window] [in a new window]   Fig. 8 Three-dimensional reconstructions of the tracer distribution in Column 4 obtained by SPECT scanning. The reconstructions represent 3-D tracer distribution at different times after injection: (a) 2 min, (b) 4 min, (c) 6 min, (d) 8 min, (e) 10 min, (f) 12 min, (g) 14 min, (h) 16 min, and (i) 18 min

Effluent Breakthrough
Figure 9 shows breakthrough in the effluent for Columns 1, 2, and 3. The shape of the breakthrough curves are quite different from one column to another. Column 1 shows a maximum concentration before 1 pore volume. Czapar et al. (1992), Bouma (1991), and Singh and Kanwar (1991) pointed out that a peak before a pore volume of 1 in an undisturbed soil column was an indication of macropore flow. The kurtosis of breakthroughs was calculated to characterize their degree of flatness. Kutosis values of 0.9, -1.2, and -1.5 were obtained for Columns 1, 2, and 3, respectively. The negative values for Columns 2 and 3 indicate that the breakthroughs are more spread out. This explains why the maximum radioactivity level is lower in Columns 2 and 3 than that in Column 1. This also suggests that flow is occurring through pores of different shapes and sizes and that there has been substantial mixing and diffusion before the tracer reached the effluent. It took about 1.15 pore volumes to reach maximum radioactivity in the effluent of Column 2 and about 1.5 pore volumes for Column 3. Therefore, this suggests that the tracer in the soil did not move preferentially. Macropores observed in the 3-D reconstructions of Columns 2 and 3 (Perret et al., 1997) are very discontinuous compared with those of Column 1. This may explain the differences obtained for breakthroughs in Columns 1, 2, and 3.

View larger version (16K): [in this window] [in a new window]   Fig. 9 Breakthrough curves observed in the effluent of Columns 1, 2, and 3

	Summary and conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

	ACKNOWLEDGMENTS

 
The authors wish to thank Dr. L. Hahn for initiating access to ^99mTc from the Foothills Medical Center, Calgary; Ingrid Koslowsky, for providing radioactive materials; and Barry Gulck, for the preparation of the radioactive tracer. The authors also gratefully acknowledge the financial support provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Environmental Science and Technology Alliance Canada (ESTAC).

Received for publication September 28, 1998.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Summary and conclusions REFERENCES

 

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