Perret Et Al.: Preferential Solute Flow Measured by Spect Scanning

ven and Germann, 1982; Bouma, 1990; Hamblin, 1985; Logsdon, 1995; McCoy et al., 1994; Steenhuis et al., Single photon emission computed tomography (SPECT) is an im1990; Singh and Kanwar, 1991; White, 1985). Macropore aging technique that is widely used in medical diagnosis. This technique has never been applied to soils. The objective of this study flow results in rapid movement of water and solutes was to investigate the capabilities of SPECT scanning for visualizing through the soil profile with important implications for preferential flow in soil. This paper describes the principle of SPECT ground water quality. Short travel time may not allow scanning and its application to tracer breakthroughs in four large contaminants, such as pesticides, to be adsorbed on and undisturbed soil columns (800-mm length 3 77-mm diam.). This new into soil particles. Consequently, most solutes in the approach allows real-time analysis of flow patterns of radioactive water will be quickly delivered to the drains or ground tracers in 2-D using planar imaging, and in 3-D using the tomographic water aquifers without being degraded by chemical and capabilities of the SPECT scanner. Not only does SPECT scanning biological actions. However, quantification and predicprovide qualitative data, but it also allows for the quantification of a tion of preferential flow has been difficult because of tracer’s spatial distribution. Our results characterized preferential flow the complexity of soil structure. 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. 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. UnfortuW and solute transport through soil macnately, progress in this area has been severely limited ropores can be quite significant and is now well by difficulties in obtaining direct and nondestructive documented (Perret et al., 1997; Perret et al., 2000; Bemeasurements of the preferential flow in a structured soil. Computer assisted tomography (CAT) scanning Johan Perret and S.O. Prasher, Dep. of Agric. and Biosyst. Eng., offers a powerful approach to the study of preferential McGill Univ., 21 111 Lakeshore Road, Ste-Anne-de-Bellevue, QC, flow (Anderson and Hopmans, 1994; Perret et al., 2000). Canada H9X-3V9; A. Kantzas, Dep. of Chem. and Petrol. Eng., Univ. of Calgary, 2500 University Dr. N.W., Calgary, AB, Canada T2NHowever, whereas CAT scanning provides high-resolu1N4; K. Hamilton, TIPM Lab., Univ. of Calgary, 2500 University Dr. tion cross-sectional images in usually more than a secN.W., Calgary, AB, Canada T2N-1N4; and C. Langford, Dep. of ond, single photon emission computed tomography Chemistry, Univ. of Calgary, 2500 University Dr. N.W., Calgary, AB, Canada T2N-1N4. Received 28 Sept. 1998. Corresponding author ([email protected]). Abbreviations: CAT, computer assisted tomography; SPECT, single photon emission computed tomography. Published in Soil Sci. Soc. Am. J. 64:469–477 (2000). 470 SOIL SCI. SOC. AM. J., VOL. 64, MARCH–APRIL 2000 (SPECT) allows longitudinal views at a sub-second and Hsu, 1984) and analyzing the radioactive ball trajectories within the charge of a rotary grinding mill (Powell level. To the best of our knowledge, SPECT has never been and Nurick, 1996). Nuclear technology has also been utilized in oil-recovapplied to studies in soil science. However, it may provide a new and exciting approach for visualizing and ery fields to visualize dynamic oil displacement in porous media. For instance, Huang and Gryte (1988) used characterizing preferential flow phenomenon. The primary objective of this paper is to investigate the capabilia gamma camera to observe immiscible displacement of oil in thin slabs of a porous medium saturated with ties of SPECT scanning for visualizing preferential flow in soil during solute transport and distribution. water. In their study, technitium-99m was used as a tracer for the water phase. The authors showed that Single photon emission computed tomography is based on detecting nuclear radiation emitted from the photon emission imaging provides a powerful approach for determining local fluid saturations in quantitative body or from an object into which a very small amount of radioactive material, called a radiopharmaceutical, terms. Lien et al. (1988) studied the one-dimensional distribution of saturation in 0.75-m long sandstone has been introduced. A special type of camera, known as a scintillation or gamma camera, is used to transform cores, operating at reservoir pressure and temperature. Information on one-dimensional fluid saturation distrithese radioactive emissions into images or data, which describe the location and intensity of these emissions. butions was obtained by labeling fluid phases with nuclear tracers and detecting radiation with a gamma camSingle photon emission computed tomography scanning is widely used as a diagnostic technique in nuclear era. They reported that the apparatus fulfilled its objective of imaging displacement processes at reservoir medicine. Ten to twelve million nuclear medicine imaging and therapeutic procedures are performed each conditions. Charlier et al. (1995) applied gamma ray absorption techniques to determine the permeability of year in the USA alone (SNM, 1999). Today, nearly all cardiac patients in developed countries receive a SPECT oil during a tertiary-gas gravity-drainage experiment. Using this technique, they were able to visualize fluid scan to detect arterial diseases or a damaged heart. Investigation of the liver, kidneys, thyroid gland, and saturation distribution in the core as a function of the injected gas’s volume. many other organs are, similarly, leading applications of SPECT (Coleman et al., 1986). It is used routinely Although the power of noninvasive and in situ SPECT scanning has been demonstrated for dynamic industrial to help diagnose restriction of blood flow to parts of the brain, as well as cancer, stroke, lung disease, and processes and for oil recovery, this technique has never been applied to soil studies, nor to the visualization many other physiological abnormalities. Scintillation camera technology has been used in inand characterization of preferential flow. This approach opens new avenues, both in 2-D and in 3-D, for tracer dustrial applications since the mid-1980s. For instance, photon emission imaging has been used to perform vistudies in porous media, such as soil. sual and radiometric scans of nuclear facilities. Sedaghat et al. (1988) developed a technique to characterize crossMATERIALS AND METHODS flow in a two sub-channel nuclear fuel assembly using This study is the first of its kind, dealing with the application a gamma camera. Gamma camera technology has also of SPECT scanning in soil hydrology. This section is divided been adapted to conduct contamination surveys inside into eight subsections. The first subsection presents the extracbuildings that are connected with nuclear production tion and preparation of the column. The second subsection (Chesnokov et al., 1997; Mottershead and Orr, 1996). explains the choice of the column size while the third one Single photon emission computed tomography has deals with breakthrough measured by SPECT scanning. In the fourth subsection, the basic principles and operation of also brought a new dimension to disciplines such as the SPECT scanner is presented. The following subsection particle tracking and flow analysis in mixing reactor presents the radioactive tracer. Finally, the last three subsecvessels. Castellana and Dudley (1984) were among the tions discuss the processing data generated by SPECT scanfirst researchers to visualize particle motion in fluid– ning, errors associated with SPECT scanning, and the use of solid systems using a gamma camera. With a similar CAT scanning as a complementary tool for soil structure charapproach, Lin et al. (1985) used a series of photomultiacterization. plier tubes to measure the solid motion of radioactive particles in gas fluidized beds. Today, radioactive-partiSoil Core Extractions and Column Preparation cle tracking continues to offer great potential for measuring recirculating phase velocities in gas-fluidized Four undisturbed soil columns (800-mm length 3 77-mm diam.) were taken from a field site at the Macdonald Campus beds and bubble columns. Scintillation cameras have of McGill University in Quebec, Canada. The soil cores were also been used to determine the hydrodynamics and obtained by driving a polyvinyl chloride (PVC) pipe into the radial distributions of velocity in vertical riser reactors soil with a backhoe. Efforts were made to reduce soil compacand mixing reactor vessels (Castellana et al., 1984; Betion by inserting the PVC pipe very slowly and in increments rker and Tulig, 1986; Legoupil et al., 1997). The potenof 0.1 m. As the pipes were inserted, the soil around them tial of SPECT scanning for imaging a gas-flow fluidizawas removed to reduce friction. The lower end of the PVC tion test rig and fluid flow in a gasification unit is pipe was sharpened to reduce compaction inside the column discussed by Jonkers et al. (1990). Other applications and to facilitate pipe insertion into the soil. The columns were to the field of engineering include measuring liquid film extracted from an uncultivated field border that had been covered for many years with a combination of quack grass thickness on the surface of a rotating disk (Castellana PERRET ET AL.: PREFERENTIAL SOLUTE FLOW MEASURED BY SPECT SCANNING 471 [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 0.4 m. 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 Fig. 1. Siemens Orbiter gamma camera at the Tomographic Imaging period of 24 h. Carbon dioxide was made to infiltrate the and Porous Media Laboratory in Calgary, AB. soil by connecting the bottom ends of soil columns to a CO2 pressurized tank. On the other ends, one-way valves were Single Photon Emission Computed installed to prevent air from reentering the soil column. After Tomography Scanning saturating the soil cavities with CO2, the undisturbed soil cores Single photon emission computed tomography is based on were slowly saturated by gradually raising the water level over detecting nuclear radiation emitted from a body or an object a 3-day period. Since CO2 dissolves in water, empty spaces into which a radiopharmaceutical has been introduced. It is were easily saturated. This technique was found to be effective different from x-ray CAT scanning in that while x-ray CAT in preventing entrapment of air bubbles in the soil. Before scanning generates transmission images, SPECT scanning procore saturation, water was deaerated in a 25-L vessel that was duces emission images (Palmer et al., 1992). In other words, connected to a vacuum pump (at 24.5 kPa) for 12 h. during x-ray CAT scanning, an external x-ray beam passes through the patient or object and the x-ray attenuation is Selection of the Size of the Columns recorded on the opposite side, whereas during SPECT scanThe maximum diameter of the column that can be scanned ning, the source of radiation is coming from inside the object and is recorded externally. Single photon emission computed by SPECT scanning depends on four factors: (i) the radioactive tomography is unique in that it documents organ function or absorption characteristics of the soil, (ii) the energy of the a dynamic process, in contrast to x-ray CAT scanning, which gamma radiation of the tracer (i.e., 140 keV for technetium is based on anatomy of structure. [Tc]), (iii) the radioactivity level of the tracer (i.e., 1.11 GBq The spatial resolution of SPECT is usually worse than that for example), and (iv) the sensitivity of the gamma camera. of a CAT scanner. For instance, the pixel resolution of a In this study, the column size was selected not only based on fourth-generation CAT scanner is normally less than 200 by the necessity of allowing the gamma radiation to emit from 200 mm, whereas the resolution of a SPECT scanner ranges the core, but also based on the need to make it large enough generally from 2 by 2 mm to 9 by 9 mm depending on the to contain preferential flow paths, yet small enough to be size of the scintillation camera and the collimator (Palmer et handled easily when full of soil. al., 1992). However, typical SPECT scanners have a temporal Given the size of the columns, the variability of macroporosresolution of about 1.5 to 3 ms (Palmer et al., 1992). Therefore, ity in the field, and the artifacts created during column extracSPECT imaging can be used in dynamic studies in which tions, the intact soil cores provide only an approximation of changes in the distribution of radiation need to be monitored the preferential flow occurring under field conditions. in a very small time scale. The time resolution of a CAT scanner is usually of the order of 1 to 2 s. Breakthrough Measured by SPECT Scanning Single photon emission computed tomography scanning can be seen as a complementary approach to CAT imaging beThe breakthrough experiment was performed on four saturated soil columns. A hydraulic head of 0.1 m was maintained cause it allows visualization of physiological functions or dynamic processes that are not usually seen by x-rays. X-ray at the upstream face of the soil columns in order to reach steady-state flow. A simple system was constructed for this CAT is primarily designed for visualizing the structure of an object or patient’s anatomy, whereas SPECT imaging, with purpose and is described in Perret et al. (1999a). A thin layer of mineral oil was maintained on top of the water to minimize the use of radiopharmaceuticals, provides a powerful technique for inspecting specific dynamic processes. gas exchange between air and de-aerated water. A tracer pulse (i.e., 60 mL of radioactive solution at ≈0.43 GBq) was initiated A Siemens Orbiter (Siemens Medical Systems, Iselin, NJ) was used for this study (Fig. 1). This equipment is located at at the upstream end of the soil columns. Twenty-milliliter samples were collected in the effluent every 30 s. The radioacthe Tomographic Imaging and Porous Media (TIPM) Laboratory in Calgary, Alberta, Canada. It consists of three main tivity level was evaluated with the gamma camera at the end of the breakthrough experiments. Sampling time was carefully components: the Orbiter detector stand assembly, the operator’s console, and the NucLear MAC computer. The latter is taken in order to account for radioactive decay and to estimate flow rate. During the tracer breakthrough, the spatial distribua Power PC with NucLear software for displaying and processing SPECT data installed on it. This computer is used to tion of radioactivity was monitored in the soil column with a SPECT scanner. A small cotton thread, saturated with a acquire, display, store, and post-process data generated by the gamma camera. To understand the basic concepts of SPECT radioactive solution, was taped around the soil columns to delimit their boundaries. scanning, let us follow the process from emission of gamma 472 SOIL SCI. SOC. AM. J., VOL. 64, MARCH–APRIL 2000 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.