47012-GB8
Microtomographic Imaging of the Air-Water Interface in Unsaturated Porous Media
Experimental Activities and Findings Student recruitment and training. Two undergraduate students were recruited to the project and have shown exceptional promise in their abilities to develop analytical and experimental methods related to the unsaturated aqueous-phase miscible displacement experiments and the X- ray microtomography imaging at the Advanced Photon Source (APS). This project has fostered the development of several important scientific skills, including how to 1) keep a defensible, highly detailed, and accurate laboratory notebook and ensure laboratory safety, 2) perform fundamental laboratory techniques related to solute transport (e.g., soil column miscible-displacement transport experiments, determination of particle-size distribution) 3) develop and test analytical methods on sophisticated laboratory instrumentation (e.g., UV-Visible Spectrometer, Gas Chromatograph, Synchrotron X-ray Microtomography), 4) to troubleshoot a system using well-reasoned 'scientific' approaches of alternating between hypotheses and evidence, and 5) extract useful and relevant information from the primary literature. Finally, the students have gained considerable experience using Linux-based computing for image analysis, including writing scripts in bash, idl, and tlc and running specialized software. Importantly, their visits to Argonne National Laboratory’s Advanced Photon Source (APS) have exposed them to cutting-edge technology that few undergraduates have the opportunity to use. Experimental Activities and Findings Development of automated REV analysis methods. Work during Award Year 1 included developing and automating procedures (tcl scripts run in Amira imaging software) for assessing whether the small volume imaged within the larger porous media system is sufficiently large to capture a representative elementary volume (REV) with respect to the air-water interfacial areas. We have applied our expanding-cube REV method to a limited set of natural and model porous media to measure interfacial area REV and determine whether/how the REV depends on system properties (e.g., grain size, grain size distribution, water saturation, etc.). Preliminary results suggest, as would be expected, that media containing larger grains have larger REV associated with them. Interestingly, interfacial area REVs are also larger for higher water content systems, due to the sparse distribution of the isolated air-filled pockets. Synchrotron X-ray Microtomographic Imaging. During Award Year 1, both students participated in two visits to Argonne National Laboratory’s Advanced Photon Source (APS) to conduct X-ray microtomography imaging of unsaturated natural sandy porous media. APS X-ray beamtime is awarded via a competitive proposal process. APS Visit 1. Our previous imaging protocols were modified to improve image resolution from ~11 to ~6 µm via “unbinning” of the detector elements. Because an average grain size of ~100 µm was previously found to be too fine for accurate information to be derived from the images, we hoped that higher resolution images would extend the applicability of our imaging methods to a wider range of porous media. Moreover, we hypothesized that image resolution would influence the magnitude of the features we sought to quantify in the images, allowing us, for example, to quantify smaller features that would otherwise be masked in lower resolution images. High resolution images were collected at multiple water saturations for multiple natural sandy porous media. In particular, natural media with finer average grain size than was previously amenable to image-based analysis were included in our sample set in order to test the advantage of high resolution imaging. Unfortunately, 32-bit computing in the PI's lab was found to be insufficient for processing of the resulting high resolution images, and we have recently complete a transition to a 64-bit system and upgraded image-processing software to accommodate this new stage of our research. APS Visit 2. The purpose of APS Visit 2 was to test and refine methods for conducting unsaturated aqueous flow experiments at the beamline, rather than solely imaging static porous media columns. Logistics governing unsaturated flow (e.g., hydraulic connectivity and negative pressure at the soil column outlet) were merged with logistics required for imaging (e.g.,small column dimensions, Xray transparent column materials, Xray dopants for the water phase). Porous media were saturated and iteratively drained to the desired steady-state water content followed by microtomographic imaging to obtain interfacial areas. The high resolution imaging methods developed during APS Visit 1 were employed allowing 12 µm resolution on significantly larger soil columns, improving by 2-fold the image resolution that would normally be achieved. Miscible displacement tracer experiments. Experimental activities at our home institution included the development of the vacuum chamber methods for unsaturated miscible displacement interfacial tracer experiments involving 1-nonanol as the interfacial tracer and pentafluorobenzoic acid (PFBA) as the nonreactive tracer. Small discrete effluent sample sizes (< 0.5 mL) typically associated with unsaturated solute transport experiments represent an analytical challenge. Accordingly, we have developed an online flow-through UV-Visible spectroscopy method for PFBA analysis (nonreactive aqueous tracer) to avoid collection and analysis of small-volume time-discrete effluent samples. We have also developed a direct- aqueous-injection method for analyzing 1-nonanol (interfacial aqueous tracer) that obviates solvent extraction of 1-nonanol from the aqueous phase.
