Graduate Thesis Or Dissertation
 

Quantitative, multi-spectral, light-transmission imaging of colloid transport in porous media at the meso scale

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  • Colloids, particles smaller than ten microns in diameter, are ubiquitous in the subsurface. Colloids have an effect on the transport of contaminants that bind to their surfaces, and can reduce the permeability of aquifer materials through deposition. Some microorganisms, including pathogens, are also transported in the subsurface as colloids. The study of colloid transport behavior is important to the improvement of contaminant transport predictions, and thus to the protection of water quality. Imaging technologies provide detailed spatial-temporal concentration data not obtainable by traditional methods, which rely on the collection and analysis of water samples at discrete locations. A quantitative, light-transmission imaging system was developed for the observation of solute and colloid transport in a chamber containing porous media. Multi-spectral LEDs and high-quality optical filters were used with a sensitive charge-coupled device camera to obtain images of a 47x57cm sand pack containing two fluorescent tracers—disodium fluorescein and red-fluorescent latex colloids. The properties of the optical components, and the selection of tracers, were optimized to allow for collection of data on three distinct properties of the system in time and space—water content, fluorescein concentration, and colloid concentration. An image processing technique was developed to normalize collected images and to remove noise. The system was calibrated through comparison of processed intensities with known fluorescein and colloid concentrations. Quantification was possible over the concentration range 10⁻² to 10²ppm for fluorescein, and 10⁻¹ to 10³ppm for colloids. This wide range of detection and the very low detection limits were made possible by the improvement in the signal-to-noise ratio that resulted from system development. Experiments were conducted to demonstrate the utility of this method to observe and quantify fluorescein and colloid transport. Model predictions compared favorably with collected data over multiple data sets, each containing approximately 300 data points. Colloid transport was reasonably fit by the advection-dispersion equation using a retardation factor and a first-order deposition coefficient to represent colloid interactions with the sand surface. In some cases, accurate description of colloid transport will require a more detailed parameterization of colloid dynamics that was beyond the scope of this work. Data presented in this thesis however demonstrate the power of this system to provide insight into the fate and transport of fluorescein and colloids in porous media. These insights facilitate prevention of groundwater pollution by colloidal contaminants, and contribute to effective design of remediation strategies for colloid-contaminated environments.
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