Serrano-Guzman, Maria F.

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    Detection and monitoring of DNAPLs in the subsurface under transient conditions using cross well radar
    (2008-04) Serrano-Guzman, Maria F.; Padilla Cestero, Ingrid Y.; College of Engineering; Hwang, Sangchul; Solís Rodríguez, Rafael A.; Harmsen, Eric; Department of Civil Engineering; Torres, Ramón
    Contamination of soils and groundwater, accidental spills, poor storage facilities, and inadequate disposal practices cause serious detriment of the environment and can pose a serious threat to human health. Common contaminants found in underground environments include many Dense Non-Aqueous Phase Liquids (DNAPLs). DNAPLs are liquids denser than water. Most DNAPLs experience only partial degradation in the subsurface, and persist for long time slowly releasing soluble organic constituents to groundwater. The most common DNAPLs are halogenated solvents, such as trichloroethylene (TCE) and tetrachloroethylene (PCE). Their heterogeneities distribution in the environments makes DNAPLs difficult to locate, characterize, and remediate. It is therefore, necessary to develop new technologies that will enhance our ability to characterize contaminated sites, locate underground contaminants, evaluate fate and transport processes, and remediate contaminated sites. The research presented herein develops and evaluates Cross Well Radar (CWR) technologies to detect and monitor DNAPLs contamination in subsurface environments under transient flow conditions. It involves systematic development and testing of sensing system, signal management and processing; and imaging technologies. Electromagnetic and flow experiments are used in conjunction with image acquisition technologies to generate critical information and evaluate the effectiveness and reliability of CWR systems. A methodology has been developed to detect electromagnetic (EM) changes caused by variable spatial and temporal distribution of fluids with different EM properties. The method used a 2D flow and electromagnetic soilBed instrumented with loop antennas. Measurements show sufficient contrast between EM properties of uncontaminated and DNAPL-contaminated soil to apply CWR for contaminant detection. The contrast is dependent on water content, frequency range of analysis, fluid movement, distribution, and heterogeneities, and the presence of physical, and fluid interfacial areas. A method was developed to estimate relative permittivity along raypath between transmitting and receiving CWR antennas from EM measurements. The method assumes lossless medium and perfectly coupled and identical radiation characteristics of the antennas. Estimates determined from water and TCE flow experiments indicate that variable and temporal distribution of fluids with different EM properties cause detectable changes in dielectric properties of the bulk soil. A sequentially algebraic reconstruction method (SART) was developed and applied to generate tomographic images of the estimated relative permittivities. The tomograms can be used to image and visualize the presence of disturbances in the medium. The tomographic method generates acceptable tomograms of under ground target elements in soils, provided that there is sufficient density of antennas array and proper grid spacing. The codes provide a tool for optimal CWR system design and can be applied to determine the number of antennas required for good resolution of a specific geometry in lossless medium. Generated images suggest slight variations of the tomograms after injection of TCE and water in the system. The tomographic results show changes caused by variable flow and fluid saturation and distribution conditions. The experimental resolution and potential measurement error, however, limit the asseverance of conclusive remarks in the system. This research also developed image acquisition and processing algorithms to analyze visual images of dyed contaminants, discriminate between regions of different amounts of DNAPLs, and assess potential relationships between electromagnetic variations and the spatially-distributed DNAPL in the soil. The results indicate that the image processing and analysis techniques developed in this research are effective in detecting changes by fluid flow and distribution. Differences in color intensity in the presence of water suggest that this technique may be applicable to monitor flow and saturation. Changes on pixel intensity during dyed TCE injection also indicate its application to monitor transport and mass of TCE in the system. The methods developed and tested in this research represent significant contributions, which move underground detection technologies closer to real applications. Recommendations addressed the limitations encountered and establish a basis for full deployment of CWR technologies for detection of underground contamination.