Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item THE DISTRIBUTION AND FUNCTION OF DENITRIFICATION GENES: EXPLORING AGRICULTURAL MANAGEMENT AND SOIL CHEMICAL IMPLICATIONS(2016) Bowen, Holly; Yarwood, Stephanie A; Environmental Science and Technology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Denitrification is a microbially-mediated process that converts nitrate (NO3-) to dinitrogen (N2) gas and has implications for soil fertility, climate change, and water quality. Using PCR, qPCR, and T-RFLP, the effects of environmental drivers and land management on the abundance and composition of functional genes were investigated. Environmental variables affecting gene abundance were soil type, soil depth, nitrogen concentrations, soil moisture, and pH, although each gene was unique in its spatial distribution and controlling factors. The inclusion of microbial variables, specifically genotype and gene abundance, improved denitrification models and highlights the benefit of including microbial data in modeling denitrification. Along with some evidence of niche selection, I show that nirS is a good predictor of denitrification enzyme activity (DEA) and N2O:N2 ratio, especially in alkaline and wetland soils. nirK was correlated to N2O production and became a stronger predictor of DEA in acidic soils, indicating that nirK and nirS are not ecologically redundant.Item EVALUATION OF THE INFLUENCE OF NITROGEN ON PRIMARY PRODUCTION USING RETROSPECTIVE DATA, REGRESSION ANALYSIS, AND MODELING(2012) Ziombra, Katherine Elizabeth Davis; Harris, Lora A; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Anthropogenic activities have negatively affected water quality in the Chesapeake Bay and its tributaries. The Potomac River (PR), the largest tributary, is a primary study site for water quality research and new management strategies. The Blue Plains Wastewater Treatment Plant (BP), located in the tidal fresh portion of the PR, is the largest total nitrogen (TN) point source. Retrospective examination of water quality data for the PR revealed relationships among discharge, N loading and concentration, light and primary production. Regression analysis revealed BP (TN) load was an important variable influencing production, coupled with local dissolved inorganic nitrogen concentrations and photic depth prior to installation of biological nutrient removal (BNR) at BP. After 100% BNR implementation, BP TN did not influence production. Four existing primary production models were evaluated for applicability to tidal fresh systems. Regression analysis demonstrated all models were significant but the BZpI0t model provided the most robust results.Item Modeling Approaches for Treatment Wetlands(2009) Carleton, James N.; Montas, Hubert J; Biological Resources Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Although treatment wetlands can reduce pollutant loads, reliably predicting their performance remains a challenge because removal processes are often complex, spatially heterogeneous, and incompletely understood. Although initially popular for characterizing wetland performance, plug flow reactor models are problematic because their parameters exhibit correlation with hydraulic loading. One-dimensional advective-dispersive-reactive (ADE) models are also inadequate because longitudinal dispersion in wetlands is often non-Fickian as a result of steep velocity gradients. Models that make use of residence time distributions have shown promise in improving performance characterization, particularly when interdependencies of stream-tube scale velocities and reaction rate coefficients are considered (the "DND" approach). However this approach is limited to steady-state conditions, and to an assumption that transverse mixing is nil. This dissertation investigates three aspects of wetland modeling and is organized in a journal paper format. The first paper describes development of a DND model which accommodates non-steady-state conditions. The model processes flow and inlet concentration time series, and calculates as output effluent concentration time series. A version of the code allows optimization of model parameters by minimization of summed squared deviations between predicted and measured effluent concentrations. In example comparisons, model results compare favorably with measured data. The second paper develops an analytical solution to a two-dimensional advective-dispersive-reactive equation, in which all flux terms are expressed as power functions of the transverse dimension. For uniform inlet concentration this idealized heterogeneity model is similar to a DND model, but with the inclusion of transverse diffusion. An example is used to illustrate the beneficial impact that transverse mixing has on reactor performance. The third paper describes development of a model based upon a stochastic interpretation of the ADE. The solution technique that is employed results in a bicontinuum model that for steady-state conditions becomes a weighted sum of two exponential decline functions. For low and intermediate degrees of mixing, model results nicely match those of the corresponding idealized heterogeneity model, and for high mixing they match results of the corresponding one-dimensional ADE. Comparisons against data suggest the bicontinuum model may represent wetland performance better than the DND model in some but not all cases.Item Dynamics of Low Immersion Milling(2008-07-14) Young, Sigmund Max; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, dynamics of low immersion milling is explored through analytical and numerical means. Using linear and nonlinear cutting force models, maps are constructed for single degree-of-freedom and two degree-of-freedom systems where the time spent cutting is "small" compared to the spindle rotation period. These maps are used to study the possibilities for different nonlinear instabilities and construct stability charts in the space of cutting depth and spindle speed. The analytical predictions are compared with numerical results as well as prior experimental results. Good agreement amongst analytical, numerical, and experimental results is seen. Limitations of the analytical and numerical approaches are discussed and extensions for future work are suggested.Item Analog System-on-a-Chip with Application to Biosensors(2005-04-28) Hodge, Angela Marie; Newcomb, Robert W.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation facilitates the design and fabrication of analog systems-on-a-chip (SoCs). In this work an analog SoC is developed with application to organic fluid analysis. The device contains a built-in self-test method for performing on-chip analysis of analog macros. The analog system-on-a-chip developed in this dissertation can be used to evaluate the properties of fluids for medical diagnoses. The research herein described covers the development of: analog SoC models, an improved set of chemical sensor arrays, a self-contained system-on-a-chip for the determination of fluid properties, and a method of performing on-chip testing of analog SoC sub-blocks.