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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2011 - 20162

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    DISSOLVED AND GASEOUS FLUXES OF CARBON AND NITROGEN FROM URBAN WATERSHEDS OF THE CHESAPEAKE BAY
    (2016) Smith, Rose Marie; Kaushal, Sujay S; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Carbon and nitrogen loading to streams and rivers contributes to eutrophication as well as greenhouse gas (GHG) production in streams, rivers and estuaries. My dissertation consists of three research chapters, which examine interactions and potential trade-offs between water quality and greenhouse gas production in urban streams of the Chesapeake Bay watershed. My first research project focused on drivers of carbon export and quality in an urbanized river. I found that watershed carbon sources (soils and leaves) contributed more than in-stream production to overall carbon export, but that periods of high in-stream productivity were important over seasonal and daily timescales. My second research chapter examined the influence of urban storm-water and sanitary infrastructure on dissolved and gaseous carbon and nitrogen concentrations in headwater streams. Gases (CO2, CH4, and N2O) were consistently super-saturated throughout the course of a year. N2O concentrations in streams draining septic systems were within the high range of previously published values. Total dissolved nitrogen concentration was positively correlated with CO2 and N2O and negatively correlated with CH4. My third research chapter examined a long-term (15-year) record of GHG emissions from soils in rural forests, urban forest, and urban lawns in Baltimore, MD. CO2, CH4, and N2O emissions showed positive correlations with temperature at each site. Lawns were a net source of CH4 + N2O, whereas forests were net sinks. Gross CO2 fluxes were also highest in lawns, in part due to elevated growing-season temperatures. While land cover influences GHG emissions from soils, the overall role of land cover on this flux is very small (< 0.5%) compared with gases released from anthropogenic sources, according to a recent GHG budget of the Baltimore metropolitan area, where this study took place.
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    DYNAMICS OF METABOLIC GASES IN GROUNDWATER AND THE VADOSE ZONE OF SOILS ON DELMARVA
    (2011) Fox, Rebecca Jane; Fisher, Thomas R; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Denitrification removes nitrogen from watersheds under reducing conditions, but N2O and CH4, both greenhouse gases, can also be produced. The overarching hypothesis of my thesis was that hydric environments accumulate N2O and CH4 in groundwater and the vadose zone. To test the hypothesis, groundwater samples were taken monthly during 2007-2009 at 64 piezometers in 10 wetlands for analysis of excess N2, N2O, CH4, and CO2. Vadose zone gas and groundwater samples were taken during 2008-2010 at two riparian buffers and a hydrologically restored wetland. The hydrology of the 10 locations was complex. A hydrologic connection across a transect was determined at one location where NO3- significantly decreased, excess N2 significantly increased, and moderate concentrations of N2O and CH4 accumulated. Within these 10 locations, three N2O and four CH4 hot spots were identified, and hot moments accounted for a large percentage of total accumulated N2O and CH4. I found evidence of CH4 ebullition, the production of CH4 bubbles in the vadose zone that strip other dissolved gases. The locations that accumulated the most dissolved CH4 and N2O were natural wetlands and riparian areas, respectively. I measured both positive and negative excess N2 concentrations in the vadose zone. Flux estimates ranged from -600 to 880 kg N ha-1 yr-1, which brackets missing N estimates at the watershed scale. These concentrations were calculated using N2/Ar, and both gases are affected by physical processes. These calculated excess N2 profiles could have been produced through either biological and/or physical mechanisms, and these processes currently cannot be distinguished. Less than 1% of the missing N on the transect scale, measured as the difference in N concentration between two piezometers, was accounted for by calculated diffusional fluxes from groundwater to the vadose zone. The primary mechanism transporting gases from the vadose zone to the atmosphere was diffusion, but convection transported 20% of the calculated median CO2 yearly flux. Increased production of N2O and CO2 was observed in the vadose zone after rainfall events. Overall, large concentrations of N2O, CH4, CO2, and excess N2 accumulated in the groundwater and vadose zone of these locations, supporting the overarching hypothesis.