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 Extending the Cover Crop Growing Season to Reduce Nitrogen Pollution(2021) Sedghi, Nathan; Weil, Ray R; Environmental Science and Technology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Maryland currently has the highest rate of cover crop use in the United States. The Cover Crop Program, started as an initiative to clean nutrients from the Chesapeake Bay, has made it a common practice to plant a cereal cover crop after cash crop harvest in fall, and kill it several weeks before cash crop planting in spring. In Maryland, this practice does not allow enough growing time with warm conditions for optimal cover crop growth. Planting earlier in fall and killing a cover crop later in spring could improve soil N cycling. We hypothesized that interseeding into a cash crop in early fall, and delaying spring cover crop termination could increase cover crop biomass, carbon accumulation, and nitrogen uptake and decrease nitrate leached. We tested these hypotheses over four years with five field experiments, consistently using a brassica-legume-cereal cover crop mix. We evaluated the relationships between cover crop planting date and fall cover crop N uptake and reduction in nitrate leaching. In spring, we tested termination timing effects on cover biomass C and N, soil mineral N concentration, soil moisture, and corn yield. We tested multiple dates for broadcast interseeding cover crops into standing soybean cash crops. We partnered with farmers on Maryland’s Eastern Shore to test if our methods are feasible at a realistic scale. We measured nitrous oxide emissions to test if our recommended cover crop practice has the negative drawback of increasing emissions of nitrous oxide, a powerful greenhouse gas. The nitrate leached under late drilled and early interseeded methods were comparable under conditions which favored late drilling, but interseeding outperformed drilling when there was adequate rainfall for seed germination. The result was lower nitrate porewater concentrations under early planted cover crops. Nitrous oxide emissions increased slightly with cover crops relative to no cover crop, but the increase was negligible when compared to the nitrous oxide produced from applying N fertilizer. Our research showed that extending the cover crop growing season of a brassica-legume-cereal mix has multiple environmental benefits and few drawbacks.Item Leaching of Contaminants from Recycled Asphalt Pavement used in Highway Construction(2017) Seybou Insa, Ousmane; Aydilek, Ahmet H; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recycled asphalt pavement (RAP) is a material resulting the removal and reprocessing of the existing asphalt pavement. Annually, more than 80 million tons of RAP is produced in the United States. Most of the RAP is landfilled, which constitutes a significant economic burden and a waste of landfill space. RAP has great potential in highway construction due to its good physical properties. However, concerns have been raised due to potential surface water and groundwater contamination by metals including some heavy metals and Polycyclic Aromatic Hydrocarbons (PAHs) present in RAP. A series of batch-level pH-dependent leaching tests and Toxicity Characteristics Leaching Procedure (TCLP) tests were conducted to investigate the leaching of metals from seven RAPs collected across the state of Maryland. The results of the pH-dependent leaching tests indicated As to be slightly above the U.S EPA Water Quality Limit (WQL). The results of a series of TCLP tests showed that two PAHs, chrysene and Indeno[1,2,3-cd]pyrene, may be present in the leachates, albeit, at concentrations very comparable to those leach from a new asphalt material. The results of the geochemical modeling indicated that the leached metals were solubility-controlled. Oxide and hydroxide minerals control the leaching of Al and Fe; whereas, leaching of Ba, Ca and Mg were controlled by carbonate and/or sulfate minerals.Item Evaluation of Leaching Protocols for the Testing of Coal Combustion Byproducts(2010) Becker, Jason Louis; Aydilek, Ahmet; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Beneficial reuse of coal combustion byproducts requires an evaluation of metal leaching potential. Reuse of high carbon fly ash in highway embankment construction was evaluated using different soil-fly ash mixtures and three common leaching tests: the water leach test (WLT), the Toxicity Characteristic Leaching Procedure (TCLP), and the column leach test (CLT). The effect of test methodology and pH on Cu, As, and Cr leaching was examined. TCLP concentrations for Cu and As exceeded those from WLTs in the majority of mixtures due to lower pH conditions, while Cr was higher in the WLT for alkaline fly ash mixtures. Peak CLT concentrations were higher than TCLP and WLT concentrations for the majority of mixtures, but usually decreased rapidly, suggesting that leachate concentrations might exceed regulatory limits, but only for a short time. Based on these results, a combined WLT and CLT leaching protocol for testing fly ash mixtures is presented.Item Release of inorganic and organic contaminants from fly ash amended permeable reactive barriers(2008-01-25) Morar, Doina Lorena; Aydilek, Ahmet H.; Seagren, Eric A.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Large quantities of fly ash are generated in the United States via coal combustion, most of which is disposed of in lagoons or landfills. The overall goal of this research was to assess the feasibility of using high carbon content (HCC) fly ashes as a reactive medium in permeable reactive barriers (PRBs) for remediation of petroleum hydrocarbon contaminated groundwater. A series of column and batch tests were performed to evaluate the leaching of selected metals from the fly ash, and adsorption/desorption of two target hydrocarbons (naphthalene and o-xylene) onto/from this PRB medium. Leaching of metals in the column experiments exhibited a first-flush, followed by a tailing slope elution pattern for all fly ashes. The naphthalene and o-xylene adsorption/desorption on/from the fly ashes were directly correlated with the organic carbon of the fly ash as measured by loss in ignition. Adsorption/desorption hysteresis was obvious in column and batch tests, suggesting that the adsorption/desorption was not completely reversible during the testing.Item Brassica Cover Crops for Nitrogen Retention in the Maryland Coastal Plain(2006-07-27) Dean, Jill Elise; Weil, Ray R; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Brassica cover crops, forage radish (Raphanus sativus L. cv 'Daichon'), oilseed radish (Raphanus sativus L. cv 'Adagio'), rape (Brassica napus L. cv 'Dwarf Essex'), and cereal rye (Secale cereale L. cv 'Wheeler') were examined for ability to decrease mineral N losses and influence organic N cycling at two Maryland Coastal Plain agricultural sites. Brassicas were similar or superior to rye regarding N uptake and soil profile NOsub3-N depletions (105-180 cm depth). Rape and rye maintained soil porewater NOsub3-N below 3 mg L to the minus 1 throughout spring while radish performed similarly on fine-textured soil, but caused porewater NOsub3-N > 10 mg L to the minus 1 on coarse-textured soil. Dissolved organic N averaged 51% of total N in porewater, but was unaffected by cover crops. Brassicas were as effective as rye in minimizing mineral N losses, but the role of cover crops in managing organic N was unclear.