Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
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 Occurrence and Removal of Polychlorinated Biphenyls (PCBs) in Urban Stormwater(2020) Cao, Siqi; Kjellerup, Birthe V.; Davis, Allen P.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polychlorinated biphenyls (PCBs) are a group of chlorinated organic compounds. They are persistent in the environment and can threaten the health of humans and wildlife. Urban stormwater runoff is considered as an important source of PCBs to aquatic environments. The objective of this study is to provide information on the occurrence and removal of PCBs in stormwater; specifically, the occurrence, concentrations, and biological transformations of stormwater PCBs were studied together with their removal. Concentrations of 209 PCB congeners were determined in surface stormwater sediments collected from various roadway sites and bioretention media. The total PCB concentrations ranged from 8.3 to 57.4 ng/g dry weight (dw), with a mean value of 29.2 ng/g dw. Land use had an impact on the concentration of PCBs, where higher stormwater sediment PCB concentrations were found in dense urban areas (average: 39.8 ± 10.5 ng/g) compared to highways passing through greenspace (average: 18.0 ± 0.4 ng/g). PCB sorption tended to increase with the concentration of total organic carbon (TOC) and smaller particle size (< 75 µm) of stormwater particulate matter. In bioretention core samples, PCB concentrations decreased with bioretention media depth (from 30.0 ± 2.0 ng/g at the surface to 21.2 ± 4.8 ng/g at 40 cm depth), and with distance from the stormwater entrance (from 38.4 ± 2.3 ng/g at the entrance to 33.2 ± 2.9 ng/g at 3 m distance). A non-Aroclor congener, PCB 11, was detected in all samples, likely originating from yellow road paint. Putative organohalide respiring bacteria within Chloroflexi and aerobic PCB degrading bacteria containing the functional genes encoding for biphenyl dioxygenase (bphA) and ring cleavage (bphC) were detected in some of the stormwater sediments and bioretention media. The presence of such bacteria and a higher level of ortho-chlorinated biphenyls indicated the potential of PCB biotransformation in these samples. The performance of an on-campus bioretention indicated that bioretention is effective in removing PCBs from stormwater, with 64–92% reduction of dissolved PCB concentrations. Overall, urban stormwater is an important environmental source of PCBs. Bioretention has the potential to remove PCBs from stormwater via adsorption and biotransformation.Item Topological Analytics for Vulnerability Enhancement and Recovery Strategy after Disruptions of Rail Networks in the United States(2020) Cao, Siqi; Ayyub, Bilal; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rail networks are real-life examples of complex networks and critical logistic and economic contributors to the wellbeing of society. Natural or human-caused hazards leading to the disruptions of rail network’s components can cause severe consequences including significant economic impacts. Therefore, analyzing rail networks and further reducing the impacts of potential disruptions are critical in order to manage risks to the performance of rail networks. Based on existing research on rail networks, this thesis proposes a methodology to analyze the rail networks with a large number of nodes, links, and complex connectivity from topological perspectives. Additionally, topology enhancement prior to failures and recovery strategies post to failures are used to reduce the impacts of potential failures based on vulnerability and resilience assessments. The analysis results of two case studies, the Amtrak and Class I rail networks, indicate that the proposed methodology is well suited to analyze and enhance the topology, vulnerability, and resilience of complex rail networks effectively and efficiently.