Browsing by Subject "Seasonality"

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    Changes in seasonal precipitation of East Central North America with connections to global climate.
    (2010-10) Hardt, Benjamin Fulper
    Research on oxygen isotopes in stalagmites collected in West Virginia caves has yielded several new insights into regional climate. Oxygen isotopes most likely represent changes in the mean annual isotopic composition of precipitation, a parameter determined locally by the seasonal distribution of precipitation (Hardt et al., 2010). Holocene samples indicate that summer precipitation represented a greater proportion of annual totals, consistent with lake level results (Shuman and Donnelly, 2006) and climate models (Braconnot et al., 2007; Diffenbaugh et al., 2006). During the Pleistocene, seasonal precipitation varies on precessional timescales, although the phasing appears unusual in that it is in-phase with September insolation rather than June. This offset could be due to several processes, most likely in conjunction with each other. These processes include: changes in Gulf of Mexico sea surface temperatures, partially modulated by glacial meltwater routed through the Mississippi (Oglesby et al., 1989; H Wang et al., 2010); changes in the mean state of the tropical Pacific ocean between to El Nino- and La Nina-like conditions (Timmermann et al., 2007); and a seasonally-lagged sea surface temperature response in the subtropical North Atlantic, which would enhance the anticyclonic circulation of the Bermuda High. These same processes also appear to influence isotopic behavior over the last glacial Termination. During the last glacial maximum, oxygen isotopic composition is enriched, consistent with model results indicating wetter summers near the southern margin of the ice sheet (Bromwich et al., 2005), but inconsistent with a temperature control. Antiphasing in summer precipitation between West Virginia and Florida support a control on seasonal precipitation by changes in the position or size of the Bermuda High (Donders and de Boer et al., 2009). Comparison with speleothems from Northeast Brazil (Cruz et al, 2009) indicates a role for the El Nino Southern Oscillation due to its effect on the position of the Bermuda High (Seager et al., 2005).
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    Sources, Transport, and Sediment-Water Distributions of Contaminants of Emerging Concern in a Mixed-Use Watershed
    (2015-06) Fairbairn, David Joel
    Understanding the sources and transport of contaminants of emerging concern (CECs) is crucial for risk assessment and mitigation. The goal of this research was to augment this knowledge by characterizing the spatiotemporal variability of a diverse set of CECs in a mixed land-use watershed. The study area (South Zumbro Watershed, Minnesota) presented a gradient of land uses that facilitated the investigation of agricultural and urban/residential sources and transport of CECs. Concentrations and loadings of CECs in water samples were analyzed in light of spatial, temporal, hydrologic, and physicochemical variables. Contaminant mass balances were analyzed to characterize the proportional sources and instream transport of CECs. Sediment-water distributions of CECs were assessed in terms of their magnitude, variability, and predictability. Three distinct CEC groups emerged with respect to sources, transport, and seasonal/temporal patterns. The first group (i.e., atrazine, acetochlor, metolachlor, and daidzein) was characterized by agricultural/upstream-dominated sources and transport. These CECs were input primarily via upstream routes with loadings and concentrations that were greatest during high flows. For the second group (i.e., acetaminophen, trimethoprim, DEET, caffeine, cotinine, and mecoprop), a mix of wastewater and runoff transport was demonstrated by frequent detections in effluent and upstream samples, with peak loadings/concentrations associated with high flows and areas of greater population density/development. For the third group (i.e., sulfamethoxazole, carbamazepine, erythromycin, tylosin, carbaryl, and 4-nonylphenol), the detections, concentrations, and loadings were explained by effluent-dominated sources and transport. These CECs showed expected trends of stable loading across events, with the greatest concentrations and detection frequencies associated with low flows and the wastewater treatment plant. Average measured sediment-water distributions exceeded equilibrium hydrophobic-based predictions for five of seven detected CECs by at least an order of magnitude. The consistency and predictability of the measured distributions improved with increasing CEC hydrophobicity and persistence. Thus, spatiotemporal analysis can be used to characterize and track CEC sources and transport, even for ubiquitous CECs. These results augment existing knowledge of CEC sources, fate, and transport by describing dominant sources, transport, and temporal patterns for different types of CECs. This will enhance monitoring, exposure/risk assessments, and management of CECs in surface water ecosystems.