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Browsing by Author "Rashash, Diana M. C."

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    An analysis of phytoplankton variations in the Occoquan Reservoir, 1970-1988
    Rashash, Diana M. C. (Virginia Tech, 1991)
    The phytoplankton data from Occoquan Reservoir surface samples collected during 1970-1988 were entered into a database. The data were analyzed to determine the relationships between changes in phytoplankton succession patterns and the various physical and chemical water-quality variables. The investigation considered the variations in both the phytoplankton and the water-quality variables with respect to sampling station location along the length of the reservoir. The variables included rainfall, season, pH, alkalinity, nitrogen, phosphorus, copper sulfate, odor intensity and type, and chlorophyll-a. The discharge from the Upper Occoquan Sewage Authority advanced wastewater treatment plant (UOSA AWT) had a noticeable influence on both the phytoplankton division dominance and the genera occurrence. Before the AWT plant was built, Chlorophycota was usually dominant; afterwards, Bacillariophycota dominance increased. The dominant genera were a subset of the genera that occurred frequently. Of the original 72 genera, an analysis of 9 prevalent genera would have provided virtually the same information. Neither the intensity nor type of odors observed were predictable on the basis of the phytoplankton genera present in the reservoir. No statistically valid conclusion could be reached about the impact that copper sulfate applications had on reservoir algae.
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    Identification and characterization of odorous metabolites produced by selected freshwater algae
    Rashash, Diana M. C. (Virginia Tech, 1994)
    The occurrence of taste-and-odor problems that are caused by algal metabolites in water supplies has been well documented. Several commonly occurring odor-producing algae were selected and cultured for this research. Initial studies involved the algal cells and cell-free media from cultures grown under fairly optimal conditions. Gas chromatography-mass spectrometry (GC-MS) and flavor profile analyses (FPA) were performed to identify the organic compounds produced by the algae and their respective odors. Three of the algal cultures underwent additional studies that investigated the effects of selected changes in culture conditions on both population growth and compound production. Experimental variables included nitrogen concentration (ammonium, nitrate, and nitrite), phosphorus concentration, light intensity, and temperature. Parametric and nonparametric analyses were performed to identify the environmental factors that had a significant effect on algal production, accumulation, and release of taste-and-odor compounds. The organic compounds were extracted from both the algal cells and the cell-free media. Continuous liquid-liquid extraction and Kuderna-Danish concentration (CLLE-KD) was an effective and reliable method for the isolation and concentration of a broad range of organic compounds. The plot of flavor profile analysis (FPA) results obtained for odor standards adhered to the Weber-Fechner Law (W-F) over the range of concentrations evaluated. The odor intensities of algal cultures were generally lower than the odor intensities predicted from the W-F plot of the compound standards. Masking of the odor associated with one compound by the odor associated with another was observed. Odors produced by young algal cultures (e.g., low population densities) were detected in FPA samples at compound concentrations below the limits of detection by GC-MS. Anabaena laxa retained most of the geosmin it produced within the algal cells. Phormidium sp. produced more 2-methylisoborneol (MIB) than geosmin, and the alga retained only a relatively small amount of either compound within the cells. Synura petersenii produced more 2t,4c,7c-decatrienal than 2t,6c-nonadienal, and large fractions of the concentrations produced were retained within the algal cells. Various combinations of nutrient reduction, early algal-bloom within-reservoir treatment, and removal of algal cells prior to oxidation were suggested as likely methods by which odor problems may be reduced.