A comprehensive study was undertaken to assess the fate of indicator organisms (fecal coliform, E. coli, and enterococci) in brackish water of the south shore of Lake Pontchartrain, a recipient of stormwater runoff from the New Orleans metropolitan area. Although significant
improvements in lake water quality have been reported over past years, the fate and behavior of indicator organisms is not completely understood. That is particularly true at the selected study sites of Lincoln Beach and Jahncke Canal. The purpose of this study was to examine the distribution
of indicator organisms in different components of the water body to achieve a fuller understanding of the fate of microbial indicators fecal coliform, E. coli, and enterococci. Knowledge gained from the study will lay the foundation for further modeling and the development of health
advisories for swimming activities. Results indicate that satisfactory water quality of Lincoln Beach was observed during dry weather periods. Concentrations of indicator organisms in both water column and sediment, however, significantly increased during and after heavy stormwater runoff,
particularly at sites near the Jahncke Canal discharge point. However, the elevated titers of indicator organisms decreased to background levels after 24 to 48 hours. Overall removal rate constants for fecal coliform, E. coli, and enterococci from the water column ranged from 0.112
to 0.124 h−1, 0.106 to 0.170 h−1, and 0.101 to 0.110 h−1, respectively. These results were confirmed by laboratory studies which evaluated microbial indicator die-off under various salinity, temperature, and sunlight conditions. Sediment die-off
sampling indicated reduction rate constants ranging from 0.013 to 0.043 h−1, 0.023 to 0.030 h−1, and 0.020 to 0.026 h−1 for fecal coliform, E. coli, and enterococci, respectively. Attachment of microbial indicators to suspended matter
and subsequent sedimentation appeared to be a significant fate mechanism. A slower rate reduction of indicator organisms in sediment further suggested that bottom sediment may act as a reservoir for prolonging indicator organism survival and adds concern of recontamination of overlying waters
due to potential solids resuspension. Additionally, field data was assessed to determine which microbial indicator may be most suitable to determine recreational water quality at the study sites. Results indicate that enterococci may be a more stable indicator than E. coli or fecal
coliform and consequently a more conservative indicator under marine water conditions.
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