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Microorgansims in Urban Stormwater (EPA/600/2-77/087) July 1977

Microbiological quantitative assays of Baltimore City urban runoff were conducted throughout a 12 month period to show the relationships to several factors such as separate or combined sewer flow, urban characteristics of drainage area, rainfall, and quantity of flow during and between rain storms. In general, there was a consistently high recovery of both pathogenic and indicator organisms throughout the study except for Shigella sp. which is believed to have been present but could not be isolated due to interferences during the culture procedure. There appeared to be little relationship between pathogen recovery and season of the year, amount of rainfall, period of the antecedent rainfall, and stream flow.

The most concentrated pathogens were Pseudomonas aeruginosa and Staphylococcus aureus. Salmonella and enteroviruses, though frequently isolated, were found. The background samples (sewage, urban streams and reservoirs) between storms gave good positive correlation between indicators and pathogens at a 95 to 99% level of confidence, whereas, the stormwater had no or poor correlation. The logical solution would point to the removal of sanitary sewage overflows rather than the disinfection of all urban runoff for removing the health hazard and improving the quality of urban runoff.

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