Assessment of Risks from Pathogens in Combined Sewer Overflow Discharges in Seattle, Washington. J. Simmonds, S. Munger, J. Rose, and C. Gibson, University of South Florida, St. Petersburg, FL; and J. Toll, Parametrix Inc., Kirkland, WA
Human health risks from exposures to pathogens in the Duwamish River and Elliott Bay were quantified, along with the risk reductions that would occur were combined sewer overflows (CSOs) that discharge to the system eliminated. The assessment relied on a comprehensive field sampling program, a fully calibrated hydrodynamic and fate and transport model of the estuary, and pathogen risk assessment techniques. Human health risks from exposures to pathogens were assessed using modeled concentrations of viruses and Giardia in the estuary resulting from CSO discharges, as well as using fecal coliform concentrations as indicators of risk. The results of this study indicated that fecal coliform concentrations in the estuary would remain substantially elevated above water quality criteria, even if CSOs were completely removed. However, substantial risk reductions were predicted based on the evaluation of viruses and Giardia in the CSO discharges. These results indicate that risks of infection to persons recreating in the estuary during CSO discharges may range up to about 1 in 100. However, these risks are predicted to decrease by greater than a factor of 10 within 6 hours of the end of the CSO discharges. CSO discharges were predicted to occur during about 7 percent of the hours in the year. Risks of infection from pathogens attributable to CSO discharges are somewhat mitigated by the infrequent use of the estuary during the winter months (the rainy season) when the majority of CSO discharges occur and a CSO signage program implemented by the Seattle-King County Health Department.
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