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Urban wet weather pollution has been identified as a major cause of impairment of Ohio River water quality. A study was undertaken to determine the extent and severity of urban, interstate, wet weather impacts on Ohio River water quality in the Cincinnati, Ohio area, identify causes
and sources of impacts, and evaluate water quality improvements based on various control scenarios. There are approximately 330 combined sewer overflows within the study area, encompassing two major municipal systems in Kentucky and Ohio. Key components of the study include: 1) Modeling
of the combined sewer systems to enable pollutant loads to be quantified under various wet weather events; 2) Development of an Ohio River model to quantify resulting pollutant loads in the Ohio River and to estimate the extent and severity of water quality impairments under various wet weather
events; 3) An extensive field sampling program to calibrate and verify model results, as well as quantify urban nonpoint source wet weather loadings; 4) Execution of the modeling system to determine the extent and severity of water quality impairments under typical conditions, and to quantify
improvements based on various combined sewer system control scenarios; and 5) Evaluate various monitoring techniques for identifying impacts from combined sewer overflows. These components combined also allow for a determination of the relative contributions of pollutants by the various wet
weather sources within the study area. Key findings of the study include: 1) In this particular case, bacteria turned out to be the only pollutant of concern; 2) The project framework, for evaluating wet weather impacts on large rivers, has been successfully transferred to other projects;
3) Ohio River bacteria levels exceed criteria for contact recreation, at times even during dry weather; 4) Ohio River bacteria criteria violations significantly increase during and after wet weather events (typically for two or more days); 5) Combined sewer overflows collectively account for
approximately 75 percent of the total annual fecal coliform load in the study area; 6) Based on modeling results for a “typical year”, the Ohio River exceeds contact recreation criteria approximately fifteen percent of the time along the center channel and as much as eighty percent
of the time along its banks; 7) The greatest benefit to water quality from CSO reductions occurs for “average” storm events. For “heavy storm events”, greater than 75 percent reduction in bacteria loads from CSOs is necessary to achieve significant improvements; 8)
Even with 100 percent control of bacteria loads from CSOs, contact recreation criteria will be exceeded on occasion. 9) Results from a special survey of giardia and cryptosporidium identified the presence of both pathogens in the Ohio River – no consistent relationship with the occurrence
of wet weather was identified.
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