Approximately 100 years ago, Evansville began building a sewer system to carry storm water away from homes, businesses, and streets.

Later, when indoor plumbing arrived, homes and businesses connected their sewage lines into these storm sewers, making them "combined" sewer systems (CSS). Evansville's two wastewater treatment plants were not constructed until the 1950's. Approximately 500 miles of sanitary and combined sewers are now under the authority of the Evansville Water and Sewer Utility (EWSU).

During dry weather, the combined sewers transport sewage to the City's two treatment plants. However, after significant rainfall, combined sewers can become overloaded with the incoming storm water. When this happens, overflows are directed into Pigeon Creek and the Ohio River, preventing the sewers from backing up into homes and onto streets. This is referred to as a combined sewer overflow event, or CSO.

Combined sewers serve mainly the older parts of town, while outlying sections of the city are served by separate sanitary and storm sewers. However,sanitary sewage from those outlying areas eventually flows through the combined sewer system before reaching a treatment plant.

Combined sewers are sanitary sewers that convey both sewage and storm runoff. During dry weather, combined sewers carry sewage from domestic, commercial, and industrial sources to the wastewater treatment plant. During wet weather events, the same sewers also convey surface runoff collected from streets, lawns, parking lots, parks, etc. to the wastewater treatment plant (WWTP). When the capacity of the WWTP is exceeded, such as during heavy rainfall events, the excess wastewater is occasionally allowed to bypass directly to surface water bodies, including the Ohio River and Pigeon Creek, through flow diversion structures. This excess wastewater is called combined sewer overflow (CSO).

Approximately half of the sanitary sewers in the City of Evansville, Indiana are combined sewers. The other half consists of separate sanitary sewers conveying sewage only. The combined sewer system in Evansville is located in the south central part of the city, bounded generally by the Ohio River on the south, Pigeon Creek on the north, Carpenter Creek on the west, and Vann Avenue on the east. Figure 1.1 shows the major interceptors and the subsystems of the City's combined sewer area.

The Clean Water Act of 1972 requires CSO communities to restore their waterways. Almost 1,000 other cities in the United States have the same requirements. Pigeon Creek and the Ohio River are valuable natural resources that deserve our protection. Evansville is working closely with state regulatory agencies to remediate its CSOs. The regulatory agencies have defined a sequential process necessary for compliance. For instance, a Stream Reach Characterization and Evaluation Report (SRCER) to establish the existing baseline conditions of the receiving stream and the CSO discharge concentrations must be completed prior to developing a Long Term Control Plan (LTCP). The LTCP is a plan to establish costeffective and practical projects to reduce the impact and frequency of CSO discharges.

Characterization of Evansville's Combined Sewer System (CSS)

In Evansville's combined sewer system, 9 CSOs discharge to Pigeon Creek, and another 14 discharge to the Ohio River. The combined sewer system (CSS) tributary to the Pigeon Creek CSO outfalls was examined during the recently completed Pigeon Creek SRCERI Watershed Diagnostic Study. The SRCER examined the Pigeon Creek CSOs for six months- fiom February 2000 to August 2000. Rainfall throughout the period and six select wet weather CSO discharge events were recorded and sampled. During those wet weather events, creek water samples were taken at five locations. Automatic samplers at two CSO outfalls each obtained sets of twelve time-separated samples.

The creek water samples were analyzed for TSS (total suspended solids), BOD (biochemical oxygen demand), E. coli (Escherichia coli), phosphorus, TKN (total Kjeldahl nitrogen), ammonia nitrogen, pH, and nitrate nitrogen. The combined sewage samples were analyzed for TSS, BOD, E. coli, phosphorus, TKN, ammonia nitrogen, arsenic, zinc, chromium, copper, lead, cadmium, and nickel. The wet weather sampling results were compared to Indiana Surface Water Standards.

The state's E. coli standard of 235 colonies/ 100 ml was exceeded in four of the five storms sampled instream, demonstrating it to be the parameter of greatest concern. It is the most widely known member of the coliform group of bacteria, and considered an indicator of pathogenic organisms. No other water quality standards, as monitored during this study, are conclusively and adversely impacted by the CSOs.