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The water quality in the Chicago Area Waterways (CAWs) has been improved in the past two decades as a result of intercepting combined sewer overflows (CSOs) by the deep tunnels that have been built under the Tunnel and Reservoir Plan (TARP) and better performance at the water reclamation plants discharging to the waterways. However, the storage capacity of the deep tunnels, of which most has been in use, is limited to under 6.9 million cubic meters (245 million cubic feet) and is insufficient for 803 square kilometers (310 square miles) of combined sewer areas, and CSO discharges via gravity CSO outfalls to the CAWs still frequently occur until the storage reservoirs are complete in about another decade. A recent Use Attainability Analysis (UAA) study for the CAWs by the Illinois Environmental Protection Agency (IEPA) required an evaluation of treating the gravity CSOs in the system and its impact on the water quality of the CAWs. An unsteady flow water quality model developed for the CAWs was used for the evaluation of eliminating gravity CSOs on the water quality of the CAWs. Two scenarios, with and without the gravity CSOs in the model, were simulated. The simulated hourly dissolved oxygen (DO) concentrations at thirty seven selected locations throughout the CAWs were analyzed and compared. The simulation results indicated that eliminating gravity CSOs increased stream DO concentrations in the entire system with different improvements at different locations. The simulated DO concentration increase was the most significant in the Upper North Shore Channel, where the stream flow was dominated by gravity CSOs during a storm. CSOs from a fairly large storm could have prolonged impact on stream DO concentrations, which could last for weeks. Even if all gravity CSOs were eliminated, which means the complete capture of the gravity CSOs to the system, the target DO value of 4 mg/L could not be satisfied 100 percent of the time at some locations in the CAWs under the summer conditions in 2001 and 2002.
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Document Type: Research Article

Publication date: 2007-01-01

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