In-stream dissolved oxygen impacts and sediment oxygen demand resulting from combined sewer overflow discharges

Combined sewer overflows (CSOs) are a major source of contamination to urban waterways. Discharges from CSOs represent a major source of chemical oxygen demand, biological oxygen demand, nutrients, toxics, and bacteria to receiving waters. The impacts of these discharges to ecosystem health include deposition of sewer sediment on the stream bed, anoxic events, and algal blooms. A case study is presented, which analyzed in-stream dissolved oxygen and sediment oxygen demand (SOD) in an urban stream in Philadelphia, Pennsylvania, to observe short-term and long-term water-column oxygen depletion caused by CSO discharges. Multiple SOD measurements were collected at six locations in a 5.61-km reach. Continuous in-stream dissolved oxygen measurements indicate that water-column dissolved oxygen concentration in the reach is depleted as it flows through the study section. This depletion appears to be the result of elevated SOD downstream of the CSO outfalls.