Five-day carbonaceous biochemical oxygen demand (CBOD₅) was measured as an indicator of organic matter present in the marshland upwelling system (MUS) that was installed on a private camp along Bayou Segnette, Louisiana within the Barataria Terrebonne National Estuary System. The MUS is used to treat decentralized wastewater from coastal dwellings. Raw wastewater was injected at a depth of 4.3 m into the surrounding intermediate marsh subsurface with an injection frequency of 30 minutes every hour at a flow rate of 2.8 L min^-1. The system was monitored to determine the treatment efficiency of CBOD₅ removal below NPDES standard levels of 25 mg L^-1. Prior to Hurricane Katrina the system yielded a removal rate constant of 2.45 m^-1 and a predicted surface concentration of 0.01 mg L^-1. CBOD₅ levels dropped below 25 mg L^‑1 within 1.14 m. Hurricane Katrina caused the cessation of active wastewater injection, which allowed subsurface conditions after deactivation of the system to be monitored. CBOD₅ levels at all monitoring wells reached steady state levels below 15 mg L^-1 by February 2006, six months after Hurricane Katrina.