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.