Active sediment capping for pollutant mixtures: control of biogenic gas production under highly intermittent flows

Sediments in ‘Bubbly Creek’, located in the south fork of the south branch of the Chicago River (Illinois, USA) were characterized for the selection of amendments for an active capping demonstration project. Bubbly Creek is a 6800 ft (2000 m) creek that starts at the Racine Avenue pumping station (RAPS), the largest sewage pumping station in the world. During large storm events, the RAPS can discharge up to 6000 ft 3 s –1 (175 m 3 s –1 ) of combined storm and wastewater into the creek. These high flow rates result in large shear flows on the sediment. Both heavy-metal levels (up to 900, 2900 and 6200 mg kg –1 of Pb, Cr and Zn, respectively) and polycyclic aromatic hydrocarbons (up to 3300 mg kg –1 ) were identified as contaminants of concern in the creek sediments. A complicating issue in the creek is the copious gas ebullition from organic matter biodegradation in sediments. These gases can open advective channels that may result in substantial pollution release and compromise cap effectiveness. After modelling the performance of several different kinds of active capping materials for a large range of sediment contaminants, a conceptual design for the active capping demonstration project was proposed. The capping is proposed to be carried out in conjunction with overlying wetlands in the shallow regions of the creek, to remove nutrients from the river and pollutants discharged from combined sewer outflows. Metal and organic contaminant sequestration, and oxidizing agents coupled with a gas collection system, will be evaluated and compared to a sand reference cap.