CONTINUOUS DISSOLVED OXYGEN MONITORING IN CHICAGO

Due to the low stream velocities and the urban pollutant loading to Chicago area waterways, dissolved oxygen (DO) concentrations in these waterways have historically been low. More than thirty years ago, the Metropolitan Water Reclamation District of Greater Chicago (District) determined that the DO standards for deep-draft waterways in the Chicago metropolitan area could not be continuously met by advanced wastewater treatment and through the capture and treatment of combined sewer overflows (CSOs). In order to improve the DO concentration in area waterways, the District designed, constructed, and began operating seven supplemental aeration stations on Chicago area deep-draft waterways during the late 1970s and early 1990s.The results of weekly DO surveys conducted at 75 stations during 1994 through 1996 showed that even with supplemental aeration, DO concentrations were still below Illinois water quality standards at selected deep-draft waterway stations. During the weekly surveys, water samples were collected in the field, fixed chemically, and returned to the laboratory for titration. Consequently, in August of 1998 the District initiated a continuous DO monitoring program at 20 locations on the deep-draft waterways. Fifteen additional stations were added to the DO monitoring network during 1999 and in 2001. The continuous DO monitoring program will continue through 2003. DO is measured hourly at 35 locations, 365 days a week, using remote, in-situ water quality monitors.The objectives of continuous DO monitoring are: (1) to identify reaches in Chicago's deep-draft waterways where the DO is below the DO standard; (2) to collect DO data for a use attainability analysis (UAA) in the Chicago metropolitan area; (3) to determine total maximum daily loads (TMDLs); (4) to determine the effects of CSOs on waterway DO concentrations; and (5) to provide DO data for calibration and verification of an unsteady state water quality model being developed for the deep-draft waterways.