VOCs IN CSOs AND CSO STORAGE FACILITIES IN THE CHICAGO METROPOLITAN AREA

Since the 1980s, deep tunnels have been used to capture and store combined sewer overflows (CSOs) in the Chicago metropolitan area. The stored CSOs are pumped to the water reclamation plants for treatment after the storm. Three CSO reservoirs have been built or are planned to be built for this region as flood and pollution control measures to capture more CSOs during large rainfall events. The presence of volatile organic compounds (VOCs) in CSOs may impact the collection, storage and treatment of CSOs. Two studies were conducted in the late 1980s and middle 1990s to gather information on the occurrence and concentrations of organic priority pollutants, including many VOCs, in freshly generated and stored CSOs. A total of 30 and 55 VOCs, respectively, were analyzed for in each CSO sample collected during these two studies.CSO samples were collected at three and nine locations in the 1980s' and 1990s' studies, respectively. Both type of CSOs, namely freshly generated and stored CSOs, were sampled in each study. All three sampling locations in the 1980s' study were also included in the 1990s' study. The sampling procedure in both studies was similar. Grab samples were collected one to four times during a rain event that typically had a cumulative rainfall of at least 0.5 inches.A total of 18 samples were collected from three sampling locations during nine rain events in the 1980s' study, and 103 samples were taken from nine locations during fifty-two rain events in the 1990s' study. The occurrence and levels of VOCs detected at the three locations used in both studies were generally less in the later study period with only a few exceptions. In the 1990s' study period, the numbers of VOCs detected and most frequently detected VOCs varied from location to location. More VOCs were found in the stored CSOs than in the freshly generated CSOs. The average number of total VOCs detected in at least one sample was 17 in the stored CSOs at the three locations, and 9 in the freshly generated CSOs at the six locations. Some of one- and two-carbon chlorinated alkanes and alkenes were the frequently detected VOCs in both studies with concentration levels at around 10 μg/L. Acetone, which was not analyzed in the former study, was the most frequently detected compound, typically at a 100-μg/L level, in the later study period with highest mean and individual concentrations among all detected VOCs.