The Thornton Composite Reservoir in the south suburbs of Chicago has reached near-full capacity for the first time since it began operations in 2015, raising concerns about flooding risks and persistent sewage odors among local residents. Following heavy rainfall over the weekend, the reservoir’s water level peaked at 94% capacity, or approximately 7.3 billion gallons, according to the Metropolitan Water Reclamation District of Greater Chicago. As of Tuesday morning, the level had slightly decreased to 93%.

The Thornton Composite Reservoir, which serves 14 communities including Chicago’s Far South Side, Calumet Park, Dolton, Harvey, Lansing, Markham, and South Holland, is the largest combined sewer reservoir in the world. It was designed to hold excess stormwater and prevent combined sewer overflows during periods of heavy rain. However, with billions of gallons of sewage-contaminated water exposed to open air, residents in nearby areas have been affected by strong foul odors.

Thornton Village Clerk Nikki Kitakis described the smell as “putrid” and said the issue worsens when the reservoir is this full. “It affects all facets of life because it smells so terrible,” she said. While the water reclamation district has implemented some odor mitigation measures, Kitakis acknowledged that managing the smell remains challenging with the reservoir at such a high level.

The district is actively working to reduce water levels at both the Thornton reservoir and the McCook Reservoir in Bedford Park, which caters to central Chicago. Water is drained from Thornton by gravity through a tunnel leading to the Calumet Water Reclamation Plant, located five miles north in Chicago. There, the water is pumped, treated, and released into the environment via the Little Calumet River. Despite current storage of about 7.3 billion gallons, the reservoir still has capacity for roughly 600 million more gallons.

However, the district cautions that the system’s ability to manage further rainfall depends on several factors, including the volume, intensity, and location of upcoming storms, as well as current water and tunnel levels. There is no specific timeline for when the reservoir will be fully drained, as conditions at the treatment plant also influence the process.

Residents in affected communities have reported flooding and property damage in recent days. At a Dolton Village Board meeting on Monday, Mayor Jason House expressed empathy for those experiencing basement flooding and noted that comprehensive flood infrastructure improvements would require an estimated $50 million investment.

Public works officials and village leadership have urged residents to remain vigilant, especially as additional rain is forecasted for the Chicago area later this week. While meteorologists predict possible showers on Thursday and Friday, officials hope that subsequent weather patterns will ease the current strain on the reservoir.

The Metropolitan Water Reclamation District has encouraged residents to report any odors related to the reservoir by calling its dedicated hotline. Meanwhile, local officials and residents await relief as efforts continue to manage the reservoir’s record-high levels and mitigate the risk of flooding and poor air quality.