One of the largest public works projects for pollution and flood control in the world, the Metropolitan Water Reclamation District of Greater Chicago’s (MWRD’s) Tunnel and Reservoir Plan (TARP) has been recognized by the Association of Environmental and Engineering Geologists (AEG) as the AEG Outstanding Environmental and Engineering Geologic Project for 2025.

Emulated around the world and known famously as the Chicago region’s “Deep Tunnel,” TARP reduces flooding, improves water quality in Chicago area waterways and protects Lake Michigan from pollution caused by sewer overflows. Since it first went into service four decades ago, TARP has prevented billions of gallons of stormwater and sewage each year from polluting local waterways and flooding homes through a vast engineered system of tunnels and reservoirs. It’s this elaborate and innovative system that was honored at the AEG’s 68th Annual Meeting held at the Westin Hotel in Chicago in September.  

“We are proud to be recognized by the Association of Environmental and Engineering Geologists with this prestigious honor,” said MWRD President Kari K. Steele. “TARP has protected our region for decades and deserves credit for why our water environment is the quality it is today. It is also the engineering and geologic understanding of our bedrock landscape that makes this system world class.”

The TARP tunnels consist of four separate large-diameter, deep-underground tunnel systems that extend for a combined 110 miles and are designed to reduce pollution and flooding caused by combined sewer overflows (CSOs), a mixture of stormwater and sewage. Before the tunnels were built, these CSOs discharged to area waterways, including Lake Michigan on occasion, and caused flooding due to basement sewer backups. 

The combined sewage is now intercepted by drop shafts and redirected to the TARP tunnels. The TARP tunnels convey this polluted water to large reservoirs to store the water during heavy rainfall events. After storm events subside, this stored combined sewage is pumped into water reclamation plants to be treated, and the clean water is then discharged back to area waterways.

The TARP system has reduced this CSO pollution, and in fact, there has not been a CSO in the Calumet Area River System since 2020, thanks in large part to the MWRD’s 7.9-billion-gallon Thornton Reservoir. The MWRD is currently expanding its McCook Reservoir to contain up to 10 billion gallons.

“We are deeply honored by this recognition from the Association of Environmental and Engineering Geologists. It highlights decades of dedicated engineering that created a vital system of tunnels and reservoirs capturing pollution, safeguarding water quality and protecting more than 3.7 million residents from flooding in communities served by combined sewer systems," said MWRD Commissioner Sharon Waller.