CHICAGO | In 1998, the Metropolitan Water Reclamation District of Greater Chicago executed an agreement with the owner of the Thornton quarry to mine the north lobe of the quarry for use as an important component of the district's Tunnel and Reservoir Plan, or TARP.

Since then, 152 billion pounds of 400 million-year-old dolomite limestone has been removed from the Thornton Composite Reservoir, and while blasting and construction continues, this project is due for completion in less than three years.

Thornton Reservoir, along Interstate 80, is designed to reduce pollution and flooding. It will have almost 8 billion gallons of combined sewage and stormwater storage capacity. Communities throughout the south suburbs will benefits once the reservoir is complete.

“Part of the MWRD’s mission is to protect our water supply source, Lake Michigan, from raw sewage pollution and protect businesses and homes from flood damage, and this project will be doing both,” said MWRD Commissioner Michael Alvarez, chairman of the MWRD’s stormwater management committee. “TARP is designed to provide an outlet for flood waters to reduce flooding.”

The second phase of TARP is to construct three reservoirs. Phase I was completed in 2006 and consists of 109.4 miles of deep, large diameter tunnels in rock with a total storage capacity of 2.3 billion gallons. Phase II consists of three reservoirs which will provide more than 18 billion gallons of storage capacity.

Besides the Thornton Reservoir, the McCook Reservoir is also under construction. Stage 1 of McCook 1 is scheduled for completion by Dec. 31, 2017, and will add 3.5 billion gallons of storage capacity. The McCook Stage 2 Reservoir is scheduled for completion in 2029 and will add 6.5 billion gallons of storage capacity. The Gloria Alitto Majewski Reservoir was completed in 1998 and provides 350 million gallons of storage.

TARP’s large tunnels and reservoirs are designed to reduce the amount of combined sewer overflows and hold the polluted water until it can be fully treated at MWRD water reclamation plants. Since the TARP tunnels went online, the average number of days with combined sewer overflows in a year has dropped to 50 from 100.

TARP was the first system of its kind to address pollution and flooding and is emulated by cities around the world, including London, Singapore, and Vienna.

The Thornton Reservoir is set to come online at the same time as disinfection facilities at the MWRD’s Calumet Water Reclamation Plant in Chicago, which also serves the county's southern area. Working in tandem, the combination will maximize water quality while minimizing flooding.

Additional information about TARP can be found at

Jeanette is a journalist with The Times Media Co. who has worked as both a reporter and editor. She has a master's degree in public affairs reporting from the University of Illinois at Springfield.