What we can learn from deep tunnel systems in Chicago and Milwaukee

By Mia Jaworski
UConn Journalism

Heavy rain over­whelms old city sewer sys­tems, spilling raw sewage into rivers and water­ways. It’s a prob­lem cities around the world have faced for decades, and one grow­ing more urgent as cli­mate change fuels more intense and fre­quent storms.

One solu­tion that’s gained trac­tion: mas­sive under­ground stor­age tun­nels built deep beneath city streets.

In Chicago and Milwaukee, where com­bined sewer over­flows used to be a reg­u­lar threat, deep tun­nel sys­tems have dras­ti­cal­ly reduced the num­ber and sever­i­ty of over­flows.

Although these tun­nels do not address the root cause of CSOs—namely, out­dat­ed and inad­e­quate 19th-cen­tu­ry infrastructure—they serve as a crit­i­cal solu­tion to pre­vent imme­di­ate envi­ron­men­tal and pub­lic health risks.

Few projects rival the scale and impact of Chicago’s Tunnel and Reservoir Plan (TARP), also known as “the Deep Tunnel.” The sys­tem spans more than 100 miles, with tun­nels as large as 33 feet in diam­e­ter and buried as deep as 350 feet.

Chicago

Launched in the 1970s and par­tial­ly acti­vat­ed in 1984, the sys­tem con­tin­ues to expand and is expect­ed to reach full capac­i­ty by 2029. According to the Metropolitan Water Reclamation

District of Greater Chicago, TARP will store about 20 bil­lion gal­lons once com­plet­ed in 2029.

TARP has held back over­flows that would have oth­er­wise sent untreat­ed waste into the Chicago River, Calumet River, and Lake Michigan.

The sys­tem includes four tun­nel sys­tems and reser­voirs like the Majewski, Thornton, and McCook Reservoirs. The Thornton Reservoir has entire­ly elim­i­nat­ed CSOs with­in its ser­vice area since 2020.

The Metropolitan Water Reclamation District of Greater Chicago claims that TARP has sig­nif­i­cant­ly reduced CSOs from an aver­age of 100 days per year to 50. The envi­ron­men­tal ben­e­fits have been clear, with the num­ber of fish species in local rivers near­ly dou­bling and water qual­i­ty improv­ing dra­mat­i­cal­ly.

“Thanks to the tun­nels and many other improve­ments,” the MMSD says, “we have cap­tured and cleaned 98.6 per­cent of all the stormwa­ter and waste­water that’s entered the region­al sewer sys­tem.”

Milwaukee

Operational since 1993, Milwaukee’s sys­tem has pre­vent­ed more than 160.2 bil­lion gal­lons of pol­lu­tion from enter­ing Lake Michigan, accord­ing to the Milwaukee Metropolitan Sewerage District (MMSD), which attrib­ut­es that suc­cess to the tun­nel.

For com­par­i­son, the nation­al goal for cities with sim­i­lar sys­tems is 85 per­cent.

The system’s begin­nings date to 1972, when the State of Illinois filed a law­suit against the city and its sew­er­age com­mis­sion for pol­lu­tion. The case even­tu­al­ly reached the

U.S. Supreme Court. Five years later, a for­mal agree­ment between the Milwaukee Metropolitan Sewerage District and the Wisconsin Department of Natural Resources (DNR) com­mit­ted the region to reduc­ing sewer over­flows.

These piv­otal moments spurred the cre­ation of the Water Pollution Abatement Program (WPAP), which MMSD cred­its with deliv­er­ing “major improve­ments to the region­al waste­water treat­ment sys­tem.

Accompanying this envi­ron­men­tal shift was a major struc­tur­al and finan­cial com­mit­ment. In 1982, the MMSD was estab­lished. Between the 1980s and early 2000s, the region invest­ed $3 bil­lion into water pol­lu­tion abate­ment, $1 bil­lion of which fund­ed the deep tun­nel sys­tem.

According to the dis­trict, those efforts reduced pol­lu­tion from an aver­age of “8 bil­lion to 9 bil­lion gal­lons per year.” By 2010, an addi­tion­al $1 bil­lion was allo­cat­ed through the Overflow Reduction Plan, expand­ing the system’s capac­i­ty and improv­ing its long-term per­for­mance.

Here’s how the colos­sal tun­nel was con­struct­ed:

In 1993, 19.4 miles of tun­nels were dug 275 to 340 feet under­ground, with diam­e­ters rang­ing from 17 to 32 feet. The tun­nels cap­tured 405 mil­lion gal­lons of stormwa­ter and sewage, pro­vid­ing an imme­di­ate boost in flood con­trol and pol­lu­tion reduc­tion. While this marked a sig­nif­i­cant mile­stone, it was just the begin­ning of the jour­ney.

The project took anoth­er step in 2006. The sys­tem grew by 7.1 miles, with tun­nels dug 120 to 165 feet under­ground and a 20-foot diam­e­ter. This expan­sion added anoth­er 89 mil­lion gal­lons of stor­age, increas­ing the system’s abil­i­ty to man­age over­flow.

By 2010, the final stretch of tun­nels was com­plet­ed, adding 2 more miles, 300 to 325 feet under­ground, with a 21-foot diam­e­ter. This sec­tion added 27 mil­lion gal­lons of stor­age, bring­ing the total capac­i­ty to 521 mil­lion gal­lons across all stages. With this last com­po­nent in place, the sys­tem became fully oper­a­tional.

It’s not just a Midwest solu­tion. Cities like Boston, Washington D.C., Atlanta, and even London have embraced tun­nel sys­tems to tack­le sewer over­flows and mod­ern­ize aging infra­struc­ture.

While each has taken a slight­ly dif­fer­ent approach, their shared goal remains the same: to store excess stormwa­ter under­ground and keep pol­lu­tion out of rivers, lakes, and coastal waters.

Hartford’s tun­nel isn’t a car­bon copy of Chicago’s or Milwaukee’s—but it doesn’t have to be. It will ben­e­fit from the insights of cities that have already dug deep—literally—to address sim­i­lar waste­water chal­lenges. If it works as planned, it could be a major step in solv­ing Hartford’s CSO prob­lem.

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TOP IMAGE: Chicago dug this reser­voir, McCook Reservoir, to hold untreat­ed waste from its deep tun­nel sys­tem. This is how the reser­voir looked on May 16, 2025. Photo from mwrd.org, Chicago’s Metropolitan Water Reclamation District.