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Beyond the Overflows

Tunnel Will Hold Waste from 17 Sewer Overflow Points

Posted on June 3, 2025

By Elijah Polance
UConn Journalism

Reducing com­bined sewer over­flows is a time-con­sum­ing and expen­sive endeav­or for the agency that man­ages Hartford’s sewage treat­ment. The Metropolitan District Commission set­tled on a solu­tion not used any­where else in Connecticut: stor­ing untreat­ed waste inside a giant tun­nel.

In 2016, the MDC signed a con­tract autho­riz­ing the cre­ation of this tun­nel, called the South Hartford Conveyance and Storage Tunnel. The tun­nel will help the MDC com­ply with the con­sent order with Connecticut’s Department of Energy and Environmental Protection, requir­ing it to cor­rect pol­lu­tion dat­ing to the mid-1990s. The tun­nel was planned to begin hold­ing back waste oper­ate in 2023, but delays have pushed that to 2026.

The tun­nel will store excess stormwa­ter and waste­water that ends up in com­bined sewer sys­tems dur­ing heavy rain­fall. The pre­cip­i­ta­tion over­loads these sys­tems, caus­ing untreat­ed sewage to drain into pub­lic water­ways and back­flow into res­i­dents’ homes.

Specifications from the MDC state the tun­nel is four miles long and 200 feet below the ground. It has a diam­e­ter of 18 feet and can store 41.5 mil­lion gal­lons of water.

“A foot­ball field is 360 feet long, 160 feet wide,” said Susan Negrelli, the MDC’s direc­tor of engi­neer­ing and plan­ning. “So pic­ture that with about 100 feet of water on it. That’s 42 mil­lion gal­lons.”

The buried stor­age tun­nel begins in West Hartford, by Talcott Road, and ends in the South Meadows area of Hartford, across Brainard Road from the Hartford Water Pollution Control Facility.

The tun­nel will main­ly elim­i­nate com­bined sewer over­flows in south­ern Hartford, espe­cial­ly 10 of the CSOs that over­flow into the South Branch of the Park River and seven oth­ers that flow into Folly Brook and then into Wethersfield Cove. The tun­nel will also take in water from sur­round­ing towns includ­ing West Hartford, Newington and Wethersfield, said Nick Salemi, spokesper­son for the MDC.

MDC Director of Engineering Susan Negrelli shows a map of the tun­nel’s route through Hartford on March 28, 2025. Photo by Julianna D’Addona

Negrelli said the MDC decid­ed to build the tun­nel in south­ern Hartford because of the con­sent order require­ment to elim­i­nate CSOs to near­by bod­ies of water, CSO issues in West Hartford, and the prox­im­i­ty to the Hartford WPCF. She called it the “log­i­cal start­ing point.”

“This is where the plant is too, so the tun­nel ends at the plant,” Negrelli said. “So any­thing you’re going to build onto that, you can go fur­ther into the sys­tem as you may.”

According to Negrelli, the tun­nel project involved five con­struc­tion con­tracts that includ­ed relo­cat­ing util­i­ties, bor­ing under­ground to build the main tun­nel and shafts, build­ing a sta­tion to pump the waste to the sewage treat­ment plant, and cre­at­ing con­duits for more waste from the east and west.

The drilling for the tun­nel began in 2019 and was car­ried out with a “hard rock tun­nel bor­ing machine” built in Germany and known as ISIS. The drilling took three years, fin­ish­ing in 2022.

The pump sta­tion will pump the stored waste to the Hartford WPCF. Because the tun­nel runs slight­ly down­hill from West Hartford to this point, grav­i­ty will Negrelli said the pump sta­tion relies on grav­i­ty to get the water mov­ing up 200 feet to get treat­ed.

The con­duits are a series of pipes that will allow water to descend 200 feet from sewer sys­tems to the tun­nel with­out let­ting too much water pres­sure build.

Salemi said that the tun­nel will not become less use­ful as the MDC sep­a­rates com­bined sewer sys­tems through­out Hartford. He said that because the process will take a long time, and because excess rain­fall can be an issue regard­less of the sewer sys­tem, it will remain valu­able.

“It’s help­ful to deal with the storms, it’s not going to be we build it and then ‘Why did we build this, we don’t need it any­more,’” Salemi said. “It’s part of the sys­tem as the way to deal with it.”

Graham Stevens, Chief of the Bureau of Water Protection and Land Reuse at DEEP, believes the tun­nel is a valu­able short-term solu­tion, but does not solve the root of the prob­lem.

“Tunnels get real­ly com­pli­cat­ed, real­ly expen­sive, and ulti­mate­ly, although it’s a solu­tion to CSOs, you’re still not get­ting to the core issue, which is the fact that you’ve got one pipe,” Stevens said.

According to MDC tun­nel plans, the need to sep­a­rate sys­tems was taken into account while plan­ning CSO elim­i­na­tion in Hartford.

In the past, there were pro­pos­als for addi­tion­al tun­nel projects to help north­ern Hartford. But the MDC opted for sep­a­ra­tion instead, because a tun­nel would be logis­ti­cal­ly chal­leng­ing because of the dis­tance to the Hartford WPCF, would not refur­nish the out­dat­ed sewer sys­tems that still need replac­ing and could end up cost­ing more.


TOP IMAGE: This illus­tra­tion from the Metropolitan District Commission’s plan­ning brochure shows the route of the waste tun­nel under­neath bedrock, head­ing slight­ly down­hill for 4 miles from West Hartford to near the sewage treat­ment plant in the South Meadows area of Hartford.

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ABOUT THIS PROJECT

Eight journalism students at the University of Connecticut spent three months reporting on the combined sewer overflow repair project in Hartford and getting to know some of the real-life, sometimes devastating impact this pollution has exacted on the people who have endured it for decades.

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