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To clean up the Potomac, engineers are digging a 2-mile tunnel under it

The Washington Post logo The Washington Post 9/12/2022 Teo Armus
Workers lower the 100-ton front shield of “Hazel,” a tunnel-boring machine, into a shaft in Alexandria on Aug. 18. © Ricky Carioti/The Washington Post Workers lower the 100-ton front shield of “Hazel,” a tunnel-boring machine, into a shaft in Alexandria on Aug. 18.

A crucial race against the clock to dig a tunnel under the Potomac River depends on a powerhouse player: a 15-foot-wide, 380-ton machine named “Hazel.”

This round metal device, a custom-made tunnel boring machine that’s decorated with colorful handprints, was lowered into a 138-foot shaft weeks ago and is now preparing to dig a two-mile sewer tunnel in Alexandria.

This mission — the largest infrastructure project undertaken in this Northern Virginia community — is meant to address the city’s most glaring pollution problem: the millions of gallons of raw sewage that it puts into the Potomac every year.

“This is really driven by the goal of improving the health of our city’s waterways,” said Justin Carl, a program manager at Alexandria Renew Enterprises, or AlexRenew, the local wastewater authority. “We’re building this mega-project in a very historic area, and we’re doing it on an unprecedented timeline.”

The vast majority of homes and businesses in Alexandria have separate pipes for storm water and sewage, but the city’s historic Old Town relies on a combined sewer system with just one pipe for both.

That means that when heavy rains strike the city — which they do about 70 times per year — those combined pipes overflow into outfalls around the city, bringing along as much as 140 million gallons of untreated human waste into the Potomac and two of its tributaries, Hooffs Run and Hunting Creek.

Engineers and workers stand at the entrance to a tunnel at the bottom of a pit where Hazel will be launched into the sewers of Old Town. © Bill O'Leary/The Washington Post Engineers and workers stand at the entrance to a tunnel at the bottom of a pit where Hazel will be launched into the sewers of Old Town.

It’s an issue faced by more than 700 other U.S. cities, which also have dense neighborhoods that urbanized before the turn of the century and depend on these combined sewer overflows (CSOs).

Environmental lawsuits and state legislation have forced many of these communities to undertake remediation efforts similar to Alexandria’s: D.C. has largely finished a 13-mile network of sewage tunnels under the Anacostia River, and cities such as Seattle; Columbus; and Pawtucket, R.I., have embarked on their own projects.

Like many other tunnel-boring machines, Hazel is named after a woman — in this case, the “mother” of the modern environmental justice movement, Chicago activist Hazel Johnson — in accordance with 16th-century mining lore of tunnel-diggers looking to Saint Barbara for protection.

But unlike other machines, Hazel faces an especially tight timeline: The machine has 14 months to connect two of the city’s outfalls back to its wastewater treatment plant, so the sewage can be captured, treated and then pumped back into the river.

Lawsuit alleges Alexandria has been polluting Potomac with coal tar

The entire remediation project must be completed by 2025, thanks to a deadline imposed by state lawmakersfew of whom knew about the issue until environmental groups began sounding the alarm.

Virginia State Sen. Scott A. Surovell (D-Fairfax), who represents an area just downstream from Alexandria, said he was “completely shocked that we were dumping raw human waste into the Potomac River on a regular basis.”

While Richmond and Lynchburg also rely on CSOs, he pointed out that Alexandria has a reputation as a community of environmentalists — it was the first in Virginia to adopt an “eco-city charter” — and enjoys one of the wealthiest populations in the commonwealth, meaning it has the money necessary to remediate the issue.

“This is something most people thought should have been taken care of 30 years ago,” Surovell said. “If the city of Alexandria couldn’t find the resources to plug the raw sewage discharge into the Potomac, I’m not clear how we could expect anybody to do it.”

Carl, the program manager working on the RiverRenew Tunnel Project, said Hazel is trying to solve what he called a “150-year-old problem.”

Justin Carl, a program manager at Alexandria Renew Enterprises, discusses Hazel as he stands over a scale model of the machine. © Bill O'Leary/The Washington Post Justin Carl, a program manager at Alexandria Renew Enterprises, discusses Hazel as he stands over a scale model of the machine.

Around the time of the Civil War, engineers in Alexandria built a CSO to direct human waste away from the homes and businesses of Old Town and into the nearby river. The system was considered a huge improvement from outdoor latrines, which discharged sewage into the ground, polluting the city’s drinking water and getting residents sick.

“When they were initially built, they were innovative. It was considered a huge leap for human health,” Carl said of CSOs. “Obviously, we’ve learned a lot since then with the impact that has on our waterways, and the fish and wildlife in our waterways.”

Although modern wastewater treatment — starting in the 1950s — has helped clean up a polluted river that newspapers once declared was “too thick to drink” but “too thin to plow,” heavy rainfall continued to overload the CSO system. That has at times led to dangerous levels of E. coli and nitrogen and phosphorous pollution.

Caitlin Feehan, AlexRenew’s director of communications and external programs, pointed out that Hazel will only address one of three flooding-related issues in Alexandria.

Outside the CSO area in Old Town, some storm water pipes — particularly in low-lying neighborhoods such as Del Ray and Rosemont — cannot handle the rain from more frequent storms and fill the streets with water. And closer to the Potomac, rising sea levels mean that king tides will sometimes flood the riverbank in Old Town.

Alexandria already needed to fix its storm pipes. But climate change is making it worse.

The $615 million project is being funded through grants from the American Rescue Plan Act and low-interest loans from a Virginia clean water fund and the Environmental Protection Agency, which will eventually need to be paid back by local taxpayers.

Monthly wastewater rates for customers in Alexandria have already increased from $45 to $57 for the average household since the RiverRenew project started and are expected to further jump to about $75 when it is complete in three years.

RiverRenew engineers and construction crews last month lowered Hazel — split into two massive round metal shields — down one of two shafts they had dug at a site near the city’s wastewater treatment plant.

Workers lower the 100-ton front shield of Hazel into the shaft. © Ricky Carioti/The Washington Post Workers lower the 100-ton front shield of Hazel into the shaft.

Hazel is expected to actually begin work on the tunnel in October, when it will dig through Potomac clay soil and send it back up to the plant while simultaneously building a concrete-lined 12-foot-wide sewer tunnel.

The machine’s destination is at an outfall at the end of North Pendleton Street in Old Town, near Oronoco Bay Park. Although the machine could have dug a tunnel under Old Town, the area’s historic status and the potential for some disruption — as well as its more difficult soil conditions — made it an easy choice to route it under the Potomac instead, Carl said.

“It’s both engineering and community-driven,” he added. “We didn’t sit here in an office and come up with a tunnel route totally based on technical decisions.”

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