The race begins to dig a 2-mile sewer tunnel under the Potomac River


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

This round metal device, a custom-made tunnel boring machine decorated with colorful handprints, was lowered into a 138-foot shaft a few 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 intended to address the city’s most pressing pollution problem: the millions of gallons of raw sewage it dumps into the Potomac.

“It’s really driven by the goal of improving the health of our city’s waterways,” said Justin Carl, 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 in unprecedented time.”

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

This means that when heavy rains hit the city – which they do about 70 times a year – these combined pipes overflow into outfalls around the city, washing up to 140 million gallons of untreated human waste into the Potomac. and two of its tributaries, Hooffs Run and Hunting Creek.

It’s a problem faced by more than 700 other US 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 those in Alexandria: DC has largely completed a 13-mile network of sewer tunnels under the Anacostia River, and towns like Seattle, Columbusand Pawtucket, RI 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 keeping with the 16th century mining tradition of tunnel diggers who look to Saint Barbara for protection.

But unlike other machines, Hazel faces a particularly tight deadline: the machine has 14 months to connect two of the city’s outfalls to its treatment plant, so that the wastewater can be captured, treated and then pumped out. in the river.

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

Virginia State Sen. Scott A. Surovell (D-Fairfax), who represents an area just downriver from Alexandria, said he was “completely shocked that we routinely dump raw human waste into the river Potomac”.

While Richmond and Lynchburg also rely on CSOs, he pointed out that Alexandria has a reputation for being a community of environmentalists — it was the first in Virginia to adopt an “eco-city charter” — and boasts one of the wealthiest populations in the Commonwealth, meaning it has the cash to solve the problem.

“It’s something that 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 discharge of raw sewage into the Potomac, I don’t see how we could expect anyone to do that.”

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

During the Civil War era, engineers in Alexandria built a CSO to direct human waste away from homes and businesses in the Old City and into the nearby river. The system was seen as a huge improvement over outdoor latrines, which dumped sewage into the ground, polluting the city’s drinking water and making residents sick.

“When they were originally built, they were innovative. It was seen as a huge leap forward for human health,” Carl said of CSOs. “Obviously we have learned a lot since then with the impact this is having on our waterways, and the fish and wildlife in our waterways.”

Although modern sewage treatment – from the 1950s – has helped clean up a polluted river that newspapers once declared ‘too thick to drink’ but ‘too thin to plow’, heavy rains have continued to overload the OSC system. This has sometimes led to dangerous levels of E. coli and nitrogen and phosphorus pollution.

Caitlin Feehan, director of communications and external programs for AlexRenew, pointed out that Hazel will only be addressing one of three flood-related issues in Alexandria.

Outside of the CSO area in Old Town, some storm drains — especially in lower neighborhoods like Del Ray and Rosemont — can’t handle the rain from more frequent thunderstorms and fill the streets with water. And closer to the Potomac, rising sea levels mean high tides sometimes flood the Old Town shoreline.

Alexandria already needed to repair its storm pipes. But climate change is making the situation worse.

The $615 million project is funded by 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 be reimbursed by local taxpayers.

Monthly sewage rates for Alexandria customers have already increased by about $12 for an average resident since the RiverRenew project began and are expected to increase to about $75 when it is completed in three years.

RiverRenew engineers and construction crews last month lowered Hazel – split into two massive, round metal shields – in one of two wells they had dug at a site near the city’s sewage treatment plant.

Hazel is slated to actually begin work on the tunnel in October, when he will dig through the Potomac’s clay soil and return it to the plant while simultaneously constructing a 12-foot-wide concrete-lined sewer tunnel.

The machine’s destination is at a weir at the end of North Pendleton Street in Old Town, near Oronoco Bay Park. Although the machine was able to tunnel under Old Town, the area’s historic status and potential for some disturbance – along with its more difficult ground conditions – made it an easy choice for routing it under the Potomac to instead, Carl said.

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


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