Almost 25 years ago, a coalition of local environmental groups filed a lawsuit against DC Water for allowing billions of gallons of untreated sewage to spill into the District’s rivers. Now, work is complete on a massive system of tunnels that will prevent 98% of sewer overflows into the Anacostia River — a major step in the ongoing restoration of the river.

“This is just a huge milestone,” says DC Water’s Moussa Wone, who is in charge of the project. “This river was really very, very, very impaired.”

On Friday, DC Water put in service the system’s final segment, the Northeast Boundary Tunnel, connecting it with an earlier phase of the project that went online in 2018.

Before the project, the city’s antiquated sewers would overflow into the Anacostia River 84 times a year, dumping an average of 2.1 billion gallons of untreated sewage directly into the waterway. With the new tunnel in operation, overflows are expected to happen only twice a year now, resulting in a dramatic improvement in water quality. Already, the first phase of the project has kept 15 billion gallons of untreated sewage and stormwater out of the river, along with 9,800 tons of trash.

The project came about as the result of a lawsuit filed in February 2000 by the Anacostia Watershed Society, Friends of the Earth, the Sierra Club and other groups.

“After years of talking with the city government and DC Water and the EPA to try to get this changed, we weren’t making any progress,” says Christopher Williams, the current president and CEO of the Anacostia Watershed Society. He says the constant sewer overflows were a “gross violation” of the federal Clean Water Act.

DC Water settled the suit, agreeing to a fix the problem by building a series of enormous tunnels, known as the Clean Rivers Project. The Potomac River and Rock Creek are also slated for similar tunnels, to be completed by 2030. The Anacostia River was the priority, though, because it received far more sewer overflows than the other waterways.

The new tunnel will also cut down on flooding in some areas, including along Rhode Island Ave., where a flood on August 14 killed 10 dogs at a doggy daycare business. Wone says the new tunnel is designed to capture stormwater and prevent flooding for up to a 15-year storm. Such a storm has a 7% chance of occurring in any given year.

Before the new tunnel opened, the sewer system could only handle a 2-to-5-year storm without flooding, meaning as much as a 50% chance of flooding in any given year.

The August 14 flood was a 20-year event, Wone says, so it would have exceeded the capacity of the new tunnel. But, he says, in such an event, the new tunnel will greatly reduce the amount of flooding.

The tunnel system was an engineering feat: a boring machine excavated and constructed the 23 ft. diameter tunnels some 100 ft. beneath the city. The entire Anacostia tunnel system runs about 12 miles, from Shaw in Northwest, to the Blue Plains sewage treatment facility in Southeast. Construction on the system began in 2011. The entire project, including the Potomac and Rock Creek portions, is slated to cost $2.7 billion, much of it paid for by DC Water customers through the Clean Rivers impervious area charge on monthly water bills.

The tunnels function by capturing and storing overflow from the city’s old sewers, and transporting it to Blue Plains to be treated. The new tunnels serve the parts of the city that have combined sewers, which carry stormwater and sewage in the same pipes. During a storm, those pipes can quickly fill up with rainwater.

Earlier this summer, work on the tunnel system during rainy weather caused a sewer overflow, forcing scheduled swim day on the Anacostia River to be cancelled. The event has been rescheduled for Sept. 23.

While the new tunnels will greatly improve water quality, Williams says there’s still a lot of work ahead to fully restore the Anacostia River. There are still high levels of bacteria in some places, possibly due to sources upstream from D.C. And there are still toxic chemicals in the river bottom — the legacy of hundreds of years of industry.

“The tunnels are a are a game changer. They’re a huge step forward, but a lot of challenges remain,” Williams says.