The District is preparing to start building underground tunnels big enough for Metrorail cars. But the only things that will commute through these pipes are stormwater and sewage.

Officials with the D.C. Water and Sewer Authority, which operates the Blue Plains Advanced Wastewater Treatment Plant, are to break ground next week on the $2.6 billion project, which is meant to end to polluted discharges into waterways.

The city will build two tunnels as part of the project. The Anacostia tunnel will extend from Blue Plains along the Anacostia River to near RFK Stadium.

Another tunnel will run 120 feet underground along M Street SE between Ninth and 14th streets. The pipes will retain stormwater from heavy rains so it can be treated to remove filth before being released into the Anacostia and Potomac rivers, as well as Rock Creek.

The District operates a single pipe to handle a combination of stormwater and sewage in a third of the city. It performs poorly during rains. Overwhelmed by a mixture of stormwater and wastewater, it threatens to bubble back up into the toilets and sinks of homes and businesses.

D.C. Water releases the untreated waste into waterways to avoid the backup. About 75 times per year, 1.5 billion gallons of untreated wastewater is dumped in the Anacostia, 850 million gallons is released into the Potomac, and Rock Creek receives about 52 million gallons of untreated overflow, according to D.C. Water.

The Anacostia is one of the nation’s dirtiest urban rivers, and the Potomac is heavily polluted. The tributaries contribute to fouling the Chesapeake Bay with nitrogen and phosphorous that create muck and “dead zones” that rob oysters, mussels and fish of oxygen.

When the new pipes are completed, the combined sewer will divert water to the M Street pipe, which will feed it to Blue Plains for treatment. Construction of that pipe will affect traffic on a busy stretch of M Street near Nationals Park. Parts of the project are scheduled to be completed in 2018, but it won’t be totally completed until 2025.