Potomac River Tunnel — DC Water Clean Rivers CSO Project

DC Water’s $819 million Potomac River Tunnel is the largest single contract the utility has ever let, and it closes out a consent-decree program that started two decades ago. A CBNA / Halmar International joint venture won it in October 2023 on lowest price with the second-highest technical score. Two tunnel boring machines are now underground.

Project Scope

The tunnel runs 5.5 miles at an 18-foot finished internal diameter, capturing combined sewer overflows along the Potomac and carrying them to the Blue Plains Advanced Wastewater Treatment Plant. Diversion facilities, drop shafts and support structures come with it. TBM “Mary” had her 21-foot cutterhead lowered underground in January 2026 and began driving that spring. The second machine, “Emily,” arrived from Germany at the West Potomac Park site on May 27, 2026 and takes the southern reach. Surface work at the Canal Road NW entrance to Georgetown University mobilized in January 2026. The permit package includes controlled underground blasting, which is not a routine allowance in a dense historic district next to a university and the C&O Canal.

Why It Matters

The performance target is a 93% cut in CSO volume to the Potomac in a typical rainfall year, dropping overflow events from roughly 74 a year to four. That’s the last big piece of DC Water’s $2.99 billion Clean Rivers Program, which already covers the Anacostia and Rock Creek. It’s also one of the few remaining U.S. megatunnel programs still awarding work, so the logistics lessons travel: staging TBM segments and spoil on constrained National Park Service and university-adjacent parcels is the kind of problem more utilities are about to inherit. Completion is set for early 2030.

Project Team & Details