The Potomac River Tunnel (PRT) is the next major phase of the DC Clean Rivers Project. The project consists of a large-diameter deep sewer tunnel, diversion facilities, drop shafts, and support structures to capture flows from existing combined sewer overflows (CSOs) along the Potomac River and convey them to the Blue Plains Advanced Wastewater Treatment Plant for treatment.

There are 8 CSO outfall locations on the Potomac River. In a year of average rainfall some 74 CSO events discharge approximately 654 million gallons of CSO into the Potomac. When complete, the PRT is estimated to reduce those events to four and reduce the volume of CSOs by 93% in an average rainfall year.

In 2019, DC Water began design work for the Potomac River Tunnel, and in 2024 began mobilization at West Potomac Park. When completed in 2030, the tunnel will improve water quality and aesthetics of the Potomac River for the benefit of all.

Watch DC Water lower Mary—the tunnel boring machine—deep underground. Check out our video.