Washington DC’s Potomac River Tunnel is the next major phase of the DC Clean Rivers Project, which is DC Water’s ongoing programme to improve the water quality of the Anacostia and Potomac rivers and Rock Creek by increasing the capacity of the sewer system.

The project comprises 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. The 8.9km-long tunnel will be completed in 2030, providing a 93% reduction in the volume of CSOs to the Potomac River in an average year of rainfall.

At present, an estimated 654 million gallons of CSOs enter the Potomac River each year by way of average rainfall.

The 5.5m-diameter tunnel will run beneath the Georgetown waterfront, along the edge of the National Mall and East Potomac Park, past Hains Point and connect by gravity to the existing Anacostia River Tunnel. Construction will require two TBMs. Starting from West Potomac Park, one machine will mine south through mostly soft ground, and another will head north to bore through rock.

In October last year DC Water awarded the design-build contract to a joint venture of CBNA and Halmar. The US$819m (€778m) contract is the largest ever awarded by the authority.

The groundbreaking ceremony was attended by city leaders and agency partners.

“Today we break ground with a sense of purpose and responsibility. This project is critical to ensure we reduce the CSOs that contribute to water quality impairment of the Potomac,” said DC Water CEO David Gadis. “Together we can shape a future where cleaner water flows and the Potomac River thrives as a beacon of environmental vitality.”