National Park Service approves DC Water's Piney Branch Tunnel to reduce sewage overflows​

The National Park Service has approved DC Water's Piney Branch Tunnel project, a significant step toward improving water quality in Washington, DC The initiative aims to address the issue of combined sewer overflows (CSOs) that have long plagued Piney Branch and Rock Creek.

Currently, approximately 40 million gallons of untreated sewage and stormwater discharge into Piney Branch annually, harming local waterways and wildlife. The Piney Branch Tunnel is designed to reduce CSO volume by ninety-six percent, decreasing overflow events from twenty-five to just one per typical year. The 2,200-foot-long underground tunnel will capture and temporarily store up to 4.2 million gallons of combined sewage and stormwater during heavy rains. Once capacity is available, the stored water will be conveyed to DC Water's Blue Plains Advanced Wastewater Treatment Plant for proper treatment. ​

This project is part of DC Water's broader Clean Rivers Project, a $2.7 billion initiative to construct over 18 miles of tunnels aimed at reducing CSOs and enhancing water quality in the Anacostia and Potomac Rivers, as well as Rock Creek. The Piney Branch Tunnel represents a critical component of this effort, targeting one of the largest CSO contributors in the area. ​

NPS conducted a thorough environmental assessment to evaluate potential impacts associated with the tunnel's construction and operation. Upon completion, the NPS issued a Finding of No Significant Impact (FONSI), allowing the project to proceed. The EA considered factors such as water quality, wetlands, wildlife, historic sites, and public spaces, ultimately determining that the project's benefits outweigh potential adverse effects.

Design work for the tunnel is currently underway, with construction expected to commence in 2026. The tunnel is anticipated to be operational by 2029, with all restoration work and related improvements completed by 2030.