Piney Branch Tunnel Project

A Cleaner Future for Piney Branch and Rock Creek
Each year, about 40 million gallons of untreated sewage and stormwater overflow into Piney Branch in Rock Creek Park during heavy rainstorms. This harms the environment, affecting water quality in Rock Creek, the Potomac River, and even the Chesapeake Bay.

The Piney Branch Tunnel Project could drastically reduce this pollution. By capturing and temporarily storing the stormwater and sewage overflow, the project will reduce untreated overflows by 96%, bringing down the number of overflow events from 25 per year to just one during typical rainfall. The stored water will be sent to DC Water's Blue Plains Advanced Wastewater Treatment Plant, where it will be treated before being safely discharged.

What’s Causing the Problem?
The issue begins with Combined Sewer Overflows (CSOs)— a system where stormwater and sewage share the same pipes. When it rains, the sewer system can’t handle the extra volume, leading to untreated sewage and stormwater being released directly into Piney Branch. The CSO 049 outfall is the largest of 23 such overflow points, contributing 40 million gallons of pollution each year.

These overflows have led the Environmental Protection Agency (EPA) to classify Piney Branch as an impaired waterbody. The proposed Piney Branch Tunnel Project will reverse this damage and meet federal clean water standards under the Clean Water Act.

How the Piney Branch Tunnel Project Will Work
A 2,200-foot-long underground tunnel will be built to store 4.2 million gallons of combined sewage and stormwater during storms. This tunnel will act as a temporary holding space until the existing sewer system can handle the extra flow.

Key Structures: A diversion structure will be added at the CSO 049 outfall to direct the overflow into the tunnel. Once the rain subsides and the sewer system can manage the flow, the stored water will be safely sent to the Blue Plains Treatment Plant.

Cleaner Water, Fewer Overflows: After treatment, the clean water will be discharged into the Potomac River. This will reduce the number of untreated sewage spills into Piney Branch from 25 times per year to just once, protecting the environment and complying with a 2005 Federal Consent Decree.

Construction Timeline
Construction of the Piney Branch Tunnel may begin in 2025.

This project marks the final major piece of DC Water’s Long Term Control Plan, also known as the DC Clean Rivers Project.

Your Voice Matters
The NPS values community involvement in shaping this important project. You are encouraged to review the EA and share your thoughts during the public comment period from October 15 to December 6. There are two ways to submit your comments:

1. Online: Visit https://parkplanning.nps.gov/dcPineyBranch, review the EA and submit comments.
2. By Mail: Send written comments to:
Superintendent
Rock Creek Park
RE: Piney Branch Tunnel EA
3545 Williamsburg Lane, NW
Washington, DC 20008

Please make sure mailed comments are postmarked by December 6. Note that all submissions, including personal details, may be publicly accessible. While requests for confidentiality will be considered, full privacy cannot be guaranteed.