Riverkeeper Flags Ongoing Fecal Pollution in Potomac Tributary

The Potomac Riverkeeper Network (PRKN) released findings March 9, 2026, indicating potential ongoing leakage of sewage-contaminated water from a culvert beneath the Chesapeake & Ohio (C&O) Canal into an unnamed tributary of the Potomac River, amid DC Water’s bypass operation diverting raw sewage through the canal due to the Potomac Interceptor failure.

PRKN staff conducted sampling over two weeks at four locations along the unnamed tributary, which flows through a stone masonry culvert between Locks 11 and 10 of the C&O Canal. This tributary previously carried more than 200 million gallons of raw sewage to the Potomac River in the initial weeks of the January 19, 2026, Potomac Interceptor collapse.

Samples were analyzed using the IDEXX Colilert-18 protocol in PRKN’s Tier III certified mobile laboratory, the SeaDog, at National Harbor, MD. Upstream samples near the culvert showed E. coli levels dropping to within the EPA recreational safety limit of 410 Most Probable Number (MPN) per 100 mL. Inside and below the culvert, levels rose sharply—often by one to two orders of magnitude—with one set of three samples on March 5 averaging 42,420 MPN. Downstream at the Potomac River confluence, levels consistently exceeded the EPA limit.

PRKN staff observed continuous dripping from 20-30 points on the culvert ceiling during all four visits, audible and present in both wet and dry conditions. The culvert floor appeared bare stone, but post-culvert areas showed orange microbes, fungus filaments, dark sludge pockets, and a stronger pungent odor compared to upstream.

Table 1 shows the E.coli bacteria levels, measured in Most Probable Number (MPN), from four sections along the unnamed tributary: above, within, and below the culvert, along with where the tributary meets the Potomac River. Bacteria levels with an * represent an average between two samples taken at the same location and time, while levels in red exceed the EPA water recreation safety limit of 410 MPN. 

The group concluded that fecal bacteria-contaminated water may be leaking from the sewage-impounded canal above, contributing to unsafe contamination in the Potomac River. This raises concerns about culvert integrity, bypass effectiveness, and potential failure of planned stream remediation without addressing the source.

PRKN called for urgent action by DC Water, federal, and state regulators to investigate the discharge source, monitor pollutants, assess the bypass and culvert, and abate contamination.

DC Water responded that it is aware of PRKN’s testing and conclusions regarding elevated E. coli near the Rock Run culvert. The utility is actively investigating bypass impacts on the culvert and tributary, with mitigation efforts ongoing. As part of approved rehabilitation, the U.S. Army Corps of Engineers (USACE) installed a containment system within and around the culvert to enable washing using National Park Service-approved methods; this work was in progress during the PRKN report and has been completed. Collected water below the culvert is pumped through the C&O Canal bypass to avoid the tributary.

DC Water reported the bypass, activated January 24, 2026, has prevented over one billion gallons of wastewater from reaching the Potomac River. Emergency repairs to the collapsed Potomac Interceptor section are nearing completion, with work progressing toward mid-March 2026 to restore full flow and remove the bypass. Environmental rehabilitation of impacted areas, including the C&O Canal, will follow.

The Potomac Interceptor collapse released an estimated 243 million gallons initially into the Potomac near the C&O Canal in Montgomery County, MD, prompting widespread advisories now largely lifted after testing showed improvements.