An Interactive Map for Tracking E. coli Levels Along the Potomac River
On January 19, 2026, a 72-inch section of DC Water’s Potomac Interceptor sewer line collapsed near the Clara Barton Parkway in Montgomery County. This event released an estimated 240 to 300 million gallons of untreated sewage directly into the Potomac River. This event is one of the largest wastewater spills in U.S. history. Emergency repairs were completed on March 14, and DC and federal health agencies lifted a recreational water advisory, but swimming remains prohibited in DC waters.
In the days that followed the spill, E. coli bacteria levels near the collapse site reached roughly 12,000 times the recommended limit of 410 MPN/100 mL for human contact. “MPN” stands for “Most Probable Number”, which is a standard measurement of bacterial concentration per 100 milliliters of water. Although authorities have lifted advisories, the “safe for recreation” designation doesn’t mean that safe conditions are uniform across the entire river.
The January spill is not the only source of E. coli in the river. Historically, Potomac river E. coli levels can vary from 10 to 5,000 MPN/100 mL. Be aware that E. coli levels also spike after rainfall. If you’re planning to paddle, fish, or spend time in the water this spring, pay attention to recent conditions, and check bacterial levels for your destination.
The sampling data is an indicator of river conditions, but not a guarantee of safety.
Using the Map and Dashboard
This interactive map uses DC Water’s official sampling data, published on Open Data DC. This map lets you see current E. coli readings at sampling sites along the river. The highest weekly value for all sites is shown in the upper left, in red if it exceeds 410 MPN/100 mL. The location of the highest reading is shown at the top of the list of sampling sites. Click on the site name to flash the location on the map. The list includes sites sampled over the last 1-2 weeks, sorted from highest E. coli to lowest. If you click on a site on the map, a popup shows the most recent sampling results for the site. Recent trends and the data sources appear beneath the map. The dashboard updates as DC Water publishes new data.
Disclaimer: This dashboard is provided for informational and educational purposes only. The data is sourced directly from DC Water and Open Data DC and is displayed as published. River and Reef does not independently verify sampling results and makes no guarantees regarding the accuracy, completeness, or timeliness of the data shown. This dashboard is not a substitute for guidance from public health officials. For health-related advice, consult the DC Department of Health, the Virginia Department of Health, the Maryland Department of the Environment, or other relevant local authorities before engaging in any recreational activity on or near the Potomac River.
For more information:
Learn more about the Potomac Interceptor spill: DC Water: Potomac Interceptor Collapse
Data source: Open Data DC
What’s in the Water? Understanding Water Quality Information/What is E. coli?
