Understanding Bacteria Sampling

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In response to the sewer overflow from the Potomac Interceptor collapse DC Water launched daily sampling on January 29, 2026, for E. coli, an indicator bacterium, at six (6) locations along the Potomac River and later expanded that to 10 sites. The locations included the overflow area as well as upstream and downstream sites. 

Following more than five months of sampling, which has indicated a return to normal conditions in the Potomac River and the lifting of the remaining shoreline and sediment health advisories, DC Water is moving to twice-a-week testing at the six Maryland locations. The transition is phased with weekend testing ending as of August 8. Weekday testing will end August 14 and sampling will be done twice weekly August 18 through September 10.

The twice-weekly schedule will include more detailed testing once a week. In addition to E. coli testing, Thursday samples will measure oxygen levels, and test for nutrients, metals, and other substances that can affect water quality.

DC Water will continue to share water quality sampling data with the public. Results for the expanded testing will be available here about one to two weeks after samples are collected.

It's important to understand that rain, snow, heat, contaminated runoff, and natural sources of E. coli such as wildlife can cause higher bacteria levels in urban rivers such as the Potomac. After heavy rain experts recommend avoiding the water for at least 48-72 hours. This is reflected in the data, where significant spikes are seen that coincide with rain and weather events but then return to normal including sites upstream of the incident. With no overflows to the river for more than a month prior to completion of emergency repairs, fluctuations in measurements are more likely due to these known factors.

A more detailed explanation is posted on the District Department of Energy and Environment (DOEE) website at doee.dc.gov/release/potomac-interceptor-update-and-faqs

DC Water Locations - Water Quality Sampling

View DOEE's Potomac Interceptor Monitoring page to view comprehensive sampling results from regional agencies and organizations in DC, Maryland, and Virginia.

*Lab results are unavailable on weekends and will be provided the following Monday (excluding holidays). 

Sampling Results

August 10 - present (Past results available here) 

410

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Consistent with public health and U.S. Environmental Protection Agency standards, swimming is not recommended when E. coli levels exceed 410 MPN/100 mL. 

For the Potomac River, historical water quality data shows E. coli levels may vary from a range as low as 10 MPN/100mL to as much as 5,000 MPN/100mL on a given day.

Enhanced Water Quality Sampling

On Saturday, April 11, DC Water began enhanced water quality sampling at popular recreation areas along the Potomac River, testing every two hours for three consecutive weeks. This proactive measure is intended to complement the daily water quality sampling already underway. It comes in response to community feedback and provides additional information to residents and recreational users about current river conditions.

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Understanding the Data and How it Compares

*Data Provided by DC Department of Energy and the Environment

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