Bacteria Monitoring

E. coli is a type of bacteria that’s found in the digestive tract of warm-blooded animals  including people. If high levels of E. coli are found in a stream or river, there’s the potential for disease-causing pathogens to also be present. In 1996, Four Mile Run was listed as an “impaired water” for E. coli and a Total Maximum Daily Load (TMDL) was established for the nontidal portions of the stream. More information about the Four Mile Run TMDL can be found on the Northern Virginia Regional Commission website.

Where Is the Bacteria Coming From?

  • Concentrated wildlife. In an urban area, wildlife often concentrates near streams.
  • Storm sewer systems. The pipes can provide a habitat for bacteria to reproduce underground. Ultraviolet light (sunlight) kills bacteria.
  • Sanitary sewer pipe breaks or leaks.
  • Pet waste. Picking up after your pet is not just a courtesy, but a public health issue.

What Do High Bacteria Levels in the Water Mean to Me?

It means there’s a greater possibility that illness-causing pathogens could be in the water. While there’s no guarantee the water will make you ill, it’s a sign that additional care should be taken. Always wash your hands after working or playing in our streams.

Should I Be Worried?

No. Activities that involve touching the water – like fishing or skipping rocks – are safe. But always thoroughly wash your hands after coming into contact with stream water and never drink it. We don’t recommend swimming or dunking your head or body into stream water. Learn more about enjoying streams safely

Below are some results from volunteer-collected E. coli data. We use the Virginia Department of Environmental Quality (DEQ)-recommended Coliscan Easygel kit with our volunteer program. While not laboratory quality, the Coliscan Easygel test results are a reliable, low-cost indicator to determine if further testing or investigation is warranted.

When Are Bacteria Levels Highest? Warm Months, After Rain

Across Arlington, our volunteer monitoring data shows that bacteria levels are highest:

  • During warm months (May to October)
  • During and 24-48 hours after rainstorms.

Those are the conditions that we tend to see peaks in bacteria levels and the most exceedances of water quality standards.

Where Are Bacteria Levels Highest?

Two charts showing data for when it is unsafe to swim or unsafe to wade.  

The charts above show how often bacteria results are considered unsafe to swim (primary contact) and unsafe to wade (secondary contact). The sites were combined into four groups and averaged over time (monthly samples from July 2013 - June 2021). 

How often is it considered unsafe to swim? Percent of samples above the water quality standard for swimming: Potomac Drainages (15%), Four Mile Run Tributaries (25%), Upper Four Mile Run (34%), and Lower Four Mile Run (38%). Arlington County recommends limiting stream activities to secondary contact and always washing hands afterwards. 

How often is it considered unsafe to wade? Percent of samples above the water quality standard for wading: Potomac Drainages (2%), Four Mile Run Tributaries (5%), Upper Four Mile Run (7%), and Lower Four Mile Run (10%).

  • Potomac Tributaries: Lowest bacteria levels. Sites include Donaldson Run, Windy Run, Gulf Branch, and Little Pimmit Run.
  • Four Mile Run Tributaries: Second lowest bacteria levels. Sites include Lubber Run (3 locations), Upper Long Branch, Doctor’s Branch and Lower Long Branch.
  • Upper Four Mile Run: Second highest average bacteria levels. Sites include Banneker Park, East Falls Church Park, Bluemont Park, and two sites in Upper Glencarlyn Park (upstream of Long Branch).
  • Lower Four Mile Run: Highest average bacteria levels. Sites include Glencarlyn dog park, Arlington Mill Community Center, Shirlington dog park, Barcroft Park, and Four Mile Run Park at Mount Vernon Avenue.

For more detail about bacteria monitoring data, see the bacteria reporting appendix of the Annual Stormwater Report.

Even when bacteria levels may be low, it’s still present. Stream users should always be careful to wash hands after playing in the stream.