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 from Four Mile Run and a few of its tributaries since 2010. 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 and Where Are Bacteria Levels Highest?
Across Arlington, our volunteer monitoring data shows that bacteria levels are highest in the warm months (May to October) and during and after rainstorms. Those are the conditions that we tend to see the highest bacteria levels and the most exceedances of water quality standards.
- Lower Four Mile Run monitoring sites tend to have the highest bacteria levels, including the Glencarlyn dog park, Shirlington dog park, Barcroft Park, Four Mile Run Park at Mount Vernon Avenue, as well as Lower Long Branch at Troy Park. Upper Four Mile Run sites have the next highest bacteria levels. These include Banneker Park, East Falls Church Park, Bluemont Park, and Upper Glencarlyn Park (upstream of Long Branch). These sites tend to exceed the secondary water quality standard at one or two of our monthly monitoring sessions a year, often during the warm season and in response to rainfall.
- The Potomac streams (Donaldson Run, Windy Run, Gulf Branch, and Little Pimmit Run) have the lowest bacteria levels, followed by the Four Mile Run tributaries (Lubber Run, Upper Long Branch, and Doctor’s Branch). These sites tend to meet the secondary water quality standard every month of the year.
For more detail about bacteria monitoring data, see Appendix M of our FY2018 Stormwater MS4 Report.
Overall, bacteria levels are trending downward in Arlington, but stream users should still always be careful to wash hands after playing in the stream.