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?
Fiscal Year 2019 and FY 2013 – 2019 Average E. coli levels by site group. FMR is Four Mile Run. WQS is water quality standard. If E. coli levels are under the secondary water quality standard, the stream is considered safe for wading. The primary water quality standard is for swimming. Arlington County recommends limiting contact to wading and always washing hands after coming into contact with the stream.
Note the December 2018 peak in response to heavy rainfall.
These results are averages of E. coli levels from 4-6 sites in each group. The orange line is the secondary water quality standard (for touching the water). The teal line is the primary water quality standard (for swimming).
- 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.
- Four Mile Run Tributaries: Second lowest bacteria levels
- Sites include Lubber Run (3 sites), Upper Long Branch, Doctor’s Branch and Lower Long Branch.
- Potomac Tributaries: Lowest bacteria levels
- Sites include Donaldson Run, Windy Run, Gulf Branch, and Little Pimmit Run.
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.