Bacteria Monitoring Program

• From April 2017 to April 2018, Stormwater Utility staff used the Level II Coliscan Easy-Gel method to measure the bacteria levels in streams across the City.
• As of April 2018, staff transitioned to using the IDEXX^® Colilert method. The Virginia Department of Environmental Quality (DEQ) recognizes the Colilert method as a Level III method.  Level III methods and associated data can be used by DEQ as if the DEQ collected and processed the samples.  
• Staff collects samples at 41 sites on the Roanoke River and its tributaries once per month, resulting in at least 492 samples collected annually.  
• As the program continues to evolve, sampling sites are added or adjusted based on new data in order to locate bacteria sources.

Preliminary Results:

Since there are many variables that affect the bacteria levels of the Roanoke River and its tributaries including: temperature, wildlife, sunlight, nutrients, turbidity, as well as nearby septic systems, potential sanitary sewer overflows, or illicit connections, it is premature to provide in-depth analysis of the data collected to date.  There are, however, observable seasonal variances, with higher bacteria in summer and fall and lower bacteria in winter, as well as higher bacteria after rainfall events.  Over time the program will establish a baseline of data to help prioritize source tracking and other remediation efforts.  

The Recreational Water Quality Standard set by the Virginia Department of Environmental Quality (DEQ) and the Virginia Department of Health (VDH) for E. coli is quantities under 235 CFU/ 100mL (colony forming units/ 100 milliliters).  For levels above 235 CFU/ 100mL, there likely is contamination by human fecal waste.

The bar graph below shows the median indicator bacteria levels for the first 15 months of the bacteria monitoring program.  The x-axis includes the sampled streams with the total number of samples taken in brackets after the stream name.  The number of sampling sites varies per watershed based on percentage of the watershed within the City’s jurisdictional boundaries, access, and other risk factors.  The y-axis represents the average fecal indicator bacteria (E. coli) levels and includes the red dashed-line depicting the DEQ/VDH threshold of a healthy stream at 235 CFUs/100ml.  While the bar graph illustrates the median or average bacteria levels, the vertical black dimension lines represent the percentage of bacteria levels above and below the 25% (bottom) and 75% (top) quartile. See a map of the different streams and watersheds in the City here. 

Next Steps:

1. Stormwater will continue to work with our colleagues at the Western Virginia Water Authority (WVWA) to track down and remove potential sewer related bacteria sources. This partnership has already resulted in successful improvements on the West Fork of Carvins Creek and on Ore Branch. WVWA is also constructing sanitary sewer upgrades which are expected to improve bacteria levels on Peters Creek and Lick Run. 
2. In 2019, Stormwater staff continues baseline monitoring in the Roanoke River and its tributaries, but also has began a source tracking program to determine potential sources of bacteria. Staff coordinates with the Hampton Roads Sanitation District to use their technology which utilizes DNA analysis to identify if the E.coli source is human or another source. This new technology allows for more advanced tracking and elimination of bacteria sources. Using this method, there have been three successful repairs made to remove human sourced bacteria sources from the City MS4 system in the Ore Branch, Roanoke River and Carvin Creek watersheds. Staff continues to track sources of bacteria with this method in Carvin Creek, Ore Branch, Murray Run, Roanoke River, and Horton Branch watersheds.