A “green” rating demonstrates that the level of E. coli at the time of sampling falls beneath the Virginia Department of Environmental Quality primary contact water quality safety standards.

The VA Primary Contact Water Quality Standards are as follows:
• E. coli single sample value = 235 MPN/100mL
• E. coli geometric mean = 126 MPN/100mL

Potomac sites that are under VA or MD jurisdiction are rated against these Virginia values. We are utilizing the Virginia standards because they are more protective of public health for individuals swimming or otherwise in primary contact with the water than Maryland’s standards. For the first 5 consecutive weeks, single sample values will be compared against the single sample standard of 235 MPN. After 5 consecutive weeks, the geometric mean will be calculated and compared to the 126 MPN value to determine a pass or fail status. Samples are rated “green” if they fall below these values. Samples over the standard are considered "red" due to their failure to meet criteria, and the increased risk of illness.

The DC Primary Contact Water Quality Standards are as follows:
• E. coli single sample value = 410 MPN/100mL
• E. coli geometric mean = 126 MPN/100mL

Potomac sites that are under DC jurisdiction are rated against these values. As requested by the DC Department of Energy and Environment, site ratings are determined by the geometric mean value, not the single sample value. This value can only be calculated after five consecutive weeks of sampling. Prior to June 5, site ratings were determined by comparing single sample values to the geometric mean value of 126 to rate sites conservatively. Beginning on June 5th, site ratings will be determined by calculating the actual geometric mean and comparing it to 126 MPN value.

A red "special status" update indicates an abnormal situation such as a severe weather event, construction activity, or a combined sewage overflow (CSO) in the past 48 hours. This status means it is extremely likely there are unsafe levels of bacteria in the river, and it is recommended to be extra cautious if you are close to the river or come in contact with the water.

Potomac Riverkeeper Network staff, interns, and volunteers monitor water quality at several locations along the Potomac River in partnership with Alliance for the Chesapeake Bay and other non-profit organizations. The monitoring effort in Virginia and Maryland waters is generously supported by the Virginia Environmental Endowment, National Harbor Marina, Peterson Companies, SP Marine Management, and Hunting Creek Garden Club. Monitoring in District of Columbia waters is supported by the DC Department of Energy and Environment through grant #RFA 2018-1805-WQD-VWQM Project #1 to Anacostia Riverkeeper.

Monitoring occurs every week from May through September. Parameters to be monitored include E.Coli, total coliform, pH, temperature, and turbidity. Detailed monitoring results can be found through the Chesapeake Monitoring Cooperative's Data Explorer, https://cmc.vims.edu/#/home

The primary parameter of concern is E. coli, a fecal coliform. E. coli is measured in terms of the Most Probable Number /100 ml water and are indicators of fecal contamination by warm-blooded animals including birds and mammals. Sampling sites along the Potomac fall within several jurisdictions and monitoring programs. Each jurisdiction has defined their own water quality standards and each program uses its own method for determining a pass or fail site status.

Water quality standards as well as the methods used to determine pass or fail status vary widely across jurisdictions and monitoring programs. We encourage users to explore the sample data in addition to checking the swim guide status. These data can be viewed through the Chesapeake Monitoring Collaborative Data Explorer, https://cmc.vims.edu/#/home