Escherichia coli (E. coli) is a bacteria that indicates fecal contamination and potential human health risks for swimming or other recreation in the water. E. coli present in the Sudbury, Assabet, and Concord rivers would indicate the presence of fecal matter from birds, rodents, pets, livestock, humans, and other warm-blooded animals. Under the Clean Water Act, the rivers should be clean enough for “recreation in and on the water” (Class B). In the United States, most swimming-associated diseases are caused by a variety of pathogens associated with fecal matter. Human fecal matter can enter river water from malfunctioning municipal wastewater treatment, failure of septic systems, illicit cross-connections between stormwater and wastewater lines, combined sewer overflows, discharge of sewage by boats, and stormwater runoff from impervious surfaces.
OARS started monitoring the rivers for E. coli in 2019, so this indicator was not included in the 2018 report card. One site in each river section is now monitored every two weeks from May to September. The Bacteria score is calculated as a swimming compliance rate, which represents the percent of samples that meet the EPA’s swimming Beach Action Value (< 235 cfu/100ml). The report card score is the average compliance rate for the previous five years. This compliance rate is similar to the EPA’s scoring method for the Charles River and Mystic River report cards, but it differs in that the swimming compliance is not averaged with a less stringent boating compliance rate. OARS chose to use only swimming compliance because our goal is to make the rivers swimmable. This means that SuAsCo bacteria grades will, by definition, be lower for similar water quality conditions in the Charles and Mystic rivers.