Bacteria and Health Safety

Is the river safe for boating or swimming? OARS is attempting to answer this with its E. coli bacteriamonitoring program. Bacteria and other pathogens can enter the rivers from leaking sewers or septic systems, storm-water runoff, or wildlife or dog waste. The EPA has linked high levels of E. coli (an indicator bacteria) in swimming water to high rates of gastro-intestinal illness.

Between June and September, OARS volunteers collect river water samples weekly at 6 locations throughout the river system. In 2020, volunteers collected additional samples in Lowell to track sources of bacteria of bacteria pollution in the lower Concord River. Samples were analyzed in a lab and E. coli bacteria counts are recorded as Colony Forming Units. The EPA has established swimming safety thresholds of 235 colony-forming-units/100mL for single samples or 126 cfu/100mL for seasonal geo-mean averages. We hope to use our sampling results to identify which sections of the river are safe for swimming and to identify pollution sources that need to be addressed.

The below map shows our sampling locations with color-coded 2019-2020 results. For more detailed results, check out the data and charts from 2019 and 2020.

We need volunteers, especially those who live near Ashland, Hudson, or Lowell, to help us collect samples in 2020. If you are interested, please contact

This work was possible because of grants from the Mass Department of Environmental Protection, Greater Lowell Community Foundation, Wild & Scenic River Stewardship Council, and donations from OARS members.