Bacteria and Health Safety

Is the river safe for boating or swimming? OARS is attempting to answer this with our E. coli bacteria monitoring 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 May and September, OARS volunteers collect river water samples bi-weekly at 6 locations throughout the river system. The samples are analyzed in a lab and E. coli bacteria counts are recorded as Most Probable Number (MPN) of colony forming units (cfu). The EPA has established swimming safety thresholds of 235 cfu/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 to track down and eliminate. See the attached Bacteria Monitoring Summary, below, for more details.

This map below shows our sampling locations with color-coded 2019-2023 results. For more detailed results, check out the data and charts from 2023.

Download OARS' 2023 Bacteria Monitoring Summary

Volunteers have also helped us conduct special studies for source-tracking bacteria in locations of concern. Between 2020 and 2022, we conducted an intensive study in Lowell to track sources of bacteria pollution in the lower Concord River. In 2023, we conducted a study in Maynard to track sources of bacteria pollution in the Assabet River in Maynard. In 2024, we plan to conduct a similar study in Ashland to identify sources there.


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