Through steamy heat, misty drizzle, and perfect summer mornings, the NRWA’s Weekly Swimming Hole Monitoring team headed out to local swimming holes to test the bacterial levels in the water in help inform water enthusiasts about the safety of taking a dip. Ten sites, six waterbodies, five communities, 30 volunteers, and156 samples collected over 13 weeks — not a single site or sample missed all season. We know scientific data, collected and analyzed with professional oversight, can help ensure the safe enjoyment of our waterways.
What data is collected and how is it used?
The water samples collected each week are used to determine the levels of E. coli bacteria in the water. E. coli is a bacteria species that, when present in water at high levels, may cause illness making our waterways unsafe for swimming. Once samples are analyzed and data is reviewed for quality control, the NRWA posts its Weekly Swimming Hole Report Card online for public viewing. NRWA also shares its findings with the Boards of Health in appropriate communities, particularly drawing attention to sites where E. coli levels are elevated to an unsafe level for recreation, encouraging the them to share that information with their community and work with the NRWA to seek answers to why the levels are high.
What did we learn in the 2026 season?
A summary of the June through August 2026 swimming season:
- Overall, E. coli results showed more frequent unsafe swimming conditions than in recent seasons. Adams Dam continued to be a site of concern, with unsafe E. coli levels recorded during 6 of the 13 sampling rounds.
- NRWA continues investigating potential bacteria sources at Adams Dam and Still River Depot through follow-up sampling, eDNA source-tracking, and groundwater testing. If specific sources are identified, we will share the findings with the public and local Boards of Health.
- Volunteers also helped document drought conditions by taking photos at monitoring sites throughout the watershed. These observations will support the Drought Management Task Force (DMTF) and strengthen NRWA’s drought outreach and documentation efforts.
View the 2026 Weekly Swimming Hole Report Card which includes a chart of the data collected, map of sites, and summary of findings.
Thank you to our volunteer team!
A successful season like this doesn’t happen without dedicated people willing to show up, week after week.
To every volunteer who hiked out to their sampling site, exercised scientific methods to collect their water samples, recorded on-site observations, transported their samples to the NRWA, and processed samples and data — all to keep the program running this summer — THANK YOU!
Every site. Every week. Every sample collected. Dedication to protecting our waterways and our communities.
Support for the NRWA Weekly Swimming Hole Monitoring Program is provided by the Greater Lowell Community Foundation, Fieldstone Foundation Inc., Bristol Myers Squibb, and the Massachusetts Department of Environmental Protection.