Bacteria Monitoring Results 2019 - 2020

The following table summarizes results for both 2019 and 2020 at our six main monitoring sites. Cells are shaded pink if they exceeded the single-sample threshold more than 25% of the time or if they exceeded the geo-mean threshold for the year.

The following charts show the full set of weekly results for 2020, organized by river system. They also show precipitation from Bedford and Worcester for reference, because bacteria levels are often linked to surface or stormwater runoff. The y-axis in the charts is in a logarithmic scale, because bacteria multiply exponentially.

Bacteria levels have generally been higher this year (2020) than last year because more sampling events have closely followed rain, and at the same time we have been in drought conditions for most of the season, which enhanced the surface runoff when rain did happen. Four of the original six sites exceeded the swimming threshold at least 25% of the time and also exceeded the geometric mean threshold for the season. CND-110 and SUD-096 have stayed within limits. The Maynard site has been completely different than last year, with 67% exceedances in 2020. The Rt. 225 site is proving to be very clean over both years.

The effects of rainfall have been interesting to observe this year. On June 22nd, there was a major bacteria spike in Lowell, even though Bedford rainfall was quite small. It turned out that there were some very localized storms on the previous day that seem to have skipped Bedford and hit Lowell. On August 3rd and 24th, the majority of the rain was in the upper watershed only. September 14th was interesting because there were clear jumps in bacteria in the Concord and Lower Sudbury but no rainfall in the previous 48 hours. It turned out that there was a major rain event 4 days prior. It seems to have already washed out of the Assabet and Upper Sudbury, but was still registering in the lower watershed.