The results for the June CWMN event are in, with much higher E. coli levels than were observed in May. Before you get too concerned, remember that bacteria levels in wet weather are always higher than levels in dry weather. The May CWMN event took place on the 5th of five consecutive days of dry weather, while the June CWMN took place on a dry day that followed 4 days of rain. This amount of precipitation had an expected effect on increasing the E. coli levels throughout the watershed on the day of sampling for June.
During rain events, stormwater runoff is generated and flows over impervious services before it reaches storm drains and eventually waterways. As it flows over pavement, it picks up a lot of contaminants along the way. Among these is pet waste, which contains a lot of E. coli. The bacteria spikes on wet weather sampling days are in response to this influx of runoff.
Massachusetts State water quality standards for bacteria in class B waters states that no single sample can exceed 235 cfu (colony-forming units) of E. coli and be considered safe for swimming. This month, 28 out of the 41 CWMN site samples exceeded this amount. The cleaner sites were at closer to the headwaters farther upstream in the Watershed. The sites with the most bacteria present were NER150, NER185, and NER125 – all on the mainstream Neponset. Since these downstream sites intercept flow from other sites further upstream, it makes sense that the bacteria levels here would be amplified.
The July CWMN event also happened during wet weather, so we can expect elevated E. coli levels to be present in those samples as well. Once we receive the results from the MWRA lab, we’ll share them with you.
We are currently in the process of transitioning to a new database system for all of our CWMN results, which is taking a bit longer than expected. With luck, we should be able to share more in depth results with you all soon.
If you have any questions, please contact our Environmental Science Fellow, Annie O’Connell, o’firstname.lastname@example.org or 781-575-0354 x306.