Sampling took place on Thursday, August 18th during dry weather conditions. Results can be found on the water quality data page and bacteria results are summarized on the map below.
Similar to July, bacteria results for August exceeded the boating and swimming limit of 235 most probable number (MPN) of E. coli for the majority our 41 sampling sites. Sites PQB036 in Canton and UNB002 in Milton were unable to be sampled due to these sites being completely dried up.
Sampling using the “direct fill” method
Although the Watershed received 0.02 inches of rain in the 72 hours leading up to this month’s sampling, sampling conditions are considered “wet” if there is greater than 0.05 inches of rain within 72 hours of sampling. During wet weather conditions, E. coli levels tend to be elevated due to stormwater runoff. However, we sampled during dry conditions and still observed unusually high E. coli results.
We are observing the opposite results than we would expect to see, and this phenomenon has been occurring all summer.
From left to right: sediment-free, medium-sediment, and heavy-sediment water samples
We hypothesized that sediment has been getting into the samples since water levels are so low, forcing volunteers to scrape the bottom of the stream in order to obtain samples. We conducted a short experiment to see if sediment in the CWMN samples could affect E. coli. Results showed a positive correlation between sediment and bacteria levels.
While this does complicate the water quality analysis of our samples, it does not change the fact that these areas will be above the swimmable limit of 235 MPN of E. coli, as it is likely sediment will be disturbed while swimming. E. coli is not harmful to humans but is an indicator that other, harmful bacteria may also be present.
As always, a big thank you to all of our volunteers!
We’ve been able to maintain water quality records throughout the watershed for over 20 years because of you. If you’re interested in becoming a Citizen’s Water Monitoring Network (CWMN) volunteer, please contact Meghan Rauber at meghan@neponset.org.
Meghan Rauber, Field Sampling Coordinator
September 2016
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