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Massachusetts Water Resources Authority



July 18, 2017

June 27, 2017


Ria Convery
(617) 788-11105,

Chestnut Hill Reservoir Algae Bloom

Update July 18, 2017

Since the cyanobacteria (or “Blue Green Algae”) bloom was detected by MWRA staff on June 26,th we have performed numerous tests on the water and communicated with our partners at Department of Environmental Protection and Department of Public Health. Since initial test results were above thresholds for water contact, we installed the signs around the reservoir warning of the ongoing cyanobacteria bloom. Recent test results have shown the bloom has subsided and the algae cell counts are now well below the thresholds. Therefore, the signs have now come down. Additionally, even better news is that testing by independent laboratories over the last two weeks has shown that the toxins associated with these algae were never detected.

What’s next? We had MWRA and consultant teams on the reservoir yesterday to collect more water and sediment samples, and also to perform an invasive plant survey. In the water samples, we are especially interested in the nutrient phosphorus which is the major nutrient in algae blooms. If we find that the phosphorus is high, we will consider performing an alum treatment as was done in 2015 following a smaller algae bloom. As I noted previously, alum is a common drinking water treatment chemical. In an open water situation such as a reservoir treatment, the alum strips the phosphorus from the water and binds it in the sediment, making it unavailable for the algae. An alum treatment could be done within the next two weeks if necessary.

 We are also looking into the sources of nutrients at the reservoir, such as the local Canada geese population. On any given day, 50 to 100 geese can be seen in and around the reservoir. A simple walking inspection along the reservoir perimeter path shows large amounts of goose droppings, which are known to be high in phosphorus and other nutrients. I should also point out that there is no other major source of nutrients getting into the reservoir. The reservoir has a very small tributary area with no storm drains or road run-off getting into the water. We have discussed control options with our partners at DCR: this is still developing. One short-term action is the installation of signs about the geese issues and warnings against feeding them. You will likely see these go up soon. 

As for the invasive plants, since we did not do our usual 10-foot winter drawdown due to last year’s drought, it’s likely that the Eurasian milfoil has increased to a level where we will need to perform a harvest operation later this summer. Now that the drought is over, we will plan to return to our winter drawdown starting in November to naturally control the invasive plants.

Update June 30, 2017, 9:30 AM

An algae bloom is ongoing at the Chestnut Hill Reservoir. This reservoir is part of MWRA’s back-up, emergency water supply and is not in use.
Testing has identified the algae as cyanobacteria. No toxins have been detected.
However, this type of algae could turn toxic. After discussion with Massachusetts Department of Public Health, we have posted the reservoir with signs saying: "No fishing, no swimming, keep animals away and rinse after contact with the water, as a precautionary measure. These activities are already prohibited because this is a back-up, emergency water supply."
Cynobacteria has been appearing in other locations throughout the state including nearby in the Charles River Basin, Mystic River and Jamaica Pond.  
We will continue to monitor the algae bloom and will post updates on our web site as warranted.

June 27, 2017

You may have noticed that a reservoir-wide algae bloom is underway that has turned the Chestnut Hill Reservoir green and is giving off a faint musty smell. MWRA has taken water quality samples that are being analyzed at the lab. This is an emergency back-up reservoir and is not part of the active water supply system.


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Updated July 18, 2017