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Massachusetts Water Resources Authority


July 29, 2021


Ria Convery
(617) 788-1105,

MWRA Treating Chestnut Hill Reservoir to Prevent Blue-Green Algae Bloom

affected area

Beginning on August 2, 2021, the Massachusetts Water Resources Authority and its consultant, Solitude Lake Management, will perform a High Dose Alum Treatment of Chestnut Hill Reservoir. This work is expected to last all week.

Alum is a common, non-toxic drinking water treatment chemical used to clarify water. Alum is not harmful to fish and wildlife. It will be used to control phosphorus release from the sediment. Phosphorus is a key nutrient that causes harmful algae blooms. Phosphorus can come from several sources including wildlife (ex: goose droppings) and plant decay. It can build up in the sediment over time and be released back into the water column.

At the Chestnut Hill Reservoir, the algae of concern is cyanobacteria (or “blue-green algae”). These blooms are of great concern to water suppliers due to their potential to produce toxins which are not readily treatable through water disinfection, and which can affect human health, Cyanobacteria can be harmful to animals, such as dogs.

For long-term prevention of cyanobacteria at Chestnut Hill Reservoir, alum will be injected by boat below the water surface in zones 15-feet deep and greater where it will settle on the bottom to inactivate the phosphorus release from the sediment. This work will be closely monitored.

Visitors to the reservoir will notice a lot of activity over the week, including a crane to load the treatment boat, several tanker deliveries, spill response equipment and staff.


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Posted July 16, 2021