Easthampton leaders posted the warning on the city's Facebook page after tests showed possible signs of the toxic algae cyanobacteria.
The Massachusetts Department of Public Health will continue to conduct tests until the bloom has gone. Until then, residents should avoid swimming, fishing, or touching the water.
Contact with the infected water can cause skin irritation, gastrointestinal distress, liver or neurological damage, and could be fatal to pets or young children.
Anyone who comes into contact with it should immediately wash it off with clean water and contact their doctor, health officials said.
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