A sealed bottle of hydrochloric acid was dropped into a tank leading to the Mount Tabor water reservoir.
It happened in May, according to the Portland Water Bureau.
The agency said a traffic cone and a plastic hose were also dropped into the 200,000-gallon tank. Each item was taken out of a storage shed by vandals.
Bureau workers said the bottle of acid was still sealed and never contaminated the water.
Tests of the water came back negative, but crews still disinfected the tank.
The incident was reported to the state, but since the water bureau said no one was ever in danger, no announcement was made at the time.
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