A river in a suburb of Buenos Aires, Argentina, has turned red, alarming residents.

Footage and images of the Sarandí watercourse, which flows into the Río de la Plata estuary, show the sudden change in color.

Local media reports indicate this is not the first time the river has changed color, raising concerns about contamination.

A lifelong resident told La Nación that the river has previously turned grey, green, violet, bluish, and very brown.

Another local told Página 12 that on some days, a thick yellow layer formed on the water’s surface, which became hard and emitted an acidic smell.

Officials have collected samples of the red water for testing, Sky News has reported.

The region’s environment ministry told La Nación that the samples will be analyzed to determine the cause of the discoloration.

Written by B.C. Begley