Blood-red waters that filled a winding waterway near the Argentine capital, Buenos Aires, raised a stink as images circulated on social media on Friday.
The area is home to tanneries and other industries that process animal hides into leather using chemicals, but along part of its banks are numerous homes and an ecological reserve.
Photos and footage raised fears that industrial chemicals had been dumped into the Sarandi Stream, which flows into the Plate River on the city's southern outskirts.
Officials in the Avellaneda municipality, some 15 kilometers (9 miles) south of the Argentine capital, said they suspected the presence of aniline, a toxic substance used in medicines and dyes.
'The smell woke us up'
The river looks like "a river of blood," resident Maraa Ducomls told the AFP news agency. "The smell woke us up. In the daytime, when we looked at this side of the river, it was completely red, all stained."
The Environment Ministry for Buenos Aires province said it had taken samples from the river to determine which substance caused the water to turn red. The ministry said the coloration could have been caused by "organic" substances.
However, local resident Ducomls said that the river had already appeared "bluish, greenish, pink, purplish, with grease on top that looks like oil" in the past.
"It's terrible, you don't have to be an inspector to see how much pollution the poor Sarandi River suffers from," he said.
