A canal in a suburb of Argentina’s capital Buenos Aires turned bright red on Thursday, alarming local residents.
Pictures and videos show the intensely coloured water flowing into an estuary, the Rio de la Plata, which borders an ecological reserve.
Local media reports suggest the colour may have been caused by the dumping of textile dye, or by chemical waste from a nearby depot.
The Environment Ministry said in a statement that water samples had been taken from the Sarandí canal to determine the cause of the colour change.
By late afternoon the colour of the water had lost some of its intensity, the AFP news agency reported.
Residents have claimed that many local companies dispose of toxic waste in the waterway, which runs through an area of leather processing and textile factories some 10km (6 miles) from the centre of the capital.
A resident, a woman called Silvia, told local news channel C5N that although it is has turned red now, “other times it was yellow, with an acidic smell that makes us sick even in the throat”.
“I live a block from the stream. Today, it has no smell. There are not many factories in the area, although there are warehouses.”
Another resident, Maria Ducomls, told AFP industries in the region dump waste in the water, and said she had seen it coloured differently in the past – “bluish, a little green, pink, a little lilac, with grease on top”.
