The only answer is “We don’t really know,” but as Reuters explains, that isn’t really good news however you look at it:
The lakes, located near an industrial park on the outskirts of the Argentine city of Trelew, sprawl across a dusty, desert-like plain and are largely undeveloped. Officials with the municipality of Trelew recently uncovered a truck dumping waste in the watershed, according to posts made by the city on social media.
Local activist Pablo Lada, a member of Argentina’s National Ecological Network (RENACE), told Reuters in an interview that the pink color could potentially be the result of a dye typically used to give prawns raised nearby their typically rose-colored hue.
“I think that the pink lagoon uncovered a …lack of treatment of this waste that has become a big problem,” Lada said.