Sulfonic acid, which can cause serious harm to human health, has leaked into the Rio Seco in Joinville, southern Brazil.
The substance – used to produce detergents and household cleaning materials – was released when a truck crashed, triggering a local state of emergency.
A local treatment station was closed on Monday morning to prevent the intake of contaminated water – potentially affecting dozens of neighbourhoods’ access to drinking water. Video on social media showed people stockpiling bottled water.