Why is this river fluorescent green?

It's not Chicago, and it's not St. Patrick's Day. So why did someone turn this river bright green?

People living on the banks of the Valira River in Spain became worried Thursday when the waters began to turn a fluorescent green. But authorities said the dying of the river was planned.

A harmless dye had intentionally been added to the river in neighboring Andorra, where the river begins, the Andorran Ministry of Health said a statement, ABC News reports.

"It is an action that has no impact beyond the visual," the ministry said, adding that the substance was harmless to both people and the natural environment and would dissolve in a few hours.

Albert Batalla, the mayor of Seu d'Urgell, a town on the banks of the Valira, issued a statement assuring residents that the dye had been used as part of an investigation at the Arinsal water bottling plant, Spain's RadioSeu reports.

The plant was reportedly linked to a gastroenteritis outbreak in Catalonia last year that left thousands of people sick after they drank from contaminated office water coolers.