Residents were in shock as the river Valira that passes through Andorra and Catalunya from the Pyrennes, turned fluorescent green earlier this week.
People feared the green colour was a result of deliberate poisoning.
The river Valira was the source of a gastroenteritis outbreak last year.
The government of Andorra released a statement clarifying that the green color was not toxic and merely due to a “harmless, non-toxic and bio-degradable dye that has been used for water research”
“The research work will last for the next few weeks, but no longer have to pour more dyes, and therefore should not be returned to produce any new color to the river”, said the statement.
The Ministry of Health was performing research to detect what the cause of last year’s pollution could have been.