Why has this river in Spain turned a luminous green?

A river in Spain turned an extremely bright green on Thursday morning, leaving local residents somewhat alarmed.

The Valira River in north east Spain flows down the Pyrennees Mountains from the tiny nation of Andorra into Spain, where it empties into the Segre River near Seu d'Urgell.

See also: Mysterious ice circles appear in Washington river

See also: Find out why this Spanish river flows with red water

Albert Batalla, the mayor of Seu d'Urgell, was quick to reassure residents that the river had been intentionally dyed green with an "entirely harmless, non-toxic, and biodegradable" substance.

The USA Today reports that the dye was added as part of an investigation at a water bottling plant to identify the source of a possible contamination.

The Local reports that the water plant was last year traced to be the source of a gastroenteritis bug that affected thousands of people across Catalonia who drank contaminated water from office water coolers.

Andorra's Ministry of Health confirmed that the harmless dye had been used to help identify the source of possible contamination and would have no environmental impact. But it made for some great pictures.

Best river cruise lines (according to Cruise Critic)
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Best river cruise lines (according to Cruise Critic)
The Loire may be France's longest river but CroisiEurope's Loire Princess is the only cruise ship to travel its waters. The efficient dual-paddlewheel design means the ship is the first to have the chance to travel the Loire.
Partnerships with Disney and Backroads Tours has cemented AmaWaterways' place as the country's best river cruise line. The line is constantly expanding and is attracting new and younger passengers, as well as families. 
Sanctuary Retreats has river ships in plenty of exotic locations, including China, Egypt and Burma. Sanctuary also boast some of the largest and most luxurious cabins in the industry, the Ananda's cabins are more than 300 square foot in size and all have balconies.
AmaWaterways collects another award thanks to its brilliant dining experience. Guests can expect to enjoy quality ingredients in the specialty restaurants on board with a range of cultures represented on the menu as well. 
Tauck River Cruising takes the title for Enrichment this year with the line dedicated to creating "authentic, enriching travel experiences that enhance people's lives". The line offers family orientated experiences ranging from scavenger hunts to cooking demonstrations.
Uniworld has gained attention as a result of its easy blend of active excursions and behind the scene looks at the destination of the day. All Uniworld excursions are broken down by activity levels which means there really is something for everyone, no matter what your fitness level is.
Not only do AmaWaterways have bikes on board their ships, they've partnered with tour company Backroads to bring a series of road biking and hiking tours to their cruises. Although cycling tours of the Danube are what the company is currently offering, trips to the Mekong in Asia is what can be expected in 2016. 
Emerald Waterways has set itself apart from the rest by providing top quality experiences for great prices. Complimentary shore excursions, transfers and free beverages are just some things you can look forward to with Emerald.
CroisiEurope offers a number of unique itineraries that cruisers won't be able to find with other lines. The company built the Loire Princess specifically to navigate the twists and turns of the Loire River and the cruise line have the Elbe Princesse ready to go for 2016 too.

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