The Rio Celeste River in Costa Rica has baffled scientists for years, who mistakenly believed its stunning turquoise color was due to chemical reactions.
Aerial view of the Rio Celeste River. (Video: YouTube.)
Scientists in Costa Rica discovered the true cause of the unusual turquoise color of the Rio Celeste, a 14 km long river in Alajuela Province, only four years ago through water sample analysis, according to Oddity Central.
Prior to this, hypotheses about the turquoise color of the Rio Celeste had been proposed over the years by the research community, but no one could provide convincing evidence to explain this natural phenomenon.
Some scholars suggested that the river’s unusual color was due to high concentrations of copper, but testing showed no copper in the water. Other hypotheses concluded that the river’s color resulted from chemicals like calcium carbonate and sulfur. Everyone was so convinced by the chemical reaction theory that no one considered the possibility that the river’s color could be an optical illusion.
The turquoise color of the Rio Celeste River. (Photo: Facebook.)
A team of scientists from the University of Costa Rica and the National University collected water samples from both the Rio Celeste and its tributaries, Sour Creek (Quebrada Agria) and Good View River (Río Buena Vista). The first thing that caught the scientists’ attention was that while the water of the Rio Celeste was a striking turquoise, the samples obtained in test tubes were completely clear.
Sour Creek and Río Buena Vista, the two rivers that merge to form the Rio Celeste, also had clear water, and analyses indicated no unusual chemical reactions. Another notable point is that the Rio Celeste only appears turquoise in a 14 km stretch, after which the water turns clear again. For this reason, the eye-catching section of the river is also known as “The Stained Stretch” (El Teñidero).
The researchers began to consider the possibility of an optical illusion. They noticed a layer of white compounds covering all the rocks at the bottom of the Rio Celeste, while examining both tributaries. Sour Creek had very little white compound, nearly negligible, while the water in Río Buena Vista was abundant.
Through electron microscopy analysis at UCR’s laboratory, the research team identified this as a type of mineral containing aluminum, silicon, and oxygen, known as aluminosilicate. When suspended in water, this compound reflects sunlight, tricking the eye into perceiving the water as turquoise.
Sunlight contains the entire spectrum of light, similar to how we see all the colors in a rainbow. In any other river, light penetrates to a certain depth, and no specific color is deflected or reflected back to the surface, making the water appear clear. In contrast, the water of the Rio Celeste allows some sunlight to pass through and reflects a blue light spectrum. Consequently, the river appears turquoise to the observer.
However, another puzzling point is that the Río Buena Vista also has a significant amount of aluminosilicate, yet its water remains clear. Scientists believe the issue lies in the size of the aluminosilicate particles. After analyzing water samples from both rivers, the research team found that the particle size of aluminosilicate in the Río Buena Vista was 184 nanometers, while in the Rio Celeste it reached up to 566 nm.
“The large size causes the scattering of sunlight, with most of the process occurring in the blue region of the visible spectrum. This is why the water of the Rio Celeste appears turquoise. It is one of nature’s wonders, a river providing minerals of one size and another river providing an acidic environment that allows those particles to grow,” said Dr. Max Chavarría Vargas.