The sky-blue waters of Rio Celeste are completely natural.
How is this possible when the two contributing streams of water are completely clear?!
In 2012, Costa Rican scientists brought their sampling tools and chemistry kits to the site of the confluence to get to the bottom of this mystery.
In the photograph on the right, you can see the confluence, where two streams come together and all the magic happens!!
From physiochemical testing both the tributaries and Rio Celeste, the researchers were able to determine key differences in the streams.
The one tributary, aptly named Quebrada Agria (or Sour Rag), is extremely acidic. The other tributary, Rio Buenavista, is neither acidic nor basic but contains aluminosilicate particles. When when the two waterways mix together at the confluence, turning into Rio Celeste, the acidity causes the particles to aggregate and the bright blue coloration appears!
. . .
Lets get technical.
Río Buenavista (aluminosilicate particles = 184nm and pH = 6.8)
+ Quebrada Agria (pH = 3.1)
= Rio Celeste (aluminosilicate particles = 570nm because pH = 5)
= vibrant sky-blue color!
The blue color comes from the light scattering caused by the suspended mineral particles in the water. The optical phenomenon is the same as that which causes rainbows in the sky! It's called Mie Scattering.
Want more detail? Check out the peer-reviewed paper yourself: < http://dx.doi.org/10.1371/journal.pone.0075165 >
Erick Castellón, María Martínez, Sergio Madrigal-Carballo, María Laura Arias, William E. Vargas & Max Chavarría. Scattering of light by colloidal aluminosilicate particles produces the unusual sky-blue color of Rio-Celeste (Tenorio Volcano Complex, Costa Rica). PLos One (2013) 8 (9):e75165. < http://dx.doi.org/10.1371/journal.pone.0075165 >