Understanding the Science Behind the Marvel: Why is the Río Celeste so Blue?
Recently, PLOS ONE published the science behind the surprising coloration of the river in the photo above. The beautiful, cloudy cyan waters may look heavily photoshopped, but they’re quite real. This is the actual color of Río Celeste, one of Costa Rica’s more unusual natural wonders. While there are other known examples of waters in such striking hues, such as Japan’s Goshiki-numa lakes, the Río Celeste is the only known river whose color forms at the junction of two crystal-clear sources. And so the question asked for centuries is, what creates this sky-blue hue?
The photo below gives you a better idea of what happens when Quebrada Agria and Río Buenavista, the waterways that merge to form Río Celeste, meet. What you see here is called Teñidero or ‘dye point’, where the transparent water suddenly turns to turquoise.
Río Celeste, similar to other examples of brilliantly colored waters, is located in a volcanic region. Hypotheses surrounding the origin of this milky blue hue include everything from thermal bacteria to the suspension of colorful minerals (such as copper) throughout the water.
Minerals suspended in water that do not dissolve are sometimes called colloidal particles, and are known for their ability to reflect and scatter light. Currently, these particles are one of the reasons for other examples of jewel-toned waters, such as Yu-gama Crater Lake in Japan. To see if they were also the cause of the sky-blue coloring in this case, researchers collected water and sediment samples from Río Celeste and the two rivers that merge to form it, and then tested the physical (size and distribution) and chemical (pH) properties of each.
The authors used a special separation technique, called ion exchange chromatography, to find out if colored metallic minerals such as copper, nickel, or cobalt were abundant enough to cause the Río Celeste’s turquoise coloration.
It turns out that there were only minute amounts of colored metal ores found in the samples—certainly not enough to cause a whole river to turn blue! However, what the researchers did find is that Río Buenavista has significant concentrations of silicon, as does Quebrada Agria, the latter of which additionally contains sulfate, chloride, and calcium.
Previous studies have shown that colloidal silica (silicon and oxygen) particles between 100- 450 nanometers (nm) in diameter could cause the blue coloration of the water because they reflect light in aparticular way. In fact, if these particles are larger than 450 nm, they can create milky hues as well— the same colors and tones found in Río Celeste.
In the figure below parts a and b show a microscope image of the sediment found at the bottom of the Río Celeste; from this, researchers concluded that the pictured clusters were created by gradual buildup of smaller particles in the water. The spectrum below a and b indicates the makeup of this sediment is mostly aluminum, silicon and oxygen, with small amounts of sulfur and iron. Based on the light-scattering properties of colloidal silica, the authors hypothesized that the presence of aluminosilicate in the rivers could have a similar light scattering effect.
So where did these particles come from? It turns out Quebrada Agria’s waters contain aluminosilicate particles too small (< 10 nm) to have an effect. However, Río Buenavista contains aluminosilicates well distributed throughout, and while these are larger (184 nm), they are not concentrated enough to actually create a noticeable scattering of light.
The key to figuring out this puzzle was that, when testing the pH, researchers found that Quebrada Agria was fairly acidic (pH of 3.1, or about as acidic as an orange), and Río Buenavista was rather neutral with a pH of 6.8.
The researchers realized that when the acidic and neutral waters of the two rivers meet, their aluminosilicate particles clump together, resulting in enough particle concentration and size to scatter light and voilà – the Río Celeste in all its turquoise glory!
Citation: Castellón E, Martínez M, Madrigal-Carballo S, Arias ML, Vargas WE, et al. (2013) Scattering of Light by Colloidal Aluminosilicate Particles Produces the Unusual Sky-Blue Color of Río Celeste (Tenorio Volcano Complex, Costa Rica). PLoS ONE 8(9): e75165. doi:10.1371/journal.pone.0075165
Images: Figure 1, 2 and 3 from the article.
Keywords: river, water, aluminosilicate, clay, light scattering, Mie scattering, Costa Rica, Río Celeste, Goshiki-numa lakes, Teñidero, Quebrada Agria, Río Buenavista, tourism, tourist sites, waterfall, lagoon, Yellowstone, thermal pools, colloidal particle, ion exchange chromatography, Yu-gama Crater Lake, thermal bacteria, silica