What makes a river sing the blues?

 

 

Articolo tratto dal The Guardian

 

The Rio Celeste in Costa Rica becomes an enchanting celestial blue at the convergence of two tributaries. Until recently scientists have been at a loss to explain this mysterious phenomenon.

 

 

On a recent trip to Costa Rica, I encountered a challenge from a local guide who took us on a hike through the rain forest to a magnificent sky-blue river, called the Rio Celeste.

Our guide took us to the origin of this 12 mile-long celestial colored waterway, which was marked by the convergence of two tributaries – each of which exhibited the usual transparent color that one comes to expect from water. He posed his question: how is it that after the two tributaries merge into Rio Celeste, the river has this remarkable blue color?

As a scientist, although admittedly way out of my comfort zone, I struggled for a rational answer. “Copper or copper sulfate – there must be some in the water – that might give it a blue hue.”

Our guide was polite. “Well, that was one of the very first ideas that scientists had, and they disproved it in 30 seconds, on site, without any fancy equipment.”

The null hypothesis. As it turned out, scooping some of the river’s water into a glass container immediately showed that the displaced water had lost its sky-blue color and appeared transparent again!

After a long discussion, our guide maintained that this was a purely optical phenomenon, caused by the scattering of light – if you will, an optical illusion of sorts. But that did not explain why the color was only visible from the point of convergence of the two tributaries, known as the “Dye Point.”

Intrigued, upon returning home and enjoying the many photos of this beautiful river, I began to look for a more precise explanation. It seems that the mystery was initially examined by researchers from Italy and more recently at the Universidad de Costa Rica and Universidad Nacional, and a plausible solution has been published in a paper in the journal PLoS ONE.

In the PLoS ONE paper, which you can read for free, Castellon and colleagues first demonstrated that a particulate mineral, Al2SiO5, made of aluminum (33.84%), silicon (10.71%) and oxygen (48.05%), was abundant in Rio Celeste, and the particles had a mean diameter of 566 nanometers. These particles were then identified in both tributaries, but were tiny (less than 10 nanometers in diameter) in Agria Quebrada, and relatively small (on average 184 nanometers in diameter) in Rio Buena Vista.

As it turns out, computational modeling experiments demonstrated that the larger Al2SiO5 particles, found only at the convergence of the two tributaries and downstream in the Rio Celeste, scatter light in the blue spectrum, whereas the smaller minerals do not.

So far, so good. But then why are the larger Al2SiO5 particles found exclusively once the two tributaries have merged?

One of the interesting features of the point of convergence is that one of the rivers, the Quebrada Agria (which flows from a nearby volcano), is quite acidic, with a pH approaching that of a lemon. The Rio Buena Vista, on the other hand, has a nearly neutral pH. But how, if at all, do these facts figure into the blue color?

It’s all in the chemistry. At the “Dye Point”, the merged waters have a mildly acidic pH that apparently facilitates the addition of protons (positively charged hydrogen atoms) to the Al2SiO5 particles, causing the generation of chemically reactive hydroxyl (oxygen-hydrogen) groups at the surface of the particles. As a result, the particles become almost neutral, losing their repulsive negative charges, and they rapidly agglomerate and sediment to the bottom of the river-bed, where theirincreased size scatter light in the sky-blue spectrum.

The natural world is full of little scientific mysteries, and having the ability to understand them only increases their beauty.