Did you know that August is (was) Water Quality Month?! NO?! Yeah, me neither. Oops! But we can still talk about it any day! After all, this is a more related topic to my MSc which I haven’t been talking about here for a while because, let’s face it, talking about snakes is just cooler (& my thesis procrastination marathon is just too good – ahem). But snakes need water! And so do we!
So, what is “water quality” anyway?!
I don’t believe there is an exact definition for this. Is “good” water quality the one that’s good for drinking? Good for making beer? Good for wildlife? Well, it is “all of that”. It is the quality needed for a specific purpose. Water quality is measured through many different physical, chemical, and biological variables, like temperature, dissolved oxygen, and algae concentration, to name a few. There are A LOT of details to unwrap from this, so we’ll leave a lot of them for another time. There’s this one thing that fascinates me though, and in my opinion is very underrated: the color of water. Where does that color come from? Can we judge a lake by its color?
I find most “basic science” fascinating. You know, those “basic” things we learn as kids like why the sky’s blue, why cookies grow, why plants look green. But we take it for granted when color is not as simple as we’d think, and I’ll use this opportunity to try to make you feel this awe for it too. Color involves so many basic concepts of physics and chemistry that I’m sure you learned at one point in life, and from so “simple” concepts (like the sun’s light and its wavelengths) so many possibilities arise.
Let’s remember the basics of color first before I shatter it down and tell you why it’s sort of wrong.
We know that the sky looks blue (most of the time) because particles in our atmosphere scatter blue light. But what does this really mean? High frequency (short wavelength) waves, like blue light, travel through the atmosphere with a higher chance of bumping into other particles. This “bumping into particles” SCATTERS (Rayleigh scattering) the light in different directions, and since this high-frequency blue light is bumping more often than other wavelengths, it is being “more scattered”, so we see the sky blue. It’s like all these particles in the atmosphere were just bombarding us with little specs of blue light.
On the contrary, low frequency (long wavelength) waves, like red light, have less chance of bumping into stuff, so they are not being scattered as much. They sort of just “pass through” the atmosphere. This is also why sunsets fall on the long-wavelength color (red): as the Earth rotates and you get further away from the sun, light must travel longer distances to reach you. The blue light will be scattered in the further-away-region from you, so most of it won’t reach our eyes, which is why we can only see the long wavelengths (red) that pass through that long distance almost uninterrupted.
This also explains why space looks black despite having so many shiny stars around: there’s no atmosphere to break down and scatter all that light.
But back to water. Is water colored?
Unlike the atmosphere (which is just “space” containing different molecules and particles), water is its own substance. (Yes, there’s other stuff mixed in it, but we’ll get to that later.) Contrary to what I was told as a kid (“water is colorless”), pure water, strictly H2O, has an intrinsic pale blue color. Now you might be thinking that when you fill a glass of water it’s just fully transparent and colorless and I’m probably lying here. But no, trust me, it’s not; it’s just a matter of volume and the distance light has to travel for enough light to be redirected back into your eyes. The bigger the volume, the “bluer” you’ll see. Try filling up your bathtub or sink – keep in mind the background color, of course; or imagine a transparent bottle of a colored detergent: when you pour a bit of it in your hand, the color is less intense than the way it looks inside the container.
You might remember that the colors we see are the ones reflected back from objects that absorb all the other colors. For example, we’re taught that plants absorb light wavelengths from most of the visible spectrum, except for green, so they “give us back” that green light that they cannot absorb. The absorption is done by electrons that vibrate in the same frequency of the incoming light. Reflection, which basically colors everything you can see, is caused by the free electrons that vibrate at different frequencies and send that light energy back. The electrons, you might recall, are in the outer part of an atom, outside its nucleus. So, based on all you know now you might have already concluded that water molecules absorb almost all colors except for blue. And you’re right! But it doesn’t happen the way we know color happens everywhere else.
Here comes the cool part.
Water molecules absorb red light the fastest and in a more efficient way than the atmospheric particles, reflecting also most of the blue spectrum. However, it’s not their electrons that absorb it! Water molecules constantly vibrate (you can picture it as if the oxygen atom is just trying to keep the hydrogen atoms in place as they try to escape, so this creates those vibrating movements). Besides these, the absorbed photons (energy from light) also promote vibrations of the water molecules at a nuclear level. All the vibrations involved here occur at low frequencies. Hence their red-spectrum-absorbance. The blue waves will both be reflected or pass through. Whatever passes through will then also be scattered by other molecules and particles in the water (Rayleigh scattering once again!). Now you might be a little disappointed and thinking that this is not really under the definition of “cool fact”. But you see, this is the ONLY molecule (that we know of) in which color is given by NUCLEAR interactions instead of electrons! (If you’re not going “WHAAAT? NO WAAAY!” right now, we cannot be friends). Basically, all the chemistry I learned throughout my life was shattered when I learned this fact and I just love it when that happens. Don’t you?
Okay, so this is just a fun chemistry-physics fact…what does that have to do with water quality?
You have probably seen already different lakes and rivers and how they don’t all look the same. Of course, some things are pretty evident, like huge algae mats that make a lake look green. But there are other shades of blue and green and other colors in lakes and rivers around the world and not all of them are due to algae, and not all colors mean there’s something wrong.
You now understand the true color of water. What we usually see is the apparent color of water. Let’s talk the obvious one first. We’ve already mentioned that algae reflect green light and the bigger number of algae in a lake, the greener it will appear to us. (There’s a lot more to this as in how algae blooms develop and when they can be considered “normal” or harmful, but I’ll leave that for another post.) Algae, as well as other microorganisms, also have different pigments other than the green chlorophyll and some species can make lakes appear pink or other “unusual” lake colors.
But it’s not only living organisms that give our waters their color
Back in Guatemala, we have a region around the central part of the country where the geology is very karstic. Where I’m currently living (Austria), most of the country is also karst. A lot of water bodies in these places look turquoise-light blue. Karst is formed from soluble rock material like limestone or dolomite, which have a lot of calcium. Calcium atoms vibrate at an orange-like frequency; they absorb orange wavelengths and reflect the complementary: turquoise-light blue. Color can also change periodically. For example, up here in spring when the glaciers start melting, the water flowing down goes eroding this limestone, thus carrying more calcium-rich water with tiny finely ground calcium-rich-rock particles that contribute to this turquoise color that gets stronger in spring/early summer.
The Amazon basin has different types of rivers. Some of the rivers in the higher parts of the basin are very clear blue waters (almost pure water with little material dissolved in it). However, you normally find pictures of the Amazon river in a lower part of the basin where it looks yellow-brown. Down here, the sedimentary rocks are rich in iron, sodium, silica, among other elements that absorb light on the smaller wavelengths (blue-green region) and reflect light from the yellow-red area (think about how iron makes our blood red too, for example). The river also carries a lot of organic material from the surrounding vegetation and other compounds which might make it look different from one section to another. Because of this the color of rivers also varies with the seasons. Whether you live where there are four seasons or down where we have rain/dry seasons, the rain will normally increase the water flow and erosion of the riverbanks which carry all these compounds that contribute to the color of water.
But there are other types of brown rivers and lakes that have nothing to do with transported sediments. Plants have some yellow-brown organic compounds called tannins, some of which are very soluble in water. Contrary to the sediment particles reflecting light, these tannins are dissolved in water and we can see through it even if the color is now less blue. You can think of this way of “brown waters” the same way your boiling water changes color with your tea. It’s still absorption and reflection processes, but different compounds and particle size.
Let’s go back to the color green
Green is not always caused by algae. In volcanic lakes that are influenced by subaquatic fumaroles in the lake bottoms (like lake Aso (Japan) or lake Quilotoa (Ecuador)), the turquoise component is usually given by sulfur and the green parts by dissolved iron ions. Iron? Green? But before I had said that iron reflects red just like our blood! Well, different states of the atoms, like with higher energy at higher temperatures, will vibrate at a different frequency, thus absorbing and reflecting different colors. Crazy, right? Colors are fascinating.
Keep in mind that the color you see is also a result of your angle of observation and the specular (mirror-like) reflections over water. When standing further away from a standing lake you might just see the reflected sky or surrounding trees, unless the water has a rougher surface (e.g. by waves).
It will come to no surprise to you now that climate change will also affect the color. With increasing temperature come changes in the rates of chemical reactions and biological activity. The electrons will vibrate at different frequencies, thus absorbing and reflecting different colors.
You can now look at a river or lake and based on the color make assumptions about its geology or the activities in the watershed. But how accurate can that be? Can color serve some other function for scientists and water quality assessments?
Poor water quality can pose a health risk for people and ecosystems, and that’s why scientific research and monitoring is important to understand the “original” state of the quality of an ecosystem and how it changes through time, either by a lake natural development or by different anthropogenic pressures. Some things can be difficult to measure or monitor, it could get too expensive, or simply for some countries, the overall lack of resources makes it impossible. One possible solution is to monitor water color through satellites (remote sensing), which is already being done in some lakes and oceans, but we still need to understand more about the interactions in our ecosystems to have better interpretations of this colorful mess.
#FunFact:
A hydrogen atom has one proton in its nucleus. Deuterium, a variant of a hydrogen atom (isotope), has one proton and one neutron. The molecule of D2O, called “heavy water”, is, as you might have already guessed it, heavier because of this extra neutron. This extra weight makes it more difficult for the molecule to shake it. It doesn’t impede the molecule to vibrate (remember the color-causing vibrations of H2O), but it does slow it down, so it vibrates at an even lower frequency that falls within the infrared spectrum and it lets all the visible spectrum go through, so this heavy water is actually colorless (for us humans at least).
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REFERENCES
- Dutkiewicz, S., Hickman, A.E., Jahn, O., Henson, S., Beaulieu, C. & Monier, E. 2019. Ocean colour signature of climate change. Nature Communications. 10(578).
- Aguilera, E., Chiodini, G., Cioni, R., Guidi, M., Marini, L. & Raco, B. 2000. Water Chemistry of Lake Quilotoa (Ecuador) and assessment of Natural Hazards. Journal of Volcanology and Geothermal Research. 97:271-285
- Klaveness, D. 2005. Photography in Limnology: Documentation of Lake Color Using a CCD Camera. Limnology 6:131-136
- Ohsawa, S., Salto, T., Yoshikawa, S., Mawatari, H., Yamada, M., Amita, K., Takamatsu, N., Sudo, Y. & Kagiyama, T. 2010. Color Change of Lake Water at the Active Crater Lake of Aso Volcano, Yudamari, Japan: is it in Response to Change in Water Quality Induced by Volcanic Activity? Limnology 11(2010):207-215
- Nibbering, E.T.J. et al. 2007. Vibrational Dynamics of Hydrogen Bonds. In: Analysis and Control of Ultrafast Photoinduced Reactions. Springer Series in Chemical Physics 87:619-687.
- Quesada, C.A., et al. 2011. Soils of Amazonia with particular reference to the RAINFOR sites. Biogeosciences. 8:1415-1440