Color of water

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Water Portal

A common misconception concerning the color of water in large bodies, such as the ocean, is that the blue is due to the reflections from the sky on its surface. This is not true, but was believed to be so decades ago. The real reason the ocean is blue is because the water, pure water, is blue. Yes, according to its frequency spectra, water is a very light shade of turquoise blue. But you need a huge amount of it to really see its color. It’s like a teaspoon of oil, it looks transparent on a white spoon, but in the bottle looks yellowish.

If the ocean owed its color to the sky, it would be a lighter shade of blue and it would be white on cloudy days. You can see clouds reflected in the surface on the sea, but they don’t completely change its color. Some constituents of sea water can influence the shade of blue you see in the ocean. This is why it can look greener or bluer in different areas. Swimming pools with white bottom, would have water that look transparent not turquoise blue, as it is observed even in indoor pools, where there’s no sky to be reflected. The scientific explanation involves the theory of radiative transfer (absorption and scattering), and material electromagnetic spectra.

[edit] Particular matter

The color of a water sample is caused by both dissolved and particulate material in water, and is measured in Hazen Units (HU). Either of these components can be deeply colored, for instance dissolved organic molecules called tannins can result in dark brown colors, or algae floating in the water ("particles") can impart a green color. But in a lot of cases water is a clear to neutral color due to a lack of pigments in the water. (e.g. the sea)

The color of a water sample can be reported as:

Apparent color
True color

Apparent color is the color of the whole water sample, and consists of color due to both dissolved and suspended components.

True color is measured by filtering the water sample to remove all suspended material, and measuring the color of the filtered water, which represents color due to dissolved components.

Testing for color can be a quick and easy test which often reflects the amount of organic material in the water (although certain inorganic components like iron or manganese can also impart color).

[edit] Water quality and color

The presence of color in water does not necessarily indicate that the water is not potable. Color-causing substances such as tannins may be harmless.

Color is not removed by typical water filters; however, slow sand filters can remove color, and the use of coagulants may also succeed in trapping the color-causing compounds within the resulting precipitate.

Other factors can affect the color we see:

Particles and solutes can absorb light, as in tea or coffee. Green algae in rivers and streams often lend a blue-green color. The red sea has occasional blooms of red Trichodesmium erythraeum algae.
Particles in water can scatter light. The Colorado river is often muddy red because of suspended reddish silt in the water. Some mountain lakes and streams with finely ground rock, such as glacial flour, are turquoise. Light scattering by suspended matter is required in order that the blue light produced by water's absorption can return to the surface and be observed. Such scattering can also shift the spectrum of the emerging photons toward the green, a color often seen when water laden with suspended particles is observed.
The surface of seas and lakes often reflect blue skylight, making them appear bluer. The relative contribution of reflected skylight and the light scattered back from the depths is strongly dependent on observation angle.