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From Gifu! Hot Spring Museum Episode 13: Various Colors of Hot Springs

The color of hot spring water varies. It can be colorless and transparent, milky white, gray, black, brown, red, blue, green, and so on. There are also cases where the color changes depending on the day, even in the same hot spring. Why do hot springs have so many different colors?

(1) Mechanism of Hot Spring Colors

Water, which should be colorless and transparent, appears green in deep rivers or blue when viewed from the beach. The color of water is caused by phenomena such as absorption and scattering of light from the sun.
The color of hot springs is also fundamentally related to this mechanism. The ions and other substances contained in the hot spring absorb light and present color, or the presence of very small particles called colloidal particles scatters light and presents color. Furthermore, relatively large substances (such as volcanic mud) or precipitates (so-called "hot spring flowers") can cause turbidity and color.
Of the sunlight, the part visible to our eyes is called visible light, with wavelengths between 380nm and 780nm.

Of the visible light, shorter wavelengths appear purple to blue, and longer wavelengths appear red. When light of all wavelengths of visible light is mixed with the same intensity, it appears white. If there are particles in the hot spring that scatter only the blue wavelength of light, other longer wavelengths of light pass through, and only the blue wavelength of light is scattered (Rayleigh scattering) and reaches our eyes. If there are large particles in the hot spring that scatter light of all wavelengths, all wavelengths of light are scattered (Mie scattering), and it appears white.
Also, if a specific color of light is absorbed in the hot spring, the hot spring appears to be the color of the remaining mixed light.

(2) Milky White Hot Springs

Most milky white hot springs are colorless and transparent when they first spring from the ground. After emerging, they change to milky white upon contact with air. This is because hydrogen sulfide contained as a component of the hot spring is oxidized by oxygen in the air, producing sulfur in the form of sulfur colloids, which are insoluble in water. As a result, sunlight is scattered by the fine particles of sulfur colloids, and all wavelengths of light are scattered, reaching our eyes and appearing white.
The chemical reaction formula for the generation of solid sulfur from hydrogen sulfide is as follows.

Even milky white hot springs may not be sufficiently cloudy when the bathwater is replaced and fresh hot spring water (source) is just added.

Hirayu Onsen "Hirayunomori" which can become milky white	Milky white hot spring (Akita Prefecture Nyuto Onsen)

(3) Blue Hot Springs

The mechanism by which some blue hot springs, which contain a lot of silicic acid, present blue color has been clarified by a research group led by Dr. Osawa of Kyoto University.
The blue hot springs they investigated are colorless and transparent when they first spring out, turn blue after 2-3 days, and change to milky white about a week later.
As small particles of silicic acid undergo dehydration condensation reactions over time and grow larger, sunlight causes Rayleigh scattering, scattering only the short-wavelength blue light, which reaches our eyes and makes the hot spring appear blue. After some time, as the silicic acid particles continue to grow through repeated dehydration condensation, sunlight causes Mie scattering, scattering light of all wavelengths, and white light reaches our eyes, making the hot spring appear milky white.
There are other blue hot springs, but the mechanism of color development is not well understood for some.

Blue hot spring (Oita Prefecture Beppu Ichinoide Onsen)

(4) Brown Hot Springs

Most brown turbid hot springs are iron-containing springs. They are colorless and transparent when they first spring from the ground, but when the hot spring emerges on the surface, it comes into contact with oxygen in the air, oxidizing the iron dissolved as ions and producing brown precipitates. This suspension of precipitates causes the brown turbidity.
Since hot springs often contain iron, even a small amount of iron can turn brown upon oxidation. Therefore, there are many hot springs nationwide with a faint brownish color.

Brown hot spring (Gero City Shimajima Onsen)	Brown hot spring (Gifu City Mitadojinbutsu Onsen)

(5) Black Hot Springs

The black hot springs, commonly seen around Tokyo and called "Kuroyu," are extracted from strata containing ancient humic substances and contain humic acid of humic origin. Humic acid is the final product formed when plants are decomposed by microorganisms, dissolving in alkali and precipitating in acid in the soil.
When humic acid is present in the hot spring, it absorbs visible light, making it appear black.
In the Tokachi region of Hokkaido, there is a transparent black-brown hot spring commonly known as "Moor Spring." "Moor" is derived from the German word "Moor" (peat soil), and like the black hot springs around Tokyo, it is caused by the absorption of light by humic acid contained in the hot spring.
Additionally, in sulfur springs of the hydrogen sulfide type, hydrogen sulfide in the hot spring water can combine with iron to precipitate black iron sulfide, causing black suspension as "hot spring flowers" or precipitates. Examples include "Sumiyu" at Shiobara Motoyu Onsen in Tochigi Prefecture. Similarly, the generation of black "hot spring flowers" is known in many hot springs, such as Goshiki Onsen in Nagano Prefecture, Kawabuki Onsen in Miyagi Prefecture, and Katsuura Onsen in Wakayama Prefecture. When the amount of "hot spring flowers" is large or the bath is stirred, the hot spring may appear gray.

Black hot spring (Tokyo Azabu Juban Onsen)	Black hot spring (Tochigi Prefecture Shiobara Motoyu Onsen)

(6) Green Hot Springs

Hot springs such as Kunimi Onsen in Iwate Prefecture and Kuma no Yu Onsen in Nagano Prefecture have a vivid green color. The mechanism by which these hot springs present green color is not clear, but it is noted that they are neutral and have a high amount of hydrogen sulfide (new findings will be published in Episode 17).
Additionally, algae (microorganisms) such as chlorella present in the hot spring can proliferate under sunlight, making the hot spring appear green.

Green hot spring (Iwate Prefecture Kunimi Onsen)	Green hot spring (Nagano Prefecture Kuma no Yu Onsen)

(7) Hot Springs with Changing Colors

As mentioned in the sections on milky white and blue hot springs, the color of hot springs is not always constant. There are known cases where the color of hot springs changes due to the passage of time after fresh source water is added, weather, changes in groundwater levels due to precipitation, and even after earthquakes.
The communal bath "Tsuboyu" at Yunomine Onsen in Wakayama Prefecture is a hot spring with a legend of color change, and even today, color changes from milky white to milky blue are observed.
In the case of "Tsuboyu," the source water springs directly from the bath, and the hot spring water, which existed in an unstable state underground, undergoes chemical changes to stabilize under surface conditions as soon as it emerges, changing the state of light scattering and resulting in color change.
Hot springs that change color over time are mostly those where fresh source water springs directly or is immediately added to the bath, especially those where the source springs directly from the bottom of the bath.
Goshiki Onsen in Nagano Prefecture is also known for its color changes, and it is known that the color of the hot spring changes with changes in the water level of the river flowing beside the hot spring.
After the 2004 earthquake off the Kii Peninsula, it was widely reported in newspapers that the color of the hot spring at Kumotori Onsen in Wakayama Prefecture, which had been light, became darker (changed to milky white).

Hot spring with changing colors (Wakayama Prefecture Yunomine Onsen)	Hot spring with changing colors (Nagano Prefecture Goshiki Onsen)