Topics

From Gifu! Hot Spring Museum Episode 17: The Mechanism of Green Hot Springs

We are resuming the "Gifu Hot Spring Museum" after a long time. We apologize for the inconvenience caused over the long period.
Well, several years have passed since the last episode 16. Naturally, research on hot springs has been conducted throughout Japan during this time, and knowledge about hot springs has deepened further.
Previously, in "Episode 13: Various Colors of Hot Springs," I mentioned that "the mechanism by which such hot springs appear green is not clear," but in recent years, the mechanism of the color of hot springs that appear vividly green like bath additives has been researched and clarified. This time, I will talk about that topic.
There are various types of green hot springs. Even colorless and transparent water can appear greenish if the bathtub is deep. As you know, when you look at a river from a distance, especially the deep parts, they appear green, and this is the same mechanism. This phenomenon occurs because the red wavelengths of sunlight are absorbed by the water. The water itself is not originally colored. Also, in hot springs where well water or river water is mixed or added, open-air baths may appear green. This is because algae and other plant plankton that originally lived in the water explosively proliferate when exposed to sunlight in open-air baths. It's the same as a pool that turns green quickly if the water is not changed. Many algae proliferate explosively due to sunlight. Since photosynthetic algae are not included as components in hot spring water that exists deep underground, it is understood that they proliferated after being discharged, unrelated to hot spring components.
On the other hand, the mechanism by which green hot springs, such as the representative Kunimi Hot Spring in Iwate Prefecture and Kuma-no-Yu Hot Spring in Nagano Prefecture, which contain hydrogen sulfide, appear green has been researched. These hot springs, which are closer to yellow-green, have been clarified to be due to a mixture of "the yellow of polysulfide ions contained as components" and "the blue caused by Rayleigh scattering by calcium carbonate or sulfur particles contained in the hot spring water." The yellow of polysulfide ions is the color of the original liquid of liquid bath additives such as the once-sold "Roku-ichi-0 Hap" or "Kusatsu Onsen Hap."
Changing the subject, it is truly regrettable that excellent bath additives like "Roku-ichi-0 Hap" or "Kusatsu Onsen Hap" have disappeared. I hope they will return to the market again.