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Find the Best Hot Springs in Gifu.
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Several decades ago, "smooth hot springs" became famous nationwide as "beauty baths." In other words, the characteristic of "smoothness" was rare and noteworthy. How about now? There are "smooth hot springs" known as beauty baths all over the country. As you know, Gifu Prefecture has a particularly large number of smooth hot springs known as beauty baths.
Among the smooth hot springs in Gifu Prefecture, those like Gero Onsen, which have naturally sprung up for a long time, are rare. Most have been newly developed in recent years through deep drilling over 1000 meters. Ikeda Onsen, Tanigumi Onsen, Bijin no Yu Shiratori, Meihou Onsen, Nakatsugawa Onsen... the list goes on.
The common points of these hot springs are as follows:
We have hypothesized the following mechanism for why hot springs in the Mino and southern Hida regions become smooth:
Calcium carbonate dissolves into the groundwater from some underground rocks (such as limestone).
If the rocks widely distributed underground contain clay minerals or zeolites, they come into contact with groundwater containing Ca2+, causing a "cation exchange" between Ca2+ and Na+.
When Ca2+ is removed from the groundwater through cation exchange, it becomes unsaturated with respect to Ca2+, causing calcium carbonate to dissolve again from the rocks. Repeating this process gradually increases Na+ and HCO3- in the groundwater, creating a Na+ HCO3- spring (bicarbonate spring) or an alkaline simple hot spring with bicarbonate as the main component.
For the reaction to continue and generate bicarbonate hot springs deep underground, groundwater must travel long distances and constantly come into contact with new rocks. Therefore, deep drilling over 1000 meters seems to meet this condition, resulting in the emergence of bicarbonate hot springs (smooth hot springs).
Fortunately, in the Mino region, there is a wide distribution of "rock bodies formed by plates accreted to the edge of the continent" known as the Mino Belt, with limestone blocks widely scattered and incorporated throughout the rock body. This limestone is considered a potential source of calcium carbonate dissolved in groundwater.
Additionally, because the rocks are accretionary, they have structures such as reverse faults and folds, with faults distributed in a network pattern, suggesting a high possibility of clay minerals existing along the groundwater pathways.
From these observations, we can consider the causes of smooth hot springs in the Mino and southern Hida regions.
As a side note, when a bicarbonate solution reaches 65°C, it transforms into a more alkaline sodium carbonate solution. In facilities that heat low-temperature hot springs, the generation of more alkaline sodium carbonate can increase the smoothness.