Japan Onsen Notebook

What the Distribution Shows

• Chloride springs (about 30%) and simple springs (about 19%) together account for nearly half of all onsen — the two most common water types nationwide.
• Sulfur springs make up about 14% overall, but in prefectures anchored around active volcanic peaks — Iwate (Sukawa, Matsukawa, Fujishichi onsen, at the foot of Mt. Kurikoma, Mt. Iwate and Hachimantai), Akita (Nyuto Onsen, between Mt. Akita-Komagatake and Mt. Nyuto), Nagano (Jigokudani and Nozawa onsen, on the Shiga Kogen plateau), Kagoshima (Maruo Onsen, in the Kirishima range), and Hokkaido (Noboribetsu and Kawayu onsen, on the Kuttara caldera and Mt. Atosanupuri) — they account for roughly 20-40% of local onsen, clearly higher than elsewhere. Since sulfur comes from volcanic gas, it tracks closely with this kind of volcanic mountain terrain.
• Chloride springs, by contrast, aren't tied to one landform. Some, like Kogane-zaki Furofushi Onsen (on the Sea of Japan coast) and Tairadate Onsen (on Mutsu Bay), both in Aomori, sit right on the coast; others, like Kashio Onsen in Nagano, lie deep in the Akaishi Mountains (the Southern Japan Alps), far from any sea. The former draw on seawater directly, the latter on ancient seawater trapped in inland rock strata — two distinct origins behind the same water type.
• Carbon dioxide, iron, and radioactive springs together make up less than a tenth of the total — rare water types found only in a handful of places with specific geological conditions, such as the granite belt of the Chugoku Mountains running through Yamaguchi and Hiroshima, where radioactive (radon) springs concentrate.