A sodium-led bicarbonate spring — the classic jūsō-sen (soda-water) style, soft on the skin. At the wellhead it runs colorless and clear with abundant gas evolution, and in the lab it carries a faint hydrogen sulfide note. Weakly alkaline at pH 8.4, it sits in the Japanese beauty-water family thanks to its bicarbonate dominance and slippery feel.
Good for
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺429mgP72 national96%
- LithiumLi⁺1.6mgP88 national1%
- Potassiumminor; matters for drinking limitsK⁺8.9mgP43 national1%
- CalciumCa²⁺2.7mgP10 national0.7%
- AmmoniumNH₄⁺1.6mgP76 national0.5%
- MagnesiumMg²⁺0.40mgP18 national0.2%
- StrontiumSr²⁺0.40mgP62 national0.1%
- BariumBa²⁺0.40mgP81 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻946mgP89 national81%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻55.5mgP94 national10%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻47.2mgP27 national7%
- Fluoridetrace; drinking limits applyF⁻7.5mgP85 national2%
- SulfateSO₄²⁻1.3mgP8 national0.0%
- Hydrogen sulfideHS⁻0.20mgP40 national0.0%
- IodideI⁻0.20mgP52 national0.0%
Measured at the source
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.