湯SimpleSodium Chloride-Bicarbonate spring
A sodium-led alkaline simple thermal spring (pH 9.19). Mineralization is light at around 0.38 g/kg — gentle on the body with little chemical edge. The alkalinity gives the slippery, soft feel typical of the Japanese 'beauty-water' family.
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.
Cationsshare of charge (mval%)
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺106mgP31 national97%
- PotassiumK⁺1.3mgP16 national0.7%
- Iron (II)Fe²⁺0.01mgP12 national0.0%
- MagnesiumMg²⁺0.01mgP4 national0.0%
- AmmoniumNH₄⁺0.01mgP13 national0.0%
- AluminumAl³⁺< —mg0.0%
- CalciumCa²⁺< —mg0.0%
- ManganeseMn²⁺< —mg0.0%
Anionsshare of charge (mval%)
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻52.3mgP28 national31%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻36.1mgP21 national25%
- Fluoridetrace; drinking limits applyF⁻3.1mgP66 national3%
Non-dissociatedmeasured by mass
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃51.6mgP30 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂15.4mgP69 national
Measured at the source
Source temperature
48.4°C
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.