A calcium- and sodium-led sodium chloride spring that also carries total sulfur. With a pH around 10 it is strongly alkaline, giving the pronounced slippery feel of the Japanese beauty-water family, and it drifts a faint hydrogen sulfide note. Moderately mineralized, it has a salty, heat-retaining body.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
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
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺216mgP86 national57%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺187mgP48 national43%
- PotassiumK⁺0.05mgP0 national0.3%
- Iron (II)Fe²⁺0.30mgP43 national0.1%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻601mgP76 national89%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻25.0mgP76 national4%
- HSiO3HSiO₃⁻38.4mgP81 national3%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻14.1mgP26 national2%
- BO2BO₂⁻11.0mgP93 national1%
- HydroxideOH⁻1.7mgP93 national0.5%
- BromideBr⁻0.40mgP39 national0.1%
- Hydrogen sulfideHS⁻0.0000mg0.0%
- ArsenicAs< —mg
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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.