A sodium-led bicarbonate spring with a chloride sub-type giving the water a gentle salty finish. At pH 8.7 it sits firmly in the alkaline 'beauty-water' family — slippery on the skin in the classic Japanese sense. The 36 °C source carries the faintest whisper of hydrogen sulfide and a barely-there sulfur note, a quiet but distinctive character.
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
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
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺318mgP63 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺5.0mgP17 national
- Potassiumminor; matters for drinking limitsK⁺4.5mgP33 national
- FeTotalFe1.8mgP79 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻415mgP72 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻147mgP48 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻54.1mgP93 national
- Fluoridetrace; drinking limits applyF⁻10.8mgP92 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻4.0mgP14 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.30mgP47 national
- Thiosulfateoxidized sulfurS₂O₃²⁻0.20mgP55 national
- BO2BO₂⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃42.2mgP23 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂15.7mgP69 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.