A sodium-led chloride spring with a bicarbonate edge — sampled as 'Ouka-sen' at 37.5 °C from a powered well in Minami-Alps. Weakly alkaline (pH 7.8 at the source, 7.97 in the lab), it carries a light salty tang and a faint metallic note. Mineralization sits at a moderate 1,145 mg/kg, enough for the heat-retaining feel typical of chloride water while the bicarbonate component softens the touch.
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺2,932mgP92 national82%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺26.3mgP43 national8%
- Magnesiummild; a laxative if you drink itMg²⁺14.2mgP65 national8%
- Potassiumminor; matters for drinking limitsK⁺10.5mgP47 national2%
- AmmoniumNH₄⁺0.30mgP37 national0.1%
- StrontiumSr²⁺0.30mgP50 national
- BariumBa²⁺0.10mgP54 national
- LithiumLi⁺0.10mgP26 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.0000mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.0000mg
- Iron (III)Fe³⁺0.0000mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.0000mg
- Manganesetrace; companion to iron springsMn²⁺0.0000mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻354mgP65 national38%
- Hydrogen phosphateHPO₄²⁻32.0mgP95 national4%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻3,183mgP89 national
- Bromidetrace, sea-originBr⁻8.9mgP85 national
- Iodideyellows in air; helps blood lipidsI⁻1.9mgP85 national
- Fluoridetrace; drinking limits applyF⁻0.90mgP47 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.0000mg
- NitrateNO₃⁻0.0000mg
- Thiosulfateoxidized sulfurS₂O₃²⁻0.0000mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂88.1mgP95 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂121mgP82 national
- CadmiumCd< —mg
- LeadPb< —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.