A sodium chloride spring of the low-temperature class (31.9 °C, heated for bathing). At pH 9.3 the water is distinctly alkaline and slippery on the skin, sitting on the edge of Japan's beauty-water family. The wellhead carries a faint sulfur note alongside salty and bitter tastes, and gas bubbles rise with the flow. Total dissolved solids of 1,953 mg/kg give it moderate mineral 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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺628mgP79 national84%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺101mgP73 national15%
- AmmoniumNH₄⁺1.8mgP79 national0.3%
- PotassiumK⁺2.2mgP23 national0.2%
- MagnesiumMg²⁺0.70mgP23 national0.2%
- StrontiumSr²⁺1.7mgP84 national0.1%
- BariumBa²⁺0.20mgP74 national0.0%
- AluminumAl³⁺0.0000mg0.0%
- Iron (II)Fe²⁺0.0000mg0.0%
- Iron (III)Fe³⁺0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- LithiumLi⁺0.0000mg0.0%
- ManganeseMn²⁺0.0000mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,121mgP81 national94%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻20.1mgP68 national2%
- Fluoridetrace; drinking limits applyF⁻10.6mgP92 national2%
- SulfateSO₄²⁻15.7mgP28 national1.0%
- BO2BO₂⁻10.1mgP92 national0.7%
- BromideBr⁻3.5mgP77 national0.1%
- HydroxideOH⁻0.30mgP74 national0.1%
- IodideI⁻0.30mgP62 national0.0%
- Hydrogen phosphateHPO₄²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃35.6mgP17 national
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.