A sodium-led alkaline simple thermal spring. It is only lightly mineralized at 206 mg/kg of dissolved solids, so it stays gentle on the body, while a notably alkaline pH of 9.5 gives the slippery, skin-smoothing feel of the Japanese beauty-water family. A generous 68.5 mg/kg of metasilicic acid leaves a thin silica film that makes skin feel noticeably softer afterwards.
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺46.2mgP12 national97%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1.1mgP3 national2%
- PotassiumK⁺0.80mgP11 national0.0%
- AluminumAl³⁺0.0000mg0.0%
- BariumBa²⁺0.0000mg0.0%
- Iron (II)Fe²⁺0.0000mg0.0%
- Iron (III)Fe³⁺0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- LithiumLi⁺0.0000mg0.0%
- MagnesiumMg²⁺0.0000mg0.0%
- ManganeseMn²⁺0.0000mg0.0%
- AmmoniumNH₄⁺0.0000mg0.0%
- StrontiumSr²⁺0.0000mg0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻25.5mgP77 national39%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻35.1mgP20 national27%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻11.4mgP13 national15%
- Hydrogen phosphateHPO₄²⁻13.9mgP95 national13%
- BO2BO₂⁻1.6mgP70 national2%
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.50mgP57 national2%
- Fluoridetrace; drinking limits applyF⁻0.60mgP38 national1%
- AsO2AsO₂⁻0.0000mg0.0%
- BromideBr⁻0.0000mg0.0%
- IodideI⁻0.0000mg0.0%
- HydroxideOH⁻0.0000mg0.0%
- ThiosulfateS₂O₃²⁻0.0000mg0.0%
- SulfateSO₄²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃68.5mgP43 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
- ChromiumCr0.13mgP75 national
- 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.
- 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.