A hot (47.6 °C) alkaline simple thermal spring. At pH 9.44 the water has the slippery, skin-smoothing feel of Japan's beauty-water family. Mineralization is light at 350 mg/kg, but a generous 58.7 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother afterwards. A faint hydrogen sulfide note hangs over the bath.
Good for
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
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⁺106mgP27 national98%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1.4mgP5 national2%
- PotassiumK⁺0.70mgP9 national0.4%
- AluminumAl³⁺0.0000mg0.0%
- BariumBa²⁺0.0000mg0.0%
- FeTotalFe0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- LithiumLi⁺0.0000mg0.0%
- MagnesiumMg²⁺0.0000mg0.0%
- ManganeseMn²⁺0.0000mg0.0%
- AmmoniumNH₄⁺0.0000mg0.0%
- StrontiumSr²⁺0.0000mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻77.3mgP34 national42%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻51.0mgP91 national33%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻33.5mgP19 national11%
- Fluoridetrace; drinking limits applyF⁻6.0mgP84 national6%
- HSiO3HSiO₃⁻12.9mgP60 national6%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻4.3mgP16 national2%
- Hydrogen sulfidesulfur character — antibacterialHS⁻1.9mgP79 national1%
- HydroxideOH⁻0.50mgP78 national0.5%
- BromideBr⁻0.10mgP16 national0.0%
- AsO2AsO₂⁻0.0000mg0.0%
- H2PO4H₂PO₄⁻0.0000mg0.0%
- Hydrogen phosphateHPO₄²⁻0.0000mg0.0%
- HSO4HSO40.0000mg0.0%
- IodideI⁻0.0000mg0.0%
- NitrateNO₃⁻0.0000mg0.0%
- SS²⁻0.0000mg0.0%
- ThiosulfateS₂O₃²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃58.7mgP36 national
- H2SO4FreeH2SO4Free0.0000mg
- 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
- 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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- 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.