A simple thermal spring led by bicarbonate, emerging hot at 43.6 °C. It is lightly mineralized at 792 mg/kg, gentle on the body, and weakly alkaline so it feels soft on the skin. A generous 117 mg/kg of metasilicic acid leaves the kind of thin silica film that makes skin feel noticeably smoother, placing it in Japan's beauty-water family.
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⁺139mgP38 national71%
- Magnesiummild; a laxative if you drink itMg²⁺14.5mgP66 national14%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺18.2mgP37 national11%
- Potassiumminor; matters for drinking limitsK⁺10.5mgP46 national3%
- AmmoniumNH₄⁺0.70mgP56 national0.5%
- LithiumLi⁺0.10mgP29 national0.1%
- HydrogenH⁺0.0000mg0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺0.10mgP54 national
- BariumBa²⁺0.10mgP53 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.10mgP30 national
- StrontiumSr²⁺0.10mgP26 national
- ZincZn²⁺0.0000mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻459mgP75 national90%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻28.8mgP21 national10%
- Hydrogen phosphateHPO₄²⁻1.0mgP82 national0.2%
- SulfateSO₄²⁻0.20mgP3 national0.1%
- AsO2AsO₂⁻0.0000mg
- Bromidetrace, sea-originBr⁻0.0000mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻0.0000mg
- Fluoridetrace; drinking limits applyF⁻0.0000mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.0000mg
- HSO4HSO40.0000mg
- Iodideyellows in air; helps blood lipidsI⁻0.0000mg
- Thiosulfateoxidized sulfurS₂O₃²⁻0.0000mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃117mgP60 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.8mgP17 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂58.1mgP71 national
- 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.
- 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.