A hot 64.2 degrees Celsius spring classified as a simple thermal source, neutral at the wellhead (pH 6.6). Sensory notes give it character: colorless and clear, with a faint tart edge on the tongue and a quiet earthy note. Total dissolved solids are light (370 mg/kg), but metasilicic acid runs to a generous 151.5 mg/kg, leaving the kind of silica film that makes skin feel noticeably smoother afterwards.
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⁺42.6mgP11 national66%
- Potassiumminor; matters for drinking limitsK⁺17.9mgP59 national16%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺5.2mgP18 national9%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺1.9mgP71 national3%
- LithiumLi⁺0.10mgP29 national0.4%
- MagnesiumMg²⁺0.10mgP11 national0.4%
- AmmoniumNH₄⁺0.10mgP24 national0.4%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- CopperCu²⁺< —mg
- Iron (III)Fe³⁺< —mg
- StrontiumSr²⁺< —mg
- ZincZn²⁺< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻56.8mgP42 national39%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻61.7mgP33 national34%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻26.4mgP20 national25%
- Fluoridetrace; drinking limits applyF⁻0.80mgP44 national1%
- ThiosulfateS₂O₃²⁻0.70mgP67 national0.3%
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.10mgP28 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃152mgP68 national
- Metaarsenious acidHAsO₂5.2mgP98 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.5mgP15 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂6.4mgP32 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.30mgP69 national
- ArsenicAs3.6mgP100 national
- CadmiumCd< —mg
- ChromiumCr< —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.