A weakly acidic, hypotonic simple thermal spring rising by natural flow from a bored well in the Oniyama district at a scalding 69.9 °C, placing it among Japan's hottest natural sources. The water is colorless and very faintly cloudy at the source, carrying the moderate rotten-egg note of hydrogen sulfide that marks the Kannawa field. It is unusually rich in metasilicic acid at 124 mg/kg, leaving the kind of thin 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⁺47.2mgP12 national49%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺25.3mgP43 national30%
- Magnesiummild; a laxative if you drink itMg²⁺6.0mgP55 national12%
- Aluminumshows up in acidic volcanic waterAl³⁺1.7mgP80 national5%
- Potassiumminor; matters for drinking limitsK⁺6.0mgP37 national4%
- Iron (II)Fe²⁺0.50mgP52 national0.5%
- Iron (III)Fe³⁺0.50mgP78 national0.5%
- StrontiumSr²⁺0.20mgP44 national0.1%
- ManganeseMn²⁺0.10mgP36 national0.1%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻113mgP57 national59%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻57.1mgP30 national40%
- NitrateNO₃⁻0.20mgP36 national0.3%
- FluorideF⁻0.01mgP2 national0.3%
- BromideBr⁻0.20mgP30 national0.0%
- ThiosulfateS₂O₃²⁻0.20mgP55 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃124mgP63 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.8mgP17 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂87.1mgP77 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.40mgP71 national
- ArsenicAs0.02mgP37 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.