A simple thermal spring on the acidic side of weakly acidic (pH 3.7), emerging at 55 °C and so classed as a hot source. The water is nearly colorless and clear at the wellhead, carrying a faint rotten-egg note of hydrogen sulfide and a trace metallic edge in both smell and taste. Total dissolved solids sit on the light side at 568.9 mg/kg, but 169.9 mg/kg of metasilicic acid lends a noticeably smooth feel against the skin.
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⁺65.0mgP18 national47%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺22.3mgP40 national19%
- Potassiumminor; matters for drinking limitsK⁺22.3mgP67 national10%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺13.3mgP92 national8%
- Magnesiummild; a laxative if you drink itMg²⁺4.9mgP50 national7%
- Aluminumshows up in acidic volcanic waterAl³⁺2.4mgP85 national4%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.20mgP65 national3%
- LithiumLi⁺0.50mgP53 national1%
- AmmoniumNH₄⁺0.30mgP37 national0.3%
- ManganeseMn²⁺0.40mgP65 national0.2%
- ZincZn²⁺0.40mgP89 national0.2%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻204mgP67 national72%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻56.2mgP28 national27%
- FluorideF⁻1.0mgP49 national0.9%
- HSO4HSO41.2mgP51 national0.2%
- BromideBr⁻0.20mgP29 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃170mgP79 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.6mgP28 national
- Metaarsenious acidHAsO₂0.70mgP88 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂149mgP85 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.60mgP69 national
- 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.