A sodium chloride spring fed from Beppu's municipal hot-spring supply network — the Kannawa line — with the certificate naming the city-owned Kannawa wells and others as its origin and taking its sample from the storage tank rather than at a wellhead. The water is weakly acidic and moderately mineralized at 4,487 mg/kg, carrying a distinctly salty tang and the heat-retaining body typical of chloride water. At 81.7 °C the source ranks among Japan's hottest. The standout figure is metasilicic acid at 685 mg/kg — an unusually generous load that leaves the kind of thin silica film that puts this water squarely in Japan's beauty-water family.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, netto 熱の湯).
cold sensitivity
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
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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⁺1,190mgP87 national87%
- Potassiumminor; matters for drinking limitsK⁺178mgP97 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺38.5mgP50 national3%
- LithiumLi⁺7.8mgP97 national2%
- MagnesiumMg²⁺4.3mgP45 national0.6%
- AmmoniumNH₄⁺1.0mgP66 national0.1%
- ManganeseMn²⁺1.2mgP80 national0.1%
- StrontiumSr²⁺0.50mgP69 national0.0%
- Iron (II)Fe²⁺0.40mgP49 national0.0%
- BariumBa²⁺0.20mgP73 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,950mgP87 national88%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻334mgP76 national11%
- FluorideF⁻2.9mgP66 national0.2%
- BromideBr⁻5.4mgP83 national0.1%
- HSO4HSO40.90mgP47 national0.0%
- IodideI⁻0.20mgP51 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃685mgP100 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂84.7mgP95 national
- Metaarsenious acidHAsO₂1.9mgP96 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂15.0mgP47 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs1.3mgP95 national
- 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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.