別府市温泉供給事業鉄輪線地獄原温泉上Beppu City Hot Spring Supply, Kannawa Line, Jigokubaru Onsen Upper
Sodium chloride water piped to Jigokubaru along Beppu City's Kannawa supply line and sampled at the storage tank rather than at the wellhead. It is weakly acidic, colorless and clear, faintly salty, and moderately mineralized at about 4.0 g/kg of dissolved solids. Even at the tank it arrives among Japan's hottest supplied waters, and it carries 515 mg/kg of metasilicic acid, a silica load high enough to leave a thin moisturizing film on the skin and to place it 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, nettō 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⁺1,060mgP86 national85%
- Potassiumminor; matters for drinking limitsK⁺150mgP95 national7%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺42.0mgP54 national4%
- LithiumLi⁺7.5mgP96 national2%
- MagnesiumMg²⁺5.9mgP54 national0.9%
- HydrogenH⁺0.30mgP61 national0.6%
- ManganeseMn²⁺1.6mgP88 national0.1%
- AmmoniumNH₄⁺0.90mgP64 national0.1%
- Iron (II)Fe²⁺1.0mgP62 national0.1%
- StrontiumSr²⁺0.30mgP55 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,760mgP85 national85%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻408mgP82 national15%
- FluorideF⁻2.4mgP62 national0.2%
- BromideBr⁻4.8mgP80 national0.1%
- HSO4HSO44.4mgP53 national0.1%
- IodideI⁻0.20mgP52 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃515mgP99 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂76.8mgP94 national
- Metaarsenious acidHAsO₂1.6mgP93 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂3.1mgP24 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs1.1mgP94 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.