龍神温泉（上の湯・下の湯）混合泉Ryujin Onsen (Kami-no-yu and Shimo-no-yu blend)
A sodium bicarbonate hot spring blending two wellheads (Kami-no-yu and Shimo-no-yu), hypotonic and alkaline, emerging at 43.3 °C. Weakly alkaline at pH 7.8 at the source and 8.0 in the lab, it sits squarely in Japan's beauty-water family. A generous 64 mg/kg of metasilicic acid leaves the kind of thin silica film that makes skin feel noticeably smoother afterwards — the trademark texture of Ryujin Onsen, long counted among the country's beauty springs.
Good for
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
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
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⁺377mgP68 national96%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺7.5mgP22 national2%
- Potassiumminor; matters for drinking limitsK⁺9.3mgP44 national1%
- MagnesiumMg²⁺1.0mgP26 national0.5%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻992mgP91 national93%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻26.8mgP20 national4%
- Fluoridetrace; drinking limits applyF⁻4.1mgP72 national1%
- CarbonateCO₃²⁻4.7mgP33 national0.9%
- SulfateSO₄²⁻0.80mgP6 national0.1%
- ThiosulfateS₂O₃²⁻0.10mgP45 national0.0%
- BO2BO₂⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃64.2mgP40 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂14.3mgP68 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂11.7mgP43 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.0010mgP6 national
- Mercurytoxic trace — part of the drinking-safety checkHg0.0005mgP51 national
- CadmiumCd< —mg
- CopperCu< —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.