龍神温泉 下の湯Ryujin Onsen Shimo-no-yu
A sodium-led bicarbonate hot spring in the classic jūsō-sen (soda-water) style. The water emerges at 46 °C, weakly alkaline (pH 7.8) and soft on the skin, sitting squarely in Japan's beauty-water family. A generous 65 mg/kg of metasilicic acid adds a thin silica film that leaves skin noticeably smoother afterwards.
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⁺361mgP66 national96%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺6.9mgP21 national2%
- Potassiumminor; matters for drinking limitsK⁺8.6mgP42 national1%
- MagnesiumMg²⁺0.80mgP24 national0.4%
- 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 smoothHCO₃⁻893mgP88 national91%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻32.0mgP23 national6%
- Fluoridetrace; drinking limits applyF⁻4.8mgP77 national2%
- SulfateSO₄²⁻4.2mgP14 national0.9%
- ThiosulfateS₂O₃²⁻2.9mgP88 national0.3%
- BO2BO₂⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃65.2mgP41 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂11.0mgP58 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- 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.
- 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.