An alkaline simple thermal spring from Gero Onsen. At pH 9.1 it sits well into the alkaline range, giving the slippery beauty-water feel that smooths the skin. The water is lightly mineralized at 340 mg/kg, so it stays gentle on the body, while a generous 65.9 mg/kg of metasilicic acid leaves a thin silica film that makes skin feel noticeably smoother afterwards.
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⁺192mgP49 national90%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺18.3mgP38 national10%
- PotassiumK⁺1.9mgP22 national0.5%
- Aluminumshows up in acidic volcanic waterAl³⁺0.09mgP37 national
- FeTotalFe< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻56.4mgP29 national30%
- Fluoridetrace; drinking limits applyF⁻10.1mgP92 national10%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻37.5mgP36 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻26.0mgP16 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻19.2mgP67 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃65.9mgP42 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.1mgP43 national
- Metaarsenious acidHAsO₂0.10mgP56 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂4.0mgP27 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —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.