A sodium-led alkaline simple thermal spring. At pH 9.6 it is strongly alkaline, giving the pronounced slippery feel of Japan's beauty-water family. A hot source at 43.4 °C, it carries 59.3 mg/kg of metasilicic acid that leaves skin noticeably smoother, and enough radon to fall within the hormesis tradition.
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⁺63.7mgP18 national92%
- Potassiumminor; matters for drinking limitsK⁺5.4mgP35 national5%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1.7mgP6 national3%
- AluminumAl³⁺0.20mgP65 national0.7%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻30.1mgP84 national32%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻51.3mgP30 national27%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻21.7mgP16 national20%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻13.7mgP26 national9%
- Fluoridetrace; drinking limits applyF⁻5.1mgP78 national9%
- BO2BO₂⁻1.5mgP64 national1%
- HydroxideOH⁻0.70mgP81 national1%
- Hydrogen sulfideHS⁻0.20mgP36 national0.6%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃59.3mgP35 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.03mgP42 national
- CadmiumCdNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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.