A blended source: water from the Kinugawa-Kawaji Onsen Kanko Kaihatsu No. 1 and No. 2 wells is combined with the Oharasawa municipal spring and sampled at the company's first distribution station rather than at a wellhead. The result is an alkaline simple thermal spring at pH 9.0 — squarely in Japan's beauty-water family, where the alkalinity gives that soft, slippery feel on the skin. Mineralization is light at 474 mg/kg, and the certificate records the water as colorless, transparent, tasteless and odorless, so nothing competes with that smooth texture; calcium and sulfate lead what little dissolved matter there is.
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺94.6mgP72 national71%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺43.7mgP11 national29%
- PotassiumK⁺0.70mgP9 national0.3%
- AluminumAl³⁺0.0000mg0.0%
- Iron (II)Fe²⁺0.0000mg0.0%
- Iron (III)Fe³⁺0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- LithiumLi⁺0.0000mg0.0%
- MagnesiumMg²⁺0.0000mg0.0%
- ManganeseMn²⁺0.0000mg0.0%
- AmmoniumNH₄⁺0.0000mg0.0%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻237mgP68 national74%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻46.0mgP26 national19%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻11.6mgP7 national3%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻4.2mgP30 national2%
- Fluoridetrace; drinking limits applyF⁻2.0mgP60 national2%
- HydroxideOH⁻0.20mgP66 national0.2%
- BromideBr⁻0.20mgP29 national0.0%
- BO2BO₂⁻0.0000mg0.0%
- Hydrogen sulfideHS⁻0.0000mg0.0%
- HSiO3HSiO₃⁻0.0000mg0.0%
- HSO4HSO40.0000mg0.0%
- NitrateNO₃⁻0.0000mg0.0%
- SS²⁻0.0000mg0.0%
- ThiosulfateS₂O₃²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃31.6mgP14 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.6mgP15 national
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.02mgP38 national
- CadmiumCdNDmg
- ChromiumCrNDmg
- CopperCuNDmg
- 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.