A cold-mineral source at 19.5 degrees in the Ryujin area of Tanabe, Wakayama, heated for bathing. The water is alkaline at pH 8.7 and lightly mineralized at 712 mg/kg dissolved solids — bicarbonate is the dominant anion (around 77 percent by mval) and sodium the dominant cation, but every value sits below the therapeutic-spring thresholds, so the certificate qualifies the source as an onsen under the Article 2 fluoride and sodium-bicarbonate provisions rather than as a therapeutic spring. Colorless, odorless and tasteless at the wellhead, with a fine sparkle of bubbles on emergence.
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
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⁺203mgP51 national98%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Potassiumminor; matters for drinking limitsK⁺< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻414mgP72 national77%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻32.4mgP84 national12%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻29.5mgP34 national7%
- Fluoridetrace; drinking limits applyF⁻4.6mgP77 national3%
- ChlorideCl⁻2.1mgP3 national0.7%
- BO2BO₂⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃18.7mgP4 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1.4mgP20 national
- 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.
- 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.