A pH 9.6 alkaline simple thermal water from Yugawa in Wakayama. Total dissolved solids are light at 195 mg/kg, but the strong alkalinity gives a pronounced slippery feel on the skin — squarely in Japan's beauty-water family. Metasilicic acid at 55.6 mg/kg leaves a thin silica film for noticeably smoother skin. At the wellhead the water carries a very faint sulfur note on the tongue.
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⁺49.7mgP13 national93%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺3.3mgP12 national7%
- PotassiumK⁺0.50mgP5 national0.4%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- FeTotalFe< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻30.0mgP82 national37%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻28.9mgP22 national30%
- Fluoridetrace; drinking limits applyF⁻8.2mgP87 national16%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻14.7mgP27 national11%
- HydroxideOH⁻2.6mgP96 national5%
- Hydrogen sulfidesulfur character — antibacterialHS⁻1.1mgP71 national1%
- BO2BO₂⁻0.50mgP50 national0.4%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻< —mg
- HSiO3HSiO₃⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃55.6mgP33 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂< —mg
- 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.
- 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.