A simple thermal spring in Yuasa-cho, Wakayama. With pH 9.5 it sits at the strongly-alkaline edge of the scale; sodium dominates the cation side (94 percent of equivalents), giving the slippery, soft-water feel associated with Japans beauty-water family. Source temperature is 30.4 C — a cold-side low-temp spring that is reheated for bathing. Mineralization is light (194 mg/kg dissolved solids), so the water is mild rather than assertive.
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⁺54.0mgP15 national94%
- Aluminumshows up in acidic volcanic waterAl³⁺0.40mgP74 national3%
- Manganesetrace; companion to iron springsMn²⁺1.1mgP78 national2%
- PotassiumK⁺0.40mgP4 national0.4%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻54.2mgP29 national37%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻24.8mgP76 national35%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻24.7mgP33 national21%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻4.7mgP6 national5%
- BO2BO₂⁻0.80mgP56 national0.8%
- ThiosulfateS₂O₃²⁻0.30mgP58 national0.4%
- FluorideF⁻0.20mgP17 national0.4%
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃27.8mgP11 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
- CopperCu0.01mgP76 national
- ArsenicAs< —mg
- CadmiumCdNDmg
- 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.