A blended spring drawing from two wells — Hamayu and Taki-no-Yu — mixed in the hotel's basement pit before reaching the baths. Classified as an alkaline simple thermal spring (pH 8.6 at the wellhead), it sits squarely in the Japanese 'beauty-water' family — soft, slippery, and gentle on the skin. A faint hydrogen sulfide note and a whisper of sulfur on the tongue come through behind a chloride-led, lightly mineralized base. A hypotonic low-temperature source (33 °C) that is reheated for bathing.
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⁺82.0mgP25 national81%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺16.5mgP35 national19%
- PotassiumK⁺1.3mgP15 national0.7%
- MagnesiumMg²⁺0.20mgP14 national0.2%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- FeTotalFe< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻138mgP46 national81%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻17.7mgP11 national6%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻12.6mgP24 national5%
- Fluoridetrace; drinking limits applyF⁻4.3mgP74 national5%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻4.8mgP34 national3%
- Hydrogen sulfideHS⁻0.20mgP40 national0.2%
- BO2BO₂⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃25.4mgP9 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.90mgP12 national
- 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.