A sodium-led bicarbonate spring — classified as jūsō-sen in the Japanese soda-water tradition. Neutral pH (6.8 at the wellhead) puts it squarely in the country's beauty-water family, with the soft, smoothing feel that comes from HCO3 chemistry on the skin. It emerges cool (25.2 degC) so it is reheated for bathing. Mineralization of 6.7 g/kg gives it real body; the water is colorless and faintly mineral-tasting at the source, turning slightly yellow and cloudy after standing in the air.
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⁺1,407mgP89 national76%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺228mgP86 national14%
- Magnesiummild; a laxative if you drink itMg²⁺87.6mgP91 national9%
- PotassiumK⁺20.2mgP63 national0.7%
- FeTotalFe5.0mgP87 national0.2%
- ManganeseMn²⁺0.50mgP64 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻4,890mgP100 national100%
- ChlorideCl⁻7.1mgP10 national0.3%
- SulfateSO₄²⁻0.50mgP5 national0.0%
- ThiosulfateS₂O₃²⁻0.20mgP55 national0.0%
- FluorideF⁻0.10mgP9 national0.0%
- BO2BO₂⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃36.8mgP19 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂28.2mgP79 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂477mgP95 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.