湯SimpleSodium Chloride spring
A sodium-led alkaline simple thermal spring (pH 9.5 at the wellhead). Mineralization is light at around 400 mg/kg, giving a gentle composition, while the alkalinity delivers the slippery beauty-water feel on the skin. Cold at the wellhead (23 °C), it 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.
Cationsshare of charge (mval%)
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺146mgP39 national88%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺11.4mgP28 national8%
- Iron (II)Fe²⁺1.2mgP63 national0.6%
- PotassiumK⁺0.80mgP11 national0.3%
- MagnesiumMg²⁺0.04mgP5 national0.1%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
Anionsshare of charge (mval%)
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻175mgP52 national79%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻9.6mgP6 national13%
- Fluoridetrace; drinking limits applyF⁻5.4mgP79 national4%
- SulfateSO₄²⁻2.9mgP12 national1.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻23.4mgP72 national
- Bromidetrace, sea-originBr⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
Non-dissociatedmeasured by mass
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃39.0mgP21 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂< —mg
Dissolved gasesmeasured by mass
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
Trace metalsmeasured by mass
- LeadPb0.01mgP46 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
Measured at the source
pH
9.50
Alkaline
Source temperature
23.0°C
Low temp.
Tonicity
0.40g/kg
Hypotonic
Flow rate
76.5L/min
Typical
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.