An alkaline simple thermal spring drawn from the Nyunokawa valley. At pH 9.3 it sits in Japan's beauty-water family — strongly enough alkaline to give skin the characteristic slippery feel. Mineralization is light at only 297 mg/kg, so the water reads as soft and gentle rather than chemically loud, with the alkaline pH carrying the sensory signature on its own.
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⁺75.9mgP24 national95%
- Potassiumminor; matters for drinking limitsK⁺4.9mgP34 national4%
- CalciumCa²⁺0.70mgP1 national0.9%
- Iron (II)Fe²⁺0.40mgP48 national0.3%
- CopperCu²⁺0.01mgP71 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻137mgP48 national64%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻27.0mgP81 national26%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻14.0mgP26 national8%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1.5mgP2 national1%
- FluorideF⁻0.30mgP24 national0.6%
- BO2BO₂⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃34.5mgP16 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.10mgP5 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
- 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.