A simple thermal spring rising in the Wanoshiro area of Kiyotaki, in the city of Nikko. It is strongly alkaline, and that shows in the feel: the water is noticeably slippery on the skin, placing it in Japan's beauty-water family. Mineralization is light, so nothing about the bath is harsh — yet at the source the water carries a faint rotten-egg note of H₂S, the one flash of character in an otherwise colorless, clear and tasteless water.
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⁺90.1mgP26 national97%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2.6mgP9 national3%
- PotassiumK⁺0.40mgP4 national0.3%
- AluminumAl³⁺< —mg0.0%
- CopperCu²⁺< —mg0.0%
- Iron (II)Fe²⁺< —mg0.0%
- Iron (III)Fe³⁺< —mg0.0%
- HydrogenH⁺< —mg0.0%
- MagnesiumMg²⁺< —mg0.0%
- ManganeseMn²⁺< —mg0.0%
- ZincZn²⁺< —mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻52.6mgP27 national36%
- Fluoridetrace; drinking limits applyF⁻17.0mgP98 national22%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻26.4mgP81 national21%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻28.1mgP18 national11%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻15.7mgP28 national8%
- BO2BO₂⁻1.2mgP65 national0.7%
- Hydrogen sulfideHS⁻0.50mgP55 national0.5%
- BromideBr⁻0.20mgP29 national0.0%
- AsO2AsO₂⁻< —mg0.0%
- H2PO4H₂PO₄⁻< —mg0.0%
- Hydrogen phosphateHPO₄²⁻< —mg0.0%
- HSiO3HSiO₃⁻< —mg0.0%
- HSO4HSO4< —mg0.0%
- NitrateNO₃⁻< —mg0.0%
- HydroxideOH⁻< —mg0.0%
- SS²⁻< —mg0.0%
- ThiosulfateS₂O₃²⁻< —mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃39.4mgP20 national
- H2SO4FreeH2SO4Free< —mg
- Phosphoric acidH₃PO₄< —mg
- Metaarsenious acidHAsO₂< —mg
- 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
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —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.