An alkaline simple thermal spring rising in Koshu's Yamato district. Its high pH gives the slippery, skin-smoothing feel of Japan's beauty-water family, and a faint hydrogen sulfide note drifts up at the source. Lightly mineralized at just 0.162 g/kg of dissolved solids, the water is gentle and soft on the skin.
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⁺48.9mgP13 national97%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1.0mgP2 national2%
- PotassiumK⁺0.30mgP3 national0.5%
- AluminumAl³⁺0.10mgP54 national0.5%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻44.2mgP90 national56%
- HSiO3HSiO₃⁻49.7mgP93 national24%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻10.3mgP21 national8%
- HydroxideOH⁻2.6mgP96 national6%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻3.7mgP6 national4%
- BO2BO₂⁻1.1mgP62 national1%
- FluorideF⁻0.20mgP17 national0.4%
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- 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.