An alkaline simple thermal spring rising at Shotokuji in Yamanashi City. At pH 9.5 it is strongly alkaline, giving the pronounced slippery feel of Japan's beauty-water family. The water is only lightly mineralized and gentle on the body, but carries a generous 126.7 mg/kg of metasilicic acid, leaving the kind of thin silica film that makes skin feel noticeably smoother. At the source it is very faintly yellow and clear, with no taste or smell.
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⁺56.8mgP16 national95%
- Potassiumminor; matters for drinking limitsK⁺3.8mgP30 national4%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺0.60mgP1 national1%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.10mgP30 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻60.1mgP94 national74%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻24.4mgP15 national15%
- Bromidetrace, sea-originBr⁻4.0mgP78 national4%
- Hydrogen phosphateHPO₄²⁻3.5mgP92 national3%
- BO2BO₂⁻1.0mgP60 national2%
- Fluoridetrace; drinking limits applyF⁻1.0mgP47 national2%
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.50mgP57 national1%
- AsO2AsO₂⁻0.40mgP94 national0.4%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃127mgP64 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.06mgP55 national
- CadmiumCd< —mg
- ChromiumCr< —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.