An alkaline simple thermal spring blended from the two Yunoyama Omoteyu wells in a 5-to-1 ratio (low-tonicity, alkaline, low-temperature class). At pH 9.1 at the wellhead it sits in Japan's beauty-water family, giving the characteristic slippery feel on the skin. Lightly mineralized at 0.19 g/kg of dissolved solids and emerging at just 26.6 degrees Celsius, it is a gentle, low-temperature source that 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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺49.2mgP13 national92%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺3.2mgP12 national7%
- PotassiumK⁺0.60mgP7 national0.9%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻70.2mgP34 national49%
- Fluoridetrace; drinking limits applyF⁻10.4mgP92 national23%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻9.0mgP47 national13%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻9.2mgP12 national11%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻4.7mgP15 national4%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃30.5mgP13 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.8mgP24 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂7.3mgP34 national
- 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.