A blended alkaline simple spring combining Yunoyama Onsen's Omote-yu No.1 and No.2 in a two-to-one ratio at the mixing tank. At pH around 9.0 the water sits firmly in Japan's beauty-water family, with a soft, slippery feel on the skin. Total dissolved solids are light at 190 mg/kg, but the source carries a notable 46.5 Bq/kg of radon — enough to place it in the radon-bearing 'hormesis' tradition. Cold at the wellhead (25.9 °C), so the bath is reheated for soaking.
Good for
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
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⁺50.0mgP14 national90%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4.0mgP15 national8%
- AmmoniumNH₄⁺0.50mgP49 national1%
- PotassiumK⁺0.60mgP7 national0.8%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻66.6mgP34 national46%
- Fluoridetrace; drinking limits applyF⁻11.4mgP93 national28%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻9.6mgP50 national14%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻9.9mgP13 national12%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻4.0mgP14 national3%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃27.7mgP10 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.0mgP24 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂5.9mgP31 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn—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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- 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.