An alkaline simple thermal spring emerging at 48.0 C. A sodium-led, lightly mineralized water (about 434 mg/kg dissolved solids) that lands firmly in Japan's beauty-water family thanks to pH 9.6 — strongly alkaline enough to give a pronounced slippery feel on the skin. A respectable 54.9 mg/kg of metasilicic acid layers in a thin silica film for noticeable smoothness, while a faint rotten-egg whisper of hydrogen sulfide and a slightly salty edge keep the water from reading as bland.
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⁺117mgP34 national87%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺14.1mgP31 national12%
- PotassiumK⁺1.3mgP15 national0.6%
- LithiumLi⁺0.10mgP29 national0.2%
- HydrogenH⁺0.0000mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻105mgP42 national48%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻121mgP58 national41%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻12.5mgP56 national7%
- BO2BO₂⁻4.9mgP81 national2%
- Hydrogen sulfideHS⁻1.5mgP75 national0.8%
- FluorideF⁻0.80mgP44 national0.7%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃54.9mgP33 national
- Metaarsenious acidHAsO₂0.40mgP81 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂43.8mgP67 national
- ArsenicAs0.26mgP85 national
- 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.
- 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.