A weakly alkaline sodium bicarbonate-chloride spring rising at a striking 81 °C — one of Japan's hottest natural sources at this site. The bicarbonate-dominant chemistry, classified as the legacy jūsō-sen (soda-water) style, gives the slippery feel Ureshino is famous for as one of the country's three great beauty-water onsens, while the chloride layer adds heat-retaining body. Metasilicic acid at 135.6 mg/kg leaves a faintly hydrating silica film, and the wellhead reads only mildly salty.
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
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
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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⁺464mgP74 national94%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺9.4mgP25 national2%
- LithiumLi⁺1.7mgP89 national1%
- PotassiumK⁺6.7mgP39 national0.8%
- AluminumAl³⁺0.70mgP76 national0.3%
- MagnesiumMg²⁺0.70mgP23 national0.2%
- AmmoniumNH₄⁺0.70mgP56 national0.2%
- ManganeseMn²⁺0.50mgP64 national0.1%
- BariumBa²⁺0.50mgP83 national0.0%
- StrontiumSr²⁺0.20mgP44 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻844mgP87 national66%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻211mgP56 national28%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻68.6mgP95 national11%
- FluorideF⁻1.9mgP57 national0.5%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃136mgP66 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂29.5mgP60 national
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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.