A blended-tank source drawing on four wells (No.27 through No.30), classified as a sodium-calcium chloride spring. The salty, heat-retaining chloride water is hypotonic and neutral in pH class, and ranks as a hot source. Moderately mineralized at about 4.0 g/kg of dissolved solids, it has noticeable body on the skin.
Good for
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
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
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⁺1,030mgP85 national65%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺457mgP92 national33%
- Potassiumminor; matters for drinking limitsK⁺47.9mgP84 national2%
- MagnesiumMg²⁺2.0mgP35 national0.2%
- StrontiumSr²⁺5.5mgP91 national0.2%
- LithiumLi⁺0.80mgP70 national0.2%
- BariumBa²⁺1.1mgP90 national0.0%
- ManganeseMn²⁺0.30mgP53 national0.0%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.04mgP17 national
- ZincZn²⁺0.01mgP41 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻2,300mgP88 national98%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻53.4mgP29 national1%
- FluorideF⁻2.4mgP62 national0.2%
- BromideBr⁻8.0mgP87 national0.1%
- IodideI⁻0.20mgP52 national0.0%
- NitrateNO₃⁻0.20mgP36 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃86.7mgP49 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.5mgP34 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂9.4mgP40 national
- ArsenicAs0.07mgP59 national
- 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.
- 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.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.