A sodium-calcium chloride spring drawn from a 25.3 °C wellhead beneath the Banpaku Expo park. Neutral pH and a salty, mineral-tinged taste reflect 6.3 g/kg of dissolved solids — moderately mineralized with noticeable body. Because the source itself is classed as a low-temperature spring (under 34 °C), the water is heated before bathing. A generous 117 mg/kg of metasilicic acid puts it in the Japanese 'beauty-water' family, leaving a thin silica film that makes skin feel noticeably smoother afterwards.
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
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
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⁺1,542mgP89 national65%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺431mgP91 national21%
- Magnesiummild; a laxative if you drink itMg²⁺139mgP94 national11%
- Potassiumminor; matters for drinking limitsK⁺105mgP92 national3%
- LithiumLi⁺2.4mgP93 national0.3%
- AmmoniumNH₄⁺3.9mgP88 national0.2%
- BariumBa²⁺1.1mgP90 national0.0%
- ManganeseMn²⁺0.40mgP59 national0.0%
- StrontiumSr²⁺0.40mgP62 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻3,026mgP90 national88%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻736mgP84 national12%
- BromideBr⁻6.5mgP84 national0.1%
- IodideI⁻2.6mgP89 national0.0%
- FluorideF⁻0.20mgP17 national0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen phosphateHPO₄²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- NitrateNO₃⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂138mgP97 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃117mgP60 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂263mgP91 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.
- 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.