A sodium-led chloride spring with bicarbonate softness layered in and an iron-bearing character — classified as a strong-salt spring. With 36.9 g/kg of dissolved solids it exceeds ocean salinity and leaves a thick mineral film on the skin. At the source the water shows a milky supernatant with a faintly salty, bitter edge; the iron content oxidizes and colors the water on contact with air. Neutral pH keeps it from stinging despite the heavy mineralization.
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⁺11,440mgP100 national89%
- Magnesiummild; a laxative if you drink itMg²⁺476mgP99 national7%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺268mgP87 national2%
- PotassiumK⁺164mgP95 national0.8%
- AmmoniumNH₄⁺36.5mgP100 national0.4%
- LithiumLi⁺11.0mgP99 national0.3%
- ManganeseMn²⁺24.6mgP100 national0.2%
- StrontiumSr²⁺15.9mgP96 national0.1%
- BariumBa²⁺3.7mgP96 national
- ZincZn²⁺0.01mgP41 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻15,380mgP99 national78%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻7,240mgP100 national21%
- BromideBr⁻37.7mgP96 national0.1%
- SulfateSO₄²⁻15.4mgP28 national0.1%
- NitrateNO₃⁻16.2mgP98 national0.1%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Fluoridetrace; drinking limits applyF⁻< —mg
- Hydrogen phosphateHPO₄²⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1,568mgP100 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃198mgP84 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂338mgP93 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- CopperCu0.02mgP87 national
- ArsenicAs0.0020mgP8 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.
- 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.