A sodium-led chloride spring with bicarbonate softness layered in, giving the water a rounder feel than salinity alone would suggest. Weakly alkaline and soft on the skin, it carries moderate mineralization with noticeable body. At the wellhead the water shows a faint metallic iron note in taste and smell, and a respectable 58.9 mg/kg of metasilicic acid leaves the kind of thin film that makes skin feel 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⁺330mgP64 national84%
- Magnesiummild; a laxative if you drink itMg²⁺14.6mgP67 national7%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺22.1mgP41 national6%
- Potassiumminor; matters for drinking limitsK⁺13.1mgP50 national2%
- AmmoniumNH₄⁺0.30mgP40 national0.1%
- StrontiumSr²⁺0.30mgP55 national0.1%
- LithiumLi⁺0.10mgP29 national0.1%
- Iron (II)Fe²⁺0.10mgP30 national0.0%
- AluminumAl³⁺0.0000mg0.0%
- BariumBa²⁺0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- ManganeseMn²⁺0.0000mg0.0%
- ZincZn²⁺0.0000mg0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻320mgP66 national53%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻396mgP71 national38%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻63.9mgP45 national8%
- FluorideF⁻0.80mgP44 national0.2%
- BromideBr⁻1.0mgP59 national0.1%
- CarbonateCO₃²⁻0.0000mg0.0%
- IodideI⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃58.9mgP35 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.0mgP46 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂30.7mgP61 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.03mgP40 national
- CadmiumCd< —mg
- CopperCu< —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.