A calcium- and sodium-led sodium chloride spring, richly mineralized at roughly 21 g/kg of dissolved solids and classified as hypertonic. At the wellhead the water is faintly yellow and slightly turbid, strongly salty with an oily odor and visible gas evolution. Because the source emerges below 25 °C as a cold-mineral spring, it is reheated for bathing.
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4,282mgP100 national58%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺3,549mgP95 national42%
- PotassiumK⁺52.2mgP85 national0.3%
- MagnesiumMg²⁺15.7mgP68 national0.3%
- ManganeseMn²⁺2.2mgP92 national0.0%
- AluminumAl³⁺0.10mgP54 national0.0%
- CopperCu²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻13,140mgP98 national100%
- BicarbonateHCO₃⁻30.9mgP19 national0.1%
- HydroxideOH⁻0.02mgP26 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂27.2mgP79 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃19.0mgP4 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂24.7mgP55 national
- ArsenicAs0.0050mgP15 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.
- 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.