A lightly mineralized water (374.4 mg/kg total dissolved solids), gentle on the body and falling in simple-thermal territory. Chloride is the leading anion, lending only a faint salty edge rather than a strong brine character. Metasilicic acid runs a touch high at 59.7 mg/kg, the kind of silica content that tends to leave skin feeling smoother.
Good for
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
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⁺110mgP32 national93%
- PotassiumK⁺1.9mgP22 national0.9%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺6.3mgP20 national
- Magnesiummild; a laxative if you drink itMg²⁺< 0.10mg
- Aluminumshows up in acidic volcanic waterAl³⁺0.09mgP37 national
- FeTotalFe< 0.02mg
- Manganesetrace; companion to iron springsMn²⁺< 0.0010mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻105mgP41 national56%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻26.0mgP79 national17%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻37.5mgP22 national12%
- SulfateSO₄²⁻0.38mgP4 national0.3%
- NitrateNO₃⁻2.7mgP93 national
- Fluoridetrace; drinking limits applyF⁻0.10mgP9 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.02mgP19 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃59.7mgP36 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.1mgP43 national
- Metaarsenious acidHAsO₂0.10mgP56 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< 0.09mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< 0.0030mg
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.
- 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.