A lightly mineralized simple thermal water in the Gero Onsen tradition, carrying just 353.9 mg/kg of dissolved solids — gentle on the body. The cations are almost entirely sodium (96.55 mval%), and with bicarbonate and carbonate ions in the mix the water is soft on the skin, of the classic Japanese beauty-water family. A generous 66.7 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother afterwards.
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⁺140mgP38 national97%
- Potassiumminor; matters for drinking limitsK⁺1.4mgP17 national3%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2.5mgP10 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.09mgP37 national
- FeTotalFe< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Fluoridetrace; drinking limits applyF⁻17.0mgP98 national18%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻38.7mgP23 national17%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻25.4mgP77 national17%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻11.0mgP22 national4%
- BO2BO₂⁻5.4mgP83 national2%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻8.1mgP11 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.30mgP47 national
- AsO2AsO₂⁻0.20mgP87 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃66.7mgP42 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —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.
- 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.