A composition table for Gero Onsen. The water is lightly mineralized, of the simple-thermal family, led by sodium and calcium with modest bicarbonate and sulfate. It carries a fair amount of metasilicic acid (about 57 mg/kg), the silica that gives skin a smoother feel, along with around 60 mg/kg of free carbon dioxide. Spring-type label, pH and temperature are not printed on this partial poster.
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⁺17.2mgP4 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺10.8mgP27 national
- Potassiumminor; matters for drinking limitsK⁺5.2mgP35 national
- Manganesetrace; companion to iron springsMn²⁺0.0060mgP15 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻37.0mgP21 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻21.0mgP32 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻13.1mgP15 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃57.3mgP34 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.0mgP46 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂60.1mgP72 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.
- 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.