Component table for Gero Onsen. Dissolved solids run light at 374 mg/kg, placing the water in simple-thermal territory — gentle on the body, low in mineral edge. The clear presence of carbonate ion (CO₃²⁻) signals an alkaline pH, and roughly 60 mg/kg of metasilicic acid contributes the soft, silica-led feel Gero is known for.
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 national95%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4.3mgP16 national4%
- PotassiumK⁺1.9mgP22 national0.9%
- Magnesiummild; a laxative if you drink itMg²⁺0.20mgP14 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.09mgP37 national
- FeTotalFe< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻105mgP42 national56%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻26.0mgP79 national17%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻37.5mgP22 national12%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻11.7mgP23 national5%
- Hydrogen sulfideHS⁻0.80mgP66 national0.4%
- NitrateNO₃⁻0.20mgP36 national
- Fluoridetrace; drinking limits applyF⁻0.11mgP9 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< —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.