A lightly mineralized simple thermal spring: the Gero Onsen composition table lists just 374 mg/kg of dissolved solids, so the water is gentle and unassertive on the body. Chloride leads the anion side, but at such low absolute concentration it reads as softness rather than salt. What stands out instead is metasilicic acid at around 60 mg/kg — enough to leave the thin silica film that puts this water squarely in Japan's beauty-water family.
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⁺110mgP31 national95%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4.3mgP15 national4%
- PotassiumK⁺1.9mgP21 national1.0%
- 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
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻105mgP42 national56%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻26.0mgP80 national17%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻37.5mgP23 national12%
- Fluoridetrace; drinking limits applyF⁻10.1mgP92 national10%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻12.7mgP25 national5%
- Hydrogen sulfideHS⁻0.70mgP60 national0.4%
- NitrateNO₃⁻0.20mgP38 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃59.7mgP36 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.1mgP42 national
- Metaarsenious acidHAsO₂0.10mgP53 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.