Wakamiro Onsen is a simple thermal spring (hypotonic, neutral, low-temperature), led by sodium with a bicarbonate slant. The wellhead reads 27.7 C, so the water is reheated for bathing. At the source it is colorless and clear with a metallic note; left to stand it picks up a faint yellow tint. A generous 115.9 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⁺130mgP36 national67%
- Magnesiummild; a laxative if you drink itMg²⁺17.0mgP70 national17%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺17.8mgP36 national11%
- Potassiumminor; matters for drinking limitsK⁺14.7mgP52 national5%
- LithiumLi⁺0.20mgP37 national0.4%
- ManganeseMn²⁺0.30mgP54 national0.1%
- Iron (II)Fe²⁺0.20mgP38 national0.1%
- AmmoniumNH₄⁺0.20mgP29 national0.1%
- BariumBa²⁺0.10mgP49 national0.0%
- StrontiumSr²⁺0.10mgP24 national0.0%
- AluminumAl³⁺0.0000mg0.0%
- Iron (III)Fe³⁺0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- ZincZn²⁺0.0000mg0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻329mgP67 national64%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻80.2mgP33 national27%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻37.0mgP38 national9%
- Hydrogen phosphateHPO₄²⁻1.2mgP82 national0.3%
- FluorideF⁻0.30mgP22 national0.2%
- IodideI⁻0.40mgP66 national0.0%
- BromideBr⁻0.20mgP28 national0.0%
- AsO2AsO₂⁻0.0000mg0.0%
- BO2BO₂⁻0.0000mg0.0%
- CarbonateCO₃²⁻0.0000mg0.0%
- Hydrogen sulfideHS⁻0.0000mg0.0%
- HSiO3HSiO₃⁻0.0000mg0.0%
- HSO4HSO40.0000mg0.0%
- ThiosulfateS₂O₃²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃116mgP59 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.0mgP18 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂53.3mgP70 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.01mgP27 national
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —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.