A simple thermal spring — colorless and clear, tasteless, and carrying a faint hydrogen sulfide note at the wellhead. Total dissolved solids come to just 321 mg/kg, so the water is lightly mineralized and gentle rather than assertive; weakly alkaline, it sits soft on the skin. What character it has comes from bicarbonate, which makes up just over 60% of the anions, while sulfur is present only in traces — the H₂S aroma reads as a hint rather than a strong smell.
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⁺82.7mgP24 national85%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺9.9mgP26 national12%
- Magnesiummild; a laxative if you drink itMg²⁺0.90mgP25 national2%
- Potassiumminor; matters for drinking limitsK⁺1.8mgP20 national1%
- AmmoniumNH₄⁺0.20mgP29 national0.3%
- StrontiumSr²⁺0.20mgP42 national0.1%
- Iron (II)Fe²⁺0.10mgP29 national0.1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻161mgP53 national63%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻35.0mgP23 national24%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻9.0mgP46 national7%
- Fluoridetrace; drinking limits applyF⁻4.9mgP77 national6%
- Hydrogen sulfideHS⁻0.20mgP36 national0.1%
- SulfateSO₄²⁻0.10mgP2 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃13.5mgP2 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.4mgP13 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —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.