A gentle simple thermal spring rising in Beppu's Shiomi-machi district, weakly alkaline and lightly mineralized at just 0.4 g/kg of dissolved solids — soft on the body with very little bite to it. What lifts it above ordinary simple water is metasilicic acid at 175 mg/kg, generous enough to leave the thin silica film that places this water in Japan's beauty-water family and makes skin feel noticeably smoother afterwards. At the wellhead it runs faintly yellow but clear, carrying a very faint hydrogen sulfide note that has already faded by the time the sample reaches the lab.
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⁺31.8mgP6 national42%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺19.2mgP37 national29%
- Magnesiummild; a laxative if you drink itMg²⁺9.1mgP60 national23%
- Potassiumminor; matters for drinking limitsK⁺6.4mgP37 national5%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺1.5mgP68 national2%
- LithiumLi⁺0.10mgP28 national0.3%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻102mgP43 national51%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻38.6mgP24 national33%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻23.8mgP32 national15%
- Hydrogen phosphateHPO₄²⁻0.40mgP68 national0.3%
- FluorideF⁻0.20mgP16 national0.3%
- BromideBr⁻0.10mgP14 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃175mgP78 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.0mgP12 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂23.3mgP54 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.0070mgP19 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.