A gentle simple thermal spring rising in the Matsubara district of Beppu. Weakly alkaline and only lightly mineralized at 910 mg/kg of dissolved solids, it nonetheless carries a generous 207 mg/kg of metasilicic acid, which puts it squarely in Japan's beauty-water family: silica that thick leaves a thin film on the skin and makes it feel noticeably smoother afterwards. The water is colorless, clear, tasteless and odorless, so there is nothing sharp or heavy about it. Note that the certificate samples at the bath inlet rather than the wellhead, so these figures describe the water as it actually reaches the tub.
Good for
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
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⁺112mgP33 national48%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺48.6mgP57 national24%
- Magnesiummild; a laxative if you drink itMg²⁺29.1mgP75 national23%
- Potassiumminor; matters for drinking limitsK⁺13.0mgP50 national3%
- LithiumLi⁺1.0mgP74 national1%
- ManganeseMn²⁺0.40mgP59 national0.1%
- StrontiumSr²⁺0.30mgP55 national0.1%
- Iron (II)Fe²⁺0.20mgP39 national0.1%
- BariumBa²⁺0.20mgP74 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻261mgP61 national42%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻148mgP48 national41%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻83.4mgP51 national17%
- BromideBr⁻0.40mgP39 national0.1%
- Hydrogen phosphateHPO₄²⁻0.40mgP69 national0.1%
- FluorideF⁻0.20mgP17 national0.1%
- ThiosulfateS₂O₃²⁻0.20mgP55 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃207mgP87 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.5mgP34 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂22.9mgP54 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.05mgP51 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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- 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.