A simple thermal spring — gentle and lightly mineralized at just 445 mg/kg of dissolved solids — that free-flows from a 250 m borehole in the Tsurumi district of Beppu at a scalding 85.4 °C, putting it among Japan's hottest natural sources. The pH is neutral and the water runs colorless and clear at the wellhead, with almost no taste or smell. What lifts it above plain hot water is metasilicic acid at 92 mg/kg — enough to place it in Japan's beauty-water family and leave the kind of thin silica film that makes skin feel smoother afterwards. A notice posted under the certificate states the source is diluted with water because it arrives too hot to enter.
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⁺64.1mgP19 national52%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺31.1mgP46 national29%
- Magnesiummild; a laxative if you drink itMg²⁺9.6mgP61 national15%
- Potassiumminor; matters for drinking limitsK⁺6.8mgP39 national3%
- StrontiumSr²⁺0.30mgP55 national0.2%
- AmmoniumNH₄⁺0.20mgP30 national0.2%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻73.4mgP33 national40%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻92.9mgP53 national37%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻65.4mgP34 national21%
- NitrateNO₃⁻4.7mgP97 national2%
- ThiosulfateS₂O₃²⁻0.60mgP66 national0.2%
- BromideBr⁻0.20mgP30 national0.0%
- Fluoridetrace; drinking limits applyF⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃92.3mgP53 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.2mgP25 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂15.4mgP48 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP68 national
- ArsenicAs< —mg
- 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.