A gently mineralized simple thermal spring from Kamegawa-Higashimachi in Beppu, carrying only about 901 mg/kg of dissolved solids — the water is mild and undemanding on the skin. It is weakly alkaline, which gives it a soft, rounded feel rather than any bite. What lifts it out of the ordinary is metasilicic acid at 237 mg/kg: that much silica leaves a thin moisturizing film and puts the spring squarely in Japan's beauty-water family. At the wellhead the water is hot enough to sit in the highest temperature class, among the hotter natural sources in the country.
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⁺172mgP43 national82%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺14.4mgP31 national8%
- Potassiumminor; matters for drinking limitsK⁺26.0mgP71 national7%
- Magnesiummild; a laxative if you drink itMg²⁺1.8mgP33 national2%
- LithiumLi⁺0.90mgP72 national1%
- ManganeseMn²⁺0.20mgP45 national0.1%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻170mgP52 national49%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻168mgP54 national28%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻102mgP55 national22%
- NitrateNO₃⁻0.80mgP70 national0.1%
- BromideBr⁻0.50mgP47 national0.1%
- FluorideF⁻0.10mgP9 national0.1%
- Hydrogen phosphateHPO₄²⁻0.20mgP54 national0.0%
- ThiosulfateS₂O₃²⁻0.10mgP45 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃237mgP93 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.0mgP46 national
- Metaarsenious acidHAsO₂0.40mgP81 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.8mgP38 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.31mgP89 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.