Umezono Onsen's own source in Motomachi, Beppu, is a simple thermal spring: neutral, hypotonic, and lightly mineralized at 856 mg/kg of dissolved solids, so it sits soft on the body rather than announcing itself. At the wellhead the water is clear with a very faint yellow tint and leaves a weak metallic note on the tongue from a trace of iron. What lifts it is the silica. Metasilicic acid runs to 213 mg/kg, generous enough to leave the thin film that makes skin feel noticeably smoother afterwards, placing it squarely in Japan's beauty-water family. Bicarbonate dominates the anion balance at 77.5 mval%, lending a soda-like softness even though the low total mineralization keeps the classification at simple spring.
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⁺101mgP30 national53%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺35.0mgP49 national21%
- Magnesiummild; a laxative if you drink itMg²⁺20.8mgP70 national21%
- Potassiumminor; matters for drinking limitsK⁺11.4mgP47 national3%
- LithiumLi⁺0.90mgP72 national2%
- Iron (II)Fe²⁺1.0mgP62 national0.5%
- StrontiumSr²⁺0.20mgP44 national0.0%
- BariumBa²⁺0.10mgP53 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻391mgP71 national78%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻52.1mgP40 national13%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻27.5mgP20 national9%
- Fluoridetrace; drinking limits applyF⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃213mgP88 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.7mgP16 national
- Metaarsenious acidHAsO₂0.10mgP56 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂57.6mgP71 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.09mgP62 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.