A simple thermal spring drawn from Beppu City's municipal hot-spring supply on its Fujimi line — hypotonic, weakly alkaline, and gentle on the body at just 779 mg/kg of dissolved solids. What lifts it above plain is the silica: metasilicic acid at 163.7 mg/kg leaves the kind of thin mineral film that puts this water squarely in Japan's beauty-water family, soft and smooth-feeling rather than assertive. The bath itself arrives hot — a handwritten newspaper posted on site notes that bathers are free to run cold water in and temper the tub to whatever suits them.
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 national81%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺14.0mgP31 national8%
- Potassiumminor; matters for drinking limitsK⁺16.6mgP56 national5%
- Magnesiummild; a laxative if you drink itMg²⁺4.2mgP44 national4%
- LithiumLi⁺1.6mgP88 national3%
- AluminumAl³⁺0.10mgP54 national0.1%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻157mgP49 national50%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻166mgP54 national31%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻78.1mgP50 national19%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃164mgP74 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.9mgP41 national
- Metaarsenious acidHAsO₂0.10mgP56 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂15.3mgP48 national
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.