A sodium-leaning simple thermal spring drawn at 56 °C from a 150 m well on Beppu's seafront. Weakly alkaline at pH 8.2, it has the soft, slightly slippery feel of Japan's beauty-water family. Total dissolved solids sit at 968 mg/kg — just shy of the threshold that would push it into a named class — and the metasilicic acid is unusually generous at 192.6 mg/kg, leaving the kind of silica film that makes skin feel noticeably smoother afterwards.
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 national63%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺38.9mgP52 national16%
- Magnesiummild; a laxative if you drink itMg²⁺—mg13%
- Potassiumminor; matters for drinking limitsK⁺28.3mgP73 national6%
- LithiumLi⁺0.12mgP30 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.08mgP19 national
- Manganesetrace; companion to iron springsMn²⁺0.05mgP22 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻275mgP61 national65%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻165mgP53 national23%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻55.0mgP41 national10%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻12.0mgP56 national3%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃193mgP83 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂8.1mgP49 national
- Metaarsenious acidHAsO₂0.30mgP76 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂10.7mgP41 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.