A bicarbonate spring drawn in the Wakakusa district of Beppu, led by sodium with magnesium and calcium alongside it. Weakly alkaline and soft on the skin, it sits squarely in Japan's beauty-water family, and a moderate 1,410 mg/kg of dissolved solids gives the water noticeable body. The standout figure is metasilicic acid at 201 mg/kg — generous enough to leave the thin silica film that makes skin feel distinctly smoother afterwards.
Good for
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
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
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⁺163mgP42 national44%
- Magnesiummild; a laxative if you drink itMg²⁺55.9mgP86 national29%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺66.0mgP63 national21%
- Potassiumminor; matters for drinking limitsK⁺35.4mgP79 national6%
- LithiumLi⁺0.40mgP50 national0.4%
- CopperCu²⁺< —mg
- ZincZn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻706mgP83 national73%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻86.6mgP37 national15%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻90.2mgP52 national12%
- Hydrogen phosphateHPO₄²⁻1.3mgP84 national0.2%
- FluorideF⁻0.20mgP17 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃201mgP85 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.7mgP28 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂16.1mgP49 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.06mgP56 national
- CadmiumCd< —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.