A bicarbonate spring led by sodium but carrying substantial magnesium and calcium — the certificate's older name, an earth-bearing soda spring, captures the blend well. Weakly alkaline and soft on the skin, it sits in Japan's beauty-water family, and at 1,498 mg/kg of dissolved solids the water has moderate, noticeable body. Metasilicic acid runs to 181 mg/kg, leaving the kind of silica film that makes skin feel distinctly smoother after a soak.
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⁺156mgP41 national38%
- Magnesiummild; a laxative if you drink itMg²⁺67.6mgP88 national31%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺92.5mgP71 national26%
- Potassiumminor; matters for drinking limitsK⁺38.2mgP80 national5%
- ManganeseMn²⁺0.80mgP73 national0.2%
- StrontiumSr²⁺0.60mgP72 national0.1%
- Iron (II)Fe²⁺0.40mgP48 national0.1%
- LithiumLi⁺0.10mgP29 national0.1%
- BariumBa²⁺0.30mgP77 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻785mgP85 national75%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻86.8mgP38 national14%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻86.5mgP52 national11%
- Hydrogen phosphateHPO₄²⁻0.40mgP69 national0.1%
- FluorideF⁻0.10mgP9 national0.1%
- BromideBr⁻0.20mgP30 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃181mgP81 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.1mgP20 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1.3mgP19 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.0010mgP6 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.