A sodium- and magnesium-led bicarbonate-chloride spring, carried on the certificate under its older name as a chloride-earth soda water — the jūsō-sen style, soft on the skin, with a salt-tinged finish from the chloride sub-type. Neutral and hypotonic, it is gentle rather than assertive, with a moderate 1,167 mg/kg of dissolved solids giving it real body. What stands out is metasilicic acid at 243 mg/kg, well into Japan's beauty-water territory: silica at that level leaves a thin film that makes skin feel noticeably smoother afterwards. Note that the sample was drawn at the bath inlet rather than at the wellhead, so these figures describe the water as it actually reaches the tub.
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
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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⁺154mgP40 national54%
- Magnesiummild; a laxative if you drink itMg²⁺37.9mgP81 national25%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺37.8mgP51 national15%
- Potassiumminor; matters for drinking limitsK⁺28.6mgP73 national6%
- LithiumLi⁺0.30mgP44 national0.3%
- StrontiumSr²⁺0.20mgP44 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻427mgP73 national55%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻127mgP45 national28%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻106mgP55 national17%
- Hydrogen phosphateHPO₄²⁻0.50mgP70 national0.1%
- FluorideF⁻0.20mgP17 national0.1%
- BromideBr⁻0.30mgP36 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃243mgP94 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.2mgP31 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂127mgP82 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.02mgP31 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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.