A sodium-dominant chloride spring — salty and heat-retaining — rising at 84.8 °C, which places it among Japan's hottest natural sources. The water is weakly acidic, colorless and clear at the tap, with a faintly salty, faintly metallic tang if tasted. Moderate mineralization (3.5 g/kg of dissolved solids) gives it noticeable body and the long-lasting warmth chloride water is known for, while an unusually generous 387 mg/kg of metasilicic acid puts it squarely in Japan's beauty-water family, leaving a thin silica film that makes skin feel smoother. The sample for this certificate was taken at the storage-tank inlet rather than at the wellhead.
Good for
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
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
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⁺965mgP84 national84%
- Potassiumminor; matters for drinking limitsK⁺147mgP95 national8%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺40.0mgP53 national4%
- LithiumLi⁺10.5mgP98 national3%
- MagnesiumMg²⁺5.5mgP52 national0.9%
- HydrogenH⁺0.10mgP54 national0.2%
- FeTotalFe1.6mgP77 national0.1%
- ManganeseMn²⁺1.3mgP83 national0.1%
- AmmoniumNH₄⁺0.90mgP64 national0.1%
- StrontiumSr²⁺0.20mgP44 national0.0%
- BariumBa²⁺0.10mgP53 national0.0%
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,446mgP84 national81%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻454mgP85 national19%
- FluorideF⁻1.8mgP56 national0.2%
- BromideBr⁻6.0mgP84 national0.2%
- HSO4HSO41.8mgP50 national0.0%
- IodideI⁻0.30mgP62 national0.0%
- H2PO4H₂PO₄⁻0.20mgP57 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃387mgP98 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂56.6mgP91 national
- Metaarsenious acidHAsO₂1.8mgP94 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂41.7mgP65 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- 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.