A sodium-led chloride spring with bicarbonate softness layered in, moderately mineralized and with noticeable body. It runs alkaline, giving the slippery feel that places it squarely in Japan's beauty-water family, and a generous 178 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother afterwards. The wellhead itself is among Japan's hottest natural sources. A notice posted at the facility says every bath here runs as aoyu — blue water — while adding that on some days the blue simply doesn't show.
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⁺415mgP71 national95%
- Potassiumminor; matters for drinking limitsK⁺27.2mgP72 national4%
- LithiumLi⁺0.60mgP61 national0.5%
- CalciumCa²⁺1.1mgP3 national0.3%
- AluminumAl³⁺0.40mgP74 national0.2%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻387mgP70 national54%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻125mgP99 national21%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻142mgP50 national12%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻101mgP55 national10%
- BO2BO₂⁻19.7mgP97 national2%
- FluorideF⁻2.0mgP59 national0.6%
- HydroxideOH⁻0.30mgP74 national0.1%
- BromideBr⁻1.1mgP62 national0.1%
- Hydrogen sulfideHS⁻0.40mgP52 national0.1%
- AsO2AsO₂⁻0.40mgP94 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃178mgP79 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.26mgP85 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.