A blend of three wells — Mitsui Kinzoku No.1, Kannon and Tenyu — sampled not at a wellhead but at the mixing-tank outlet, after dilution. Bicarbonate accounts for 81% of the anions, yet with only 0.86 g/kg of dissolved solids the certificate files it as a simple thermal spring, hypotonic and neutral at pH 6.4. It leaves the ground at 67.7 °C, among Japan's hotter natural sources, faintly yellow and clear with pale yellow-white flecks drifting in it. Metasilicic acid at 153 mg/kg is generous enough to leave the thin silica film that puts this water in Japan's beauty-water family.
Good for
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
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⁺161mgP42 national76%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺23.9mgP41 national13%
- Potassiumminor; matters for drinking limitsK⁺19.9mgP62 national5%
- Magnesiummild; a laxative if you drink itMg²⁺4.2mgP45 national4%
- LithiumLi⁺1.4mgP85 national2%
- Iron (II)Fe²⁺0.60mgP57 national0.2%
- StrontiumSr²⁺0.20mgP41 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻427mgP74 national81%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻32.7mgP22 national11%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻29.0mgP35 national7%
- Fluoridetrace; drinking limits applyF⁻2.0mgP59 national1%
- NitriteNO₂⁻0.30mgP57 national0.1%
- NitrateNO₃⁻0.20mgP42 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃153mgP69 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.6mgP48 national
- Metaarsenious acidHAsO₂0.10mgP53 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂188mgP88 national
- CadmiumCd< —mg
- ChromiumCr< —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.