A hot (72.5 °C) spring in the Kurokawa Onsen district, hypotonic and weakly acidic. With total dissolved solids of 682.7 mg/kg it falls into the simple-thermal classification — lightly mineralized rather than punchy. The standout figure is metasilicic acid at 169.8 mg/kg, well into the high-silica range, which leaves the kind of fine silica film that puts the water squarely 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
Take care
- 急性疾患(特に熱のある場合)
- Active tuberculosis
- Malignant tumor
- 重い心臓病
- 呼吸不全腎不全
- Bleeding disorders
- Severe anemia
- その他一般に病勢進行中の疾患
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⁺194mgP48 national73%
- Potassiumminor; matters for drinking limitsK⁺61.3mgP86 national14%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺14.6mgP33 national6%
- Magnesiummild; a laxative if you drink itMg²⁺4.8mgP50 national3%
- LithiumLi⁺0.90mgP72 national1%
- Iron (II)Fe²⁺1.5mgP69 national0.5%
- AluminumAl³⁺0.40mgP78 national0.4%
- ManganeseMn²⁺0.90mgP79 national0.3%
- AmmoniumNH₄⁺0.34mgP39 national0.1%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.40mgP71 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻176mgP52 national51%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻226mgP71 national48%
- FluorideF⁻1.0mgP49 national0.5%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃170mgP78 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂11.1mgP56 national
- Radonfaint radon — studied for joint pain (hormesis)Rn< —mg
Measured at the source
On the certificate
- Chronic digestive disorders
- Hemorrhoids
- Cold sensitivity
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.