金具屋第1ボーリングKanaguya No. 1 Borehole
A sulfur-bearing sodium-calcium chloride-sulfate spring that rises under its own pressure at 97 °C — among Japan's hottest natural sources, and necessarily cooled before it reaches a bath. At the wellhead the water is almost colorless and clear, yet it carries a frank sulfurous taste over a light saltiness and the unmistakable rotten-egg note of H₂S. Weakly alkaline and soft on the skin, it also runs a generous 219.6 mg/kg of metasilicic acid, putting it squarely in Japan's beauty-water family, while the chloride and sulfate load gives the moderate mineralization a warmth that lingers well after the bath.
Good for
Skin conditions
Sulfur and acidic waters are antibacterial and keratolytic, studied for atopic dermatitis and psoriasis.
atopic skin, eczema
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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, netto 熱の湯).
cold sensitivity
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
Take care
Sensitive-skin caution
Strongly acidic or sulfur-rich water can sting broken, atopic-flare, or dry elderly skin. Ease in slowly and rinse off if it stings.
broken or dry skin
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⁺364mgP68 national70%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺110mgP76 national24%
- Potassiumminor; matters for drinking limitsK⁺39.6mgP81 national4%
- LithiumLi⁺1.0mgP74 national0.6%
- MagnesiumMg²⁺0.20mgP14 national0.1%
- StrontiumSr²⁺0.50mgP68 national0.0%
- AluminumAl³⁺0.04mgP28 national0.0%
- ManganeseMn²⁺0.05mgP22 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻497mgP76 national63%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻342mgP77 national32%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻50.4mgP29 national4%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻7.1mgP40 national1%
- Hydrogen sulfideHS⁻2.4mgP79 national0.3%
- FluorideF⁻1.4mgP53 national0.3%
- BromideBr⁻2.6mgP75 national0.1%
- ThiosulfateS₂O₃²⁻1.5mgP77 national0.1%
- IodideI⁻0.70mgP76 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃220mgP89 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂74.7mgP94 national
- Metaarsenious acidHAsO₂1.7mgP93 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.70mgP15 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP65 national
- CadmiumCdNDmg
- ChromiumCrNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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.
- Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
- Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Akiyama H et al. Antimicrobial effects of acidic hot-spring water on S. aureus. J. Dermatological Science 24(3):193–200, 2000.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- 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.
- Ministry of the Environment, Japan. 禁忌症及び入浴又は飲用上の注意事項 (2014 notice).official