A simple thermal spring blended from two wells in Kawakami, Yufuin — drilled to 248 m and 200 m — whose water meets before reaching the storage tank, which is where the sample for this certificate was drawn. Weakly alkaline and lightly mineralized at roughly 0.7 g/kg of dissolved solids, it is gentle water with little bite to it. What lifts it is the silica: 220 mg/kg of metasilicic acid puts it squarely in Japan's beauty-water family, leaving the thin film that makes skin feel noticeably smoother afterwards.
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⁺107mgP30 national73%
- Potassiumminor; matters for drinking limitsK⁺25.0mgP70 national10%
- Magnesiummild; a laxative if you drink itMg²⁺6.9mgP57 national9%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺9.6mgP25 national8%
- LithiumLi⁺0.20mgP38 national0.5%
- ManganeseMn²⁺0.10mgP36 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻211mgP59 national54%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻75.8mgP33 national33%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻39.0mgP37 national13%
- Hydrogen phosphateHPO₄²⁻0.80mgP78 national0.3%
- FluorideF⁻0.30mgP22 national0.3%
- NitrateNO₃⁻0.60mgP66 national0.1%
- BromideBr⁻0.20mgP29 national0.0%
- ThiosulfateS₂O₃²⁻0.10mgP43 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃220mgP90 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.9mgP28 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂20.2mgP52 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.06mgP55 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.
- 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.