Yuzawa Park Onsen is a hot (42.5 °C at the source) simple thermal spring on the alkaline end of the scale (pH 8.7 at the wellhead, 9.2 in the lab). It sits squarely in Japan's classic 'beauty-water' family — soft, slippery feel on the skin from the high pH. Lightly mineralized at 454 mg/kg of dissolved solids, so the body of the water stays gentle.
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺72.6mgP63 national62%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺50.3mgP14 national38%
- PotassiumK⁺1.1mgP14 national0.5%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.0000mg
- Magnesiummild; a laxative if you drink itMg²⁺0.0000mg
- Manganesetrace; companion to iron springsMn²⁺0.0000mg
- AmmoniumNH₄⁺0.0000mg
- ZincZn²⁺0.0000mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻237mgP67 national82%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻21.3mgP17 national10%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻9.0mgP47 national5%
- Fluoridetrace; drinking limits applyF⁻2.0mgP60 national2%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻5.5mgP3 national2%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃54.6mgP32 national
- ArsenicAs< —mg
- CadmiumCd< —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.