Osawa Onsen Well No. 5 is a simple thermal spring in Matsuzaki, Shizuoka. Weakly alkaline (pH 8.1), it is soft on the skin and sits near the gentle 'beauty-water' family. With light mineralization of just 0.787 g/kg of dissolved solids, it is a mild, low-stimulus water. Because the source runs cool, the bath is reheated in winter.
Good for
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⁺128mgP78 national
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺102mgP30 national
- Potassiumminor; matters for drinking limitsK⁺2.6mgP25 national
- StrontiumSr²⁺0.60mgP72 national
- Magnesiummild; a laxative if you drink itMg²⁺0.20mgP14 national
- AmmoniumNH₄⁺0.10mgP24 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻480mgP87 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻20.2mgP13 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻15.9mgP16 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻3.3mgP29 national
- Fluoridetrace; drinking limits applyF⁻0.40mgP31 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃33.6mgP16 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.60mgP10 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.
- 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.