A cold-mineral spring drawn at 24 °C from Awara Onsen's Well No. 40. The chemistry is sodium-chloride leaning but lightly mineralized at just 831 mg/kg, so it falls short of the curative-spring threshold and carries no formal spring-quality name; it qualifies as an onsen under Article 2 of the Hot Spring Law by virtue of its fluoride content. Weakly alkaline at pH 8.4 with a very faint hydrogen sulfide whiff at the wellhead, hypotonic, and heated for bathing.
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺231mgP56 national81%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺43.1mgP55 national17%
- Potassiumminor; matters for drinking limitsK⁺7.9mgP41 national2%
- MagnesiumMg²⁺0.40mgP18 national0.2%
- StrontiumSr²⁺0.80mgP76 national0.2%
- CopperCu²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻309mgP65 national71%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻99.0mgP54 national17%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻81.7mgP39 national11%
- Fluoridetrace; drinking limits applyF⁻2.5mgP63 national1%
- CarbonateCO₃²⁻0.80mgP18 national0.2%
- BromideBr⁻1.0mgP59 national0.1%
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- SS²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃49.2mgP28 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.8mgP36 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- ArsenicAs< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn—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.