A cold-mineral spring (20.6 °C) drawn from beneath Kinokawa, Wakayama, where sodium leads and fluoride, metaborate, and total sulfur each push the chemistry above the Hot Spring Act's qualifying thresholds. The water is strongly alkaline at pH 9.6 — it carries a faint H₂S note at the wellhead and the classic slippery 'beauty-water' feel of high-pH springs. Mineralization is light (0.25 g/kg total solids), and the source is heated for bathing. It does not meet the stricter 'therapeutic spring' bar, but the bath is a stop on the Kii-Izumi onsen pilgrimage circuit.
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⁺77.8mgP24 national97%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1.3mgP4 national2%
- AluminumAl³⁺0.20mgP64 national0.6%
- PotassiumK⁺0.20mgP2 national0.3%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- HSiO3HSiO₃⁻—mg43%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻58.7mgP94 national27%
- Fluoridetrace; drinking limits applyF⁻6.7mgP83 national10%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻13.9mgP9 national8%
- BO2BO₂⁻12.0mgP94 national8%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5.3mgP8 national4%
- Hydrogen sulfideHS⁻1.0mgP70 national0.8%
- SulfateSO₄²⁻0.10mgP2 national0.0%
- HydroxideOH⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃26.5mgP10 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —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.