An alkaline simple thermal spring rising from the 'Ichinotaki No. 2' source at Katsuura Onsen in Wakayama. At pH 8.7 it is distinctly alkaline, giving the slippery, soft-on-the-skin feel of Japan's beauty-water family. The water is colorless and transparent with no taste, though a faint hydrogen sulfide note is detectable at the wellhead. Lightly mineralized at 455 mg/kg of dissolved solids, it is gentle on the body, and the inn serves it as untreated free-flow with no dilution, heating, recirculation, or disinfection.
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
Take care
- acute illness (especially with fever)
- active tuberculosis
- advanced malignant tumors
- severe anemia
- any other progressing illness in general
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⁺121mgP35 national75%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺33.6mgP48 national24%
- PotassiumK⁺2.1mgP23 national0.8%
- LithiumLi⁺0.10mgP29 national0.6%
- MagnesiumMg²⁺0.20mgP14 national0.2%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- AmmoniumNH₄⁺< —mg
- StrontiumSr²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻227mgP57 national86%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻9.6mgP50 national4%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻13.4mgP8 national3%
- Fluoridetrace; drinking limits applyF⁻4.1mgP72 national3%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻9.9mgP21 national3%
- Hydrogen sulfideHS⁻< —mg0.6%
- ThiosulfateS₂O₃²⁻1.4mgP78 national0.3%
- Bromidetrace, sea-originBr⁻0.70mgP51 national
- Hydrogen phosphateHPO₄²⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- NitrateNO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃28.5mgP11 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.2mgP13 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- stress-related symptoms
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.