A weakly alkaline simple thermal spring led by sodium, qualifying on its fluoride and sodium-bicarbonate content. At pH 8.35 it has the soft, slippery touch of the Japanese beauty-water family. Lightly mineralized at 631 mg/kg of dissolved solids, it is gentle on the body; as a cold-mineral source below 25 °C it is reheated 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⁺180mgP47 national96%
- CalciumCa²⁺1.6mgP6 national1.0%
- PotassiumK⁺2.8mgP25 national0.9%
- AmmoniumNH₄⁺0.70mgP56 national0.5%
- MagnesiumMg²⁺0.30mgP16 national0.3%
- StrontiumSr²⁺0.30mgP55 national0.1%
- ZincZn²⁺0.02mgP48 national
- CopperCu²⁺0.0040mgP57 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- LithiumLi⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻363mgP69 national72%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻46.4mgP27 national16%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻21.0mgP69 national9%
- Fluoridetrace; drinking limits applyF⁻4.8mgP77 national3%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻0.10mgP2 national
- Bromidetrace, sea-originBr⁻0.0000mg
- Hydrogen phosphateHPO₄²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- NitrateNO₃⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃6.5mgP1 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.2mgP25 national
- 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
- 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.