A simple thermal spring led by sodium and bicarbonate. At pH 7.8 it is mildly alkaline and soft on the skin, with the slightly slippery feel that places it near Japan's beauty-water family. Lightly mineralized at 361 mg/kg of dissolved solids, it is gentle on the body and easy for long, unhurried soaks.
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⁺75.7mgP23 national83%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺8.5mgP23 national11%
- Magnesiummild; a laxative if you drink itMg²⁺1.7mgP31 national4%
- AmmoniumNH₄⁺0.70mgP56 national1%
- PotassiumK⁺1.3mgP15 national0.8%
- Iron (II)Fe²⁺0.60mgP56 national0.5%
- AluminumAl³⁺0.20mgP64 national0.5%
- Manganesetrace; companion to iron springsMn²⁺< —mg
- ZincZn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻192mgP57 national80%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻26.6mgP20 national19%
- CarbonateCO₃²⁻0.60mgP17 national0.5%
- Hydrogen phosphateHPO₄²⁻0.40mgP69 national0.3%
- SulfateSO₄²⁻0.30mgP4 national0.3%
- FluorideF⁻0.20mgP17 national0.1%
- Bromidetrace, sea-originBr⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃50.0mgP29 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂2.0mgP19 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂3.1mgP24 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- 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.