An alkaline simple thermal spring — gentle in composition but striking in pH. At pH 9.5 the water sits at the strongly alkaline end of the scale, where carbonate and hydroxide ions give it the slippery, soap-like touch that places it squarely in Japan's beauty-water family. Total dissolved solids come to just 0.22 g/kg, so the water is light and low in irritation, with fluoride and chloride the most noticeable of the minor ions and a modest 23.9 mg/kg of metasilicic acid rounding out the profile.
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⁺68.7mgP20 national94%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺3.5mgP13 national5%
- PotassiumK⁺0.40mgP4 national0.3%
- Aluminumshows up in acidic volcanic waterAl³⁺0.03mgP25 national
- FeTotalFe< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻39.5mgP24 national32%
- Fluoridetrace; drinking limits applyF⁻15.5mgP96 national24%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻21.0mgP68 national20%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻33.6mgP21 national16%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻5.2mgP15 national3%
- BO2BO₂⁻4.4mgP75 national3%
- HydroxideOH⁻0.50mgP77 national0.9%
- Hydrogen sulfideHS⁻0.50mgP55 national0.6%
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃23.9mgP8 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
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.