A simple thermal spring rising in Iwakubo, Kofu. At the wellhead the water is brown and turbid with a metallic taste from its iron content, and orange sediment forms on contact with air. Weakly alkaline and lightly mineralized at about 0.5 g/kg of dissolved solids, it makes for a gentle, undemanding soak.
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺85.0mgP68 national65%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺33.9mgP7 national22%
- Magnesiummild; a laxative if you drink itMg²⁺7.2mgP57 national9%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺5.3mgP81 national3%
- PotassiumK⁺1.5mgP18 national0.6%
- StrontiumSr²⁺0.50mgP68 national0.1%
- ZincZn²⁺0.20mgP84 national0.1%
- Manganesetrace; companion to iron springsMn²⁺0.10mgP36 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.0000mg
- BariumBa²⁺0.0000mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.0000mg
- LithiumLi⁺0.0000mg
- AmmoniumNH₄⁺0.0000mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻183mgP63 national61%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻143mgP50 national37%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻4.3mgP6 national2%
- AsO2AsO₂⁻0.0000mg
- BO2BO₂⁻0.0000mg
- Bromidetrace, sea-originBr⁻0.0000mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻0.0000mg
- Fluoridetrace; drinking limits applyF⁻0.0000mg
- Hydrogen phosphateHPO₄²⁻0.0000mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.0000mg
- HSiO3HSiO₃⁻0.0000mg
- HSO4HSO40.0000mg
- Iodideyellows in air; helps blood lipidsI⁻0.0000mg
- HydroxideOH⁻0.0000mg
- SS²⁻0.0000mg
- Thiosulfateoxidized sulfurS₂O₃²⁻0.0000mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃34.4mgP16 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂12.9mgP44 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- 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.