A gentle alkaline simple thermal spring rising from the Shin-chōyusō tank in Isawa, Fuefuki. With a pH around 9 it has the slippery feel of Japan's beauty-water family, smooth on the skin. The water is colorless, clear, tasteless and odorless, and lightly mineralized for an easy, undemanding soak.
Good for
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
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⁺110mgP33 national90%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺8.8mgP24 national8%
- PotassiumK⁺1.8mgP20 national0.9%
- LithiumLi⁺0.10mgP29 national0.2%
- Aluminumshows up in acidic volcanic waterAl³⁺0.0000mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.0000mg
- Iron (III)Fe³⁺0.0000mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.0000mg
- Magnesiummild; a laxative if you drink itMg²⁺0.0000mg
- Manganesetrace; companion to iron springsMn²⁺0.0000mg
- StrontiumSr²⁺0.0000mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻107mgP42 national57%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻57.9mgP43 national23%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻18.3mgP65 national11%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻23.8mgP14 national7%
- Fluoridetrace; drinking limits applyF⁻1.3mgP52 national1%
- BO2BO₂⁻0.0000mg
- Hydrogen phosphateHPO₄²⁻0.0000mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.0000mg
- HSiO3HSiO₃⁻0.0000mg
- HSO4HSO40.0000mg
- HydroxideOH⁻0.0000mg
- Thiosulfateoxidized sulfurS₂O₃²⁻0.0000mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃59.9mgP37 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.0mgP42 national
- Metaarsenious acidHAsO₂0.40mgP81 national
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.27mgP87 national
- ChromiumCr< —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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- 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.