A blended spring drawing on three wells — Oka No. 191, No. 152, and No. 137 — sampled from the storage tank in Ito's Suehiro neighborhood. The wellhead temperature is a hot 44.2 °C, but the total dissolved solids sit at a light 808 mg/kg, placing it firmly in simple thermal territory. Weakly alkaline at pH 8.2, the water reads soft on the skin, with sodium leading the cations and chloride and sulfate sharing the anion column almost evenly.
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⁺164mgP42 national59%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺95.4mgP72 national39%
- Magnesiummild; a laxative if you drink itMg²⁺1.7mgP31 national1%
- PotassiumK⁺2.2mgP23 national0.5%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻252mgP59 national58%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻219mgP67 national37%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻21.4mgP13 national3%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻6.0mgP39 national2%
- IodideI⁻0.20mgP52 national0.0%
- Hydrogen phosphateHPO₄²⁻0.10mgP34 national
- Bromidetrace, sea-originBr⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃45.3mgP25 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.5mgP15 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.8mgP38 national
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.