A sodium-led chloride spring with bicarbonate softness layered in, drawn from a well sunk 1,492 metres beneath the site. It is neutral in pH and moderately mineralized at 4,146 mg/kg — the water looks clear and colorless, but it tastes strongly salty with a very faint sulfur note behind it. At 39.7 °C the source arrives just short of comfortable bathing warmth, so the bath is reheated and served through recirculating filtration with disinfection rather than as fresh free-flow.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, netto 熱の湯).
cold sensitivity
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
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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⁺1,324mgP89 national95%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺31.1mgP46 national3%
- Magnesiummild; a laxative if you drink itMg²⁺12.6mgP64 national2%
- Potassiumminor; matters for drinking limitsK⁺25.4mgP71 national1%
- Iron (II)Fe²⁺0.30mgP43 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺0.02mgP22 national
- Manganesetrace; companion to iron springsMn²⁺0.02mgP17 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,442mgP85 national66%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,239mgP92 national33%
- FluorideF⁻4.4mgP74 national0.4%
- SulfateSO₄²⁻0.20mgP3 national0.0%
- NitrateNO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃34.2mgP16 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂32.4mgP81 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂56.0mgP70 national
- 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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.