A magnesium-sodium spring that carries both dissolved CO₂ and bicarbonate. With a generous 1,130 mg/kg of free CO₂ it sits in the rare carbonated-spring class — fine bubbles cling to the skin on entry. Neutral and moderately mineralized with noticeable body, it also holds 190 mg/kg of metasilicic acid, leaving a thin silica film for noticeably smoother skin. At the source the water tastes moderately carbonated with a faint metallic edge.
Good for
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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.
- Magnesiummild; a laxative if you drink itMg²⁺330mgP98 national50%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺394mgP70 national32%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺162mgP82 national15%
- Potassiumminor; matters for drinking limitsK⁺49.1mgP84 national2%
- Iron (II)Fe²⁺4.7mgP80 national0.3%
- LithiumLi⁺1.2mgP79 national0.3%
- StrontiumSr²⁺1.4mgP82 national0.1%
- BariumBa²⁺0.80mgP84 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻2,730mgP99 national85%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻278mgP61 national15%
- BromideBr⁻0.90mgP57 national0.0%
- FluorideF⁻0.20mgP17 national0.0%
- H2PO4H₂PO₄⁻0.20mgP57 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃190mgP83 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂31.1mgP80 national
- Metaarsenious acidHAsO₂NDmg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1,130mgP100 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- CadmiumCd< —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.
- Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
- Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- 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.