A carbonated spring in the full legal sense: free CO₂ measures 1,042 mg/kg, past the threshold for the rare carbonated classification, so fine bubbles cling to the skin on entry. Beneath the fizz it is a sodium-calcium bicarbonate water — neutral in pH, moderately mineralized, with the soft, rounded feel bicarbonate gives. Metasilicic acid is generous enough to place it in Japan's beauty-water family, leaving skin noticeably smoother. The certificate records that the water, colorless, clear and faintly fizzy on the tongue at the spout, had turned slightly yellow-white and dropped a yellow-brown sediment within 24 hours of sampling — proof of how live the dissolved gas and iron still are when they meet air.
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺495mgP77 national60%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺146mgP81 national20%
- Magnesiummild; a laxative if you drink itMg²⁺76.1mgP90 national17%
- PotassiumK⁺12.5mgP49 national0.9%
- LithiumLi⁺1.2mgP79 national0.5%
- Iron (II)Fe²⁺2.7mgP77 national0.3%
- StrontiumSr²⁺3.4mgP88 national0.2%
- BariumBa²⁺4.3mgP96 national0.2%
- ManganeseMn²⁺0.20mgP47 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,893mgP96 national89%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻85.0mgP37 national7%
- Fluoridetrace; drinking limits applyF⁻16.2mgP97 national2%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻31.4mgP36 national2%
- ThiosulfateS₂O₃²⁻0.20mgP52 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃158mgP71 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂19.2mgP73 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂1,042mgP99 national
- ArsenicAs0.0030mgP10 national
- CadmiumCd< —mg
- ChromiumCr< —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.