A calcium-led sodium chloride spring with sulfate adding a quiet, settled body — the certificate prints it as a Ca·Na−Cl·SO₄ blend. The wellhead is hot (49.2 °C) and cooled to a comfortable 42 °C at the bath, while pH 8.7 puts it firmly in the alkaline range with the slippery 'beauty-water' feel that locals associate with smoother skin. Dissolved solids land at 5,423 mg/kg — moderately mineralized, enough to leave a faint salty tang on the lips and a thin mineral film as you towel off. The bath is served straight from the source: no dilution, no reheating, no recirculation, only a chlorine-based disinfection step kept in for hygiene.
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, nettō 熱の湯).
cold sensitivity
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺999mgP95 national57%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺858mgP83 national43%
- StrontiumSr²⁺6.7mgP92 national0.2%
- PotassiumK⁺4.4mgP33 national0.1%
- LithiumLi⁺0.60mgP61 national0.1%
- MagnesiumMg²⁺0.10mgP11 national0.0%
- 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
- AmmoniumNH₄⁺< —mg
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻2,318mgP89 national72%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,173mgP97 national27%
- CarbonateCO₃²⁻6.8mgP41 national0.3%
- FluorideF⁻2.4mgP62 national0.1%
- BromideBr⁻2.9mgP75 national0.0%
- BicarbonateHCO₃⁻0.50mgP1 national0.0%
- Hydrogen phosphateHPO₄²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- NitrateNO₃⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃34.7mgP16 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂15.4mgP69 national
- H2SO4FreeH2SO4Free< —mg
- Metaarsenious acidHAsO₂< —mg
- ArsenicAs< —mg
- 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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.
- 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.