The bath water here is a blend of two wells — Sugi-2 and the geothermal power station source — sampled at the hotel's own mixing valve rather than at a wellhead. It is a sodium chloride spring with sulfate adding quiet body, moderately mineralized at 1,875 mg/kg and leaving a faint salty tang on the lips. At 91.4 °C it ranks among Japan's hottest natural sources, and its alkalinity gives the slippery feel that places it squarely in Japan's beauty-water family. A generous 379.2 mg/kg of metasilicic acid leaves the kind of silica film that makes skin noticeably smoother afterwards.
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺485mgP76 national90%
- Potassiumminor; matters for drinking limitsK⁺50.8mgP85 national6%
- LithiumLi⁺5.4mgP95 national3%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4.8mgP17 national1%
- AmmoniumNH₄⁺0.30mgP40 national0.1%
- AluminumAl³⁺0.20mgP64 national0.1%
- MagnesiumMg²⁺0.10mgP11 national0.0%
- StrontiumSr²⁺0.10mgP26 national0.0%
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻546mgP76 national64%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻247mgP69 national21%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻78.0mgP97 national11%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻48.8mgP27 national3%
- FluorideF⁻1.5mgP54 national0.3%
- BromideBr⁻2.7mgP75 national0.1%
- Hydrogen phosphateHPO₄²⁻0.20mgP54 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃379mgP98 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂24.1mgP77 national
- Metaarsenious acidHAsO₂0.80mgP89 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- CadmiumCd< —mg
- CopperCu< —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.