The source rises on the hotel's own grounds in the Yumura district of Kofu: a sodium-calcium chloride water, salty and heat-retaining, with sulfate adding quiet body behind it. It is mildly alkaline and soft on the skin, and a faint salty tang is the one thing that registers on the tongue — otherwise the water runs colorless, clear and odorless. With 55.8 mg/kg of metasilicic acid and 17.3 mg/kg of metaboric acid it sits in Japan's beauty-water family, leaving the kind of thin silica film that makes skin feel smoother afterwards. Dissolved solids of 1,671 mg/kg give it noticeable body without heaviness.
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⁺430mgP72 national74%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺122mgP77 national24%
- Potassiumminor; matters for drinking limitsK⁺10.5mgP46 national1%
- LithiumLi⁺0.80mgP70 national0.5%
- StrontiumSr²⁺0.90mgP77 national0.1%
- MagnesiumMg²⁺0.30mgP16 national0.1%
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺NDmg
- Iron (III)Fe³⁺NDmg
- Manganesetrace; companion to iron springsMn²⁺NDmg
- AmmoniumNH₄⁺NDmg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻623mgP77 national67%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻374mgP80 national30%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻27.1mgP16 national2%
- FluorideF⁻3.2mgP67 national0.7%
- CarbonateCO₃²⁻2.5mgP27 national0.3%
- BromideBr⁻1.7mgP69 national0.1%
- ThiosulfateS₂O₃²⁻0.70mgP67 national0.0%
- Hydrogen sulfideHS⁻0.40mgP52 national0.0%
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- NitrateNO₃⁻NDmg
- SS²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃55.8mgP33 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂17.3mgP72 national
- Metaarsenious acidHAsO₂NDmg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.20mgP13 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- CopperCu0.01mgP76 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
- Radonfaint radon — studied for joint pain (hormesis)RnNDmg
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.