A blended spring drawing on Fuji Kawaguchiko Onsen Village's No. 3 and No. 7 wells — a calcium-and-sodium sulfate-chloride water. Sulfate gives it a settled, mineral-rich body, with a salty edge from the chloride share. It is neutral in pH and moderately mineralized at 1,893 mg/kg of dissolved solids, classified as a low-temperature spring.
Good for
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
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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
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⁺287mgP88 national
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺276mgP59 national
- Magnesiummild; a laxative if you drink itMg²⁺21.8mgP71 national
- Potassiumminor; matters for drinking limitsK⁺9.8mgP45 national
- StrontiumSr²⁺1.4mgP82 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻673mgP91 national49%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻397mgP71 national39%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻191mgP56 national11%
- Metaarsenious acidHAsO₂40.3mgP100 national
- LeadPb0.0020mgP24 national
- ArsenicAs< —mg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
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.