Shibu Onsen's combined source blends three wells — Hira-no-yu, Yakushi-no-yu and Bitonbi-no-yu — and is drawn on the grounds of Onsenji temple. It is a sodium-calcium sulfate-chloride water: sulfate gives it a settled, mineral body with a salty edge layered on top. At 65.0 °C it sits among Japan's hottest natural sources, yet the weakly acidic pH keeps the touch gentle rather than sharp. The water runs faintly yellow and clear at the spout, odorless, with a faint iron taste on the tongue. Metasilicic acid at 151.4 mg/kg puts it in Japan's beauty-water family, leaving the kind of thin silica film that makes skin feel noticeably smoother.
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, netto 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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺196mgP52 national55%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺108mgP75 national34%
- Magnesiummild; a laxative if you drink itMg²⁺9.7mgP61 national5%
- Potassiumminor; matters for drinking limitsK⁺17.7mgP59 national3%
- Aluminumshows up in acidic volcanic waterAl³⁺2.8mgP82 national2%
- LithiumLi⁺0.50mgP52 national0.5%
- Iron (II)Fe²⁺0.90mgP62 national0.2%
- ManganeseMn²⁺0.50mgP66 national0.1%
- StrontiumSr²⁺0.50mgP68 national0.1%
- AmmoniumNH₄⁺0.10mgP23 national0.0%
- ZincZn²⁺0.03mgP56 national0.0%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻397mgP82 national53%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻250mgP60 national45%
- BicarbonateHCO₃⁻5.5mgP3 national0.6%
- FluorideF⁻1.3mgP52 national0.5%
- BromideBr⁻1.3mgP66 national0.1%
- H2PO4H₂PO₄⁻0.30mgP73 national0.0%
- HSO4HSO40.30mgP42 national0.0%
- IodideI⁻0.30mgP61 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃151mgP69 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂35.6mgP83 national
- Metaarsenious acidHAsO₂0.30mgP74 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂30.7mgP60 national
- CadmiumCdNDmg
- ChromiumCrNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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.