A sodium-led chloride and bicarbonate spring rising at a punchy 75.8 °C — among Japan's hotter natural sources, with the salty, heat-retaining body typical of Shirahama. Moderately mineralized at 7.1 g/kg, weakly alkaline and clear at the wellhead. Metasilicic acid at 125.7 mg/kg leaves the kind of silica film that places it squarely in Japan's beauty-water family.
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
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⁺2,094mgP91 national91%
- Magnesiummild; a laxative if you drink itMg²⁺41.1mgP84 national3%
- Potassiumminor; matters for drinking limitsK⁺107mgP93 national3%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺46.8mgP57 national2%
- AluminumAl³⁺0.40mgP74 national0.0%
- Iron (II)Fe²⁺0.20mgP39 national0.0%
- ManganeseMn²⁺0.10mgP36 national0.0%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,971mgP87 national54%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻2,379mgP98 national38%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻180mgP100 national6%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻108mgP56 national2%
- FluorideF⁻4.3mgP74 national0.2%
- ThiosulfateS₂O₃²⁻1.2mgP76 national0.0%
- BO2BO₂⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃126mgP64 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂63.1mgP92 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂91.4mgP78 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- CadmiumCd0.06mgP100 national
- ArsenicAs< —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.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- 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.