A sodium-led bicarbonate-chloride spring, with calcium and magnesium filling out the cation side. Weakly alkaline and soft on the skin, it sits in jūsō-sen (soda-water) territory and squarely inside Japan's beauty-water family. Moderate mineralization of 1,347 mg/kg gives the water noticeable body, while metasilicic acid runs to 156 mg/kg — the kind of silica load that leaves a thin film and makes skin feel smoother afterwards. This certificate was sampled at the bath inlet rather than the wellhead, so the readings describe the water as it actually reaches bathers.
Good for
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
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
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
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⁺193mgP50 national48%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺92.4mgP71 national26%
- Magnesiummild; a laxative if you drink itMg²⁺46.0mgP85 national22%
- Potassiumminor; matters for drinking limitsK⁺30.6mgP75 national4%
- Iron (III)Fe³⁺0.20mgP66 national0.1%
- ManganeseMn²⁺0.20mgP45 national0.0%
- AluminumAl³⁺0.0000mg0.0%
- Iron (II)Fe²⁺0.0000mg0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻501mgP76 national48%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻218mgP56 national36%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻48.1mgP91 national9%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻56.5mgP42 national7%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃156mgP70 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.1mgP37 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂88.1mgP78 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.12mgP68 national
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.