天神泉源Tenjin Gensen
Tenjin Gensen — one of the wellheads that feeds Kin no Yu in Arima. A sodium-chloride hypersaline spring: at roughly 36 g/kg dissolved solids it exceeds ocean salinity and leaves a thick mineral coating on the skin. The wellhead emerges at 99.4 °C, putting it among Japan's hottest natural sources; its iron content oxidizes reddish-brown on air contact, which is what gives Arima's celebrated 'kinsen' (gold water) its colour. A respectable 121 mg/kg of metasilicic acid adds a layer of silica smoothness beneath the salt.
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⁺10,470mgP100 national76%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,725mgP97 national14%
- Potassiumminor; matters for drinking limitsK⁺2,005mgP100 national9%
- Iron (II)Fe²⁺40.5mgP97 national0.2%
- MagnesiumMg²⁺13.8mgP65 national0.2%
- ManganeseMn²⁺13.7mgP99 national0.1%
- AmmoniumNH₄⁺3.7mgP88 national0.0%
- ZincZn²⁺0.30mgP86 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻21,130mgP100 national99%
- BicarbonateHCO₃⁻244mgP60 national0.7%
- BromideBr⁻38.8mgP97 national0.1%
- SulfateSO₄²⁻20.8mgP32 national0.1%
- IodideI⁻4.9mgP94 national0.0%
- FluorideF⁻1.6mgP54 national0.0%
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂257mgP99 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃121mgP62 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂189mgP88 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn< —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.