The Shikei source of Arima Onsen — an iron-rich sodium chloride strong-salt spring. At 44 g/kg of dissolved solids it far exceeds ocean salinity, leaving a thick mineral film on the skin and ranking as a hypertonic water. It carries 67 mg/kg of iron and oxidizes reddish-brown on contact with air, which is the origin of the 'Kinsen' (golden spring) name: colorless at the wellhead, it turns tea-brown as it stands. At 98.6 °C it is among Japan's hottest natural sources, strongly salty and exceptionally heat-retaining.
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
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⁺11,580mgP100 national77%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2,049mgP98 national14%
- Potassiumminor; matters for drinking limitsK⁺2,520mgP100 national9%
- Iron (II)Fe²⁺67.0mgP99 national0.3%
- MagnesiumMg²⁺17.0mgP69 national0.2%
- ManganeseMn²⁺19.1mgP100 national0.1%
- AmmoniumNH₄⁺6.1mgP92 national0.1%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻25,710mgP100 national99%
- BicarbonateHCO₃⁻175mgP54 national0.4%
- BromideBr⁻54.9mgP99 national0.1%
- SulfateSO₄²⁻8.4mgP20 national0.0%
- IodideI⁻10.6mgP97 national0.0%
- FluorideF⁻1.8mgP57 national0.0%
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂302mgP99 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃170mgP76 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂217mgP89 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- ArsenicAs0.0050mgP16 national
- Mercurytoxic trace — part of the drinking-safety checkHg0.0040mgP95 national
- CadmiumCd< —mg
- CopperCu< —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.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.