An iron-bearing sodium chloride strong-salt spring drawn from Arima's Urawari source. At 37.6 g/kg dissolved solids it is hypersaline — exceeding ocean salinity and leaving a thick mineral coating on the skin. It is iron-rich (67 mg/kg) and oxidizes reddish-brown on contact with air, and with a wellhead temperature above 100 °C it ranks among Japan's hottest natural sources.
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
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⁺10,840mgP100 national76%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,825mgP98 national15%
- Potassiumminor; matters for drinking limitsK⁺2,045mgP100 national8%
- Iron (II)Fe²⁺67.0mgP100 national0.4%
- ManganeseMn²⁺17.1mgP99 national0.1%
- AmmoniumNH₄⁺4.0mgP89 national0.0%
- Magnesiummild; a laxative if you drink itMg²⁺13.4mgP65 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻21,980mgP100 national99%
- BicarbonateHCO₃⁻298mgP65 national0.8%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻39.6mgP38 national
- Bromidetrace, sea-originBr⁻38.7mgP97 national
- Iodideyellows in air; helps blood lipidsI⁻5.1mgP95 national
- Fluoridetrace; drinking limits applyF⁻0.20mgP16 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻0.10mgP10 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.10mgP25 national
- Thiosulfateoxidized sulfurS₂O₃²⁻0.10mgP42 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂272mgP99 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃160mgP74 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂136mgP84 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.10mgP58 national
- ArsenicAs< —mg
- CadmiumCd< —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.