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
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⁺10,840mgP100 national76%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,825mgP98 national15%
- Potassiumminor; matters for drinking limitsK⁺2,045mgP100 national8%
- Iron (II)Fe²⁺67.0mgP99 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 healingSO₄²⁻39.6mgP37 national
- Bromidetrace, sea-originBr⁻38.7mgP97 national
- Iodideyellows in air; helps blood lipidsI⁻5.1mgP95 national
- Fluoridetrace; drinking limits applyF⁻0.20mgP17 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻0.10mgP11 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.10mgP28 national
- Thiosulfateoxidized sulfurS₂O₃²⁻0.10mgP45 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂272mgP99 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃160mgP73 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂136mgP84 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.10mgP61 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.