The source at Ryokan Yumotoso in Kurokawa Onsen is classified as a simple iron spring of the sulfate sub-type — iron-rich water that oxidizes reddish-brown once it meets the air. The certificate's older name for it, "simple melanterite spring", describes the chemistry literally: iron paired with sulfate, which dominates the anion side. Dissolved solids are light and the water is only weakly acidic, so it feels gentle rather than sharp, yet iron runs to 35 mg/kg — well past the threshold at which a spring counts as iron-bearing at all. Metasilicic acid is generous too, leaving the thin silica film that makes skin feel noticeably smoother afterwards.
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
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
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⁺73.5mgP21 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺33.1mgP97 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺20.5mgP38 national
- Potassiumminor; matters for drinking limitsK⁺17.9mgP59 national
- Aluminumshows up in acidic volcanic waterAl³⁺4.6mgP84 national
- Magnesiummild; a laxative if you drink itMg²⁺4.3mgP46 national
- Iron (III)Fe³⁺2.2mgP90 national
- ZincZn²⁺0.80mgP94 national
- Manganesetrace; companion to iron springsMn²⁺0.70mgP73 national
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.50mgP63 national
- LithiumLi⁺0.50mgP52 national
- AmmoniumNH₄⁺0.40mgP43 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻282mgP72 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻65.2mgP30 national
- Fluoridetrace; drinking limits applyF⁻2.3mgP60 national
- Bromidetrace, sea-originBr⁻0.20mgP28 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃118mgP60 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.6mgP26 national
- Metaarsenious acidHAsO₂0.80mgP87 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂2.3mgP22 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.20mgP65 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.
- 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.