A calcium- and sodium-led sulfate spring of settled, mineral-rich character. Weakly alkaline at pH 7.8, the water is soft on the skin; at the source it carries a pale brown, slightly turbid look and a faint metallic note from a trace of iron. Moderate mineralization at 1,042 mg/kg with sulfate making up over 93% of the anions — a textbook Ca-Na-SO₄ profile with the quiet body that sulfate springs are known for.
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
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺188mgP84 national68%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺90.2mgP27 national28%
- Magnesiummild; a laxative if you drink itMg²⁺3.9mgP43 national2%
- StrontiumSr²⁺4.1mgP88 national0.7%
- Iron (II)Fe²⁺2.2mgP73 national0.6%
- PotassiumK⁺2.5mgP25 national0.4%
- ManganeseMn²⁺0.20mgP45 national0.1%
- AmmoniumNH₄⁺0.10mgP24 national0.1%
- ZincZn²⁺< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻659mgP91 national93%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻39.7mgP23 national4%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻10.5mgP13 national2%
- FluorideF⁻0.30mgP24 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃41.4mgP22 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.30mgP7 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂6.6mgP33 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.0040mgP11 national
- 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.
- Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
- Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- 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.