A sodium-led bicarbonate spring with a salty chloride edge, classified as a hot spring (47 °C at the wellhead). It belongs to the jūsō-sen (soda-water) family — weakly alkaline and soft on the skin, with the slippery feel of Japan's beauty-water tradition. A solid 74.8 mg/kg of metasilicic acid leaves a thin silica film that makes skin feel noticeably smoother. The water is colorless and transparent with a faint metallic note in taste and smell.
Good for
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
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, nettō 熱の湯).
cold sensitivity
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⁺765mgP82 national91%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺20.5mgP39 national3%
- Magnesiummild; a laxative if you drink itMg²⁺12.0mgP63 national3%
- Potassiumminor; matters for drinking limitsK⁺23.9mgP68 national2%
- LithiumLi⁺2.3mgP92 national0.9%
- StrontiumSr²⁺2.1mgP85 national0.2%
- HydrogenH⁺0.0000mg0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺0.10mgP54 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.10mgP30 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,239mgP92 national53%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻496mgP74 national37%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻178mgP63 national10%
- FluorideF⁻1.5mgP54 national0.2%
- BromideBr⁻1.6mgP68 national0.1%
- Iodideyellows in air; helps blood lipidsI⁻0.40mgP66 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃74.8mgP46 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂47.0mgP86 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂115mgP82 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- 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.