A simple thermal spring, classified hypotonic, neutral and hot. Total dissolved solids sit at a light 952 mg/kg — under the threshold that would earn the water a mineral class name — yet bicarbonate carries roughly seven-tenths of the anion balance and a real load of free carbon dioxide stays in solution. Metasilicic acid at 148 mg/kg is the standout figure: the kind of silica film that puts a water in Japan's beauty-water family. Worth knowing what the certificate actually measured — not the wellhead, but the open-air bath inlet, after the source had been cut with mountain water. Weakly salty on the tongue, a faint rotten-egg note of hydrogen sulfide, delivered at 47.3 °C.
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⁺130mgP36 national54%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺45.9mgP56 national22%
- Potassiumminor; matters for drinking limitsK⁺47.0mgP83 national11%
- Magnesiummild; a laxative if you drink itMg²⁺13.4mgP65 national11%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺4.0mgP80 national1%
- LithiumLi⁺0.60mgP59 national0.8%
- ManganeseMn²⁺0.30mgP54 national0.1%
- StrontiumSr²⁺0.40mgP61 national0.1%
- BariumBa²⁺0.20mgP71 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- CopperCu²⁺NDmg
- Iron (III)Fe³⁺NDmg
- AmmoniumNH₄⁺NDmg
- ZincZn²⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻436mgP74 national69%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻113mgP44 national31%
- FluorideF⁻0.60mgP35 national0.3%
- SulfateSO₄²⁻1.4mgP9 national0.3%
- BromideBr⁻0.20mgP28 national0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- H2PO4H₂PO₄⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- HSO4HSO4NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- NitrateNO₃⁻NDmg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃148mgP68 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂11.6mgP60 national
- Metaarsenious acidHAsO₂NDmg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂457mgP95 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.10mgP57 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —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.
- 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.