A blended source: the certificate lists ten wells around Tochio in the Okuhida hot-spring district, pooled and sampled at the relay tank before distribution. Chemically it is a simple thermal spring — neutral pH, colorless, tasteless and odorless, and lightly mineralized at 0.62 g/kg of dissolved solids, so it reads as gentle rather than assertive. What stands out is the heat: 69.9 °C at the tank puts it among Japan's hottest natural sources, and a notice posted alongside the analysis states that mountain water is added to bring the bath down to a usable temperature.
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
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⁺140mgP38 national82%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺17.6mgP36 national12%
- Potassiumminor; matters for drinking limitsK⁺12.0mgP48 national4%
- LithiumLi⁺0.90mgP71 national2%
- MagnesiumMg²⁺0.70mgP23 national0.7%
- ManganeseMn²⁺0.20mgP47 national0.2%
- StrontiumSr²⁺0.10mgP24 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- BariumBa²⁺NDmg
- CopperCu²⁺NDmg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺NDmg
- Iron (III)Fe³⁺NDmg
- AmmoniumNH₄⁺NDmg
- ZincZn²⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻274mgP63 national57%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻97.5mgP40 national35%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻17.9mgP30 national5%
- Fluoridetrace; drinking limits applyF⁻5.0mgP77 national3%
- BromideBr⁻0.30mgP37 national0.1%
- NitrateNO₃⁻0.20mgP42 national0.0%
- AsO2AsO₂⁻NDmg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Hydrogen phosphateHPO₄²⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- HSO4HSO4NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- NitriteNO₂⁻NDmg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃42.0mgP23 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂8.6mgP50 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂102mgP79 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- ArsenicAs0.04mgP46 national
- 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.
- 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.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.