A simple thermal spring blended from four wells at Hirayu — Horai-no-yu, Shin-Kodakara-no-yu, Yunotaira-no-yu and Yunoue-no-yu. Neutral and only lightly mineralized at 926 mg/kg of dissolved solids, it is gentle on the body rather than assertive. At the wellhead the water carries a faint yellow tint, drifting flakes of yu-no-hana mineral bloom and a slight milky cloudiness; by the time it reached the lab bench a day later it had settled clear, leaving a yellow sediment behind. The sample was drawn at the end of the storage-tank overflow pipe rather than at the wellhead itself.
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⁺151mgP40 national53%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺68.1mgP64 national28%
- Magnesiummild; a laxative if you drink itMg²⁺19.5mgP70 national13%
- Potassiumminor; matters for drinking limitsK⁺22.6mgP67 national5%
- LithiumLi⁺1.0mgP74 national1%
- StrontiumSr²⁺0.70mgP74 national0.1%
- Iron (II)Fe²⁺0.30mgP43 national0.1%
- BariumBa²⁺0.30mgP75 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- CopperCu²⁺NDmg
- Iron (III)Fe³⁺NDmg
- Manganesetrace; companion to iron springsMn²⁺NDmg
- AmmoniumNH₄⁺NDmg
- ZincZn²⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻474mgP75 national66%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻121mgP46 national29%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻30.7mgP36 national5%
- FluorideF⁻0.40mgP28 national0.2%
- BromideBr⁻0.60mgP50 national0.1%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- H2PO4H₂PO₄⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- HSO4HSO4NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- NitriteNO₂⁻NDmg
- NitrateNO₃⁻NDmg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃24.0mgP9 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂12.1mgP61 national
- Metaarsenious acidHAsO₂NDmg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂274mgP91 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- 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.
- 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.