Hot (45.6 °C) simple thermal spring at Kamifunabara, with total dissolved solids of just 708 mg/kg keeping it inside the 'simple' classification despite a sulfate-dominant anion profile (about two-thirds of equivalent anions are SO₄). The wellhead pH of 7.4 sits squarely in neutral territory; the water reads as nearly colorless and clear, tasteless, with only a faint hydrogen-sulfide whisper. Lightly mineralized and gentle on the body, with a modest dose of metasilicic acid (37 mg/kg) softening the feel on the skin.
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⁺172mgP43 national78%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺39.2mgP52 national21%
- PotassiumK⁺2.9mgP26 national0.8%
- MagnesiumMg²⁺0.40mgP18 national0.3%
- StrontiumSr²⁺0.20mgP44 national0.0%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻302mgP74 national64%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻91.8mgP38 national26%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻56.2mgP30 national9%
- FluorideF⁻1.3mgP52 national0.7%
- BromideBr⁻0.20mgP30 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃36.9mgP19 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.7mgP35 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂12.3mgP43 national
- ArsenicAs0.02mgP35 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.
- 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.