An alkaline simple thermal spring from the Gero supply — gentle in composition, with dissolved solids of just 374 mg/kg, so it sits light on the body rather than pressing minerals into the skin. The alkalinity gives it the slippery feel that places it squarely in Japan's beauty-water family, and close to 60 mg/kg of metasilicic acid leaves a thin silica film that makes skin feel noticeably smoother afterwards. Carbonate and fluoride stand out in an otherwise sparse ion table, lending the water a soft body unusual for a simple spring.
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⁺110mgP32 national95%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4.3mgP16 national4%
- PotassiumK⁺1.9mgP22 national1.0%
- Aluminumshows up in acidic volcanic waterAl³⁺0.09mgP39 national
- FeTotalFe< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻105mgP43 national56%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻26.0mgP80 national17%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻37.5mgP23 national12%
- Fluoridetrace; drinking limits applyF⁻10.1mgP90 national10%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻12.7mgP26 national5%
- Hydrogen sulfideHS⁻0.70mgP62 national0.4%
- NitrateNO₃⁻0.20mgP42 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃59.7mgP36 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.1mgP41 national
- Metaarsenious acidHAsO₂0.10mgP54 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —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.