A gentle simple thermal spring, alkaline enough to give the slippery feel that puts it squarely in Japan's beauty-water family. Dissolved solids come to just 387 mg/kg, so the water sits light on the body rather than heavily mineralized, and sodium accounts for almost the entire cation side, well short of anything you would taste as salt. What lifts it is metasilicic acid at 57.3 mg/kg, enough silica to leave the thin film that makes skin feel noticeably smoother afterwards. The posted table records the sample as taken at Gero's hot-water delivery pump station rather than at a wellhead.
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⁺118mgP34 national95%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4.4mgP16 national4%
- Potassiumminor; matters for drinking limitsK⁺2.2mgP23 national1%
- Aluminumshows up in acidic volcanic waterAl³⁺0.06mgP32 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —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⁻113mgP44 national59%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻24.1mgP73 national15%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻37.5mgP23 national11%
- Fluoridetrace; drinking limits applyF⁻10.5mgP91 national10%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻12.2mgP24 national5%
- NitrateNO₃⁻0.40mgP55 national0.1%
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃57.3mgP34 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.0mgP45 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.