An alkaline simple thermal spring (pH 8.9), sampled at the Gero Onsen hot-water pump station that distributes the town's supply. Mineralization is light — 387 mg/kg of dissolved solids — so the water is gentle on the body, while the alkalinity leaves the slippery, soft-skinned feel that places it in Japan's beauty-water family. A helpful 57.3 mg/kg of metasilicic acid adds to that, the kind of thin silica film that makes skin feel noticeably smoother afterwards. The posted analysis names autonomic nervous system instability, insomnia and depressed mood among the spring-type indications.
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
Take care
From a certificate analyzed in 2013.
- Active stage of any illness (especially with fever)
- Marked physical debilitation, such as active tuberculosis, advanced malignant tumor or severe anemia
- Severe heart or lung disease causing breathlessness on slight exertion, or kidney disease with edema
- Gastrointestinal bleeding or any visible bleeding
- Acute exacerbation of a chronic illness
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⁺0.06mgP33 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 smoothHCO₃⁻37.5mgP24 national11%
- Fluoridetrace; drinking limits applyF⁻10.3mgP91 national10%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻12.4mgP24 national5%
- NitrateNO₃⁻0.50mgP64 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 acidHAsO₂0.10mgP53 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂4.0mgP26 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
Measured at the source
On the certificate
From a certificate analyzed in 2013.
- Chronic muscle or joint pain and stiffness (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains — chronic phase)
- Cold sensitivity, peripheral circulation disorders
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Fatigue recovery, general health improvement
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.