An alkaline simple thermal spring fed from the Koden pump station of the Gero hot-spring supply. At pH 9.4 the water sits near the top of the alkaline band and has the slippery, soft-handed feel that places it in Japan's beauty-water family, while the overall load stays light at 353.9 mg/kg of dissolved solids — gentle rather than assertive. What it does carry is metasilicic acid at 66.7 mg/kg, enough to leave the thin silica film that makes skin feel noticeably smoother after a soak. The posted analysis names the spring-type indications for this water as autonomic nervous system instability, insomnia and depressed mood, over the general list of chronic muscle and joint pain and stiffness.
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 2018.
- Active stage of any illness (especially with fever)
- Active tuberculosis, advanced malignant tumor, severe anemia or other markedly debilitated states
- Heart or lung disease severe enough to cause breathlessness on the slightest exertion
- Renal failure 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⁺104mgP30 national97%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2.5mgP9 national3%
- PotassiumK⁺1.4mgP16 national0.8%
- 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⁻81.2mgP35 national46%
- Fluoridetrace; drinking limits applyF⁻17.0mgP98 national18%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻25.4mgP76 national17%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻38.7mgP25 national13%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻11.0mgP22 national
- BO2BO₂⁻5.4mgP85 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.30mgP45 national
- AsO2AsO₂⁻0.20mgP91 national
- NitrateNO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃66.7mgP42 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
On the certificate
From a certificate analyzed in 2018.
- Chronic muscle or joint pain and stiffness (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains — chronic phase)
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
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.