A simple alkaline thermal water supplied from Gero Onsen's Koda pump station. The alkalinity gives the slippery, soft-on-the-skin feel that places it in Japan's beauty-water family, and a generous load of metasilicic acid reinforces it, leaving skin noticeably smoother after a soak. Dissolved solids are light, so the water is gentle on the body and suits a long, unhurried bath rather than a short hot plunge. The posted analysis lists the spring-type indications for this water as autonomic nervous system instability, insomnia and depressed mood.
Good for
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
- Active stage of any illness (especially with fever)
- Active tuberculosis, advanced malignant tumor, severe anemia or other states of physical debility
- Severe heart or lung disease causing breathlessness on slight exertion; 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⁺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 national5%
- BO2BO₂⁻5.4mgP85 national2%
- Hydrogen sulfideHS⁻0.30mgP45 national0.0%
- AsO2AsO₂⁻0.20mgP91 national0.0%
- 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
- Chronic muscle or joint pain and stiffness (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains — chronic phase)
- Muscle stiffness associated with motor paralysis
- Cold sensitivity and peripheral circulation disorders
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Fatigue recovery and 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.
- 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.