An alkaline simple thermal spring supplied from Gero Onsen's Koda pump station. Lightly mineralized at 354 mg/kg of dissolved solids, it is gentle on the body, yet the alkalinity gives it the slippery, soft-on-the-skin feel that places it squarely in Japan's beauty-water family. A generous 66.7 mg/kg of metasilicic acid backs that up, leaving a thin silica film that makes skin feel smoother afterwards. The posted analysis lists this spring type as documented for autonomic nervous system instability, insomnia and depressed mood, on top of the usual chronic muscle and joint complaints.
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)
- Marked physical debilitation such as active tuberculosis, advanced malignant tumor or severe anemia
- Severe heart or lung disease causing breathlessness on slight exertion
- 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⁺104mgP29 national97%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2.5mgP10 national3%
- PotassiumK⁺1.4mgP17 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 feelCO₃²⁻25.4mgP77 national17%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻38.7mgP25 national13%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻11.0mgP23 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, 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.
- 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.