A gentle alkaline simple thermal spring, sampled at Gero Onsen's hot-water delivery pump station. It is lightly mineralized and easy on the body, yet the alkalinity gives it the slippery feel that places it in Japan's beauty-water family — helped along by about 60 mg/kg of metasilicic acid, which leaves a thin silica film and noticeably smoother skin. The posted analysis names 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)
- Marked physical debilitation, such as active tuberculosis, 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⁺110mgP32 national95%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4.3mgP16 national4%
- PotassiumK⁺1.9mgP22 national1.0%
- 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⁻105mgP43 national56%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻26.0mgP80 national17%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻37.5mgP23 national12%
- Fluoridetrace; drinking limits applyF⁻10.2mgP91 national10%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻12.7mgP26 national5%
- Hydrogen sulfideHS⁻0.70mgP62 national0.4%
- NitrateNO₃⁻0.20mgP42 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃59.7mgP36 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.1mgP41 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
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
- 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.
- 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.