An alkaline simple thermal spring, sampled at Gero Onsen's central delivery pump station rather than at a wellhead. It is alkaline enough to feel distinctly slippery and soft on the skin, which places it squarely in Japan's beauty-water family, while the water itself stays light — 374 mg/kg of dissolved solids, with nothing sharp or overwhelming in it. A solid 59.7 mg/kg of metasilicic acid leaves the kind of thin silica film that makes skin feel smoother afterwards. The certificate lists autonomic nervous system instability, insomnia and depressed mood among its spring-type indications, which is a fair description of what it is for: a long, unhurried soak rather than a bracing one.
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
From a certificate analyzed in 2018.
- Active stage of any illness (especially with fever)
- Active tuberculosis, advanced malignant tumor, severe anemia or other marked physical debilitation
- 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⁻105mgP42 national56%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻26.0mgP80 national17%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻37.5mgP24 national12%
- Fluoridetrace; drinking limits applyF⁻10.1mgP90 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 acidHAsO₂0.10mgP53 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)
- 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
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.